Maximisation of the ratio of microhardness to the Young's modulus of Ti-12Mo-13Nb alloy through microstructure changes

Alloys for orthopaedic and dentistry applications require high mechanical strength and a low Young's modulus to avoid stress shielding. Metastable β titanium alloys appear to fulfil these requirements. This study investigated the correlation of phases precipitated in a Ti-12Mo-13Nb alloy with changes in hardness and the Young's modulus. The alloy was produced by arc melting under an argon atmosphere, after which, it was heat treated and cold forged. Two different routes of heat treatment were employed. Phase transformations were studied by employing X-ray diffraction and transmission electron microscopy. Property characterisation was based on Vickers microhardness tests and Young's modulus measurements. The highest ratio of microhardness to the Young's modulus was obtained using thermomechanical treatment, which consists of heating at 1000 C for 24 h, water quenching, cold forging to reduce 80% of the area, and ageing at 500 C for 24 h, where the final microstructure consisted of an α phase dispersed in a β matrix. The α phase appeared in two different forms: as fine lamellas (with 240 ± 100 nm length) and massive particles of 200-500 nm size. © 2013 Elsevier B.V.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Observations on dynamic strain aging manifestation in inconel 718 superalloy

The manifestation of dynamic strain aging (DSA) in Inconel 718 is reported in this work. Analysis were performed in the material with different microstructures resulting from solution anneal and aging treatment. Tensile tests were made under secondary vacuum with temperature ranging between 200 and 950°C and strain rates of 3.2 x 10-3 to 3.2 x 10-5 s-1. Results showed the range of DSA occurrence. Analysis indicates that at lower temperatures, from approximately 200 to 450°C, serrations are controlled by the diffusion of carbon. At higher temperatures, until 800°C, DSA coincided with the occurrence of other thermally activated phenomena: dynamic precipitation, especially γ’’, and Oxidation Assisted Intergranular Cracking (OAIC). It was observed that competitive phenomena affect DSA manifestation directly due to the availability of niobium in solid solution.SCOPUS: cp.kinfo:eu-repo/semantics/publishe

The Effect of d Phase on the Mechanical Properties of an Inconel 718 Superalloy

The relationship between the δ phase content and the mechanical properties of Inconel 718 superalloy is still uncertain in the scientific literature. This study investigated the effects of the amount of δ phase and of the grain size on the mechanical properties of an aged γ-matrix with γ′ and γ′″ precipitates. The material in as-received condition in the form of a forged bar was solution-treated in different conditions and aged according to UNS7718 standard. The microstructures were characterized using optical, scanning, and transmission electron microscopy. Hardness and tensile tests were also conducted. After solution treatment, γ′ and γ′Prime; phases are dissolved and δ phase volume fraction is reduced to a minimum amount only observed by TEM, resulting in an increase of the grain size and a decrease of hardness and strength. After aging, the precipitation of γ′ and γ′″ occurs and the amount of δ phase increases. The volume fraction of d phase varying from 0.30 to 1.38% and the grain size varying from ASTM 7 to 5 do not have a significant effect on the tensile properties and hardness. © ASM International.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Oxidation assisted intergranular cracking under loading at dynamic strain aging temperatures in Inconel 718 superalloy

It is well established that 718 superalloy exhibits brittle intergranular cracking when deformed under tension at temperatures above 600°C. This embrittlement effect is related with grain boundary penetration by oxygen (Oxygen Assisted Intergranular Cracking - OAIC). Simultaneously, impacting on its mechanical properties, the precipitation of coherent γ′ and γ″ phases occur above 650°C and Dynamic Strain Aging (DSA) occurs in the temperature range between 200 and 800°C. Although literature indicates that OAIC is the mechanism that controls mechanical properties at high temperatures, its interactions with DSA and precipitation are still under discussion. The objective of this work is to investigate the interactions between the embrittlement phenomena (OAIC and DSA) and the hardening mechanism of γ′ and γ″ precipitation on the mechanical properties of an annealed 718 superalloy. Tensile tests were performed at a strain rate of 3.2 × 10-4 s-1 under secondary vacuum, in temperatures ranging from 200 to 800°C. Fracture surfaces were observed by scanning electron microscopy (SEM) and precipitation by transmission electron microscopy (TEM). The effect of DSA and precipitation on the strength and of OAIC on the ductility was verified.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Work hardening and aging contribution on the mechanical properties of X-750 nickel-based superalloy

The influence of work hardening and aging on the mechanical properties of the X-750 nickel-based superalloy was investigated. The degree of cold working by wire drawing ranges from 0% to 30% and the aging heat treatment was performed at 1005. K for 16. h. The results have shown that the yield strength is proportional to the degree of area reduction, but the increase of yield strength becomes less important after aging. The static recovery plays an important role on the final properties of the alloy, reducing the contribution of γ' precipitates during aging. As a consequence of this recovery there is an increase of the ductility for the samples with area reduction superior to 20%. Empirical equations were proposed correlating the yield strength with the degree of cold working. © 2012 Elsevier B.V.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Mechanical characterization of Ti-12Mo-13Nb alloy for biomedical application hot swaged and aged

