超塑性・ナノクリスタルの疲労

金沢大学工学部本研究課題では,室温において超塑性変形をしめす超微細結晶材料の疲労変形機構を明らかにすることを最終的な目標として,(1)単結晶材料,(2)Equal Channel Angular Extrusion(ECAE)加工された超微細結晶粒銅,および(3)EACE加工されたPb-62%SnとZn2-2%Al合金の繰返し応力-ひずみ応答を調査した.銅,Fe-Cr,Fe-Cr-Niなどの単結晶や三重結晶を用いて結晶性材料の繰返し応答の基礎的な特徴を調査した.単結晶材料では特定の塑性ひずみ振幅域において固執すべり帯(PSB)が形成され,その形成にともなってバウシンガ効果が大きく変化することが明らかになった.また三重結晶を用いた研究では,結晶間の残留応力がバウシンガ効果の原因の一つであることを実験的に示した.ナノクリスタルの繰返し変形の基礎的側面を調査するために行った超微細結晶粒銅の繰返し変形試験では以下のようなことが明らかになった.ECAE加工された銅の応力振幅は250MPaにも到達し,バウシンガ効果も非常に高かった.しかしながら,微細結晶粒銅のそれらの特性は200℃程度の熱処理で劇的に変化した.ECAE加工されたPb-SnおよびZn-Al合金では,室温においても大きな伸びと流動応力の大きなひずみ速度依存性が一方向変形において観察された.繰返し変形試験では,初期段階から応力振幅は飽和状態に達した.Zn-Al合金では飽和応力振幅およびバウシンガ効果はひずみ速度が増加するにつれて増加した.一方,一方向変形おいて流動応力のひずみ速度依存性が特に大きかったPb-Sn合金では逆に応力振幅およびバウシンガ効果にひずみ速度依存性はほとんど見られず,一方向変形とは異なる傾向を示した.応力振幅およびバウシンガ効果の繰返し変形でみられたひずみ速度依存性は,Pb-Snでは超塑性変形に必要な熱活性化過程が作用しないこと,Zn-Alでは転位密度が増加することからそれぞれ理解することは可能であった.In order to investigate fatigue mechanisms of ultrafine grained materials showing superplasticity at room temperature, we have examined cyclic stress-strain response of (1) single crystal specimens, (2) ultrafine grained copper produced by "Equal Channel Angular Extrusion (ECAE)" method, and (3) Pb-62%Sn and Zn-22%AI alloy fine-grained by the ECAE method.Fundamental aspects of the cyclic response in crystalline materials were investigated in single- and tr-crystal specimens of copper, Fe-Cr alloy and Fe-Ni-Cr alloy. The single crystal experiments revealed that fatigue deformation gives rise to formations of "persistent slip bands" (PSBs) at a certain range of plastic strain amplitude. The formation of the PSBs induced a drastic changes in Bauschinger energy parameter. In the fatigue tests on the tricrystal specimens, it was experimentally shown that residual stresses between adjoining grains is one-of sources for the Bauschinger effect in the crystalline materials.in the cyclic deformation tests on ultrafine grained copper to characterize basic feature of fatigued nano-crystals, following results were obtained. The stress amplitude of the copper fined-grained by the ECAE method became up to 250 MPa. The Bauschinger effect was also significantly high. However, this kind of excellent property of the ultrafine grained copper was very sensitive to heat treatment : both the Stress amplitude and the Bauschinger effect were apparently reduced by an annealing at 200ﾟC.In the monotonic tensile tests, both the Pb-Sn and Zn-Al specimens showed large elongations even at room temperature. These two materials also showed significant strain-rate dependence of flow stress. In the cyclic deformation tests, the stress amplitudes in the Pb-Sn and Zn-Al alloys were almost saturated from the beginning of the experiments, although a cyclic hardening curve of the annealed copper was divided clearly into the hardening and saturation stages. The stress amplitude and the Bauschinger effect in the Zn-Al alloy apparently increased with increasing strain rate. On the other hand, the Pb-Sn alloy revealed no strain rate dependence of the stress amplitude and the Bauschinger effect. This tendency obtained in the cyclically deformed Pb-Sn alloy is completely different from the result in the monotonic- test. (The cyclic response of the annealed copper was -. independent of the strain rate as well as the yield stress under monotonic deformation.) The absence of the strain-rate dependence in the cyclically deformed Pb-Sn alloy can be attributed to the missing of thermally activated processes which are dominant in the monotonic deformation : the applied plastic strain was probably accommodated substantially by dislocation motion because the plastic strain amplitude is very low with compared to the monotonic deformation. On the other hand, the cyclic deformation in the Zn-Al alloy may involve the thermally activated processes such as dislocation climb and grain boundary diffusion. It is feasible that the plastic strain can not be accommodated only by the dislocation motion because the grain size of the Zn-Al was very small.研究課題/領域番号:09650715, 研究期間(年度):1997 – 1998出典：研究課題「超塑性・ナノクリスタルの疲労 」課題番号09650715（KAKEN：科学研究費助成事業データベース（国立情報学研究所）） （https://kaken.nii.ac.jp/ja/report/KAKENHI-PROJECT-09650715/096507151998kenkyu_seika_hokoku_gaiyo/）を加工して作

