Deformation mechanisms of pure Ti during equal channel angular pressing

The microstructural development of commercially pure titanium was investigated to elucidate the mechanisms of grain refinement and strain accommodation during equal channel angular pressing. The samples were processed at 623 K via route C, in which the sample was rotated 180° around its longitudinal axis between the passes. TEM micrographs of the sample undergoing the first pass revealed that the strain imposed by the pressing is accommodated mainly by {1011} deformation twinning. During the second pass, the deformation mechanism was changed to dislocation slip. TEM analysis indicated that the slip system consisted of alternating twin bands containing dislocations ofa slip on a prismatic plane and ofa+c slip on a pyramidal plane. Microstructural evolution in commercially pure titanium subjected to equal channel angular pressing was discussed based on the preferred orientation formed during the first pass and resolved shear stress for the slip systems.     

Role of strain path change in grain refinement by severe plastic deformation: A case study of equal channel angular extrusion

Equal-channel angular extrusion (ECAE) provides exciting opportunities to explore the role of strain path change (SPC) in grain refinement by severe plastic deformation (SPD). In this study, crystal plasticity simulations were carried out using a viscoplastic self-consistent model for a face-centered cubic model material processed via an extended range of processing routes and with two die angles (90° and 120°). Each processing route was defined according to the interpass billet rotation angle (χ), which varied from 0° to 180° at intervals of 15°. Based on a statistical analysis of the simulated slip activities, it is proposed that differences in grain refinement among these cases can be best correlated to key differences in the slip activities, i.e. the significance of newly activated slip systems at pass-to-pass transitions corresponding to macroscopic SPCs. Accordingly, grain refinement is anticipated to be most efficient for routes with χ near 75° for the 90° die or 0–45° for the 120° die, and least efficient with χ near 180° for both dies. The relative grain refinement efficiencies thus predicted are in good overall agreement with those indicated by the generation of high-angle boundaries and reduction of grain size in pure copper measured by electron back-scatter diffraction. It is suggested that the effect of SPC and the resulting characteristic slip activities should be incorporated in understanding the effectiveness of grain refinement and unpinning the underlying grain subdivision mechanisms in SPD with different SPCs.     

Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy

By means of surface mechanical attrition treatment, nanometer-sized grains (with an average size of 30 ± 5 nm) were generated in the surface layer of a single-phase AZ91D alloy. Transmission electron microscopy investigations showed that the strain-induced grain refinement process in AZ91D alloy includes three steps. At the initial stage twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets. With increasing strain, double twins and stacking faults form and a number of dislocation slip systems are activated, including basal plane systems, prismatic plane systems and pyramidal plane systems. As a result of the dislocation slip along these systems and of the cross slips, high-density dislocation arrays are formed which further subdivide the twin platelets into subgrains. Obvious evidence of dynamic recrystallization were identified within the high-strain-energy subgrains with a further increase of strain, leading to the formation of nano-sized grains in the surface layer.     

Twin-Induced Plasticity of an ECAP-Processed TWIP Steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.     

等通道挤压变形奥氏体不锈钢中孪晶细化机理

采用SEM和TEM分析了等通道挤压奥氏体不锈钢中孪晶的细化过程.结果表明,一道次变形后,原退火孪晶受剪切断裂,并在一些区域形成小的形变孪晶;随着挤压道次增加,孪晶通过孪生和滑移的方式进一步变形,滑移由晶界开始并向晶粒内部扩展,最后将大的孪晶破碎,在孪晶层状结构内部通过孪生方式形成二次孪晶,在随后的变形过程中,逐渐形成微米级孪晶组织.八道次挤压后形成纳米级的晶粒和细小的微孪晶组织.     

冲击接触载荷下45钢的微观塑性变形特征与损伤

选取退火４５钢为研究对象，考察冲击接触载荷下亚表层微观形变特征与损伤，结果表明，冲击接触载荷下，铁素体的变形以多滑移为特征；滑移在晶界受阻产生应力集中并产生力偶使晶粒发生相对转动，从而在晶界产生裂纹；亚晶界和亚晶粒的形成是不同区域的不同滑移系统的交互作用的结果。     

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic SteelWithout Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide-pearlite steel has been investigated. Particular attention has been given to a quantitative characterization of changes in the ferrite grains. Electron back-scattered diffraction and transmission electron microscopy observations show that the ultrafine ferrite (~388 nm) can be produced through low equivalent strain cold rolling without severe plastic deformation (SPD) and annealing. The average grain size of ferrite depends on the volume fraction, shape and distribution of granular carbides as well as interlamellar spacing of pearlite. A general explanation of granular carbides-assisted grain refinement is that the embedded carbides between natural barrier will significantly facilitate dislocation nucleation during cold rolling. Dislocation reaction occurs more drastically and quickly near these granular carbides. Such reactions promote the formation of high-angle grain boundaries. The formation of ultrafine ferrite grains and subgrains in steel after cold rolling to ε=1.4 strain makes the strength and ductility increased simultaneously compared with ε=0.6 cold-rolled steel. The results suggest a new material design strategy to obtain ultrafine-grained structure via the granular carbides assistance.     

