Ti3Al-Nb合金超塑压缩变形的研究

研究了Ｔｉ－２４Ａｌ－１４Ｎｂ－３Ｖ－０．５Ｍｏ合金压缩时的超塑变形行为和变形过程中的组织变化。结果表明：所研究合金在ｔ＝９８０℃、ε＝７．２×１０－３ｓ－１的条件下ｍ值为０．３４,压缩时的真应变可大于１．６。在合金的超塑变形初期,Ｏ相发生动态再结晶和等轴化过程。随着变形过程的进行,新形成的Ｏ相晶粒在Ｂ２相中滑动和转动,Ｂ２相中的位错运动和界面扩散起调节作用     

优质GH738合金热变形过程中的再结晶机制

摘要：结合MTS压缩实验,分析了不同变形温度、应变速率、变形量及变形后保温时间对优质GH738合金再结晶的影响规律;进而,利用光学金相显微镜（OM）、透射电子显微镜（TEM）和电子背散射衍射（EBSD）分析,系统研究了该合金热变形前后的合金的晶粒组织、晶内亚结构、晶粒取向差异和弯曲晶界。结果表明：在实验参数范围内,优质GH738合金在热压缩过程中的再结晶以晶界弓弯方式形核的非连续动态再结晶机制为主,以亚晶旋转方式形核的连续动态再结晶机制为辅;热变形及之后的保温过程中形成的弯曲晶界大部分为普通晶界,少部分为孪晶界;弯曲晶界是取向微偏离CSL点阵的临位晶界发生小面化的结果,其本质是借助晶界位错运动形成的。     

Ti3Al基金属间化合物低周疲劳研究

利用ＳＥＭ和ＴＥＭ研究了Ｔｉ３Ａｌ基合金室温和６５０℃下恒应力低周疲劳的断口形貌和变形行为。研究表明，疲劳裂纹均起源于试样表面缺口处，室温下断口呈解理特征；６５０℃下断口中可观察到延性疲劳条带，且有沿晶的二次裂纹。合金的变形集中于β转变组织，β转变组织主要通过位错运动及形变孪晶的产生来协调变形，初生α２相晶内亚晶协调变形，６５０℃时非基面弯曲位错的开动使塑性变形更加有利。     

TiAl基合金超塑性变形中的位错运动

研究了结晶Ｔｉ－３３Ａｌ－３Ｃｒ－０．５Ｍｏ合金在超塑性变形过程中的位错运动。透射电镜分析结果表明，超塑性变形时，合金中存在大量位错运动，在晶界附近和晶粒内形成了位错网，并在晶界上出现位错塞积。大多数位错是从晶界及α２相粒子处发射出来的。     

优质GH738合金热变形过程中的再结晶机制

结合MTS压缩实验,分析了不同变形温度、应变速率、变形量及变形后保温时间对优质GH738合金再结晶的影响规律;进而,利用光学金相显微镜(OM)、透射电子显微镜(TEM)和电子背散射衍射(EBSD)分析,系统研究了该合金热变形前后的合金的晶粒组织、晶内亚结构、晶粒取向差异和弯曲晶界。结果表明:在实验参数范围内,优质GH738合金在热压缩过程中的再结晶以晶界弓弯方式形核的非连续动态再结晶机制为主,以亚晶旋转方式形核的连续动态再结晶机制为辅;热变形及之后的保温过程中形成的弯曲晶界大部分为普通晶界,少部分为孪晶界;弯曲晶界是取向微偏离CSL点阵的临位晶界发生小面化的结果,其本质是借助晶界位错运动形成的。     

脉冲电流对Zn－22％Al合金组织及断裂行为的影响

本文借助于透射电镜和扫描电镜，观察了Ｚｎ－２２％Ａｌ合金超塑变形时，脉冲电流对其组织和断裂行为的影响。研究表明，施加脉冲电流，合金中各部分晶粒变形是不均匀的，局部发生动态回复和动态再结晶，同时保留着纤维状或放射状的变形组织。合金的断裂是通过晶界开裂、扩展、最终断裂，表现出沿晶断裂机制。     

脉冲电流对Zn—22%Al合金组织及断裂行为的影响

本文借助于透射电镜和扫描电镜，观察了Ｚｎ－２２％Ａｌ合金超塑变形时，脉冲电流对其组织和断裂行为的影响。研究表明，施加脉冲电流，合金中各部分晶粒变形是不均匀的，局部发生动态回复和动态再结晶，同时保留着纤维状或放射状的变形组织。合金的断裂是通过晶界开裂、扩展、最终断裂，表现出沿晶断裂机制。     

