Influence of Processing Parameters on Microstructural Evolution and Tensile Properties for 7075 Al Alloy Prepared by an ECAPBased SIMA Process

A modified strain-induced melt activation(SIMA) process consisting of homogenization, equal-channel angular pressing(ECAP) and subsequent heating to the semisolid temperatures was introduced to prepare the 7075 aluminum alloy with superior thixotropic behaviors. The effects of both the homogenization and the number of ECAP passes, as well as the isothermal temperatures on the microstructural evolution, were investigated. The results indicate that ideal microstructure wherein fine and globular solid grains surrounded by uniform liquid films can be achieved through ECAP deformation–recrystallization mechanism. Increasing the number of ECAP passes accelerates the recrystallization of strained grains,thus reducing the average grain size and improving the grain sphericity. Moreover, higher holding temperatures and prolonged soaking time can improve the growth of the solid grains. Two main coarsening mechanisms, viz. coalescence and Ostwald ripening, contribute to the growth of the solid grains simultaneously and independently. The tensile strength of the 7075 alloys after four-pass ECAP-based SIMA and T6 heat treatment is relatively lower than the as-received billet,while the elongation of SIMA processed samples is much higher than that of as-received ones. Increasing the number of ECAP passes improves the tensile strength for alloys with and without T6 treatment due to the fine grain strengthening mechanism.     

Microstructure and corrosion behaviour of Mg–2Gd–1Y–1Zn–0·2Zr (at-%) alloy processed by equal channel angular pressing

Significant grain refinement was achieved in a new Mg–2Gd–1Y–1Zn–0·2Zr (at-%) alloy through multipass equal channel angular pressing (ECAP) at 623 K. Corrosion behaviours of the ECAPed alloy were investigated by hydrogen evolution and electrochemical measurement in NaCl solution at room temperature. The results showed that a large number of intergranular phases were stretched and gradually broken above four ECAP passes, but the fine grained α-Mg phase was much easier to grow after 12 ECAP passes for dynamic recrystallisation. The corrosion resistance of the ECAPed Mg alloy in a fine grained state considerably increases, compared with that in the as cast state. After four ECAP passes, the corrosion potential, the pitting potential and the resistance value achieved ?1·55 V, ?1·39 V and 2·08 KΩ respectively. However, excessive ECAP passes reduced the corrosion resistance of the fine grained Mg alloy, due to grain coarsening and the gradual loss of barrier effect of intergranular phases.     

What is behind the inverse Hall–Petch effect in nanocrystalline materials?

An inverse Hall–Petch effect has been observed for nanocrystalline materials by a large number of researchers. This effect implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation-based models, diffusion-based models, grain-boundary-shearing models and two-phase-based models. In this paper, we report an explanation for the inverse Hall–Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is dependent on strain rate and temperature and deviates from the Hall–Petch relationship below a critical grain size.     

Ultrafine-grained pure Ti recycled by equal channel angular pressing with high strength and good ductility

Machining chips of commercially pure Ti (ASTM grade 2) were consolidated into full density by equal channel angular pressing (ECAP) with an average grain size as low as 0.8 μm, yield strength up to 650 MPa, and ductility of ～16%. Effect of recycling condition on the microstructure and mechanical properties were investigated in terms of ECAP temperature, number of passes and chemical composition. Using electron backscatter diffraction it is evident that continuous dynamic recrystallization (however, which is purely a phenomenological terminology) plays a significant role in grain (with misorientation ≥15°) formation, whilst benefitting from high stacking fault energy, this continuous conversion of subgrain (<15°) into grain can be essentially considered as an extended recovery with a substantial presence of low angle grain boundaries in the recycled Ti. The Hall–Petch relationship is adapted to explain the strengthening of the recycled Ti. Additionally, using scanning electron microscopy fractography, the ductility was analyzed by a modified Griffith criterion. Last, superior energy efficiency of ECAP reduces environmental impact when comparing to conventional melting/casting. ECAP develops an innovative solid-state process for improving the recycling value of waste Ti.     

