Retardation of grain growth and cavitation by an electric field during superplastic deformation of ultrafine-grained 3Y-TZP at 1,450–1,600 °C

The influence of a continuous dc electric field applied orthogonal to the tensile direction on the flow stress, grain growth, and cavitation during superplastic deformation (SPD) of ultrafine-grained 3Y-TZP at 1,450–1,600 °C was determined. The field gave a significant reduction in the level of the stress-strain curve, and reduced grain growth and cavitation. The decrease in flow stress by the field was attributed mainly to the retardation of grain growth. The decrease in cavitation correlated with the retardation of grain growth and was attributed largely to the reduction in flow stress by the field.     

喷射成形超高碳钢超塑性变形后的微观组织

研究了喷射成形超高碳钢的超塑性及其变形前后的显微组织.变形前,喷射态超高碳钢的组织为典型珠光体组织,而变形后,珠光体中的条状碳化物逐渐发生碎化和球化,并弥散分布于晶界处,此外,在铁素体基体中以及碳化物颗粒周围出现了高密度位错亚结构,而基体铁素体晶粒也有所伸长.喷射成形超高碳钢超塑性微观机制是以晶界滑动为主,晶内变形以及位错蠕变起了协调作用.     

Some effects of an electric field on the plastic deformation of metals and ceramics

 The external parameters generally considered in the plastic deformation of metals and ceramics are the temperature, pressure or stress and time. Usually neglected are the effects of electric and magnetic fields. However, such fields can often have a significant influence, especially when applied concurrently with the more common parameters. Some examples of the effects of an electric field on the plastic deformation of metals and ceramics found by the author and his coworkers are presented. Included are the following: (a) the influence of electropulsing on the flow stress of metals at 78–300 K, (b) the effect of an external electric field (surface charge) on the superplastic deformation of the 74754 Al alloy, (c) the influence of an electric field on the flow stress and ductility of polycrystalline NaCl at 0.28–0.75 T_M and (d) the effect of an electric field on the superplastic deformation of 3Y-TZP. Mechanisms responsible for the observed effects are considered. Received: 1 January 1998 / Accepted: 1 March 1998     

Deformation mechanism maps for micro-grained, ultrafine-grained, and nano-grained materials

There are several deformation mechanisms that depend on grain size and are controlled by grain boundary diffusion. These mechanisms include: Coble creep, superplastic flow (micrograin superplastic flow and high-strain rate superplastic flow), and nanograin deformation. By combining the rate-controlling equations of these mechanisms and by making assumptions regarding triple-junction creep, a deformation map based on grain size was constructed. It is demonstrated that this map can account for the locations of experimental data representing three types of deformation behavior: micrograin superplasticity, high-strain rate superplasticity, and nanograin deformation.     

铍青铜(QBe2)超塑变形中的空洞形成与晶粒重排的试验研究

本文对铍青铜(QBe2)超塑变形中的空洞形成过程和晶粒重排进行了研究。结果表明,空洞的形成直接影响着晶粒重排过程。晶粒重排以多重方式进行,而不是单一方式,形成空洞是其中的一个步骤。对于含有第二相粒子的 QBe2合金,其超塑变形中所产生的位错与第二相粒子的交互作用是导致空洞形成的重要原因。空洞的形成并不直接导致材料的断裂,而断裂的真正原因是空洞的连结。文中给出了描述晶粒重排和空洞形成与连结的示意图。     

Investigation of Subboundaries Evolution in Superplastically Deformed NiAl by Positron Lifetime

In order to study the subboundaries evolution in superplastically deformed NiAl,the positron lifetime change during superplastic deformation process was measured.It is shown that the superplastic deformation of NiAl has not influence on its τ2,the newly recrystallized grain boundaries formed during entire superplastic deformation process belong to the calegory of subboundaries and have not contribution to the superplastic strain.     