A New Method of Low Amplitude Signal Detection and Its Application in Acoustic Emission

A novel methodology is proposed to enhance the reliability of detection of low amplitude transients in a noisy time series. Such time series often arise in a wide range of practical situations where different sensors are used for condition monitoring of mechanical systems, integrity assessment of industrial facilities and/or microseismicity studies. In all these cases, the early and reliable detection of possible damage is of paramount importance and is practically limited by detectability of transient signals on the background of random noise. The proposed triggering algorithm is based on a logarithmic derivative of the power spectral density function. It was tested on the synthetic data, which mimics the actual ultrasonic acoustic emission signal recorded continuously with different signal-to-noise ratios (SNR). Considerable advantages of the proposed method over established fixed amplitude threshold and STA/LTA (Short Time Average / Long Time Average) techniques are demonstrated in comparative tests.publishedVersion© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

Phenomenological approach towards modelling the acoustic emission due to plastic deformation in metals

Considering acoustic emission (AE) as a phenomenon reflecting the elastic energy dissipation process in deforming metals, a simple yet self-consistent model is proposed to account for the AE behaviour accompanying microstructure evolution during uniform plastic deformation of metals. The relationship between the AE power, mobile dislocation density and plastic flow characteristic parameters - the strain hardening rate and flow stress - is derived and experimentally verified for face centred cubic metals with different stacking fault energies (SFE). Despite its simplicity, the proposed purely phenomenological relation captures most of the salient features of the AE behaviour of early deformation stages in metals.acceptedVersion© 2019. This is the authors’ accepted and refereed manuscript to the article. Locked until 16.6.2021 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

Novel method for in situ damage monitoring during ultrasonic fatigue testing by the advanced acoustic emission technique

Ultrasonic fatigue testing (USFT) is an effective method for the rapid characterisation of the high cycle fatigue properties of structural materials. However, the process of initiation and progression of fatigue damage remains uncertain in this way of testing due to the limitations of existing measuring techniques. The acoustic emission (AE) method was developed in the present work to pave a new way to monitor the fatigue process during USFT. The proposed new methodology revealed the AE activity related to fatigue damage, allowing to distinguish between surface and internal fatigue crack initiation and to follow the development of fatigue damage.acceptedVersio

Structure, Texture and Strength of Mg-5.8Zn-0.65Zr Alloy after Hot-to-Warm Multi-Step Isothermal Forging and Isothermal Rolling to Large Strains

Aiming at improving deformability of wrought Mg alloys, the commercial hot-pressed alloy MA14 (ZK60) was subjected to hot/warm severe plastic deformation (SPD) to a total true strain of 9. The processing schedule involved a combination of two-stage multi-step isothermal forging performed at 400 and 300 °C and subsequent isothermal rolling at 300 °C. The evolution of microstructure, texture and mechanical properties were investigated. SPD resulted in a highly homogeneous microstructure with the size of equiaxed grain (subgrain) reduced down ~ 2.5 µm (1.5 µm). The strong initial fiber texture was transformed into the weak pyramidal texture during forging operation and then to the typical basal texture during further rolling. Changes in the distribution of excess phases are also reported. In response to microstructural changes induced by SPD, the ductility of the alloy enhanced substantially without compromising the strength. Unlike conventional semi-products having a high anisotropy of mechanical properties, the proposed processing method resulted in the isotropic yield stress and elongation to failure. The nature of grain refinement and the structure-property relations emphasizing the effects of the grain structure and texture on the tensile strength and its anisotropy are discussed.acceptedVersion© 2017. This is the authors’ accepted and refereed manuscript to the article. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

