On Plastic Deformation Behavior of Cryorolled AA8090 Alloy

AA8090 alloy was rolled up to 50 and 75 % reductions at both liquid nitrogen (LNR) and room temperatures (RTR). Both hardness and tensile behavior were evaluated on rolled samples. Optical microscope, TEM and EBSD were used for detailed microstructural examination of rolled samples. Williamson-Hall peak broadening analysis on X-ray diffraction data was made to evaluate crystallite size, lattice strain and dislocation density. An enhanced tensile strength was evidenced in LNR samples when compared to RTR samples without sacrificing ductility, which was ascribed to the higher density of dislocations in LNR samples than RTR samples. A large number of dislocation tangled regions along with ultrafine grain structure were evidenced through TEM and EBSD. Significant fraction of special boundaries in combination with increased fraction of texture components like S, Brass, Cu and Goss would be another reason for enhanced properties in LNR conditions than that of RTR. These components were observed to be strengthened with increased rolling reduction. Work hardening behavior clearly evidenced the variation in amount of work hardening and recovery phenomenon. It showed large variation in recovery in the case of 50 % reduction than that of 75 % reduction, which was attributed to significantly higher density of dislocations in 75 % rolled samples in RTR and LNR.     

Role of Tool Shoulder Diameter in Friction Stir Welding: An Analysis of the Temperature and Plastic Deformation of AA 2014 Aluminium Alloy

The influence of tool shoulder diameter and its rotational speed on the high temperature plastic deformation of the material during friction stir welding of AA 2014 aluminum alloy is investigated, using the principles of hot working. The soundness of weld and defect formation are analyzed using the Zener–Hollomon parameter ‘Z’ to describe the high temperature plastic deformation behaviour of material, under the simultaneous influence of temperature and strain-rate. The observed hot deformation behaviour is correlated with the deformation processing map for the first time. At a given rotational speed, the volume of shoulder driven flow reduces with increasing shoulder diameter.     

Cu-Cr-Zr-Ce合金高温流变应力的研究

利用Gleeble-1500D热模拟试验机对Cu-Cr-Zr-Ce合金在变形温度为600～800℃、应变速率为0.01～5 S-1条件下进行了热压缩试验,测定了其应力-应变曲线,并通过光学显微镜观察了其热压缩过程中的微观组织.结合两者分析了动态回复和再结晶机制.结果表明,动态再结晶是该合金软化的主要机制.     

A Study of Plastic Deformation Behavior of AA1050 Aluminum Alloy during Pure Shear Extrusion with Back Pressure

In the present study, the effects of back pressure on the filling fraction of die and the effective strain distribution throughout severely deformed material during pure shear extrusion, a novel severe plastic deformation process, are investigated by finite element analysis. A pure shear extrusion process found in the literature is employed and the predicted forming load is compared with experiments. A good agreement is observed between the results of the simulation with Coulomb friction of 0.12 and experiments. Various back pressures are applied to plunger at the exit channel of the die, and their influence on the filling fraction of the die and the effective strain in severely deformed billets are studied, indicating that the homogeneity of the effective strain on the cross-section of the deformed billet is decreased slightly. It is also found that the filling fraction of the die exit channel as well as average strain on the cross-section of the billet are increased.     

A Coupled Crystal Plasticity and Anisotropic Yield Function Model to Identify the Anisotropic Plastic Properties and Friction Behavior of an AA 3003 Alloy

A multi-scale simulation of the tip test, developed to determine the tribological characteristics of the back-extrusion process, was conducted on an AA 3003 alloy. A microstructure-level simulation, coupled with crystal plasticity finite element (CPFE) analysis, was utilized to characterize the macro-mechanical properties of the AA 3003. Owing to the limited size of the material provided, we performed CPFE analyses rather than multiple mechanical tests to determine the plastic anisotropy characteristics of the AA 3003 alloy. A three-dimensional finite element (FE) model of the tip test was developed using two different yield functions, namely the generalized von Mises yield function and Hill’s (1948) yield function, with material parameters identified from the CPFE analyses. The results revealed the following: 1. The directionality observed during the tip test is governed by the plastic anisotropy, rather than the frictional conditions. 2. The plastic anisotropy results in different Coulomb friction values. Therefore, the anisotropy should be carefully addressed in the tip test.