超塑性形变中的晶界形态变化及再结晶的作用

为了研究晶界形态及动态再结晶在超塑性形变中的作用,采用光学显微镜、扫描电镜、透射电镜,对硬铝ＬＹ１２ 的超塑性形变过程进行了观察、分析．提出金属材料的超塑性主要依靠晶界流态化区的粘性变形来实现,动态再结晶不是超塑性的一种机制．     

SiCp/Al复合材料超塑性变形的断口特征

对SiCp/2024Al复合材料在不同温度下超塑性变形后的断口形貌进行了分析，结果表明，变形温度愈高，晶界结合强度愈低，沿晶断裂愈明显，晶界滑动愈易实现；超塑性变形需要强度适中的晶界结合；典型超塑变形条件下的断口呈晶界圆滑型沿晶断裂。     

Grain boundary sliding and accommodation mechanisms during superplastic deformation of ZK40 alloy processed by ECAP

Controlling mechanism during superplastic deformation of ZK40 alloy processed by ECAP was identified. Effects of twinning and dynamic strain ageing (DSA) on superplasticity were analyzed. Amplitude in stress oscillation was correlated with solute atom concentration theoretically. Twinning can be an enhancing factor in grain boundary sliding and DSA had apparent influence on stress fluctuation; they were accommodation mechanisms for superplastic deformation through grain reorientation and interaction between solute atoms and dislocations, respectively. The interaction between mobile and forest dislocations played a dominant role for the occurrence of DSA, when dislocation density was relatively low in large grains. The effect of DSA became more active with increasing temperature, although grain boundary sliding (GBS) was the controlling mechanism throughout the whole process of superplastic deformation under elevated temperatures.     

7B04铝合金超塑性变形行为

针对7B04铝合金开展了变形温度为470~530℃,应变速率为0.0003~0.01s~(-1)的高温超塑性拉伸实验,研究了材料的超塑性变形行为和变形机制。结果表明,7B04铝合金的流动应力随着变形温度的升高和应变速率的降低而逐渐减小,伸长率随之增加;在变形温度为530℃,应变速率为0.0003s~(-1)时,7B04铝合金的伸长率达到最大1105%,超塑性能最佳;应变速率敏感性指数m值均大于0.3,且随变形温度的升高而增加;在500~530℃的变形温度范围内,m值大于0.5,表明7B04铝合金超塑性变形以晶界滑动为主要变形机制;变形激活能Q为190kJ/mol,表明7B04铝合金的超塑性变形主要受晶内扩散控制;7B04铝合金超塑性变形中在晶界附近有液相产生,且适量的液相有利于提高材料的超塑性能。     

Superplastic deformation of the Pb-Sn eutectic

Dynamic observations of grain-boundary sliding during superplastic flow of the Pb-Sn eutectic are reported. These observations confirm the postulate that the dominant deformation mode during superplastic flow is grain-boundary sliding with localized deformation necessary to maintain grain coherency. Extensive grain-boundary sliding is also observed when the strain-rate and/or grain size is outside the superplastic flow regime. Stress relaxation tests were also carried out on the Pb-Sn eutectic. These tests provide data on the activation energy (45±5 kJ mol^–1), grain-size dependence (d ^–3), and stress dependence of superplastic flow in this alloy. A threshold stress of 1.3×10⁶N m^–2 for the onset of superplastic deformation is also observed.     

In situ TEM investigation of microstructural behavior of superplastic Al–Mg–Sc alloy

Dynamic changes in microstructure of the superplastic ultrafine-grained Al–3Mg–0.2Sc (wt.%) alloy refined by equal-channel angular pressing (ECAP). were observed by in situ transmission electron microscopy at temperatures up to 300 °C (annealing and tensile deformation) in order to simulate the initial stages of superplastic testing. It was found that the microstructure changes significantly during the preheating before the superplastic deformation, which was accompanied by decreased microhardness. During the deformation at 300 °C, high dislocation activity as well as motion of low-angle grain boundaries was observed while high-angle grain boundaries did not move due to the presence of scandium in the alloy.     

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodal γ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 ℃ was systematically investigated in the temperature range of 750-900 ℃ under an initial strain rate of 10-4 s-1.A bimodal γ grain-distribution microstructure of TiA1 alloy sheet,with abundant nano-scale or sub-micron γ laths embed-ded inside β matrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m =0.27 at 800 ℃,which is the lowest temperature of superplastic deformation for TiAl alloys attained so far.The maximum elongation reaches ～360％ at 900 ℃ with an initial strain rate of 2.0 × 10-4 s-1.To elucidate the softening mechanism of the disordered β phase during superplastic deformation,the changes of phase composi-tion were investigated up to 1000 ℃ using in situ high-temperature X-ray diffraction (XRD) in this study.The results indicate that β phase does not undergo the transformation from an ordered L20 structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of both y and β dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD),the superplastic deformation mecha-nism can be explained by the combination of DR and grain boundary slipping (GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic de-formation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy.     

