Wear behaviour of Cr-Ni steels in H2SO4 solution

Bu çalışmada, Fe-0.4C-27Cr-5Ni ve Fe-0.4C-18Cr-9Ni dökme çeliklerin konsantrasyonu % 0-10 arasında değişen H2SO4 çözeltisi içerisindeki aşınma davranışları incelenmiştir. Aşınma deneyleri, ileri-geri aşınma deney cihazında, 275 gr sabit yük altında ve 26 mm/s deney hızında 10 mm çapındaki Al2O3 bilyenin numune yüzeylerine sürtünmesi ile gerçekleştirilmiştir. Deney sonrası numune yüzeyinde oluşan aşınma izleri, yüzey profilometresi yardımı ile incelenmiş ve deney sonuçları aşınma iz alanları ölçülerek mm2 cinsinden alan kaybına göre değerlendirilmiştir. Deney sonuçları, Fe-0.4C-27Cr-5Ni çeliğinin Fe-0.4C-18Cr-9Ni çeliğine nazaran daha fazla aşındığını göstermiştir. Aşınma deneylerinin yapıldığı çözeltinin konsantrasyonunun artması çeliklerin aşınma direncini ve sürtünme katsayısını azaltmıştır. Anahtar Kelimler :Cr-Ni çelikleri, aşınma, korozif aşınma, korozyon.In this study, wear performances of Fe-0.4C-27Cr-5Ni and Fe-0.4C-18Cr-9Ni steels were examined. Microstructural characterization of the as-cast steels was made by microscopic examinations and X-ray diffraction analysis. Mechanical properties of the steels were determined by hardness measurements and Charpy impact tests at room temperature. Wear tests were performed on a reciprocating wear tester in        %0-10 H2SO4 solutions. The tests were made under normal load of 275 g by rubbing a 10 mm diameter Al2O3 ball on the surface of the samples with a sliding velocity of 26 mm/s. After the wear tests, wear tracks developed on the worn surfaces were investigated by a profilometer and the results of the wear tests were quantified in the units of mm2 according to wear track area measurements. The matrixes of the Fe-0.4C-27Cr-5Ni and Fe-0.4C-18Cr-9Ni were identified as ferritic and austenitic, respectively. In the microstructures of both steels dominant carbide was Cr23C6. Fe-0.4C-27Cr-5Ni steel exhibited higher hardness and lower toughness than Fe-0.4C-18Cr-9Ni steel. In H2SO4 solutions, Fe-0.4C-27Cr-5Ni steel exhibited higher wear resistance than Fe-0.4C-18Cr-9Ni steel. Increase of concentration of H2SO4 solution resulted in decrease of wear resistance and friction coefficient of both steels.Keywords: Cr-Ni steels, wear, corrosive wear, corrosion