Beta titanium alloys were developed for biomedical applications due to the combination of its mechanical properties including low elasticity modulus, high strength, fatigue resistance, good ductility and with excellent corrosion resistance. With this perspective a metastable beta titanium alloy Ti-12Mo-13Nb was developed with the replacement of both vanadium and aluminum from the traditional alloy Ti-6Al-4V. This paper presents the microstructure, mechanical properties of the Ti-12Mo-13Nb hot swaged and aged at 500 °C for 24 h under high vacuum and then water quenched. The alloy structure was characterized by X-ray diffraction and transmission electron microscopy. Tensile tests were carried out at room temperature. The results show a microstructure consisting of a fine dispersed α phase in a β matrix and good mechanical properties including low elastic modulus. The results indicate that Ti-12Mo-13Nb alloy can be a promising alternative for biomedical application.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Control of the microhardness to young modulus ratio by mechanical processing of a Ti-10Mo-20Nb alloy

β-metastable titanium alloys with high strength to Young's modulus ratio have been developed by different authors aiming their use as biomaterials for hard tissue replacement. However, it is not easy to combine low elastic modulus with high mechanical strength. In this work, a β-Ti alloy (Ti-10Mo-20Nb) was produced by arc-melting, then homogenized, cold swaged and aged in order to obtain fine α-Ti precipitates in a β-Ti matrix. The microstructures were characterized by transmission electron microscopy and X-ray diffraction. Mechanical properties characterization was based on Vickers microhardness tests and Young's modulus measurements. The cold swaged material subjected to an ageing treatment at 500 °C for 4 h showed the highest hardness/Young's modulus ratio, associated to very fine α phase precipitates in a β-Ti matrix.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Influence of the Chemical Composition on the Phase Stability and Mechanical Properties of Biomedical Ti-Nb-Mo-Zr Alloys

A new generation of titanium alloys with non-toxic, non-allergenic elements and lower Young’s modulus (YM) have been developed, presenting modulus values close to that of bone. In titanium alloys, the value of the Young’s modulus is strongly dependent on the chemical composition. Young’s modulus also depends on the present phases and on the crystallographic texture related to the thermomechanical processing. A lower YM is normally attributed to the formation of the α″ phase into the β matrix, but there is no consensus for this assumption. In the present work, four alloys were designed and melted, based on the Ti-Nb-Mo-Zr system and heat-treated to favor the formation of the β phase. The alloys were produced by arc melting under argon atmosphere and heat-treated at 1000 °C for 24 h under high vacuum, being subsequently quenched in water to room temperature. Alloys were then characterized by optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Young’s modulus was determined by the impulse excitation technique and Vickers microhardness. The purpose of the study was to define an optimal chemical composition for the further production on a semi-industrial scale of a new Ti-Nb-Mo-Zr alloy for orthopedic implant manufacturing. The results showed that all of the four studied alloys are potential candidates for biomedical applications. Among them, the Ti-24Nb-4Mo-6Zr alloy has the lowest Young’s modulus and the highest microhardness. So, this alloy presents the highest HV/YM ratio, which is a key indicator in order to evaluate the mechanical performance of metallic biomaterials for orthopedic implants.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Effect of hot swaging on microstructure and properties of aged Ti-10Mo-20Nb alloy

Mechanical properties of metastable β-Ti alloys are highly dependent on the final microstructure, which is controlled by the thermomechanical processing. These alloys are used for biomedical applications and require a high mechanical strength as well as a low Young’s modulus to avoid stress shielding. Previous work on the development of cold swaged Ti-10Mo-20Nb alloy showed that the best compromise strength and Young’s modulus was obtained when the forming is followed by an aging heat treatment at 500 °C. In this work, Ti-10Mo-20Nb alloy was hot swaged and aged at 500 °C for 10 min, 4h and 24h. The microstructure was analyzed by X-ray diffraction, optical microscopy and transmission electron microscopy. Mechanical characterization was based on Vickers microhardness tests and Young’s modulus measurements. Aging at 500°C for 10 min after hot swaging resulted in a nearly 100% β phase microstructure while aging at 500°C for 4h or 24h led to a bimodal microstructure consisting on α precipitates dispersed in the β matrix. The higher hardness to Young’s modulus ratio was obtained for the sample aged at 500 °C for 4h. This value was higher than those obtained for the Ti-6Al-4V alloy and commercially pure Ti.SCOPUS: cp.kinfo:eu-repo/semantics/publishe

Influence of Nb Addition on α″ and ω Phase Stability and on Mechanical Properties in the Ti-12Mo-xNb Stoichiometric System

Metastable β-Ti alloys have become one of the most attractive implant materials due to their high biocorrosion resistance, biocompatibility, and mechanical properties, including lower Young’s modulus values. Mechanical properties of these alloys are strongly dependent on the final microstructure, which is controlled by thermomechanical treatment processing, in particular the Young’s modulus and hardness. The aim of this work was to analyze the influence of phase precipitations in heat-treated Ti-12Mo-xNb (x = 0, 3, 8, 13, 17, and 20) alloys. The alloys were prepared via arc melting and treated at 950 °C/1 h, and then quenched in water. The microstructures were analyzed by optical microscopy, transmission electron microscopy, and X-ray diffraction. Mechanical properties were based on Vickers microhardness tests and Young’s modulus measurements. Microstructural characterization showed that α″ and ω stability is a function of Nb content for the Ti–12Mo base alloy. Nb addition resulted in the suppression of the α″ phase and decrease in the ω phase volume fraction. Although the ω phase decreased with higher Nb contents, ω particles with ellipsoidal morphology were still observed in the Ti–12Mo–20Nb alloy. The α″ phase suppression by Nb addition caused a marked increase in the Young’s modulus, which decreased back to lower values with higher Nb concentrations. On other hand, the decrease in the ω phase continuously reduced alloy hardness. The study of the effect of chemical composition in controlling the volume fraction of these phases is an important step for the development of β-Ti alloys with functional properties.SCOPUS: ar.jinfo:eu-repo/semantics/publishe