Effect of grain refinement on corrosion of ferritic–martensitic steels in supercritical water environment

The effects of grain refinement on the corrosion behavior of three ferritic–martensitic (F/M) steels, HT9, T91, and NF616, and two binary model alloys Fe‐15%Cr and Fe‐18%Cr in supercritical water (SCW) have been investigated. Grain refinement down to a size of about one micron in the surface regions, was achieved by introducing severe plastic deformation by shot peening. After exposure to SCW with 25 ppb oxygen at 500 °C for up to 3000 h, an improvement in corrosion resistance was observed in grain‐refined samples because of the enhanced diffusion of chromium on the surface, through a high density of grain boundaries. The chromium content in the steels and the exposure durations in SCW were determined to be important factors influencing the efficacy of the grain refinement effects. These results are supported by both experimental evidence and theoretical predictions. Another approach for grain refinement, equal channel angular pressing (ECAP), was also investigated for T91 steel. ECAP resulted in lower weight gain due to corrosion compared to the untreated samples, but exhibited a slightly higher weight gain compared to the shot‐peened samples after long‐term exposures in SCW which is probably caused by different fractions of high‐angle grain boundaries in grain‐refined regions, introduced by different grain refinement techniques.     

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.     

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.     

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Ai——Ⅰ.SLIP CHARACTERISTICS

Comparing with ordinary ploycrystalline materials sized to μm grade,the slip morphology ofthe coarse grained polycrystalline pure Al is characterized by:(1)several slip domains occurin a grain,and in same domain,several slip systems operate at same time or one after anotherintensely,a beautiful and neat slip pattern is forming on the specimen surface;(2)for highΣ-value coincident and random grain boundaries,the grain boundary affecting zone(GBAZ),bout 50—120μm wide,is favourable site to form intergranular crack at early fa-tigue life easily,and migration or slide of the boundaries were often observed.While lowΣ-value near-coincident grain boundaries show a higher degree of slip continuity and straincompatibility than high Σ-value ones.Intergranular crack is not easily nucleated at lowΣ-value near-coincident boundaries;and(3)due to suppression of grain boundary slip attriple grain boundary node,the high Σ-value and random grain boundary among the threeboundaries of tricrystal crack easily during cyclic deformation.     

等通道球形转角挤压过程中工业纯铝的微观组织演变与力学性能

突破传统ECAP变形全过程通道等截面思路,提出一种耦合剪切应变和正应变于一体的新型等通道球形转角挤压(equal channel angular extrusion with spherical cavity,ECAE-SC)工艺。在自行研制的模具上对工业纯铝进行室温单道次ECAE-SC挤压实验,采用OM、EBSD和TEM等技术手段,研究了ECAE-SC变形过程中工业纯铝微观组织的演变规律,并测试了变形后试样的显微硬度。结果表明,在ECAE-SC工艺剧烈简单剪切变形诱导下,工业纯铝仅需1道次挤压变形即可获得等轴、细小、均匀的超细晶组织,平均晶粒尺寸约为400 nm;工业纯铝室温ECAE-SC变形以位错滑移为主并伴有不完全连续动态再结晶,其微观组织经历了剪切带→位错胞→小角度亚晶→大角度等轴晶粒等动态演化过程。1道次ECAE-SC变形后,工业纯铝组织以{110}001高斯织构为主,同时存在部分{111}112铜型织构;材料显微硬度值大幅提升,由初始289.4 MPa提高到565.3 MPa,增幅高达95.33%,且分布均匀性良好。     

超声滚压剧烈塑性变形诱导金属材料结构演变的研究进展

首先,对表面完整性的基本概念和内涵进行了概述,同时简要介绍了超声实现滚压技术的基本原理及其优点。随后,对比分析了不同剧烈塑性变形方法的特点和局限性,引出了实现表面完整性的相关剧烈塑性变形协调机制。在此基础上,随后结合其他剧烈塑性变形强化工艺,重点总结了超声滚压剧烈塑性变形对金属材料表面微观结构演变的影响。具体探讨了剧烈塑性变形诱导晶粒细化机制、晶粒生长机制以及合金元素偏聚机制等,主要分别论述了不同层错能的面心立方、体心立方以及密排六方等不同金属晶体结构的晶粒细化机制(以位错滑移、变形孪晶为主导)、晶粒长大机制(以晶界迁移、晶粒旋转为主要)与合金元素偏聚机制(晶界偏聚、位错核心偏聚)等。最后,对以上内容进行了综合总结,并针对超声滚压技术研究中存在的问题给出进一步研究和发展的建议,从而为实现超声滚压金属材料的表面完整性的主动精准控制及提高其服役寿命与可靠性提供一定的参考。     

Microstructure and evolution of mechanically-induced ultrafine grain in surface layer of AL-alloy subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.     