     

TA15钛合金高温塑性变形流变应力行为研究

通过热/力模拟实验对TA15钛合金高温塑性变形流变应力进行了研究.实验结果表明:应变速率和变形温度的变化显著影响合金流变应力的大小,流变应力随变形温度的升高而降低,随应变速率的提高而增大.根据实验数据,计算了合金的高温塑性本构方程常数,并采用Zener-Hollomon参数的双曲正弦函数形式描述了合金的流变应力行为.     

Sub-zero Temperature Dependence of Tensile Response of Friction Stir Welded Al-Cu-Li (AA2198) Alloy

Mechanical properties at ambient and cryogenic temperatures of Al-Cu-Li alloy are required for design and fabrication of liquid hydrogen and liquid oxygen tanks of satellite launch vehicles. In the present work, bead-on-sheet, friction stir welding was carried out with three different rotation speeds. The yield and strain hardening behaviors of the welds were evaluated in temperature range of 20 K to 298 K. Both yield stress and strain hardening ability in the specimen increased with decrease in testing temperature. The dependence of yield stress on temperature was modeled on the basis of thermally activated dislocation mobility, while that of strain hardening was modeled on the temperature dependence of dynamic recovery rate parameter. The recovery parameter followed an Arrhenius-type relationship with temperature. The model parameters determined from the experimental data were further used to simulate the stress–strain curves at different sub-zero temperatures for the friction stir welds.

     

Producing of AA5083/ZrO2 Nanocomposite by Friction Stir Processing (FSP)

In this study, friction stir processing (FSP) was used to produce AA5083/ZrO₂ nanocomposite layer. Optical microscopy and SEM were used to probe the microstructures formed in the composite layer. In addition, the mechanical properties of each sample are characterized using both tensile and hardness tests. Results showed that FSP is an effective process to fabricate AA5083/ZrO₂ nanocomposite layer with uniform distribution of ZrO₂ particles, good interfacial integrity, and significant grain refinement. On processing, in the proper combination of process parameters, the metal matrix composite layer was observed to have increased tensile and hardness properties.     

Modelling Corrosion Behavior of Friction Stir Processed Aluminium Alloy 5083 Using Polynomial: Radial Basis Function

Aluminium alloy 5083, widely used in marine applications, undergoes accelerated corrosion in sea water due to the aggressive reaction of chloride ions with the secondary phase particles and other intermetallics present in the alloy matrix. The corrosion rate of the alloy is also influenced by the temperature difference between the alloy and its environment. Friction stir processing (FSP) is a recent solid state processing technique for improving the surface properties of metals and alloys. In this study, an attempt has been made to explore the possibility of improving the corrosion resistance of AA5083 by FSP. FSP trials were performed by varying the tool rotation speed, tool traverse speed and shoulder diameter of the tool, as per face centered central composite design. The corrosion potential and the corrosion rate of friction stir processed AA5083 was studied using potentiodynamic polarization studies, at three different temperatures. Mathematical models based on polynomial—radial basis function were developed and used to study the effect of process parameters on the corrosion potential and the corrosion rate of friction stir processed AA5083. FSP resulted in refinement of the grain structure, dispersion and partial dissolution of secondary phase particles in the matrix, which increased the corrosion resistance of the alloy.     

基于摩擦修正的TB8合金热压缩流变应力行为分析

采用Gieeble-1500热模拟试验机对TB8(Ti-15Mo-2.7Nb-3Al-0.2Si)合金进行了等温热压缩变形试验,温度范围为750-1100℃,应变速率范围为0.01～1s-1.在热压缩过程中由于摩擦影响导致流变应力不能真实反映材料的高温变形行为.采取一种简便的方法对实验数据进行了摩擦修正,研究了TBS合金热变形流变应力行为,并对合金的变形机制进行了初步探讨.结果表明:热压缩过程中摩擦对于流动应力的影响十分显著,采取的修正方法降低了实验中摩擦引起的误差;TB8合金的热变形行为具有高度的变形温度和应变速率敏感性,随着变形温度的提高和应变速率的降低,真应力显著降低;动态回复和动态再结晶是TB8高温变形时主要软化机制.     

