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.     

NiAl超塑性变形的正电子寿命研究

研究了B2型多晶NiAl的晶界结构对缺陷态正电子寿命的影响，并结合正电子谱探讨了热挤压及超塑性变形过程后的晶界结构，热挤压的NiAl合金具有有完全的再结晶组织，而超塑性变形过程中的动态回复和再结晶是一种不完全再结晶过程，所得到的晶界仍属于亚晶界或小角度晶界范畴，再结晶晶界在超塑性变形过程中不产生滑动。     

NiAl合金的超塑性行为及其变形机制

研究了等原子比NiAl合金的NiAl-25Cr,NiAl-20.4Fe-Y,Ce,NiAl-30Fe-Y合金的超塑性行为及其变形机制,结果表明,几种合金在一定条件下均表现出超塑性行为,单相NiAl超塑性变形源于变形过程中所发生的动态回夏及再结晶,两相及多相NiAl合金的超塑性变形机制则是晶粒的转动和界面的滑动。     

微量磷(P)对等原子比NiAl的微观组织与力学性能的影响

研究了微量P对挤压态等原子比NiAl的微观组织与高温力学性能的影响.结果表明:微量P的添加对NiAl的晶格常数有一定的影响,P偏聚于NiAl晶界处;并对其高温延伸率有重要影响.P偏聚于晶界阻碍了合金变形过程中的动态回复和再结晶,加剧了晶界处孔洞的形成,造成了NiAl-P合金与二元NiAl合金高温力学性能的显著差异,主要表现在:应力-应变曲线经历了较长的加工硬化阶段;最大延伸率明显下降;变形激活能升高,应变速率敏感指数下降.NiAl-P合金的高温变形机制为变形过程中位错的滑移与攀移共同作用.     

Microstructure and Elevated Temperature Tensile Behavior of Directionally Solidified Ni-rich NiAl-Mo(Hf) Alloy

The microstructure, high strain rate superplasticity and tensile creep behavior of directionally solidified (DS) NiAl-Mo(Hf) alloy have been investigated. The alloy exhibits dendritic structure, where dendritic arm is NiAl phase, interdendritic region is Ni3Al phase, and Mo-rich phase distributes in the NiAl and Ni3Al phases. The alloy exhibits high strain rate superplastic deformation behavior, and the maximum elongation is 104.2% at 1373 K and strain rate of 1.04×10-2 s-1. The balance between strain hardening (by dislocation glide) and strain softening (by dynamic recovery and recrystallization) is responsible for the superplastic deformation. All the creep curves of the DS NiAl-Mo(Hf) alloy have similar shape of a short primary creep and dominant steady creep stages, and the creep strain is great. The possible creep deformation mechanism was also discussed. The creep fracture data follow the Monkman-Grant relationship.     

A study of the strength of P/M 6061Al and composites during high strain rate superplastic deformation

Metal Matrix Composites (MMCs) are potential candidate materials in the aerospace and automobile industries because of its attractive properties, in particular, their high specific properties, and Superplastic forming (SPF) is a good solution to the problems in the forming process of MMCs due to their low ductility resulting from the incorporation of reinforcement. High strain rate superplasticity (HSRS) is attractive for industrial applications because superplastic forming at high strain rates can reduce forming time greatly. The strength of P/M 6061 Al and 6061 Al/SiC_p (3 m) composites during superplastic deformation at temperatures of 853 K–871 K and a high strain rate of 0.1 s^–1 has been studied in this paper. Experimental results presented a softening effect by the SiC_p reinforcement. Mechanical and microstructural analyses show that the decrease in the strength during high strain rate superlastic (HSRS) deformation is associated with the decreased grain size of the Al matrix with increase of the SiC_p volume fraction or the extrusion ratio, and the occurrence of liquid phase. The formation of the liquid phase was related to segregation of the solute atom during HSRS deformation.     

