Superplastic behaviour of Ti-48at.%Al two-phase titanium aluminide compacts made from mechanically alloyed powder

An HIP compact of MA-processed powder having a nominal composition of Ti-48at.% Al was produced. The compact consisted of a large amount of TiAl(λ) and a small amount of Ti₃Al (₂), in a completely ultra-fine equiaxed grain structure. This two-phase compact showed typical superplastic deformation behaviour. A maximum elongation of 550% was obtained. A strain exponent, n = 2, and grain size exponent, p = 2, were determined from the results of a strain-rate-change test and a creep test at constant initial stress using samples having various grain sizes, respectively. The activation energy for creep, Q_c at constant stress was calculated to be 350 kJ/mole. It is concluded that the superplastic deformation mechanism of the material under study is grain boundary sliding controlled by lattice diffusion in the TiAl phase.     

On the applicability of a mesoscopic interface sliding controlled model for understanding superplastic flow in bulk metallic glasses

It is demonstrated that a mesoscopic interface sliding controlled flow model, which has already been shown to account for superplastic deformation in different types of crystalline materials, is also capable of describing superplastic flow in bulk metallic glasses. The only difference is that the random high-angle grain boundaries in crystalline materials along which deformation is concentrated, have to be replaced by the transient interfaces which are formed by interconnecting shear transformation zones in the region of homogeneous flow in bulk metallic glasses. Comparison with experimental results concerning superplastic flow in eight bulk metallic glasses shows that the numerical solutions obtained in the paper for the transcendental stress–strain rate equation of superplastic deformation lead to accurate predictions.     

Superplasticity of fine-grained 7475 Al alloy and a proposed new deformation mechanism

A study has been made to investigate the superplastic deformation mechanisms of 7475 Al alloy in relation to the variation of grain size ranging between 5.5 μm and 13 μm. The strain-rate sensitivity (m) was increased with decreasing grain size in the superplastic deformation regime. Microstructural investigation after tension tests revealed that the dispersoid free zones were produced mostly at the grain boundaries normal to the tensile direction. A new model for describing the deformation behavior of the 7475 Al alloy has been proposed based on the assumption that the grain boundary sliding was accommodated by both diffusional flow and slip. This new model well predicts many aspects of experimental results.     

Fe_3Al－Ti超塑性变形过程中晶粒形态演化规律

对Ｆｅ_３Ａｌ－Ｔｉ合金超塑性变形中不同应变量下的晶粒形态进行了研究。发现Ｆｅ_３Ａｌ－Ｔｉ超塑性行为与连续再结晶有关，随变形量增大，晶粒逐渐细化，但晶粒形状变化不大，并沿拉伸方向有所伸长。ＴＥＭ分析表明，晶粒细化过程与超塑性过程中亚晶界向大角晶界的演化有关。本文对Ｆｅ３Ａｌ－Ｔｉ合会超塑性变形机理进行了初步的探讨。     

Tensile superplastic deformation of YBa2Cu3O7−x high-TC superconductors

The superplastic behavior of polycrystalline YBa₂Cu₃O_7−x was investigated. Tensile elongations above 85% were achieved for fine-grained (<1 μm) microstructures tested in the temperature range of 800–875°C and strain rates varying from 6×10⁻⁶ to 10⁻³/s. It is suggested that the dominant superplastic deformation mechanism is grain boundary sliding accommodated and controlled by interface reaction, characterized by a stress exponent of n=2, a grain size exponent of p=1.5, and an activation energy of Q_sp=515±104 kJ/mol. A Langdon–Mohamed deformation mechanism map was constructed. Overlaying the available experimental data onto the map, including the results from creep studies, gives an insight into the deformation mechanisms involved in high-temperature deformation of YBa₂Cu₃O_7−x.     

Er、Zr微合金化5083铝合金的超塑性

针对5E83合金(Er、Zr微合金化5083合金),采用超塑性拉伸试验、扫描电镜(SEM)、电子背散射衍射(EBSD)和透射电镜(TEM),探究了Er、Zr微合金元素、晶粒尺寸、变形温度、应变速率对合金超塑性的影响。通过再结晶退火、空冷和水冷的搅拌摩擦加工(FSP),分别获得了晶粒尺寸为7.4、5.2、3.4μm的完全再结晶组织,作为初始状态进行超塑性拉伸。结果表明,初始晶粒尺寸越细小,超塑性伸长率越高。当晶粒尺寸>5μm时,超塑性变形过程晶粒粗化缓慢,细化初始晶粒可显著提高超塑性;而当晶粒尺寸<5μm时,超塑性变形过程晶粒粗化严重,进一步细化初始晶粒对超塑性的提高有限。不同变形温度、应变速率的超塑性拉伸结果显示在变形温度为450～540℃、应变速率为1.67×10^-4～1.67×10^-1 s^-1,超塑性伸长率随变形温度和应变速率的提高呈现先上升后下降再上升的趋势;变形温度为520℃、应变速率为1.67×10^-3 s^-1条件下,水冷FSP态合金获得最大伸长率330%...     