Grain refinement mechanism of multiple laser shock processing impacts on ANSI 304 stainless steel

Micro-structural evolution and grain refinement in ANSI 304 stainless steel subjected to multiple laser shock processing (LSP) impacts were investigated by means of cross-sectional optical microscopy and transmission electron microscopy observations. The plastic strain-induced grain refinement mechanism of the face-centered cubic (fcc) materials with very low stacking fault energy was identified. The micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface was about 50–200 nm. Multidirectional mechanical twin matrix (MT)–MT intersections led to grain subdivision at the top surface during multiple LSP impacts. Furthermore, a novel structure with submicron triangular blocks was found at the top surface subjected to three LSP impacts. The grain refinement process along the depth direction after multiple LSP impacts can be described as follows: (i) formation of planar dislocation arrays (PDAs) and stacking faults along multiple directions due to the pile up of dislocation lines; (ii) formation of submicron triangular blocks (or irregularly shaped blocks) by the intersection of MT–MT (or MT–PDA or PDA–PDA) along multiple directions; (iii) transformation of MTs into subgrain boundaries; (iv) evolution by continuous dynamic recrystallization of subgrain boundaries to refined grain boundaries. The experimental results and analyses indicate that a high strain with an ultra-high strain rate play a crucial role in the grain refinement process of fcc materials subjected to multiple LSP impacts.     

Relationship between local deformation behavior and crystallographic features of as-quenched lath martensite during uniaxial tensile deformation

Electron backscattering diffraction patterns were used to investigate the relationship between local deformation behavior and the crystallographic features of as-quenched lath martensite of low-carbon steel during uniform elongation in tensile tests. The slip system operating during the deformation up to a strain of 20% was estimated by comparing the crystal rotation of each martensite block after deformation of 20% strain with predictions by the Taylor and Sachs models. The results indicate that the in-lath-plane slip system was preferentially activated compared to the out-of-lath-plane system up to this strain level. Further detailed analysis of crystal rotation at intervals of approximately 5% strain confirmed that the constraint on the operative slip system by the lath structure begins at a strain of 8% and that the local strain hardening of the primary slip systems occurred at approximately 15% strain.     

An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment

By means of surface mechanical attrition (SMA), a nanostructured surface layer was formed on a pure Fe plate. Microstructure features of various sections in the surface layer, from the strain-free matrix to the treated top surface, were systematically characterized by using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the SMA treatment in Fe was proposed. It involves formation of dense dislocation walls (DDWs) and dislocation tangles (DTs) in original grains and in the refined cells (under further straining) as well, transformation of DDWs and DTs into subboundaries with small misorientations separating individual cells or subgrains, and evolution of subboundaries to highly misoriented grain boundaries. Experimental evidences and analysis of the grain refinement mechanism indicate that high strains with a high strain rate are necessary for formation of nanocrystallites during plastic deformation of metals.     

Microstructures and compression properties of copper processed by high-pressure torsion

通过高压扭转对铜试样施加不同程度的变形, 研究了样品扭转面(ND面)和纵截面(TD面)上微观组织特征. 对ND面, 在较小的剪应变下, 原始晶粒形貌模糊, 晶粒内部形成等轴状的位错胞及亚晶结构; 随变形量的增大, 亚晶 间取向差及亚晶内部的位错密度增大, 最后形成亚微米尺度的等轴晶粒. 对TD面, 变形初期原始晶粒被拉长, 晶粒内 部为位错墙分割成的层状结构, 层内为拉长的位错胞; 随变形程度的增大, 拉长晶粒的宽度减小, 与剪切方向的夹 角减小, 晶内层状组织间距减小, 并逐渐演化成拉长的亚晶组织; 进一步增大变形, 晶粒拉长痕迹消失, 变形组织 与ND面相似, 为等轴状亚微米晶粒. 压缩实验表明, 经16圈扭转后, 整个试样上的压缩性能基本均匀, σ0.2达到385 MPa, 应变率敏感性指数增大至0.021.     

Effect of microstructural features on the corrosion behavior of severely deformed Al–Mg–Si alloy

The aim of this study was to determine the influence of severe plastic deformation processing and the changes in microstructure resulting therefrom on the corrosion resistance of an Al–Mg–Si alloy. The alloy was processed using incremental equal channel angular pressing, which caused a reduction in grain size from 15 to 0.9 µm. The grain refinement was accompanied by an increase in the number of grain boundaries and dislocations, and by changes in grain orientation. However, there was no change in the size and number of intermetallic particles, which presumably resulted in a constant number of galvanic couplings. Electrochemical experiments revealed only slight differences between the samples before and after processing. Higher potential transients/oscillations upon immersion and increased corrosion currents in the vicinity of corrosion potential point to slightly higher reactivity of the most refined material. This indicates that intermetallic particles are the most crucial microstructural elements in terms of corrosion resistance. Their impact exceeds that of grain boundaries, in particular, at the stage of corrosion initiation. The development of corrosion attack is controlled more by the microstructure of the matrix as the grain refinement resulted in a less pronounced corrosion attack in comparison with the coarse-grained sample.