690合金高温连续变形动态再结晶行为

应用液压机对690合金圆锥试样在3种不同温度下(1100、1140和1180℃)进行连续压缩变形实验,利用光学显微镜和背散射衍射技术研究690合金在热加工过程的动态再结晶行为.研究发现:在连续热压缩变形过程中动态再结晶以三叉晶界形核-原始晶界形核-孪晶形核(孪晶界和孪晶碎化)-晶内形核的顺序发展,而孪晶促进了690合金的再结晶过程.      

铌对Ti_3Al价电子结构及其脆性的影响

应用固体与分子经验电子理论计算了Ｔｉ３Ａｌ 及加入铌后各相的价电子结构， 并从均匀变形因子α、解理能Ｇｃ 及位错行为等方面分析了铌对Ｔｉ３Ａｌ 脆性的影响。铌使Ｔｉ３Ａｌ 合金的α及Ｇｃ 提高； 同时铌也减弱了ＴｉＴｉ 共价键， 增加了基面滑移， 从而导致Ｔｉ３Ａｌ 脆性有本质改善。     

变形条件对GH625合金高温变形动态再结晶的影响

采用Gleeble高温压缩实验研究了变形条件对GH625合金高温变形动态再结晶的影响,结果表明:当变形程度较小时,原始晶粒内部出现大量孪晶,晶界呈现锯齿状凸出;随变形程度的增加,在晶界弓出部位开始形核,形成大量再结晶晶粒,随变形程度进一步增加,GH625合金动态再结晶体积分数增大,但是再结晶晶粒尺寸无明显变化;GH625合金动态再结晶是一个受变形温度和应变速率控制的过程,变形温度越高,动态再结晶越容易形核,应变速率越小,动态再结晶过程进行得越充分。在低应变速率条件下,GH625合金获得完全动态再结晶组织的温度随变形速率的升高而升高,而在高应变速率条件下必须考虑变形热效应对合金变形组织的影响。     

Microtexture and grain boundary evolution during microstructural refinement processes in SUPRAL 2004

Electron backscatter pattern (EBSP) analysis of as-processed, processed and annealed, and superplastically deformed specimens of commercially processed SUPRAL 2004 material has been employed to reveal the boundary misorientation distribution and evolution. Earlier studies using X-ray diffraction (XRD) and transmission electron microscopy on this alloy have attributed the transition to microstructures capable of supporting extensive superplastic flow to continuous recrystallization occurring early in the deformation process. The micro- and mesotextural data of the present study show that the deformation texture evident in the as-processed material persists without the formation of recrystallization texture components and remains up to the apparent onset of the grain boundary sliding (GBS) regime. Comparison of the correlated and uncorrelated boundary misorientation data illustrates that the development of boundaries misoriented by ∼5 to 15 deg is not random in nature. There is no evidence of recrystallization involving the formation and migration of high-angle boundaries during the refinement process. Microtextural and boundary data from this study provide evidence that the microstructural transition enabling superplastic mechanical behavior of SUPRAL 2004 may be described by a recovery-dominated, continuous process involving the development of moderately misoriented boundaries and leading to a refined microstructure with a boundary distribution of low interfacial energy character.     

A Constitutive Equation for Grain Boundary Sliding: An Experimental Approach

Although grain boundary sliding (GBS) has been recognized as an important process during high-temperature deformation in crystalline materials, there is paucity in experimental data for characterizing a constitutive equation for GBS. High-temperature tensile creep experiments were conducted, together with measurements of GBS at different strains, stresses, grain sizes, and temperatures. Experimental data obtained on a Mg AZ31 alloy demonstrate that, for the first time, dynamic recrystallization during creep does not alter the contribution of GBS to creep during high-temperature deformation. The experimentally observed invariance of the sliding contribution with strain was used together with the creep data for developing a constitutive equation for GBS in a manner similar to the standard creep equation. Using this new approach, it is demonstrated that the stress, grain size, and temperature dependence for creep and GBS are identical. This is rationalized by a model based on GBS controlled by dislocations, within grains or near-grain boundaries.     