Fatigue behavior of Ti6Al7Nb titanium alloy

Titanyum ve alaşımları sahip oldukları üstün mekanik özellikleri, düşük yoğunlukları ve yüksek korozyon dirençleri sayesinde havacılık, otomotiv, kimya ve biyomedikal endüstrilerinde yaygın olarak kullanılmaktadırlar. Titanyum ve alaşımlarının yüksek sıcaklıkta oksijene olan yüksek afiniteleri kullanımları sınırlamaktadır. Titanyum ve alaşımlarının yüksek sıcaklıkta oksijene maruz tutulmaları yüzeyde oksit tabakasının ve hemen altında ise oksijen difüzyon bölgesinin oluşmasını sağlamaktadır. Bu işleme termal oksidasyon adı verilmektedir. Bu çalışmada aşınma ve korozyon direncinde önemli artışlar sağlayan termal oksidasyon işleminin Ti6Al7Nb alaşımının yorulma davranışı üzerine etkisinin incelenmesi amaçlanmıştır. Bu amaçla 600 C’de 60 saat tutulmasıyla gerçekleştirilen termal oksidasyon işlemi sonrasında numunelerin yüzey karakterizasyonu, kesit mikroyapısının incelemesi ve yüzey mikrosertlik değerinin ölçülmesi ile yapılmıştır. Numune yüzeylerinin mikrosertlik değerleri, termal oksidasyon öncesi ve sonrasında, numuneler çekme deneyine tabi tutularak, akma ve çekme dayanımı ile kopma uzaması ve kesit daralması değerleri belirlenmiştir. Dönel eğmeli yorulma deney düzeneği ile orijinal ve termal oksidasyon uygulanmış numuneler, 25 Hz frekansta farklı gerilme genliği değerlerinde yorulma deneylerine tabi tutularak S-N eğrileri elde edilmiştir. Dönel eğmeli yorulma deneyleri sonrasında, numunelerin kırık yüzeyleri stereo mikroskop ile incelenmiştir. Termal oksidasyon sonucunda Ti6Al7Nb alaşımının yüzeyinde ince bir oksit tabakası ve hemen altında oksijen difüzyon bölgesi oluşmuştur. Termal oksidasyon sonucu yüzey sertliğinin Ti6Al7Nb alaşımında % 170 gibi yüksek bir oranda arttığı belirlenmiştir. Termal oksidasyon işlemi sonrasında Ti6Al7Nb alaşımının mekanik özelliklerinde kayda değer bir değişiklik görülmemiştir. Termal oksidasyon sonrasında Ti6Al7Nb alaşımının yorulma dayanım sınırı düşmüştür. Anahtar Kelimeler: Oksidasyon, titanyum, yorulma.Musculoskeletal system diseases cost in the coun-tries around the globe a significant amount of money annually. The occurrence of bone fractures has also increased due to an increase of the number of traffic accidents and the increase of life expectancy almost twice as compared with past centuries. It is expected that one-third of European citizens will be soon over the age of 60. Hence, healthcare costs will be an increasing burden for society. Therefore, the need for economically feasible biomaterials for fracture healing will increase. For decades, researchers have focused on devoloping a viable and cost effective alternative materials for the dental applications instead of nickel-chromium and cobalt-chromium alloys which have been popular for the denture frameworks since 1970s. Usage of alloys containing nickel was limited due to doubts related with biological safety. Titanium, which eliminates persisting doubts as to biological safety of alloys containing nickel, was introduced in the 1970s. Titanium alloys are attractive materials for many engineering applications, which require excellent combination of mechanical properties, corrosion resistance and biocompatibility along with a low weight. Their excellent corrosion resistance and biocompatibility are assumed to be due to the formation of a dense and stable TiO2 layer, which rebuilds spontaneously after being damaged, even in solutions with low oxygen contents. Unfortunately, in applications where contacting motion of counterparts is maintained, titanium and its alloys have limited usage owing to their poor tribological performance. Poor wear resistances of titanium and its alloys can be enhanced by surface modification techniques. Among the surface modification techniques, thermal oxidation is one of the simplest and cheapest process and appears as it is very promising way to produce hard surfaces on titanium and its alloys. Surface modifications like surface roughening, oxidation or coating techniques also often improve the bioadhesion and the corrosion behaviors of titanium implants. Although many types of titanium alloys are commercially available, since when compared with commercially pure titanium Ti6Al4V alloys have superior physical and mechanical properties and almost same of nickel-chromium and cobalt-chromium alloys, alfa/beta alloy Ti6Al4V is the most commonly used in the biomedical applications. Sometimes usage of Ti6Al4V alloy can be problematic due to the toxicity of vanadium in the body. In order to eliminate this negative effect of vanadium, in the 1980s new vanadium-free alloy containing niboium Ti6Al7Nb was devoloped. The aim of this study was to examine the effect of thermal oxidation on rotating bending fatigue behavior Ti6Al7Nb alloy. Thermal oxidation was conducted at 600°C for 60 h in air. Characterization of the oxidized alloy was made by microstructure examinations, hardness measurements, tensile and fatigue tests. Micro-structure survey was conducted on the cross sections of the oxidized alloy by an optical light microscope after etching with 2% HF. Hardness measurements were made on the surfaces of the samples by a micro hardness tester in the unit of Vickers hardness under different indentation loads. Before and after thermal oxidation, tensile tests were performed in order to measure the yield and tensile strengths, percent elongation and reduction in area. Fatigue tests were performed under rotating bending condition at a frequency of 25 Hz. Fractured surfaces obtained from rotating bending fatigue tests were investigated utilizing a stereo microscope. In this study, the effect of thermal oxidation on the rotating bending fatigue behavior of Ti6Al7Nb alloy was investigated. Thermal oxidation, which was performed at 600 C for 60 h in an air furnace, resulted in the formation of 0.7 µm thick oxide layer with a 7 µm oxygen diffusion zone beneath it. Thermal oxidation con-siderably improved surface hardness to 929 HV0.1. Among the tensile properties only the yield strength was affected from the thermal oxidation. Yield strength of the examined alloy increased from 947 MPa to 1019 MPa, which corresponds to about 8 % improvement. Other tensile properties including tensile strength, elongation at fracture and reduction in area did not show any significant variation with thermal oxidation. Rotating bending endurance limit at 5x106 cycles decreased from about 560 MPa to about 500 MPa upon oxidation. The loss in fatigue resistance corresponds to 12%. Keywords: Fatigue, oxidation, titanium