Multi billet extrusion technology for manufacturing bi-layered components

The development of the multi billet extrusion technology for manufacturing bi-layered aluminum based semi-products for heat radiators is the subject of this work. To obtain increased strength of the product, ultra fine grained ECAP billets were prepared and used during the extrusion. Appropriate deformation route to get desirable level of grain refinement and billet quality were identified by a series of experiments and numerical simulations performed with the implemented microstructure evolution model. Similar combination of experimental and numerical research was used within the paper for identification of dies geometry and process parameters during development of the multi billet extrusion technology.     

Inverse Hall–Petch behavior in diamantane stabilized bulk nanocrystalline aluminum

The mechanical behavior of bulk cryomilled aluminum stabilized by diamantane nanoparticles is compared with that of bulk cryomilled commercial purity (CP) aluminum with no diamantane. Bulk cryomilled aluminum was prepared by consolidating the cryomilled aluminum powders via hot isostatic pressing followed by trap extrusion (high strain extrusion) at room temperature. The results revealed that, while the grain size of cryomilled CP aluminum (155 nm) was larger than that of cryomilled aluminum containing diamantane (68–95 nm), the strength of the former is higher than of the latter. This finding is explained in terms of the occurrence of nanoscale softening, which signifies a transition from conventional Hall–Petch behavior (strengthening with decreasing grain size) to inverse Hall–Petch behavior (softening with decreasing grain size).     

��ϸ����̼�����������е�Hall��Petch��ϵ�о�

 研究了355MPa级超细晶低碳钢热连轧板拉伸性能和晶粒尺寸之间的关系，结果表明屈服强度σs与晶粒尺寸d成Hall Petch关系；经验证，对晶粒尺寸在3μm～10μm范围的低碳钢热连轧板，其屈服强度σs的计算值与测量值偏差较小，Hall Petch关系具有很好的适用性。     

等径弯曲通道变形制备超细晶铜的热稳定性

室温下采用等径弯曲通道变形(Equal Channel Angular Pressing,ECAP)C方式进行了纯铜(99.95%)12道次挤压变形。通过等温和等时退火,研究ECAP变形后铜的退火行为,并研究了等径弯曲通道变形和退火后纯铜的显微硬度和显微结构变化。分析了ECAP应变量、退火时间和退火温度对超细晶铜的再结晶行为、抗软化性能的影响。结果表明:ECAP变形后的超细晶铜在退火过程中,表现出不连续再结晶现象;ECAP降低了铜的热稳定性,变形道次越高再结晶温度越低。退火后稳态晶粒尺寸随变形道次的增加而细化,硬度值随变形道次的增加而增大,回归分析表明,晶粒尺寸与硬度之间的关系符合Hall-Petch公式。     

Influences of processing routine on mechanical properties and structures of 7075 aluminum alloy thick-plates

1　INTRODUCTIONAluminumalloy 70 75 ,havingsuperiormechani calpropertieswithahighspecificstrength ,isoneoftheattractiveengineeringmaterialsforapplicationinaircraftstructural partsandotherhighlystressedstructuralapplicationswhereveryhighstrengthandgoodresistancetocorrosionarerequired[1,2 ] .Itmayperform greatdifferentlywithdifferentworkprocess ing .Investigationsonthisalloyhavealwaysbeengo ingoninordertotapitslatentpowerofservice[37] .Alloying ,grainrefiningandsolution agingarethemainmethod…     

Analytic treatment of metallic multilayer strength at all length scales: Influence of dislocation sources

Metallic multilayers exhibit a very pronounced size effect where the mechanical strength depends on the layer thickness. Traditionally the Hall–Petch relation is used to account for the size effect. However, rigorous application of dislocation pileup theory predicts significant deviation from the Hall–Petch relation due to elastic inhomogeneity, discreteness of dislocations and dislocation source operation. Elastic inhomogeneity leads to anomalous scaling where the scaling exponent deviates from 1/2 of the classical Hall–Petch relation. The discrete dislocation effect is properly accounted for by a piecewise approach that can be applied at all length scales. In this article, a key step in the formulation is taken: the dislocation source characteristics are taken into consideration. Thus, all the three effects are accounted for. Analytic formulas linking yield stress to microscopic interface strength, dislocation source activation stress and other easily obtainable parameters (the Burgers vector, the elastic constants of constituent materials, crystal structure and layer thickness) are provided for all length scales. The model is then applied to Cu/Ni multilayers and the predicted strength is compared with experimental data.     

