Microstructural evolution and mechanical properties of Cu–Al alloys subjected to equal channel angular pressing

Ultrafine-grained (UFG) or nanocrystalline (NC) Cu–Al alloys were prepared using equal-channel angular pressing (ECAP) to investigate the influence of stacking fault energy (SFE) on the microstructural evolution during deformation and the corresponding mechanical properties. The grain refinement mechanism was gradually transformed from dislocation subdivision to twin fragmentation by tailoring the SFE of alloys. Meanwhile, homogeneous microstructures and nanoscale grains were readily achieved in the low-SFE Cu–Al alloys and the equilibrium grain size was decreased by lowering the SFE. Moreover, in the Cu–Al alloy with extremely low SFE, shear fracture occurred during ECAP at strain levels higher than two due to the formation of macroscopic shear bands. In addition, the normalized deformation conditions at large strain were qualitatively discussed. More significantly, the strength and uniform elongation were simultaneously improved by lowering the SFE. This simultaneity results from the formation of profuse deformation twins and microscale shear bands, and their extensive intersections.     

Correlation between stacking fault energy and deformation microstructure in high-interstitial-alloyed austenitic steels

The correlation between stacking fault energy (SFE) and deformation microstructure of high-interstitial-alloyed austenitic Fe–18Cr–10Mn–(N or N + C) alloys was investigated. As the content of the interstitial elements increased, the deformation microstructure changed in a sequence strain-induced martensitic transformation, mixture of martensite and twin, and finally deformation twin. The SFE, playing an important role in the transition of deformation microstructure, was evaluated by the Rietveld whole-profile fitting combined with the double-Voigt size–strain analysis for neutron diffraction profiles of tensile-strained bulk samples. At fixed N + C content, the ratio of mean-squared strain to stacking fault probability remained constant regardless of the accumulated strain, whereas the ratio gradually increased with increasing N + C content. Almost linear dependence of measured SFE on N + C content could be established. According to the SFE, deformation bands exhibited distinct substructures, and their particular intersecting behavior resulted in the formation of different types of products (secondary ε martensite, α′ martensite and secondary twin) at the intersecting regions.     

Some observations on deformation-related discontinuous precipitation in an Al-14.6at.%Zn alloy

The discontinuous precipitation (DP) in a supersaturated Al-14.6at.%Zn alloy in relation to different forms of deformation structures has been investigated with optical and electron microscopy. It has been found that intense surface scribing, followed by short-term ageing at 65 °C, resulted in a recrystallized duplex structure with nanoscale equiaxed β-Zn particles and α-Al grains. In the absence of recrystallization and shear banding, moderate surface grinding increased the transformation kinetics of DP on the alloy surface by an order of magnitude compared with that of the undeformed counterpart. The enhanced transformation kinetics is attributed to intragranular nucleation and growth of DP colonies associated plausibly with dislocation cell wall structures induced by the surface strain. In contrast, bulk deformation by means of cold-rolling (13-66% reduction) and in situ stress-ageing (∼1% strain) both suppressed the development of DP in the alloy. The role of deformation bands as nucleation sites of DP and the driving force determining the development of DP colonies in deformed matrices are discussed.     

In situ AFM observation of crack propagation in CuNiAl shape memory alloy

The nucleation and propagation behaviors of cracks in CuNiAl shape memory alloy were observed using in situ technique under atom force microscope (AFM). The results indicated that, in CuNiAl shape memory alloy, the martensitic transformation first occurred at crack tip upon loading, and then the microcrack nucleated and propagated along martensite interface. The slip bands appeared only after considerable crack propagation. After that, the crack usually propagated alternately along martensite interface or slip bands.     