The effect of SiC size on the wear behaviour of 60 vol % SiC-Al composites

Bu çalışmada, basınçlı infiltrasyon yöntemi ile üretilmiş olan aluminyum matrisli kompozitlerin eğme mukavemeti ile darbe ve aşınma direncine takviye SiC partikül boyutunun etkisi incelenmiştir. Kompozitlerin oda sıcaklığı mekanik özellikleri eğme ve darbe deneyleri ile belirlenmiştir. % 60 SiC içeren aluminyum matrisli kompozitlerin eğme mukavemeti ve darbe direnci SiC boyutu arttıkça azalmıştır. Kompozitlerin aşınma davranışları metal-metal aşınma deneyleri ile incelenmiştir. Metal-metal aşınma deneyleri kuru kayma koşullarında M2 kalite takım çeliği üzerinde yapılmıştır. Deneyler, numunelere 28 N yük uygulatarak gerçekleştirilmiştir. Aşınma deney sonuçları, iri SiC (37 mm) ile takviye edilmiş kompozitlerin, ince SiC (13 mm) partikülleri ile takviye edilen kompozitlere nazaran daha yüksek aşınma direnci gösterdiğini ortaya çıkarmıştır. Anahtar Kelimeler: Alüminyum, kompozit, SiC ve mekanik özellikler. In this study, the effect of the reinforcing particle size on bending strength, impact and wear resistance of SiC reinforced aluminum matrix composites produced by the pressure infiltration technique has been investigated. Room temperature mechanical properties of the composites were determined by three point bending and impact tests. Bending strength and impact resistance of Al-60 vol.% SiC composites decreased with increasing reinforcing SiC particle size. The wear behaviour of the composites were examined by metal-metal wear tests. Results of the wear tests were evaluated according to the weight loss of the composites. The metal-metal wear tests were performed on an unlubricated M2 quality tool steel disc. The tests were carried out by applying a normal load of 28 N to the composites. Wear tests conducted on M2 quality tool steel disc under dry sliding conditions revealed that composites reinforced with coarse SiC particles exhibit higher wear resistance than those with fine SiC particle. After the wear tests, worn surfaces of the composites were examined by a scanning electron microscope. Wear surfaces of the composites worn on M2 quality tool steel have a characteristic view of typical mild wear. Energy dispersive spectroscopy analysis revealed material transfer from M2 tool steel to the composite during testing.Keywords: Aluminum, composite, SiC and mechanical properties

Microstructure-mechanical property characterization of a line pipe steel containing 0.03 % Nb and 0.05 % V

Petrol ve doğal gaz taşımacılığında kullanılan borular, API (American Petroleum Institute) standartları doğrultusunda yüksek mukavemetli, tok ve yüksek kaynaklanabilirlik kabiliyetine sahip çeliklerden üretilmektedir. Bu çelikler, Yüksek Mukavemetli Düşük Alaşımlı (YMDA) çelikler olup, termomekanik haddeleme yöntemiyle, ilgili standartlardan daha üstün özelliklerde üretilmektedir. Petrol ve gaz boru hattı çeliğini üreten bütün çelik firmaları, proses parametrelerini kendi işletme şartlarına göre tespit ederek üretimlerini gerçekleştirmektedir. Bu çalışmada, Ar-Ge faaliyetleri kapsamında Ereğli Demir ve Çelik Fabrikaları T.A.Ş.’de (ERDEMİR) üretilen API X60 kalite boru hattı çeliğinin, mikroyapı-mekanik özellik ilişkisi incelenmiştir. Ferrit tane boyutu küçüldükçe; akma ve çekme mukavemeti ile sertliğin arttığı, uzamanın ise değişmediği görülmüştür.Anahtar Kelimeler: Mekanik özellikler, boru hattı çeliği, Yüksek Mukavemetli Düşük Alaşımlı (YMDA) çelikler, Hall-Petch bağıntısı, ferrit tane boyutu.Line pipes used in the oil and natural gas transportation are produced from high strength steels, which should exhibit high toughness with better weldability according to API (American Petroleum Institute) standards. These steels are typically classified as High Strength Low Alloy (HSLA) steels and produced controlling chemical composition and microstructure (i.e. thermomechanical rolling process). High strength is achieved by the small additions of carbide forming alloying elements to low carbon steel such as V, Nb and Ti. These microalloying elements retard the growth of the austenite grains and encourage development of fine-grained ferritic microstructure upon cooling. Steel producers first establish their processing parameters according to their mill conditions, and then realize commercial productions by taking into account the relevant standards. In this study, the relationship between microstructure and mechanical properties of API X60 quality line pipe steel produced in Eregli Iron and Steel Works Co. (ERDEMIR) under research activities have been investigated. The composition of the examined steel was 0.09 % C, 1.24 % Mn, 0.22 % Si, 0.06 % Al, 0.006 % S, 0.001 % P, 0.03 % Nb and 0.05% V. After continuous casting, 200 mm thick slabs were hot rolled to 8, 12, 15 and 17.5 mm thickness. It was seen that yield strength, tensile strength and hardness increased, however percent elongation did not change with decreasing grain size of ferrite.Keywords: Mechanical properties, line pipe steel, High Strength Low Alloy (HSLA) steels, Hall-Petch relationship, ferrite grain size