体外浸泡纯钛的表面腐蚀和ECAP的影响

采用在Ringer模拟体液静态体外浸泡的方法,研究工业纯钛试片经过一定时间的浸泡后的失重和表面形貌的变化。采用等通道径角挤压(ECAP)方法处理工业纯钛并与粗晶纯钛进行对比,对ECAP处理的TA9和粗晶TA9也做了对比。结果表明:纯钛在400℃ECAP处理后的微观组织形成具有明显方向性的板条状组织,ECAP纯钛表面沉积的Na Cl晶体数量大于粗晶Ti,TA9也表现出相似结果。纯钛的腐蚀机制是一种受到电偶腐蚀控制的均匀腐蚀,细晶组织导致电偶的数量增加。     

The Hall–Petch breakdown in nanocrystalline metals: A crossover to glass-like deformation

The deformation behavior of nanocrystalline Ni–W alloys is evaluated by nanoindentation techniques for grain sizes of 3–150 nm, spanning both the range of classical Hall–Petch behavior as well as the regime where deviations from the Hall–Petch trend are observed. The breakdown in strength scaling, observed at a grain size of 10–20 nm, is accompanied by a marked transition to inhomogeneous, glass-like flow (i.e. shear banding) at the finest grain sizes approaching the amorphous limit. As a consequence of this mechanistic crossover, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of deformation at approximately the same grain size as the onset of the Hall–Petch breakdown. These data experimentally connect the mechanical properties of nanocrystalline alloys to the well-known behavior of amorphous metals.     

Translations: BISITS—British Industrial and Scientific International Translation Service

This paper described the mechanical properties and corrosion behaviour of new designed Mg–Gd–Nd–Zn–Zr alloy processed by equal channel angular pressing (ECAP) at 375°C. An attractive phenomenon was observed. Both strength and ductility of ultrafine grained Mg–Gd–Nd–Zn–Zr alloy were improved after multipass ECAP. The microstructure of the alloys became much finer and more homogeneous with increasing ECAP passes. The yield strength, ultimate tensile strength and elongation of the alloys under eight-pass ECAP process were over 223?MPa, 270?MPa and 36% respectively, showing desirable mechanical properties of equal channel angular pressed Mg–Gd–Nd–Zn–Zr alloy. The equal channel angular pressed alloy displayed a lower corrosion resistance immersed in Hank's solution due to the crystalline defects as well as the galvanic corrosion induced by precipitation of ultrafine β phase particles.     

等径角挤压模具对超细晶材料显微组织的影响

论述了等径角挤压（ECAP）制备超细晶材料工艺的研究进展，介绍了ECAP的技术原理和工艺特点，着重分析了ECAP模具对ECAP细晶材料的显微组织的影响．优化ECAP模具结构。     