Crystallographic analysis of the FCC → BCC martensitic transformation in high-carbon steel

Based on the phenomenological crystallographic theory of martensitic transformations, the following crystallographic characteristics of the tetragonal martensite in high-carbon steel have been calculated: orientation relationships between the crystal lattices of the bct martensite and fcc austenite; the magnitude and direction of the macroscopic shear; the habit plane; the angle and the axis of rotation of the crystal lattice of the martensite. The calculation was performed for three variants of lattice deformation: Bain deformation; two-shear Kurdjumov-Sachs deformation; and the deformation we suggested upon the analysis of the fcc-bcc transformation. In the last variant, a minimum rotation of the crystal lattice of martensite is required; consequently, this variant is closest to the real mechanism of the martensitic transformation. An expression has been derived that describes the interrelation between the degree of tetragonality of the crystal lattice of martensite and the magnitude of the shear deformation of the lattice. It has been shown that the 12 crystal-lographically equivalent variants of shear upon the formation of the lattice of the tetragonal martensite form three groups in each of which the martensite has the same tetragonality axis. For each variant of the shear, we have two equivalent variants of deformation of the martensite with invariant lattice. This results in 24 variants of orientation relationships.     

Strain-induced martensitic transformation in stainless steels: A three-dimensional phase-field study

A three-dimensional elastoplastic phase-field model is developed to study the microstructure evolution during strain-induced martensitic transformation in stainless steels under different stress states. The model also incorporates linear isotropic strain hardening. The input simulation data is acquired from different sources, such as CALPHAD, ab initio calculations and experimental measurements. The results indicate that certain stress states, namely uniaxial tensile, biaxial compressive and shear strain loadings, lead to single variant formation in the entire grain, whereas others, such as uniaxial compressive, biaxial tensile and triaxial strain loadings, lead to multivariant microstructure formation. The effects of stress states, strain rate as well as temperature on the mechanical behavior of steels are also studied. The material exhibits different yield stresses and hardening behavior under different stress states. The equivalent stress is higher at low strain rate, whereas a higher elongation is obtained at high strain rate. The deformation temperature mainly affects the hardening behavior of the material as well as the transformation, i.e. martensite volume fraction decreases with increasing temperature. Some of the typical characteristics of strain-induced martensite, such as the formation of thin elongated martensite laths, shear band formation and nucleation of martensite in highly plasticized areas, as well as at shear band intersections, are also observed.     

Cu—Ni—Al形状记忆合金形变、相变以及裂纹形核的相互关系

采用金相显微镜和扫描电镜原位拉伸技术研究了Cu-Ni-Al形状记忆合金中的马氏体形变、马氏体相变以及微裂纹形核三者之间的关系。结果表明,拉伸时缺口前方首先形成可逆变的马氏体而不出现滑移带,微裂纹沿马氏体和母相界面形核,只有当裂纹扩展－段距离且应力集中足够大后,才有可能出现形变带,此时微裂纹将沿形变带和马氏体界面交替形核。     

热轧双相钢塑性变形和断裂行为的研究

文章利用SEM、TEM研究了热轧双相钢单向拉伸过程中不同变形量下的形变位错结构和断口组织特征。结果表明,双相钢中铁素体的塑性变形是由位错增殖、位错运动和交互作用、滑移带形成、位错缠结产生及其密度的增加、位错胞出现及细化等一系列演变构成;变形10%时,板条马氏体已产生塑性变形,其板条中出现形变带,并且随马氏体变形的继续,形变带不断形成和发展。变形颈缩时,双相钢中的微孔主要通过相界面结合力的丧失和马氏体断裂产生,而马氏体断裂主要出现在马氏体的尖角处。     

用原子力显微镜研究形状记忆合金中裂纹形核和扩展

运用原子力显微镜（AFM）对形状记忆合金CuNiAl中裂纹扩展行为进行了观察。观察表明、拉伸时、裂纹尖端首先产生马氏体。纳米微裂纹择优沿马氏体相界面形核。但当外力较大，且裂纹扩展距离较长使裂尖应力集中足够大时，不但能诱发马氏体相变。也可出现滑移带。一旦出现滑移带，裂纹也可沿马氏体或滑移带交替扩展。     

Moderation of the recrystallization texture by nucleation at copper-type shear bands in Al-1Mg