Effect of Hf addition on reciprocating wear performance of ZrN coatings

Bu çalışmada, ZrN ve (Zr, %12 Hf)N kaplamaların aşınma davranışları incelenmiştir. Kaplamalar Zr ve %21Hf alaşımlı Zr katotların kombinasyonu ile üretilmiştir. Üretilen kaplamaların aşınma deneyleri, karşılıklı kazımalı aşınma deney cihazında, 1.5 N sabit yük altında ve 0.02 mm/s deney hızında 10 mm çapındaki Al2O3 bilyenin numune yüzeylerine sürtünmesi ile gerçekleştirilmiştir. Deney sonrası numune yüzeyinde oluşan aşınma izleri, yüzey profilometresi yardımı ile incelenmiş ve deney sonuçları aşınma iz alanları ölçülerek mm2 cinsinden alan kaybına göre değerlendirilmiştir. Deney sonuçları, (Zr, %12 Hf)N kaplamanın ZrN kaplamaya göre daha az aşındığını ve Hf ilavesinin aşınmaya karşı direnci artırdığını  göstermiştir.Anahtar Kelimler: Kaplama, FBB, ZrN, Hf, aşınma.In this study, a commercial ZrN and new (Zr, Hf)N coatings by alloying ZrN with Hf were deposited on hardened AISI D2 quality cold work tool steel plates by arc PVD technique. The coatings were prepared using combination of two cathodes under the equvalent PVD process conditions. ZrN and (Zr, %12 Hf)N coatings were obtained by utilising two Zr cathodes and one Zr and one Zr+21% Hf cothodes, respectively. The characterisation of the coatings were made by Scanning Electron Microscopic (SEM) examinations, X-ray diffraction analyses (XRD), hardness and adhesion tests as well as roughness and thickness measurements. Hardness measurements were carried out ultramicrohardness tester with a Vickers pyramid indenter. The indentation procedure consisted of 60 steps, with a waiting period between consecutive steps of 1 s. During the tests, load (P)-indentation depth (h)-time (t) data were recorded. Hardness measurements were performed under constant load of 25 mN. In order to investigate adhesion properties of the ZrN and the (Zr, 12% Hf)N coatings, scratch tests were used. The coatings were scratched up to 100 N normal load by using 10 mm/min scratching speed and 100 N/min loading speed. The dry sliding wear behaviour of ZrN and (Zr,12%Hf)N were examined by a reciprocating wear tester in ambient atmoshper. Alloying of ZrN coating with 12% Hf did not change the hardness significantly, but achieved an improvement on adhesion strength and wear resistance.Keywords: Coating, PVD, ZrN, Hf, wear

The effect of RRA treatment on the mechanical properties of 7075 alloy

Bu çalışmada 0.85 mm kalınlığındaki 7075 kalite aluminyum alaşımı sacın, sertlik, mukavemet ve darbe direnci gibi mekanik özelliklerine, retrogresyon ve yeniden yaşlandırma ısıl işleminin etkisi incelenmiştir. Başlangıçta T6 temper durumunda olan alaşıma, 220°C’de 15 saniye – 60 dakika arasında retrogresyon uygulanmış, yeniden yaşlandırma 120°C’de 24 saat süreyle yapılmıştır. Retrogresyon ve yeniden yaşlandırma işlemi sonunda, sertlik önce artarak en yüksek değerine ulaşmış, daha uzun retrogresyon sürelerinde ise azalmıştır. Akma ve çekme mukavemeti ise retrogresyon süresine bağlı olarak azalırken, kopma uzaması ve darbe direncinde, retrogresyon süresine bağlı olarak sürekli bir artış görülmüştür. Sonuç olarak, retrogresyon ve yeniden yaşlandırma ısıl işlemiyle, belirli retrogresyon sürelerinde, T6 temper durumu seviyesine eşit ya da daha yüksek sertlik, mukavemet, süneklik ve tokluk elde edilmiştir.Anahtar Kelimeler: Aluminyum, retrogresyon ve yeniden yaşlandırma, çökelme sertleşmesi.In this study, the effect of retrogression and reageing heat treatment on mechanical properties such as hardness, strength and impact resistance of 0.85 mm thick 7075 quality aluminium alloy sheet has been investigated. The alloy, which was initially in T6 temper condition, was retrogressed at 220°C for various times between 15 seconds and 60 minutes, prior to reageing at 120°C for 24 hours. Hardness and electrical conductivity were monitored as an indicator of the microstructural changes during retrogression or reageing stages. Hardness was initially decreased giving a minimum as the retrogression time increases. Further retrogression times beyond increment in hardness resulted in a decrement of hardness again. Electrical conductivity was progressively increased as the retrogression time increased. Hardness of retrogressed and reaged samples initially increased as the retrogression time increased, exhibiting their maximum values, then finally decreased. Yield and tensile strength progressively decreased while elongation at fracture and impact resistance increased with increasing retrogression time. It is concluded that, equal or higher values of hardness, strength, ductility and toughness than those of T6 temper can be obtained with retrogression and reageing heat treatment. Improvement in hardness can be attributed to the increased number of precipitates in retrogressed and reaged microstructure. On the other hand, higher values of impact resistance are accompanied with overageing, which commences in longer retrogression time.Keywords: Aluminium, retrogression and reageing, precipitation hardening