Deformation characteristics of an Al−6Cu−O. 4Zr superplastic alloy

The results of a mechanical property and TEM microstructural investigation of a superplastic Al-Cu-Zr alloy are reported. The conventional 3 stage ln σvs ln ∈ curve is observed, but a high value ofm is retained to low strain-rates and suggests that any operative creep yield stress is low. Marked mechanical property changes occur in the initial stages of deformation at the same time as dynamic recrystallization removes the initially directional microstructure of highly elongated grains. Following deformation at the higher strain-rates, Regions II and III, individual dislocations, dislocation networks, and low angle boundaries are observed. After deformation in Region I little evidence of any dislocation activity is seen. It is shown that two distinct recrystallization behaviors occur. In samples statically annealed for long periods recrystallization takes place in a discontinuous manner, high angle boundaries sweeping out from the ρ-particles, which act as nuclei. In the case of superplastically deformed samples the dynamic recrystallization is continuous in nature, and not related to the ρ-particles present. The current theories explaining the beneficial effects of ternary additions to Al-Zr alloys are examined in the light of the above observations. R. H. BRICKNELL, formerly with Department of Metallurgy and Materials Science, University of Cambridge J. W. EDINGTON, formerly with the Department of Metallurgy and Materials Science, University of Cambridge     

Recrystallization Phenomena During Friction Stir Processing of Hypereutectic Aluminum-Silicon Alloy

Microstructural evolution and related dynamic recrystallization phenomena were investigated in overlapping multipass friction stir processing (FSP) of hypereutectic Al-30 pct Si alloy. FSP resulted in the elimination of porosities along with the refinement of primary silicon particles and alpha aluminum grains. These alpha aluminum grains predominantly exhibit high angle boundaries with various degrees of recovered substructure and dislocation densities. The substructure and grain formation during FSP take place primarily by annihilation and reorganization of dislocations in the grain interior and at low angle grain boundary. During multipass overlap FSP, small second phase particles were observed to form, which are accountable for pinning the grain boundaries and thus restricting their growth. During the multipass overlap FSP, the microstructure undergoes continuous dynamic recrystallization by formation of the subgrain boundary and subgrain growth to the grain structure comprising of mostly high angle grain boundaries.     

Effects of microstructural evolution on superplastic deformation characteristics of a rapidly solidified Al-Li alloy

This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10⁻²/s to 10⁻¹/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.     

Deformation characterization of superplastic AA7475 alloy

AA7475 alloy was deformed up to 25% elongation in INSTRON at 788K. The grain boundary sliding due to this superplastic deformation was measured by Scanning Electron Microscope. The microstructure and texture development due to this deformation at elevated temperature was analyzed from the Orientation Image Microstructures i.e. the Electron Back Scattered Diffraction Image. The Orientation Image Microstructures revealed that superplastic deformation was associated with recovery and recrystallization in-situ process.     

Deformation Behavior and Dynamic Recrystallization of Biomedical Co-Cr-W-Ni (L-605) Alloy

The mechanical behavior of Co-20Cr-15W-10Ni alloy is studied by compression tests at high temperature. Microstructures after deformation are evaluated using SEM-EBSD. Significant grain refinement occurs by dynamic recrystallization for high temperatures and low strain rates [T > 1373 K (1100 °C), strain rate <0.1 s^?1], and at high strain rates (strain rate ~10 s^?1). Dynamic recrystallization is discontinuous and occurs by nucleation of grain boundaries, leading to a necklace-like structure. The nucleation mechanism is most likely bulging of grain boundaries. However, recrystallization occurs also by rotation of annealing twins, which can bulge as well. Modeling of the observed mechanical behavior gives a fair quantification of flow softening due to dynamic recrystallization, indicating the progress of dynamic recrystallization with deformation.     

Near-Surface Deformed Layers on Rolled Aluminum Alloys

Near-surface deformed layers, which are characterized by nano-sized fine grains, are generated in aluminum alloys by hot and cold rolling. During the rolling processes, the alloy surface and near-surface regions experience a high level of shear deformation that results in significant microstructure refinement, leading to formation of near-surface deformed layers with microstructures different from that of the underlying bulk alloy. Two types of near-surface deformed layers are observed. Type A is characterized by fine grains with grain boundaries decorated by oxide particles; type B is characterized also by fine grains but with the grain boundaries free of oxide particles. The high levels of shear deformation result in dynamic recrystallization. Together with mechanical alloying, this is responsible for the formation of the near-surface deformed layer. Furthermore, the structure in the near-surface deformed layer can survive the typical annealing process particularly if the grain boundaries are pinned by oxide particles.