定向凝固 NiAl/Cr(Mo,Hf)合金的微观组织及力学性能

研究了定向凝固NiAl-28Cr-5.85Mo-0.15Hf合金的微观组织与在293～1373K温度范围内的力学性能.结果表明:合金是由NiAl枝晶轴和枝晶间区(NiAl和Cr(Mo)相的共晶)组成的.经过长期固溶时效处理NiAl/Cr(Mo)合金析出少量弥散分布的Huesler相,其余Hf以固溶体方式存在.DSNiAl-28Cr-5.85Mo-0.15Hf合金具有明显的韧脆转变行为,韧脆转变温度依赖于应变速率.室温拉伸断口呈现明显的解理断裂,而韧脆转变温度在以上时,合金具有较大的变形量,断口上有许多韧窝,呈现明显的塑性断裂特征.     

3004铝合金中的PLC效应

通过对3004铝合金进行系列温度拉伸试验,研究了其应力-应变关系曲线上出现锯齿屈服的现象,并结合动态应变时效机制对其进行了分析.试验结果表明:在3004铝合金发生DSA的温区内,低温区出现的正常的PLC效应是由于合金中Mg原子与位错交互作用造成的;而在高温区,随着大量沉淀相的析出,出现了反常的PLC效应.     

TC4-DT合金的超塑性变形及其本构方程

采用CMT4104电子万能拉伸试验机分别进行温度为870℃,应变速率为3.3×10-4s-1的恒应变速率和温度为850~890℃,应变速率为3.3×10-5~3.3×10-3s-1的应变速率循环法超塑性拉伸实验。结果表明:在变形过程中存在动态回复与动态再结晶现象,并采用Avrami方程描述了动态再结晶动力学行为;基于应变速率循环法获得了TC4-DT合金的本构模型,再通过1stopt软件加以回归拟合,得到较为精确的TC4-DT合金超塑性变形本构方程。     

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.     

低合金汽车结构钢动态响应模型研究

目的 研究低合金汽车结构钢不同应变速率下的动态响应行为,并提供一种简单准确的动态增长因子的预测方法,为内高压成形工艺提供参考.方法 进行高应变速率拉伸(动态拉伸)实验,并用高速摄像机和扫描电子显微镜对材料的拉伸行为和断口形貌进行表征,利用实验数据建立一系列结构钢的动态响应模型,研究不同应变速率下的响应特性.结果 随着应变速率的不断增大,应力-应变曲线出现周期性波动衰减,通过模型分析,发现在高应变速率下材料变形具有两阶段特征,与应力波传播的往返次数特征相似.结论 与Cowper-Symonds(C-S)模型拟合结果对比发现,所提出的两段式模型得到的实际结构钢屈服强度动态增长因子拟合度更高,可更好地对材料屈服强度进行预测.在实际内高压成形过程中应保证应变速率不宜过高,避免因应变速率过高导致的不均匀变形,影响材料成形质量.     

Zn-4%Al合金的超塑性变形行为

本文研究了经不同微细化处理后(～1μm)的Zn—4％Al合金超塑性变形行为和显微组织的变化。结果表明:热轧、淬火和冷轧三种状态只要晶粒达到微细化,都能呈现超塑性;三种状态的合金以V卡=lmm/mim的速度拉伸时延伸率随温度的变化出现两个峰值。微观组织观察表明,经超塑性拉伸变形后晶粒长大,并趋于等轴化。晶粒的长大除了该温度下的热长大外,应变对晶粒的长大也有显著的影响。Zn—4％Al合金在超塑性变形中产生空洞,多数空洞平行于拉伸轴方向被拉长,最后由于空洞的连接引起试样的断裂。     

钨合金材料的动态拉伸实验研究

采用旋转盘式间接杆-杆型冲击拉伸装置(SHTB)对颗粒度分别为2, 5μm和20μm的三种91%(质量分数)细化钨合金材料在动态冲击载荷作用下的力学性能进行了实验研究,分析了三种钨合金在应变率为0.001, 200, 500s-1时动态力学性能,给出了颗粒度大小与材料屈服强度的关系;采用扫描电子显微镜(SEM)对动态拉伸实验中回收的试件断口进行断口分析,研究钨合金在不同应变率状态下材料的破坏特征,在动态拉伸载荷作用下钨合金材料呈现出一种混合破坏模式包括钨颗粒的劈裂以及颗粒与基体界面的开裂.     