Deformation behavior of dual-phase steels

Tez (Doktora)-- İTÜ Fen Bil. Enst., 1988.Bu çalışmada, "Ferrit + Martensit" mikroyapısı kazandırmak ama cıyla, A1-A3 sıcaklıkları arasındaki beş ayrı sıcaklıktan su verilen a- laşımsız düşük karbonlu (%0.074 C) çelik saçın oda sıcaklığındaki meka nik özellikleri ve deformasyon davranışları incelenmiştir. Çift faz mikroyapısına (ferrit + martensit) sahip olan ve su verme sıcaklığı arttıkça, martensit içeriklerinin de arttığı tesbit e- dilen alaşımsız %0.074 C'lu çeliklerin, mekanik özelliklerinin belirlen mesi amacıyla yapılan sertlik ve çekme deneylerinden, mikroyapıda bulu nan martensit miktarının artmasıyla, sertlik ve mukavemetin arttığı, sü- nekliğin ise azaldığı saptanmıştır. Bu ısıl işlem koşullarında optimum mekanik özellikler, mikroyapısında %17'den daha az oranlarda martensit içeren çift bazlı çeliklerden elde edilmiştir. Alaşımsız az karbonlu çelikten üretilen çift fazlı çelikle rin deformasyon davranışlarının belirlenmesi amacıyla, plastik defor masyon bölgesinde yapılan gerilme gevşemesi, deformasyon hızını değiş tirme ve yükleme-boşaltma deneylerinden, bu çeliklerin deformasyon dav ranışlarını kontrol eden iki bölgenin mevcut olduğu tesbit edilmiştir. Gerilme gevşemesi ve deformasyon hızını değiştirme deneyleri yardımıy la, bu bölgelerdeki etkin dislokasyon mekanizmalarının, dislokasyon ke sişmesi (I. Bölge) ve Peierls-Nobarro gerilme engellerinin aşılması (II. Bölge) olduğu saptanmıştır. Yükleme-boşaltma deneylerinden ise, plastik def ormasyonun, serbest dislokasyonların bağlanması sonucu alt yapıda mevcut dislokasyon engellerinin kabaşlamasma neden olduğu ve söz konusu engeller arasındaki mesafenin artan deformasyonla önceleri azalıp, sonra sabit kaldığı tesbit edilmiştir. Çekme deneyi esnasında, elastik def ormasyondan plastik defor- masyona geçerken sürekli akma gösteren, alaşımsız çift fazlı çelikle rin, gerilme gevşemesi deneylerinden sonra tekrar yüklenmesi, defor masyon hızını değiştirme deneylerinde deformasyon hızının aniden artı rılması ve yükleme-boşaltma deneylerinde yükün boşaltılıp derhal yeni den uygulanması sonucu, gerilme-birim şekil değiştirme eğrilerinde sü reksiz akma olayının ortaya çıktığı gözlenmiştir. Bu olayı açıklamak amacıyla yapılan incelemelerde, süreksiz akmanın, gerilme gevşemesi deneylerinde, gerilme gevşemesi esnasında serbest dislokasyon yoğunlu ğunun azalmasından, deformasyon, hızını değiştirme deneylerinde, defor masyon hızı artırıldığında serbest dislokasyon yoğunluğunun da artma sından ve yükleme-boşaltma deneylerinde, yükün boşaltılması sırasında serbest dislokasyon yoğunluğunun azalmasından kaynaklandığı sonuçları na varılmıştır. Ayrıca gerilme gevşemesi ve deformasyon hızını değiş tirme deneyleriyle, çift fazlı çeliklerin oda sıcaklığında, gerilme altında deformasyon yaşlanmasına uğradıkları da saptanmıştır. Dual-phase steels are a new class of low carbon high strength sheet steels characterized by a micros tructure consisting of a disper sion of about 20 percent of hard martens ite particles in a soft ducti le ferrite matrix. The term "dual-phase" refers to the presence of essentially two phases, ferrite and martensite, although small amounts of bainite, pearlite or retained austenite may also be present in the micros tructure. These steels have a number of unique properties, which include; continuous yielding, i.e. no sharp yield point, and a relati vely low yield-to-tensile stress ratio together with a rapid rate of work hardening and high elongations which gives excellent formability compared to traditional high strength steels at the same strength level. As the automotive industry shows a strong tendency to use of high strength sheet steels in order to reduce automotive weight and thus improve its fuel economy, dual-phase steels found weight reduction applications in this area. The simplest way of achieving dual-phase micros tructure from low carbon (the overall carbon content required is 0"1 percent or lower) f erritic-pearlitic steels (plain carbon or High Strength Low Alloy Steels) is to use intercritical annealing, in which the steel is heated to the "ferrite + austenite" field between A^ and A3 and held for several minutes to form small regions of austenite. Depending on the hardenability of austenite grains and/or cooling rate, marten- site forms in the microstructure on cooling. The transformation of the austenite phase into martensite in dual-phase steels occurs at low tempuratures, so that, the ferrite must plastically deform to ac comodate the volume expension (^2 to ^4 percent) arising from austeni te to martensite transformation. As a result, both high mobile dislo cation density and residual stresses are generated in the ferrite grains. The continuous yielding and low yield stress behaviour of dual-phase steels is a consequence of this mobile dislocations and residual stresses surrounding the martensite particles. The excellent combination of strength and ductility displayed by dual-phase steels results from its composite microstructure which is a mixture of ferrite and martensite. To obtain, appropriate micro- structure; cooling rate and temperature of intercritical annealing are critical factors. Depending on cooling rate, microstructure might be a lot complex, due