氢对TC4钛合金室温变形行为的影响

应用压缩试验研究了置氢TC4钛合金的室温变形性能; 利用金相显微镜、 X射线衍射仪和扫描电镜等手段分析了置氢后组织演变、相组成和断口特征. 结果表明: 氢处理改善了材料的相组成, 促进了塑性相α″马氏体和亚稳β相的生成, 氢处理后变形极限在低氢时没有较大提高, 随后氢含量超过0.45%后, 塑性大幅度提高, 变形极限较原始提高了90%. 在压缩试验中, 流变应力对氢有较高的敏感性, 随着变形速度和氢含量的增加, 速度引起的加工硬化降低.     

氮含量对Cr12N合金高温形变行为的影响

Cr12N合金（%:2.03～2.26C、0.16～0.33Si、0.29～0.37Mn、11.86～11.96Cr、0.09～0.28N）由10kg真空感应炉熔炼并通过充氮和加氮化硅进行N合金化。通过Gleeble-1500热模拟试验机将不同N含量的Cr12N合金在真空下以10℃/s加热至1180℃3min,再以10℃/s冷却至1100～900℃20s,以1～10²s^-1进行30%压缩变形,得出真应力-应变曲线和氮对该合金高温力学性能的影响。结果表明,N抑制合金动态再结晶发生,Cr12N合金在高温变形时,动态回复是最主要的软化机制;与0.09%N合金和0.28%N合金比较,0.18%N合金具有较高的峰值应力和屈服应力,回复激活能最低。     

氢含量对新锆合金室温循环变形行为的影响

NZ2是我国自行研制的具有自主知识产权的新型Zr-Sn-Nb系锫合金,其堆外性能,尤其是耐蚀性能已达国外新合金同等实验条件下的相同水平,是十分具有应用前景的新锆合金.锆在反应堆中服役时会与冷却剂反应而吸氢,从而使锆合金的性能降低,导致失效.通过应变控制的对称拉-压(εR=εmin/εmax=-1)疲劳实验研究了不同氢含量(未渗氢,200,450.730μg·g-1)对NZ2新锆合金的室温低周疲劳性能以及氢含量对材料循环硬化及软化的影响,发现随着氢含量的增加,相同应变下材料的低周疲劳寿命下降,但达到某一饱和值的时侯材料的低周疲劳性能不再降低,随着氢含量的增加,材料有循环软化趋势,NZ2锫合金台氢试样比无氢试样更容易出现循环软化现象.     

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.     

NiAl/Cr(Mo)-Hf 合金的高温变形行为

研究了铸造及热等静压 (HIP)处理后的两种NiAl/Cr(Mo) Hf合金的微观组织 ,分析发现合金主要由NiAl相和Cr(Mo)相以及铪固溶体和Heusler相组成。对合金的高温变形行为和断裂特征进行研究 ,结果表明该合金具有良好的高温压缩性能 ,并运用线性回归方法计算了应力指数n和变形激活能Q。     

变形温度对TC11合金低应变速率下的变形行为及组织的影响

采用THERMECMASTOR-Z热模拟试验机研究了TC11钛合金在变形温度780～1080℃,应变速率0.001～1 s-1范围的热变形行为,并采用金相显微镜研究了变形温度对TC11钛合金组织的影响,主要研究结果如下:变形温度对TC11钛合金的流动应力有显著影响,在较高温度或较低应变速率时,变形呈稳态流动特征;在较低温度或较高应变速率时,变形呈流变软化特征.在β单相区,当应变速率为1 s-1时,组织主要为拉长的β晶粒和少量的动态再结晶晶粒;当应变速率为O.01～0.1 s-1时,变形机制主要为动态再结晶;当应变速率在0.001 s-1附近时,变形机制为动态回复.在(α+β)两相区,变形温度870～960℃,应变速率0.001 s-1附近时,变形机制为超塑性.