Experimental prediction of deformation mechanism after continuous dynamic recrystallization in superplastic P/M7475

The deformation mechanism in high-strain-rate superplastic P/M7475 before and after continuous dynamic recrystallization (CDRX) was investigated. The recrystallization process in P/M7475 differed from that in conventional superplastic material, I/M7475. In I/M7475, the fine-grained microstructure was obtained by static recrystallization before deformation. On the other hand, the substructure in P/M7475 evolved into fine grains during deformation by CDRX. The percentage of high-angle and random boundaries was low at an initial stage of deformation. However, it increased with strain in P/M7475. The microstructural change in P/M7475 influenced a deformation mechanism and affected grain boundary sliding (GBS). The ratio of contribution of GBS to total elongation was low at an early stage of deformation in P/M7475. However, it increased with deformation progressed. It is suggested that the deformation behavior in P/M7475 changed from dislocation creep to superplasticity as the dominant deformation mechanism changed to GBS. The activation energy for superplastic flow in P/M7475 was close to that for lattice self-diffusion in pure aluminum. It is therefore concluded that the dominant deformation mechanism after CDRX in P/M7475 is GBS accommodated by dislocation movement controlled by lattice self-diffusion, similar to that in I/M7475.     

Cavitation during superplastic flow of ternary alloys based on microduplex Pb-Sn eutectic

The effect of phases having a range of hardnesses on the superplastic tensile behaviour of microduplex Pb-Sn eutectic has been studied. The presence of relatively hard particles induced cavitation at particle/matrix interfaces during deformation in an otherwise noncavitating system, and the growth and interlinkage of cavities led to brittle superplastic fractures. Density measurements showed that cavitation increased as the volume fraction, hardness and size of intermetallic particles was increased. Increasing strain rate and decreasing deformation temperature also led to an increased level of cavitation. Cavity nucleation was attributed to the limited ability of the relatively hard phases to contribute to the accommodation processes occurring during superplastic flow.     

TC21合金在形变热处理工艺下的组织特征

对TC21钛合金在不同条件下超塑拉伸后,进行双重退火热处理,研究热加工工艺对TC21合金显微组织演变的影响。结果表明,当变形温度在890～960℃时,TC21合金的伸长率随变形温度的增加先增加后减少,最佳超塑性变形温度为910℃;TC21合金在α+β相区超塑变形,然后在α+β相区双重退火处理后得到双态组织;在β区进行超塑变形、α+β相区双重退火处理后得到网篮组织。     

Mechanisms of elevated-temperature deformation in the B2 aluminides NiAl and CoAl

A strain rate change technique, developed previously for distinguishing between pure-metal and alloy-type creep behaviour, was used to study the elevated-temperature deformation behaviour of the intermetallic compounds NiAl and CoAl. Tests on NiAl were conducted at temperatures between 1100 and 1300 K while tests on CoAl were performed at temperatures ranging from 1200 to 1400 K. NiAl exhibits pure-metal type behaviour over the entire temperature range studied. CoAl, however, undergoes a transition from pure-metal to alloy-type deformation behaviour as the temperature is decreased from 1400 to 1200 K. Slip appears to be inherently more difficult in CoAl than in NiAl, with lattice friction effects limiting the mobility of dislocations at a much higher temperature in CoAl than in NiAl. The superior strength of CoAl at elevated temperatures may therefore be related to a greater lattice friction strengthening effect in CoAl than in NiAl.     

The effect of layer number on the superplasticity of laminate 7475/2091Al alloy

The layer number is one of the important structure parameters in laminate material. For laminate 7475/2091, the laminate with same thickness but different layer number from 2 to 20 were processed and studied. Experiment results show that the layer number of the laminate has a strong influence on the superplasticity of the laminate. Microscopy observations indicate that a metallurgical bonding at interface is obtained between two components in the laminates. There is a diffusion effect region on each side of a bonding interface. Cavities nucleate and grow there, especially at bonding interfaces in optimum superplastic deformation condition. The cavity evolution during superplastic deformation is an important factor that affects total elongation rate of the laminate.     