to the formation of retained austenite, bainite, pearlite and carbides besides martensite. The temperature of intercritical annealing is of great importance, as it will deter mine the amount of martensite in the microstructure. The increase in martensite volume fraction increases strength but decreases ductility of dual-phase steels. Optimum mechanical properties can be obtained if the martensite content is less than 20 percent. The features of ferrite affects the properties of these steels, as well. For a given martensite volume fraction, optimum strength/ductility combination can be obtained if ferrite grains are small, polygonal and clean. Another microstructural constituent which is believed to improve ductility of dual-phase steels is retained austenite. A mild rate of cooling after intercritical annealing causes to increase the amount of retained austenite which may be as high as 10 percent in some dual phase steels. During tensile deformation, most of the retained austenite transforms to martensite with a few percent of plastic strain and it is claimed that this transformation improves the uniform ductility. The automobile parts produced from steel sheets, such as outher body panels, require high resistance to denting besides excellent for- mability. The denting resistance is a function of sheet thickness and the yield strength of the formed part. In dual-phase steels, high rate of work hardening results rapid increase in yield strength after a few percent plastic deformation,, On the other hand, it is possible to increase the yield strength of prestrained dual-phase steels (espe cially water quenched and tempered ones) by baking at low temperatures carried out after painting operation, due to strain aging. The increase in yield strength after prestraining and aging at low tempera tures (^200°C) is primarily related to the total carbon content of the ferrite, but interaction of carbon with microalloying elements may also be important. The decrease in martensite volume fraction and/or ferrite grain size accelerates the rate of strain aging. Because of their superior mechanical properties than those of similar steels with f errite-pearlite micros true tur e, work hardening behaviour of dual-phase steels became very attractive» In studies made on the work hardening behaviour of these steels the stress-strain curves separated into three regions according to Crussuard-Jauol analy sis and the changes in the substructure at each region are examined with various microscopes. In the first region of deformation, ferrite matrix homogenously deforms by the movement of mobile dislocations surrounding the martensite particles and rapid rate of work hardening is present because of the elemination of residual stresses introduced by martensitic transformation. Second region, covers a region on stress-strain curve where work hardening rate of ferrite is reduced as the plastic flow of the ferrite is constrained by the hard undefor- ming martensite particles and by the effect of "transformation of retained austenite to martensite. In this region, ferrite matrix, builts up a dislocation density higher than that for pure ferrite phase, and, generally cell structure begins to form near martensite particles. Finally, in the third region well developed cell structure forms in the ferrite matrix but the cell size is smaller near marten site than away from martensite. Further deformation does not change the sizes of the cells and causes yielding of the martensite particles. The plastic deformation behaviour of metals can also be inter- prated with mechanical tests which are called "indirect methods"in this study. The two common indirect methods are; stress relaxation and strain rate change tests. In both of these tests, Taylor-Orowan equation is utilized fundementally and deformation parameters are vi measured. Another indirect method is loading-uploading test which is not as popular as stress relaxation and strain rate change tests and based on different concept, but can be successively used in defiermitiinp the effect of def°rmati-on. on substructures. These indirect methods are briefly described below. I. Stress Relaxation Test: Stress relaxation is a time depen dent decrease in stress in a body, which is constrained to a certain fixed deformation at a constant tempurature. The decrease in stress is the result of dislocation moving, to overcome localized barriers,, The major assumption necessary to analyse stress relaxation, which have been widely used to obtain fundemental parameters of microdeformation modes in metals, such as internal and effective stress, activation volume and dislocation velocity stress exponent, is that the mobile dislocation density (pm) is unchanged during the test» But it is sho wed that, in some metals this is not the case and mobile dislocation density changes with time, during stress relaxation. If relaxed stress- logarithmic relaxation time graph plotted, to determine the value of activation volume, is linear with a single slop, one can claim that, density of mobile dislocations is constant. The simplest method that takes into account the change in mobile dislocation density with rela xation time, is the determination of sequential relaxation curves from the referance