Shear Localization in Dynamic Deformation: Microstructural Evolution

Investigations made by the authors and collaborators into the microstructural aspects of adiabatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiC_p composites. The principal findings are the following: (a) there is a strain-rate-dependent critical strain for the development of shear bands; (b) deformed bands and white-etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

Evolution of Sand Microstructure during Shear

Quantitative measurements of the local void ratio distribution are used to demonstrate how the microstructure throughout dilatant triaxial specimens of uniform fine quartz sand evolves during drained axial compression loading. Shear-induced increases in the mean of the local void ratio distribution initiate at the center of the specimen and migrate toward the ends of the specimen as axial strain increases. At any given strain, the mean of the local void ratio distribution is largest near the center of the specimen, reflecting the influence of end platen and membrane restraining effects. The results provide direct quantitative microstructure-based evidence that global or macro response, as conventionally used in interpreting specimen behavior, can be misleading as to the true material response. Implications of the test results on practical issues such as the location of local strain measurement systems are noted.     

Effect of Strain Rate on Evolution of the Deformation Microstructure and Texture in Polycrystalline Copper and Nickel

The evolution of crystallographic texture in polycrystalline copper and nickel has been studied. The deformation texture evolution in these two materials over seven orders of magnitude of strain rate from 3 × 10⁻⁴ to ~2.0 × 10⁺³ s⁻¹ show little dependence on the stacking fault energy (SFE) and the amount of deformation. Higher strain rate deformation in nickel leads to weaker á 101 ñ \left\langle {101} \right\rangle texture because of extensive microband formation and grain fragmentation. This behavior, in turn, causes less plastic spin and hence retards texture evolution. Copper maintains the stable end á 101 ñ \left\langle {101} \right\rangle component over large strain rates (from 3 × 10⁻⁴ to 10⁺² s⁻¹) because of its higher strain-hardening rate that resists formation of deformation heterogeneities. At higher strain rates of the order of 2 × 10⁺³ s⁻¹, the adiabatic temperature rise assists in continuous dynamic recrystallization that leads to an increase in the volume fraction of the á 101 ñ \left\langle {101} \right\rangle component. Thus, strain-hardening behavior plays a significant role in the texture evolution of face-centered cubic materials. In addition, factors governing the onset of restoration mechanisms like purity and melting point govern texture evolution at high strain rates. SFE may play a secondary role by governing the propensity of cross slip that in turn helps in the activation of restoration processes.     

Microstructure and Microtexture Evolution of Pure Titanium during Single Direction Torsion and Alternating Cyclic Torsion

Metallurgical and Materials Transactions A - Systematic experimental studies of microstructure and crystallographic texture of pure titanium during the Single Direction Torsion (SDT) and...     

纯铜连续挤压全过程组织演变的实验研究

采用光学显微镜对纯铜连续挤压过程中组织的演变进行了分析。结果表明,纯铜在孔型轧制区和摩擦剪切区晶粒沿变形方向被拉长,镦粗区晶粒呈胞状结构,粘着区出现了剪切带,形成变形亚结构,直角弯曲区晶粒成纤维状,开始发生动态再结晶,扩展成形区晶粒发生完全动态再结晶,形成细小的再结晶晶粒。     

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

TC21合金的高温变形行为及其组织演变

通过Gleeble压缩试验,系统地研究了TC21合金在α+β两相区、β单相区的热变形行为及其组织演变.对流动应力的分析表明,峰值应力与变形温度和应变速率间符合双曲正弦形式的Arrhenius本构关系.变形死区的组织观察表明,随着变形温度的升高,合金中细小α相首先转变为β相;进入单相区后,取向相近的β晶粒间相互合并而迅速长大.应变速率对微观组织的形貌和尺寸有一定影响,在低应变速率时等轴α相的形貌并没有明显变化,变形以β相为主;随着应变速率的升高,α相沿轴向方向被压扁,β相呈现纷乱不均的分布;进入单相区后,粗大的β晶粒被压扁,并在晶界处生成新的等轴β晶粒.     

TC11钛合金等轴组织热变形行为与组织演变

采用等温恒应变速率压缩试验研究了TC11钛合金等轴组织两相区980～800℃,应变速率O.001～O.1 s-1,变形程度50%条件下的变形行为,分析了变形参数对应力-应变曲线、微观组织演变机制和规律的影响,建立了该合金两相区变形的热加工图,并采用EBSD技术测试了热变形组织的晶界特征.结果表明:(1)980℃变形,β相是主要变形相,O.001～O.1 s-1之间的功率耗散效率值在动态回复和再结晶范围内;α相经历了变形促进下的溶解(高应变速率)和聚集粗化(低应变速率)的过程,即α晶粒尺寸和相含量随着应变速率的加快明显减小.(2)950～900℃变形,O.001～O.01 s-1之间的功率耗散效率值在超塑性变形范围内;变形主要是软基体的β相和界面的变形行为;变形过程中,α晶粒尺寸和相含量基本不变.(3)850～800℃变形,α相是主要变形相,发生了连续动态再结晶过程;β相起晶界协调变形的作用.     