High temperature mechanical characteristics of superplastic yttria-stabilized zirconia. An examination of the flow process

Extensive high temperature creep data are available in the literature for superplastic yttria-stabilized tetragonal zirconia polycrystals containing from 2 to 4 mol% of Y₂O₃ (termed YSZP). The different models developed to account for these experimental data are critically analyzed. It is shown that grain boundary sliding with threshold stress can predict the mechanical (experimental data found for n, p and Q) and microstructural features in high purity YSZP over the range of stresses, grain sizes and temperatures examined experimentally. The physical origin of the threshold stress is discussed. This formalism is essentially similar to that used for explaining the superplastic behavior of metals.     

Superplastic response in Al-Mg sheet alloys

The ability to achieve large strains to failure coupled with extremely low flow stresses makes superplastic forming (SPF) an attractive option in the automotive industry for the manufacture of complex parts from aluminum (Al) sheet. However, a barrier to increased usage is the cost penalty associated with superplastic alloys, which are specially processed to have a small and stable grain size. In this article, high-temperature tensile tests are used to compare the superplastic performance of two different Al-Mg alloys that were specially processed for SPF with that of a conventionally processed Al-Mg alloy. The results of the tensile tests and optical microscopy are used to highlight the mechanisms that control deformation in each of these alloys under different test conditions. Failure in both types of materials was found to change from internal cavitation to external necking with increases in strain rate. The specially processed alloys experienced minimal grain growth or grain elongation during forming, and therefore it was assumed that deformation was controlled by grain boundary sliding. Contrary to this, the conventionally processed alloy experienced significant grain growth at the higher test temperatures, and hence it was concluded that deformation was at least partially controlled by some mechanism other than grain boundary sliding. The different deformation characteristics resulted in a different set of optimal forming conditions for the two types of materials. The SPF alloys displayed higher strains to failure at the slower strain rates and higher temperatures, while the conventionally processed alloy displayed higher strains to failure at the faster strain rates and lower temperatures.     

Creep deformation mechanisms in fine-grained niobium

Creep rates in fine-grained Nb were measured at 600 °C using free-standing Cu/Nb polycrystalline multilayered foils. For specimens with layer thicknesses ranging from 0.5 to 5 μm and Nb grain sizes ranging from 0.43 ± 0.05 to 1.87 ± 0.13 μm, two distinct regimes were observed. At high stresses, the stress dependence, grain size dependence and activation energy for creep are consistent with power-law creep, with an average stress exponent of 3.5. At low stresses, creep rates exhibited a linear dependence on stress and an inverse linear dependence on grain size. A model is presented for a vacancy generation-controlled creep mechanism, whereby deformation rates are controlled by the rate of vacancy generation at or near grain boundaries, not by their diffusion. The proposed model is consistent with experimental observations of stress and grain size dependence, as well as the measured activation energy for creep.     

2091铝锂合金超塑变形的晶内断裂

研究了２０９１铝锂合金超塑变形的断裂行为。扫描电镜观察表明,２０９１铝锂合金超塑变形中存在晶内断裂,光学金相观察表明断裂试样存在较活跃的动态再结晶和较多的大轴径比晶粒,透射电镜观察发现断裂试样内部存在较多的晶内位错。研究表明,由于再结晶缩小了大轴径比晶粒的横截面使其在超塑变形的晶粒转动中受到很大的弯曲应力而造成了晶内断裂。     

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

The mechanical behaviors of consolidated iron with average grain sizes from tens of nanometers to tens of microns have been systematically studied under uniaxial compression over a wide range of strain rates. In addition to the well-known strengthening due to grain size refinement, grain size dependence is observed for several other key properties of plastic deformation. In contrast with conventional coarse-grained Fe, high-strength nanocrystalline and submicron-grained Fe exhibit diminished effective strain rate sensitivity of the flow stress. The observed reduction in effective rate sensitivity is shown to be a natural consequence of low-temperature plastic deformation mechanisms in bcc metals through the application of a constitutive model for the behavior of bcc Fe in this strain rate and temperature regime. The deformation mode also changes, with shear localization replacing uniform deformation as the dominant deformation mode from the onset of plastic deformation at both low and high strain rates. The evolution and multiplication of shear bands have been monitored as a function of plastic strain. The grain size dependence is discussed with respect to possible enhanced propensity for plastic instabilities at small grain sizes.     