stress for a fixed relaxation time. The decreas in the amount of relaxed stress with relaxation cycle indicates that, mobile dislocation density is decreasing during stress relaxation» The decrease in density of mobile dislocations might cause changes in the macroscopic flow stress of the metal in a way of discontinuous yielding (which is known as transient yielding) by reloading after stress rela xation. II. Strain Rate Change Test : The strain rate applied to a specimen has an important influence on the flow stress of metals. During tensile or compression tests, performed at a constant tempera ture, it is possible to determine the deformation parameters such as strain rate sensitivity exponent, internal and effective stress and activation volume, by changing the strain rate suddenly (strain rate change test) and measuring instantaneous change in the stress. At low tempuratures, a change in the density of mobile dislocation during strain rate change test might be very possibly occur. One might expect to observe transient yield point due to an increase in the dislocation density when the strain rate is increased. However, there are some other explanations in the litarature on this type of yielding. III. Loading-Unloading Test : In the mathematical treatment of elastisity, certain assumptions are made, one of which is that the state of the system has time to follow the load variation. However, even in the region below the propartional limit, metals are not per fectly elastic, since this would require coincidence of the curves for loading and unloading. The important point is that the direct relationship between stress and strain is disturbed and a hysterisis loop occurs on the stress-strain diagram. Loading-unloading cycle performed at the plastic deformation region also modifies the stress- strain diagram. That is, if the tensile specimen is stressed beyond vii the macroscopic yield stress value, then unloaded and reloaded without appreciable delay (this kind of interrupted tensile test is named as "loading-unloading test" in this study), strain enegry is dissipated in the f.öım.of heat produced by internal friction. The loss of strain energy causes formation of a hysterisis loop in the plastic deformation region of the stress-strain curve. The area of the hysterisis loop is a measure of the internal friction. The important source of internal friction is the reverse motion and/or reverse curvature of dislocation during unloading. By relo ading after unloading, in the course of plastic deformation, a transient yield point has been occured an the flow stress of some metals. The majority of mechanisms about this kind of yield points are based on the immobilization of mobile dislocations during unlo ading, but some researchers proposed different models. The purpose of the present investigation was to study the plastic deformation behaviour of as-quenched dual-phase steels by utilizing indirect methods. It should be mentioned that, according to the our litarature survey, up to date, indirect methods (stress relaxation, strain rate change and loading-unloading tests) have not been applied to dual-phase steels, yet. The material used in this investigation was temper rolled, about 1.0 mm thick plain carbon sheet steels with a composition of 0.074 %C, 0.41 %Mn, 0.03 %Si, 0.071 %Cu, 0.073 %A1, 0.026 %P and 0.021 %S. Tensile test specimens were machined from as-received sheet, according to ASTM E8 standart. To eliminate the effect of temper rolling and provide normalized starting microstructure, all specimens were annealed 1 hr at 960°C and cooled in air. Normali zed specimens were then intercritically annealed at five different tempuratures between 740-780°C for 9,Q;s,in salt bath and water quenched. By this heat treatment, f erritic-martensitic microstruc ture with five different martensite volume fractions were obtained. The mechanical properties of as-quenced dual-phase steels which contain different amounts of martensite, were determined with tensile test at an initial strain rate of 1.3x10"^ l/s. To interprete the plastic deformation behaviour of these steels, stress relaxation, strain rate change and loading-unloading tests were applied at various plastic strains until necking. For stress relaxation, selected initial strain rate was 1.3x10-4 i/s. strain rate change test were performed by decreasing the strain rate from 1.3x10"^ l/s to 1.3xl0~5 l/s or by increasing the strain rate from 1.3xl0-5 l/s to 1.3x10-4 i/s. in lo ading-unloading tests, the utilized initial strain rate was also 1.3xl0"4 l/s and unloading was done until all the stress on the speci men has been removed. All of these test were carried out with Instron Universal Testing machine at room tempurature, using extansometer. At each plastic strain, transient yield effect was observed by reloading after stress relaxation, increasing the strain rate and reloading after unloading in these as-quenched dual-phase steels. These trainsient yield effects were preciously determined by utilizing the "Ten Step Zero Load Suppresion" unit of the Instron Universal Testing Machine. It should be emphasized that, no attempt was made to improve the mecha nical properties of these steels. viii The following results were obtained from the experiments, 1) The mar tens ite content of dual-phase steels, produced from plain carbon (0.074 %C) sheet steels by intercritiçal annealing and water quenching, increases with annealing tempurature parabol icly. Increase in martensite volume fraction causes linear increase of yield and tensile strengths and parabolic decrease of uniform and total elongations of as-quenched dual-phase steels. For this production method, optimum mechanical properties of as-quenched dual-phase steels can be obtained if martensite volume fraction is less than 17 percent. 