The critical dependency of copper type shear band formation on deformation variables in Al-1Mg has been described (Acta mater., 49 (2001) 2739). The influence of copper type shear bands on recrystallization behaviour during post-deformation annealing in Al-1Mg is now discussed. Local orientation measurements and orientation images of grains nucleating at shear bands have been made using the electron back-scatter diffraction (EBSD) technique. Bulk recrystallization texture is investigated from X-ray diffraction (XRD) measurements. Shear bands are shown to be potent nucleation sites during annealing and a weak, almost random, texture associated with nucleation at shear bands has been inferred. Most significantly, shear band formation is found to have a pronounced moderating influence on the strength of the potentially dominant cube component of the recrystallization texture.     

等径弯曲通道制备的超细晶铜的疲劳性能

研究了等径弯曲通道(ECAP)变形后的超细晶T3铜在恒应力幅控制条件下的疲劳寿命和循环形变行为.通过扫描电镜观察了疲劳试样表面的滑移带,并利用电子背散射技术观察了疲劳前、后晶粒尺寸的变化.结果表明,超细晶T3铜具有较高的疲劳极限(σ-1=153 Mpa),是粗晶铜疲劳极限的2倍.在低周疲劳域内表现出疲劳软化,而在高周疲劳域内表现比较稳定的疲劳行为,甚至出现疲劳硬化.类似驻留滑移带(PSB)的剪切带与最后一次挤压的剪切面一致,剪切带的形成和晶界滑移是疲劳裂纹形核和疲劳断裂的主要原因.     

Nucleation kinetics of the α→γM massive transformation in a Ti-47.5 at.% Al alloy

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements and quantitative stereological analysis of microstructures, coupled with calculations based on the classical theory of nucleation, were performed to study the nucleation kinetics of the α→γ_M massive transformation in a Ti-47.5 at.% Al alloy. Using previously determined thermodynamic data on the reaction temperature, time, undercooling and driving force for the massive transformation, the free energies for critical nucleus formation and the nucleation rates were computed for different nucleus shape models, including incoherent grain face, edge and corner nuclei and faceted grain face nuclei, and compared with experimentally determined nucleation rates. The results indicate that nucleation of singly faceted grain face nuclei and incoherent grain corner and grain edge nuclei are highly likely. Good agreement was also obtained between the calculated and experimental nucleation rates at these various sites. Based on these results, possible nucleation mechanisms during the massive transformation are discussed in light of current thinking on the nature of this transformation.     

Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys

During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M_s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions.     

两相区轧制后低碳中锰钢中残留奥氏体在单向拉伸过程中的演变行为

通过对轧制后低碳中锰钢薄板在不同预拉伸变形量下的组织进行了分析,并研究了单向拉伸过程中残留奥氏体组织演变的规律。试验结果表明,残留奥氏体向马氏体的相变主要发生在局部塑性变形阶段即屈服阶段,屈服阶段结束后,组织中的残留奥氏体基本全部转变为马氏体,试样进入均匀塑性变形阶段。拉伸过程中位错在局部的大量堆积和逐步迁移,在宏观上表现为吕德斯带的产生和移动,导致局部区域组织中的残留奥氏体发生相变,并且这种现象大多是沿着轧制方向进行的,并不会重复发生。     

Nucleation of phase transformations at intragranular inclusions in steel

The control of phase transformation nucleation in steel by inclusions has attracted considerable attention for a few decades. The potency as a transformation catalyst appears to depend on the species of ~nclusions and so, they may not simply act as an inert heterogeneous nucleation site. The formation of a solute depletion zone in surrounding austenite and the provision of nucleation sites which can form low energy nucleuslinclusion interfaces are considered to be a principal mechanism. These are discussed on the basis of simple models and available experimental data to relate nucleation potency with thermodynamic and other material properties of inclusions.     