AZ61镁合金薄板超塑性变形特征的SEM研究

利用扫描电镜(SEM)和超塑性拉伸实验对一次热挤压加工成型的AZ61镁合金薄板(晶粒尺寸～12μm)超塑性变形特征进行了研究.结果显示,在最佳的变形温度(623K)和应变速率(1×10-4s-1)条件下,可获得的最大的超塑性形变量为920%.在523～673 K实验温度和1×10-2～1×10-5s-1应变速率范围内,材料的应变速率敏感指数(m值)随实验温度升高和应变速率的降低而增加.较高的m值(0.42～0.46)对应于晶界滑动机制(GBS),而较低的m值(0.22～0.25)则对应于位错滑移机制.变形温度和应变速率是影响超塑性变形量和变量机制的主要因素.     

MICROSTRUCTURAL MODELING AND NUMERICAL ANALYSIS FOR THE SUPERPLASTIC FORMING PROCESS

Superplastic forming is a manufacturing process during which a sheet is blow formed into a die to produce lightweight and strong components. In this paper, the microstructural mechanism of grain growth during superplastic deformation is studied. A new model, which considers grain growth, is proposed and applied to conventional superplastic materials. The relationships among the strain, strain rate, test temperature, initial grain size, and grain growth in superplastic materials are discussed. According to the proposed model, theoretical predictions for superplastic forming processes are presented, and comparison with experimental data is given. The new constitutive equation of superplasticity is introduced into a finite element method program to study superplastic blow forming. The effects of the geometric shape parameters of the die on the superplastic blow forming process are investigated, and the inhomogeneity in the thickness distribution of the specimen is analyzed.     

Investigation of the deformation behavior of samples with superplastic layers

The tensile deformation behavior of samples made of magnesium and titanium alloys with superplastic layer(s) was investigated. It was observed that deformation took place by means of layer-by-layer shear in the superplastic region. Traces of such shear were visible to the unaided eye on the surface of the deformed specimens. The spacing of these shear surfaces was about 6–8 grains. It is suggested that superplastic deformation proceeds by cooperative grain shear along two intersecting grain boundary systems oriented at approximately 45°–60° to the tensile axis. Using optical microscopy and back-scattered electron imaging on a scanning electron microscope, traces of shear systems were also observed on the prepolished surfaces of deformed magnesium alloy samples and the etched surfaces of deformed titanium alloy samples.     

Cavitation and cavity-induced fracture during superplastic deformation

The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.     

TC18钛合金的超塑性行为与变形机制

通过高温拉伸实验研究TC18钛合金在温度为720~950℃,初始应变速率为6.7×10~(-5)~3.3×10~(-1)s~(-1)时的超塑性拉伸行为和变形机制。结果表明:TC18钛合金在最佳超塑性变形条件下(890℃,3.3×10~(-4)s~(-1)),最大伸长率为470%,峰值应力为17.93MPa,晶粒大小均匀。在相变点Tβ(872℃)以下拉伸,伸长率先升高后下降,在温度为830℃,初始应变速率为3.3×10~(-4)s~(-1)时取得极大值373%,峰值应力为31.45MPa。TC18钛合金在两相区的超塑性变形机制为晶粒转动与晶界滑移,变形协调机制为晶内位错滑移与攀移;在单相区的超塑性变形机制为晶内位错运动,变形协调机制为动态回复和动态再结晶。     

脉中电流对2091铝锂合金动态再结晶动力学的影响

研究了脉冲电流对2091铝锂合金超塑变形中动态再结晶及动力学的影响结果表明,脉冲电流加速动态再结晶,减小形核时的平均晶粒直径.脉冲电流能加快位错墙的形成并使其角度增大,使再结晶形核率提高.脉冲电流加快位错在晶界上的攀移及消失、减小形核界面两边的能量差,降低形核界面的迁移速率及再结晶形核的长大速率分析了脉冲电流作用下的动态再结晶动力学行为