拉伸速率对Ni40 Ti50 Fe10合金环境氢脆的影响

研究了不同应变速率对Ni40Ti50Fe10合金环境氢脆的影响,并用扫描电子显微镜观察了拉伸断口的形貌.研究结果表明:Ni40Ti50Fe10合金在真空中的拉伸行为与应变速率无关,断口形貌主要是韧窝塑性断口.Ni40Ti50Fe10合金在空气中的塑性随着应变速率的降低而减小,断口形貌也从韧窝塑性断口转变为韧窝塑性断口和解理脆性断口的混合断口.Ni40Ti50Fe10合金在氢气中低应变速率拉伸时表现出明显的脆性,断口形貌主要为解理脆性断口.在高应变速率下(2×10-1s-1),Ni40Ti50Fe10合金在真空、空气和氢气中的塑性相近,说明在该应变速率下可以有效地抑制环境氢脆.Ni40Ti50Fe10合金的屈服强度不受应变速率和环境的影响.     

Pb-Sn共晶合金超塑性变形三维力学行为图

本文的研究首次表明,可以通过分析恒定拉伸速度得到的拉伸试验数据来建立超塑性变形的应力、应变速率及应变的三维力学行为图。在此基础上,对Pb—So共晶合金的三维力学行为进行分析,可以获得应变速率敏感性指数m值作为应变和应变速率的二元函数的变化(应变速率和应变的对数坐标系上的等m值曲线)。由此可以给出超塑性变形时应变和应变速率区的限度,有益于指导实际的超塑性成型。     

孔径对第三代镍基单晶高温合金DD33板式试样室温拉伸性能的影响

本文研究了孔径(φ=0.5、0.7及0.9 mm)对第三代镍基单晶高温合金DD33板式试样室温拉伸性能的影响。采用ARAMIS-光学动态应变测量系统原位观测了拉伸过程中不同孔径孔周围的应变分布,并利用扫描电镜观察了拉伸试样的断口形貌。结果表明:随着孔径的增大,试样的屈服强度和抗拉强度均降低。拉伸过程中孔周围产生应变集中,而且只在孔边很小的范围。弹性变形阶段,孔边应变增长缓慢;一旦发生塑性变形,应变快速增长。孔径越大,孔周围的应变集中越严重,应变梯度越大。上述研究结果对理解含冷却孔单晶叶片的变形行为,优化叶片结构,从而提高叶片寿命具有重要意义。     

Some guidelines for promoting high temperature deformation of metallic alloys

Capacity to reach large deformation at high temperature is an important issue in many forming processes of metallic alloys. It is well known that a low value of the stress exponent (or a concomitant high value of the strain rate sensitivity parameter) is a key point for controlling resistance to necking. A first way for decreasing the stress exponent is to get superplastic properties but it frequently requires dealing with fine microstructures which can be difficult to produce and to preserve. Moreover, in the case of single phase alloys, like aluminum or magnesium alloys, superplastic deformation generally induces damage which can result in premature fracture or damaged components after forming. The aim of this paper is to give some guidelines for promoting high temperature deformation of metallic alloys, with a particular attention given to superplastic forming. The possibility to reduce the temperature of superplastic forming (SPF) for titanium alloys, the capacity to get a better understanding of the specificities of damage process in the case of superplastic deformation and the ability to get large strains to fracture avoiding the production of fine grains before strain are more specifically discussed.     

Effect of strain rate sensitivity and cavity growth rate on failure of superplastic material

Abstract

Necking development and fracture strain of superplastic material under tensile load are analysed by introducing a model of cavity growth into the long wavelength approximation analysis which can describe the external neck development of specimens during deformation. The results show that both strain rate sensitivity m and cavity growth rate η have an important influence on the fracture strain of superplastic material. According to these results, a fracture diagram is presented in m–η coordinates, which is divided into three: a region in which material fails by macroscopic external necking, a region where cavity growth is predominant leading to fracture without pronounced external necking, and an intermediate region where both fracture modes occur. The prediction of fracture strain for various superplastic alloys exhibiting cavity growth during deformation is in good agreement with experimental results. The present analysis thus enables quantitative prediction of the effects of both strain rate sensitivity and cavity growth on superplastic fracture under uniaxial tension.