Ti-15-3合金超塑性变形及微观组织演变

研究了Ti-15-3合金以不同的应变速率进行一段拉伸与二段拉伸后的力学性能与微观组织. 试样在超塑性变形过程中发生了不同程度的连续动态再结晶, 第二相粒子弥散分布在β相基体中, 晶粒尺寸随变形量的增加而减小, 而晶界取向差则随变形量的增加而增加. 非理想组织的Ti-15-3合金在二段拉伸条件下表现出更好的超塑性.     

片层组织TC17钛合金高温变形行为研究

通过热压缩试验研究了具有初始片层组织的TC17钛合金在780~860℃和应变速率0.001~10 s-1范围内的热变形行为和组织演变。分析了该合金在两相区变形的应力-应变曲线特征,其流变应力本构关系可以用双曲正弦方程和Zener-Hollomon参数描述,得到TC17合金在两相区变形的平均激活能为488.86 kJ.mol-1。显微组织分析发现:TC17合金在两相区变形时组织演变的主要特征是片层组织球化;热变形参数严重影响片层组织球化过程的进行,加大变形量、降低应变速率以及提高变形温度可以提高片状组织的动态球化程度。     

Microstructure and Property of High Carbonic-Chromium Cast Steel with Different Hot Deformation Ratio

The microstructure and properties of high carbonic-chromium cast steel subjected to different hot deformation ratios were studied. The experimental results show that the microstructure and properties of high carbonic-chromium cast steel are obviously improved after hot deformation, and the best mechanical properties of the cast steel can be obtained under hot deformation ratio of 40 %-50 %, which leads to the morphology change of eutectic carbide and the precipitation of granular carbides.     

Microstructure and Texture Evolution of Cold-Drawn 〈111〉 Single-Crystal Copper

The evolution of microstructure and texture in drawn 〈111〉 single-crystal copper with strains from 0.28 to 4.12 was analyzed. The texture analysis shows that in drawn 〈111〉 single-crystal copper, grain subdivision takes place, and at high strains, fiber textures consist of 〈111〉 and 〈100〉. However, the distribution of fiber textures is inhomogeneous along the radial direction. 〈111〉 is located in the center of samples and 〈100〉 is near the surface. Comparison of the texture evolution of drawn 〈111〉 and 〈100〉 single-crystal coppers indicates that the initial orientation is an important factor affecting the ratio of fiber texture 〈111〉 to 〈100〉. The results of microstructure show that at strains lower than 0.28, microstructure can be characterized as dislocation cells and few microbands. When strains are more than 0.58, a large number of extended planar dislocation boundaries appear. At strains more than 1.96, most of the extended planar boundaries are parallel to the drawn direction.     

高碳珠光体钢在温变形过程中的组织变化

用SEM、TEM等方法研究了一种原始组织为层片状珠光体的高碳钢在多向温变形(楔横轧)和单向温变形(压缩)条件下试样心部与表层显微组织的演变情况.结果表明,在多向温变形条件下,试样表层可获得铁素体晶粒与渗碳体颗粒尺寸均在0.3 μm以下的超微细(α θ)复相组织,心部渗碳体片虽已碎断并部分球化,但其排布形态与变形珠光体基本保持一致,而单向温变形条件下的情况则恰好相反;变形过程中试样所受应力、应变状态的不同是引起组织差异的根本原因.     

强变形过程中铁镍合金的微观结构演化机制

 采用透射电镜观察了铁镍(Fe 32%Ni)合金在形变温度500 ℃(＜05Tm)、形变速率10-2 s-1的变形条件下多轴锻造变形过程中的微观结构演变。结果表明,低温多轴锻造强变形可明显细化晶粒,细化过程为：首先,位错墙、位错缠绕等结构通过大量位错滑移运动在原始晶粒内形成;其次,不同方向的变形导致不同方向的滑移系开动,从而致使不同方向的位错墙互相交叉,将原始粗晶粒细分成小尺寸的胞块结构,当变形量达到一定程度时,位错墙和位错缠绕结构内的位错开始重新排列,形成小角度晶界,导致亚晶粒形成;由于变形量不断增加强迫大量的位错在亚晶界处积聚、重排,同时不同方向的变形造成亚晶发生转动,位错重新规则排列及亚晶转动使小角度的亚晶界转变为大角度晶界,从而形成细小的新晶粒。