马氏体在位错偶上形核长大的分子动力学模拟研究

应用分子动力学方法和ＮｉＡｌ合金的嵌入原子势,模拟研究了位错偶上马氏体形核长大的过程和微观机理。计算结果表明,马氏体形核的位置与位错应力场的分布有关 。     

车用铝合金在剪切应力状态下变形及损伤机理研究

采用Iosipescu和双缺口拉伸实验,对车用铝合金(5052)在剪切和准剪切应力条件下的大变形和损伤机制进行研究。断口分析和有限元三向应力度计算表明纯剪切条件下材料主要以剪切滑移带变形并萌生微裂纹,基本不产生孔洞损伤。而准剪切条件下材料损伤存在有微孔洞和剪切带混合并发机制。高三向应力度区首先出现微孔洞裂纹起始,但随着损伤向剪切应力区过渡,三向应力度不断降低,孔洞的长大聚合受到抑止,转而出现剪切滑移带中的平行微裂纹,材料最终主要以剪切滑移失效。     

Influences of cyclic loading on martensite transformation of TRIP steels

While austenite transformation into martensite induces increasing of the crack initiation life and restraining of the growth of fatigue cracks in cyclic-loading processes, TRIP-assisted steels have a better fatigue life than the AHSS (Advance High Strength Steels). As two key parameters in the cyclic loading process, strain amplitude and cyclic frequency are used in a kinetic transformation model to reasonably evaluate the phase transformation from austenite into martensite with the shear-band intersections theory, in which strain amplitude and cyclic frequency are related to the rate of shear-band intersection formation and the driving force of phase transformation. The results revealed that the martensite volume fraction increased and the rate of phase transformation decrease while the number of cycles increased, and the martensite volume fraction was almost constant after the number of cycles was more than 2000 times. Higher strain amplitude promotes martensite transformation and higher cyclic frequency impedes phase transformation, which are interpreted by temperature increment, the driving force of phase transformation and the rate of shearband intersection formation.     

铜晶体的疲劳损伤微观机制

对Cu单晶体、双晶体和多晶体疲劳损伤微观机制的总结结果表明：在中、低应变范围Cu单晶体的疲劳裂纹主要沿驻留滑移带萌生,而在高应变范围则沿粗大形变带萌生;Cu双晶体中疲劳裂纹总是优先沿大角度晶界萌生和扩展,而小角度晶界则不萌生疲劳裂纹;对于Cu多晶体,疲劳裂纹主要沿大角度晶界萌生,有时也沿驻留滑移带萌生,而孪晶界面两侧由于滑移系具有相容的变形特征而未观察到疲劳裂纹萌生．     

Effect of crystallographic orientation and grain boundary character on fatigue cracking behaviors of coaxial copper bicrystals

Four coaxial copper bicrystals were employed to study the slip morphologies and fatigue cracking behaviors during cyclic deformation. Three of them had high-angle grain boundaries (GBs) with nearly the same misorientation and one bicrystal had a twin boundary (TB). Different slip bands (SBs) operated near the GBs and TB, generating different dislocation arrangements, which are mainly determined by the crystallographic orientations of the component grains. The GBs suffered impingement or shear damage caused by slip difference from both sides. It is suggested that there is an energy increase in the interfaces between matrix and persistent slip bands (PSBs), GBs and TBs per cycle during cyclic deformation due to the accumulation of lattice defects, which would make the interface unstable. After a certain number of cycles, fatigue cracks initiated firstly at GBs for some bicrystals while fatigue cracking occurred preferentially at PSBs for the others. It is confirmed that the energy growth rate is an increasing function of the shear stress, strain amplitude and strain incompatibility, which results from slip differences on both sides of the interfaces. Interfaces with different energies and strain incompatibilities have different fatigue cracking resistance. It is found that GBs with defective and complex structure, and hence high interfacial energy accompanied by high modulus of the residual GB dislocation (GBD), are preferential sites for fatigue cracking, while the fatigue cracking appeared predominantly at PSBs when the modulus of the residual GBD is lower than that of a perfect dislocation with simple GB structure and low interfacial energy. The present model for the energy can predict well which kind of interface would form cracks preferentially during cyclic deformation in one coaxial bicrystal and which GB would need more cycles to initiate fatigue cracking between coaxial bicrystals with different GB characters.