Kinetics of cyclically-induced mechanical twinning in γ-TiAl unveiled by a combination of acoustic emission, neutron diffraction and electron microscopy

The cyclic response and the microstructure evolution of the near γ-TiAl alloy are investigated by a blend of contemporary experimental techniques centred around in-situ acoustic emission (AE) measurements reflecting the cyclically-induced structural changes in the real-time scale. TEM and SEM – ECCI/EBSD examinations provided an adequate qualitative description of microstructural features associated with populations of dislocations and mechanical twins evolving concurrently in the course of cyclic deformation. Since the TEM offers only local and post-mortem information, the in-situ neutron diffraction technique scanning a large part of the gauge length was employed to characterise the lattice strain distributions with cycling. The volume fraction of twins as a function of loading cycles was obtained after loading or unloading half-cycles. The processes controlling the cyclic strain hardening during each deformation cycle were assessed by (i) the statistical analysis of the shape of the hysteresis loop aiming at the characterisation of distribution internal stress barriers for deformation mechanisms involved and (ii) the spectral and statistical analysis of AE data providing information on the kinetics of these mechanisms. Each of the used experimental methods brings its own set of advantages and limitations in terms of characterisation and interpretation. Their unique combination ensures a host of benefits that promote a comprehensive understanding of primary deformation mechanisms - deformation twinning, dislocation slip and detwinning. It is shown that the cyclic mechanical behaviour of the TiAl alloy can be comprehensively explained by the interplay between these mechanisms co-operating during each loading cycle. This interplay governs the behaviour of underlying mechanisms in the early stages of the fatigue damage evolution and likely determines the overall fatigue response of near γ-TiAl alloy at room temperature.acceptedVersio

Fractographic features of technically pure magnesium, AZ31 and ZK60 alloys subjected to stress corrosion cracking

Using quantitative fractography, the present study clarifies the effect of chemical composition and preliminary plastic deformation on the fracture mode of structural magnesium alloys experienced stress corrosion cracking (SCC). It is demonstrated that the fracture surfaces of alloys ZK60 and AZ31 failed during slow strain rate testing (SSRT) under SCC conditions are commonly featured by (i) the close-to-side surface region of typically brittle fracture initiation composing of intergranular and cleavage facets, which gradually transforms to (ii) the region of transgranular fluted facets optionally followed by the regions of (iii) fluted facets with secondary cracks, (iv) flat dimpled rapture and (v) slant dimpled rapture. Counterintuitively, the preliminary plastic strain introduced in air results in the increase of the fraction of the ductile mode on the fracture surface, elongation and stress at fracture of both alloys SSRT tested in corrosive media. The positive effect of pre-straining can be likely explained from the mechanistic viewpoint due to the increase of the yield stress, which has to be overcome to trigger plastic deformation needed to break the surface protective film. Features of the SCC mechanisms affecting the fracture surface appearance are quantified and discussed.acceptedVersion"© 2019. This is the authors’ accepted and refereed manuscript to the article. Locked until 28.11.2021 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Inhibiting stress corrosion cracking by removing corrosion products from the Mg-Zn-Zr alloy pre-exposed to corrosion solutions