Strengthening an austenitic Fe–Mn steel using nanotwinned austenitic grains

An austenitic Fe–25Mn steel with a low stacking fault energy was subjected to dynamic plastic deformation (DPD) followed by thermal annealing. The as-DPD sample is structurally characterized by a mixed nanostructure consisting of nanosized grains with an average size of 43 nm and bundles of nanoscale twins (with an average twin/matrix lamella thickness of 5 nm), as well as some dislocation structures, which exhibits a high yield strength of about 1470 MPa but a limited tensile ductility. Thermal annealing leads to static recrystallization (SRX) of the nanostructures forming a hierarchical structure of nanotwinned grains embedded in microsized SRX grains, owing to the higher thermal stability of the nanotwinned bundles than that of nanosized grains. With an increasing number of SRX grains the yield strength and ultimate tensile strength drop while the tensile ductility increases. The calculated yield strength of the nanotwinned grains is about 1.5 GPa, much lower than that determined from Hall–Petch strengthening extrapolated to the nanoscale. Work hardening rates of the nanotwin grains, comparable with that of the microsized grains, are higher than that of the original coarse grained sample. The micrograined austenitic Fe–Mn samples strengthened by nanotwinned grains exhibit enhanced strength–ductility synergy in comparison with the deformed samples. A combination of a 977 MPa yield strength with a uniform elongation of 21% is achieved in the annealed samples, well above that of the deformed samples.     

The relationships of microstructure and properties of a fully lamellar TiAl alloy

The relationship between the yield strength and microstructure parameters of a fully lamellar TiAl alloy has been studied systematically. The grain size and the lamellar spacing were chosen as microstructure parameters. The experimental results showed that the yield strength increases with the decrease of grain size and more obviously with the decrease of the lamellar spacing. The relationship between yield strength and grain size and lamellar spacing can be approximately described by Hall–Petch relation.     

扩展路径低温ECAP及时效热处理对Cu0.6Cr合金的组织和性能影响

采用光学显微镜和X射线衍射仪对Cu0.6Cr合金经低温扩展路径等通道转角挤压（ECAP）后的组织演变规律进行了研究。采用扫描电子显微镜和能谱仪研究了Cu0.6Cr合金经不同时效热处理条件后的晶粒大小、析出相分布规律和断裂特征。并且分别测试了合金经低温ECAP和低温ECAP+时效热处理后的抗拉伸强度、硬度和导电率。结果表明,Cu0.6Cr合金经低温ECAP变形后形成明显细化且相互交割的纤维组织,并且合金在变形中始终保持（111）面的择优取向。时效热处理的合金变形量越大,析出相的数目和尺寸就越大,第二相析出速率也越快。5道次合金经450 ℃时效2 h后的抗拉伸强度为568.1 MPa,维氏硬度为1624.8 MPa,导电率为82%IACS。     

Grain refinement and hydrogen embrittlement in iron aluminide alloy FA129

Abstract

Iron aluminides are susceptible to hydrogen cold cracking during gas tungsten arc welding. Fine grained base materials have been shown to be more resistant to environmental embrittlement when tested in the presence of water vapour than coarse grained base materials. To study the effect of fusion zone grain size on cold cracking susceptibility, welds were produced using magnetic arc oscillation to refine the fusion zone grain structures. Tensile tests were conducted in varying water vapour atmospheres, on weldments with average fusion zone grain sizes ranging between 115 and 530 μm. Fracture strength data followed Hall–Petch behaviour and the effect of water vapour concentration was also incorporated into the traditional plot. The results of the tensile tests showed the finer grain size fusion zones were less susceptible to hydrogen cracking and more tolerant of high hydrogen concentration than coarse fusion zone grain structures.     

The effects of triple junctions and grain boundaries on hardness and Young’s modulus in nanostructured Ni–P

Hardness and Young’s modulus were measured on a series of nanocrystalline Ni–P samples. With decreasing grain size, a transition from regular to inverse Hall–Petch relationship and a reduction in Young’s modulus at the smallest grain sizes was observed, which can be attributed to grain boundary and triple junction effect.     

高层错能7003铝合金ECAP晶粒细化位错机制研究

在室温对完全时效7003铝合金进行4道次ECAP(Bc)挤压,成功获得大约200nm至几百纳米的超细晶粒。借助透射电镜(TEM)观察ECAP变形过程中的微观组织特征,如位错缠绕区(DTZ)、位错胞结构、孤立位错胞(IDC)、稠密位错墙(DDW)、胞块(CB)、显微带(MB)和S带等。7003铝合金在变形过程中的细化机制主要是位错分割机制。