2) It is possible to seperate the stress-strain curves of as- quenched dual-phase steels into three regions according to Crussuard- Jauol analysis, which is used to determine the deformation behaviour of dual-phase steels, from the view point of changes in the substruc ture. Increase in martensite volume fraction, makes the seperation of I. and II. regions difficult, but in general, causes a decrease in the critical transition strains from region I to II and region II to III. 3) According to stress relaxation, strain rate change and loa ding-unloading tests, the deformation behaviour of as-quenched dual- phase steel is controlled by two regions. The strain ranges which correspond to these two regions depend on the martensite content of the dual-phase steels. In the first region, dislocation interaction and in the second region, overcoming Peierls-Nobarro stress barrier are the effective dislocation mechanisms. The coarsening of disloca tion obstacles occur during plastic deformation by binding of mobile dislocations. The distance between these obstacles decreases and then stays constant with increasing deformation. The effect of de formation on the distance between dislocation obstacles is in accord with the change in cell size with strain. 4) Reloading after stress relaxation or increasing the rate of straining suddenly, during strain rate change test or reloading after removing all of the stress on the specimen during tensile test cause the occurance of transient yield effect on a small region of stress- strain curves of dual-phase steels, which exhibit continuous yielding during transition from elastic to plastic deformation. Ip all these tests, observed transient yield condition, exhibit three characteristic stages: a yield point with increment in stress (Aay ı) f followed by a yield drop (AOyyd.) and/or a plateau and finally the return to the ori jinal trajectory o 5) For each test, the value of increment in stress (Aay £), even if different in magnitude, generally increase with plastic strain and become constant when a stable cell structure is formed. The in crease of martensite content of dual-phase steels, decreases the va lue of strain at which Aay £ becomes constant. On the other hand, yield drop (Aoyyd)' is not observed until a critical plastic strain (Ep c) is reached. The effect of martensite volume fraction on the ix value of ep c depends on the type of testing method. That is, for stress relaxation and strain rate change tests, ep>c increases, but for loading-unloading test, epjC decreases with increasing martensite content of dual-phase steels. 6) Transient yield points which arise from reloading after stress relaxation performed at plastic deformation region are, due to the decrease of mobile dislocation density during the test. Pinning of mobile dislocations by solute atoms is the main mechanism. Thus, it is concluded that as-quenched dual-phase steels are exposed to strain aging under stress, at room tempurature. 7) The reason of transient yield point observed in strain rate change test is, pinning of dislocations and effective dislocation sour ces by solute atoms at low strain rates and the sudden increase in dis location density to compensate to high rates of strain when the strain rate is increased. 8) Transient yield point, occured by reloading after unloading to zero stress level in the plastic deformation region of as-quenched dual-phase steels, is attributed to the decrease of mobile dislocation density during unloading. The decrease in the density of mobile dislo cations is due to the interaction of mobile dislocations with other dislocations and/or dislocation obstacles. Strain aging phenomenan is not effective for such transient yielding which arises from reloa ding after unloading.DoktoraPh.D

Characterization and In Vitro Bioactivity of Calcium and Phosphorous Containing Titania Layer on Ti6Al4V Alloy

Calcium and phosphorous containing titania layers on Ti6Al4V biomedical alloy were formed by micro arc oxidation (MAO) in an electrolyte containing calcium acetate and sodium phosphate, and then subjected to hydrothermal treatment (HT) in order to achieve improved biocompability with modified titania layer. Samples were hydrothermally treated in water solution whose pH was adjusted to 11.0-11.5 by adding NaOH, at 230 °C for 10 h and cooled in the autoclave. Surface morphology, microstructure, and phase composition of titania layer were investigated systematically before and after HT. Their biomimetic apatite inducing ability in a simulated body fluid (SBF) was investigated. The bioactivity tests of modified MAO surface on Ti6Al4V alloy showed a considerable improvement compared to the unmodified MAO surfac