Oxidation Behaviour of Sputtered Ni-3Cr-20Al Alloy

1. IntroductionNi-base superalloys are the important materialsused at high temperature and greatly rely on the presence of Cr and/or Al to form a protective scale ofCrook or Allot. Therefore, the investigation of oxidation of the simple Ni-Cr-Al ternary alloys is beneficial to the understanding of oxidation patterns of complex Ni-base superalloys. Some works have been conducted on the Ni-Cr-Al alloys with normal grain sizeand the results indicated that different oxides couldbe formed accor…     

Ni-Al系金属间化合物超塑性的研究进展

在一定组织形态和外界条件下,单相或多相的镍铝金属间化合物及其合金表现出超塑性变形行为,其中多相挤压态NiAl合金及单相NisAl合金的超塑性变形机制是晶界滑移(动态回复和再结晶协调变形);单相NiAl的超塑性变形则来自于变形过程中发生动态回复和再结晶;而定向凝固多相NiAl合金的超塑性则是在拉伸过程中动态再结晶与应变硬化平衡的结果.超塑性变形机制多样性源于镍铝金属间化合物晶体结构独特的物理性质.     

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.     

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

The superplastic behavior of medical magnesium alloys is reviewed in this overview article. Firstly, the basics of superplasticity and superplastic forming via grain boundary sliding (GBS) as the main deformation mechanism are discussed. Subsequently, the biomedical Mg alloys and their properties are tabulated. Afterwards, the superplasticity of biocompatible Mg-Al, Mg-Zn, Mg-Li, and Mg-RE (rare earth) alloys is critically discussed, where the influence of grain size, hot deformation temperature, and strain rate on the tensile ductility (elongation to failure) is assessed. Moreover, the thermomechanical processing routes (e.g. by dynamic recrystallization (DRX)) and severe plastic deformation (SPD) methods for grain refinement and superplasticity in each alloying system are introduced. The importance of thermal stability (thermostability) of the microstructure against the grain coarsening (grain growth) is emphasized, where the addition of alloying elements for the formation of thermally stable pinning particles and segregation of solutes at grain boundaries are found to be major controlling factors. It is revealed that superplasticity at very high temperatures can be achieved in the presence of stable rare-earth intermetallics. On the other hand, the high-strain-rate superplasticity and low-temperature superplasticity in Mg alloys with great potential for industrial applications are summarized. In this regard, it is shown that the ultrafine-grained (UFG) duplex Mg-Li alloys might show remarkable superplasticity at low temperatures. Finally, the future prospects and distinct research suggestions are summarized. Accordingly, this paper presents the opportunities that superplastic Mg alloys can offer for the biomedical industries.     

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。     

超塑处理对喷射成形GCr15钢超塑性的影响

研究了超塑处理对喷射成形GCr15钢超塑性的影响，超塑性拉实验结果表明，未经超塑处理的铸态试样，延伸率为119％，经过二次油淬和二次油淬＋高温回火超塑处理的两种试样，延伸率分别为328％和671％，变形温度和应变速率对GCr15钢的超塑性有一定的影响，但材料的微观组织对其超塑性具有决定性作用，超塑处理改变了材料试样的微观组织，导致其超塑性发生变化，经过二次油淬＋高温回火超塑处理后试样具有球化组织，其超塑性最好，未经超塑处理的铸态试样具有珠光体组织，超塑性最差，经过二次油淬超塑处理后试样的组织是马氏体和少量碳体物的混合，其超塑性介于上述两种试样之间，喷射成形工艺使GCr15钢获得均匀细化的稳定组织，这对于细晶超塑性是必要的，超塑处理材料的超塑性得到更大的提高。     

外应力场下NiAl合金微裂纹动态扩展的分子动力学模拟

目的 对NiAl合金中不同晶体取向的裂纹扩展动力学行为进行原子尺度研究,明晰在塑性变形过程     

The fracture process in a fine-grained superplastic 7475 Al alloy

In order to contribute towards alloy design and therefore an improvement in fracture toughness of engineering materials in general, the effect of temperature, strain rate and strain level on the superplastic deformation, cavity nucleation and growth, and fracture behaviour are studied in an important rate-sensitive structural engineering material, 7475 Al, in the light of current models and thinking. The efficacy of hydrostatic pressure in reducing cavitation during superplastic deformation is considered.     

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.