Magnesium and its alloys are susceptible to stress-corrosion cracking (SCC), which can manifest itself during the slow-strain rate tensile (SSRT) testing in air if the specimens were pre-exposed to corrosive media. This phenomenon is generally associated with hydrogen embrittlement (HE) which is believed to be related to diffusible hydrogen penetrating into the metal during the pre-exposure. In the present study, we show that the corrosion product layer deposited on the surface of the pre-exposed specimens is crucial in the SCC mechanism. The specimens of the alloy ZK60 were SSRT tested in air, in corrosive media, in air after pre-exposure to corrosive media as well as after pre-exposure and removal of corrosion products. To vary the severity of SCC, four NaCl-based corrosion solutions were utilised. The embrittlement resulted in a marked decrease in ductility and the concurrent appearance of multiple side-surface cracks as well as brittle fragments on the fracture surface. The most striking finding of the present study is that the removal of corrosion products from the surface of the pre-exposed specimens results in complete recovery of the mechanical response and in the elimination of all harmful embrittling features regardless of the corrosive solution used. This effect is found to be inconsistent with the common viewpoint where the SCC mechanism is governed entirely by diffusible hydrogen. The obtained results shed new light on the role of diffusible hydrogen, corrosion products and irreversible corrosion damage in the SCC mechanism of Mg alloys pre-exposed to corrosive media.acceptedVersio

Effect of grain size on mechanical properties and hydrogen occluding capacity of pure magnesium and alloy MA14 subjected to stress-corrosion cracking

The high susceptibility of magnesium alloys to stress corrosion cracking (SCC) is an urgent problem that restricts their widespread application in the aerospace and automotive industries, as well as in medicine. The development of scientifically based ways towards increasing the resistance of magnesium alloys to SCC requires a deep understanding of the physical nature of this phenomenon. Such an understanding has not been achieved as yet. In particular, the contradicting results have been reported on the role of hydrogen in the SCC mechanism. According to the most common viewpoint, the driving force of the SCC is diffusible hydrogen, which dissolves in a metal in an atomic form. The concentration of diffusible hydrogen, and therefore its significance in SCC, should strongly depend on grain size. In the present study, the specimens of technically pure magnesium and the MA14 alloy having different grain sizes were tested in tension at a low strain rate in air as well as in corrosive media to assess their susceptibility to SCC. Immediately after the end of the test, the concentration of hydrogen in the fractured specimens was analyzed by hot extraction in a carrier gas flux method. It is found that the concentration of hydrogen in the MA14 alloy specimens, from which corrosion products were removed before the gas analysis, does not depend on the grain size. In the case of pure magnesium with removed corrosion products, a higher concentration of hydrogen was found in the coarse grained specimens. It is shown that after removing corrosion products, hydrogen extraction from all the investigated materials commences at the temperature of 300°C that indicates an insignificant concentration of diffusible hydrogen in the base metal. It is concluded that the occlusion of diffusible hydrogen in the specimens of pure magnesium and the MA14 alloy during SCC is limited and, therefore, diffusible hydrogen does not play the key role in the SCC mechanism of these materials.publishedVersio

Effect of deformation processing of the dilute Mg-1Zn-0.2Ca alloy on the mechanical properties and corrosion rate in a simulated body fluid

Magnesium and its alloys have several competitive advantages due to their low density and the highest specific strength among modern structural metallic materials. However, the relatively low ductility and poor corrosion resistance hinder their broader use in industry. Unlike many engineering applications, the ability of magnesium alloys to dissolve in chlorine-containing media is attractive for their applications as temporary implants. In the present work, the influence of thermomechanical processing on mechanical properties and corrosion resistance of the alloy Mg-1Zn-0.2Ca intended for biomedical applications is investigated. The low content of alloying elements permitted grain boundary hardening to be realised to a large extent during deformation processing. Severe plastic deformation through the multi-axis isothermal forging at relatively high homological temperatures in combination with isothermal rolling gave rise to the significantly refined to the micrometre scale homogeneous microstructure with an excellent balance of the tensile strength and ductility (the yield stress and ultimate tensile strength are in excess of 210 and 260 MPa, respectively, and the elongation at break is over 20 %) and corrosion resistance in the simulated body fluid (SBF) in vitro. With the pH value of the SBF maintained at 7.4 throughout the test, the corrosion rate assessed by the hydrogen evolution and gravimetric methods was found to be nearly constant without signatures of saturation for the deformation-processed specimens. The rate of hydrogen desorption of 0.5 ml / cm2 / day was found to be far below the amount that could be accommodated by the human body without adverse effects.publishedVersio