MST/491     

The effect of the second-phase volume fraction on the grain size stability and flow stress during superplastic flow of binary alloys

This paper considers to what extent the second-phase volume fraction in superplastic binary alloys affect the matrix grain size stability during deformation and, through it, the flow stress at constant temperature and strain rate. It is shown for five different superplastic binary alloy systems, that at constant temperature and strain rate the flow stress will increase with the deviation of the second-phase volume fraction in the alloys from that required for maximum matrix grain size stability. A new parameter (Z) which quantifies these deviations has been introduced in this paper. The possible errors in determining the pertinent parameters in the rate equation for superplastic flow by testing alloys withZ is discussed.     

异步轧制AZ31镁合金板材的超塑性工艺及变形机制

经过异步轧制工艺获得AZ31镁合金薄板。在300~450℃范围内,分别通过5×10-3,1×10-3s-1和5×10-4s-1不同应变速率进行高温拉伸实验研究其超塑性变形行为,计算应变速率敏感指数m值、超塑性变形激活能Q及门槛应力σ0值。通过EBSD分析和扫描电镜观察拉伸断裂后的断口形貌,分析AZ31镁合金的超塑性变形机制。结果表明:AZ31镁合金的塑性变形能力随着变形温度的升高及应变速率的降低而增强。当拉伸温度为400℃、m=0.72、应变速率为5×10-4s-1时,AZ31具有良好的超塑性,伸长率最大为206%。温度为400℃时,异步轧制AZ31镁合金的超塑性变形是以晶格扩散控制的晶界滑移和基面滑移共同完成的。     

Superplastic deformation and postformed mechanical properties of high temperature titanium alloy IMI 834

Abstract

Superplastic forming is particularly attractive for high temperature Ti alloys because of the much lower forming stresses compared with those encountered during forging. The superplastic deformation parameters of IMI 834 sheet were obtained at 900, 940, and 990°C. At 990°C, IMI 834 shows low flow stresses, high values of strain rate sensitivity, and minimum strain anisotropy, however, 300% superplastic elongation was readily obtained at the lower forming temperature of 940°C but with a higher flow stress. A reduction in the room temperature and 600°C tensile properties with superplastic strain resulted from strain enhanced grain growth during superplastic deformation; this effect was greatest at 990°C. Aging of post 990°C superplastically formed material was studied. The creep performance of IMI 834 was found to be slightly reduced by superplastic forming. These properties and the changes in the microstructure and texture are compared with other Ti alloys under superplastic conditions.

MST/1822     

Superplastic power-law creep of Sn–40%Pb–2.5%Sb peritectic alloy

The power law-creep behavior of superplastic Sn–40Pb–2.5Sb alloys with different grain sizes has been investigated at room temperature. Stress exponent values for these alloys have been determined by indentation creep, conventional creep and uniaxial tension tests in order to evaluate the correspondence of indentation creep results with conventional tests. In all cases, the indentation results were in good agreement with each other and with those of the tensile and conventional creep tests. The average stress exponent values of about 2.6 and 3.0 corresponding to the strain rate sensitivity (SRS) indices of 0.33–0.39, depending on the grain size of the materials, indicate that the grain boundary sliding is the possible mechanism during creep deformation of Sn–Pb–Sb alloys. Within limits, the indentation tests are thus considered useful to acquire information on the creep behavior of small specimens of these soft tin–lead–antimony alloys at room temperature. It is also demonstrated that the indentation creep test provides a convenient method to measure SRS and thereby to assess the ability of a material to undergo superplastic deformation.     

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.     

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.     

Threshold stress in dispersionally strengthened superplastic Al alloys

It is important for practical applications that some commercial alloys with stabilized finegrained structure should exhibit superplastic behaviour at high temperatures. In this paper the results of impression creep tests conducted on AlMgZn alloys are reported and the strain rate sensitivity and activation enthalpy were determined. The mechanical behaviour of the alloys as a function of the strain rate sensitivity can be divided into three regions. At low and high stresses the strain rate sensitivity parameter is low and the deformation process is not superplastic. Superplastic deformation takes place only at intermediate stresses. The microstructural interpretation of these processes involves, in general, the change of the micromechanisms controlling the different deformation processes. It was determined that by the supposition of a threshold stress depending strongly on temperature, the two regions due to low and intermediate stresses of the deformation can be described by the same constitutive equation.     

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.