5A90铝锂合金电子束焊接头超塑性变形组织演变

对5A90铝锂合金电子束焊接头进行高温拉伸试验,使用光学显微镜观察试样变形过程中的组织演变,并对变形机理进行分析.结果表明,超塑性变形初期,接头超塑性变形机制以扩散导致的晶界迁移为主,焊缝细小等轴晶粒迅速长大.当应变大于100%时,接头中大晶粒开始发生动态再结晶,超塑性变形机制转变为动态再结晶机制.在超塑性变形过程中热影响区平均晶粒尺寸与焊缝平均晶粒尺寸逐渐接近,组织存在耦合均匀化过程.提出采用均匀化系数K来表征焊缝与热影响区的组织均匀化程度,随着变形的进行,K值逐渐升高.     

定向凝固Ni3Al合金高温变形后的显微组织特征

利用金相,扫描电镜和透射电子显微镜研究了定向凝固Ｎｉ３Ａｌ合金高温变形后的显微组织特征。研究结果表明,当变形速率较快时,原始柱状晶晶界无明显变化,晶内无明显结构存在;当变形速率较慢,合金呈现塑变形时,变形初期柱状晶晶界呈现“锯齿状”,后期原始柱状晶界消失,代之以晶粒尺度约为１５ｍｍ的晶粒带,晶粒带中既有小角度晶界,也有大角度晶界。     

A mesoscale model to predict the effect of microstructural heterogeneities on superplastic deformation

A two-dimensional approach is proposed to describe superplastic deformation at a mesoscopic scale in order to predict the effect on the mechanical behaviour of microstructural heterogeneities, like grain size distribution or spatial distribution of grains. The grains in the microstructure may slide and plastically deform, the extent of each mechanism depending on the applied conditions. The degree of homogeneity of deformation through the material is estimated by the predictions of differences in shear rates along bands specifically oriented in reference to the stress axis. It is shown that the superplastic behaviour not only depends on the grain size distribution but also on the degree of dispersion of the large grains through the microstructure, despite a same mean grain size. This dependence may concern the macroscopic behaviour as well as the extent of localisation of deformation through the sample.     

Superplasticity in fine-grained AZ61 magnesium alloy

The effect of grain refinement on the superplasticity of ingot-processed magnesium alloy was investigated. From the AZ61 material with a linear intercept grain size of 5 μrn, which was obtained by the multi-rolling process at an elevated temperature, tensile elongation over 400% could be achieved at 10^-3s^-1 at 400°C with a maximum value of 560% at 2x10^-4s^-1 at the same temperature. It was found that grain boundary diffusion controlled grain boundary sliding and pipe diffusion controlled slip creep govern the plastic flow at low and high strain rate ranges, respectively. A deformation map for pure magnesium was constructed to examine the effect of grain size and flow stress on deformation behavior at elevated temperature. The superplastic formability of Mg alloys was demonstrated by forming an AZ61 sheet into a hemi-sphere.     

纳米面心立方金属的变形机制及影响其力学性能的因素

综述了纳米面心立方金属的变形机制随晶粒尺寸的减小而发生的变化,即变形机制由晶界处发射不全位错、形成孪晶转变为晶界滑移、晶粒转动.当变形机制为晶界处发射不全位错、形成孪晶时,存在最佳孪晶形成晶粒尺寸范围,此时的孪晶形核应力最小.另一方面,随着晶粒尺寸的减小,在变形机制发生转变的临界晶粒尺寸附近存在韧-脆断裂方式的转变.提高孪晶密度、在纳米晶材料中加入微米晶相形成双峰晶粒材料可以提高纳米晶材料的塑性,得到更好的综合机械性能.     

SUPERPLASTICITY OF A SiC_w/2024 Al COMPOSITE MADE BY PRESSURE INFILTRATION

The superplastic characteristics of the β-SiC whisker reinforced 2024 aluminum composite, fabricated by pressure infiltration and hot-rolling after extrusion, were investigated. The composite has a fine grain size of about 1μm, and exhibits a maximum tensile elongation of 370% in the initial strain rate of 3.3×10-3s-1 at 788K.The superplastic deformation mechanism of the composite is thought to be grain boundary (interface) sliding accommodated by grain boundary diffusion of aluminum atom and an appropriate amount of liquid phase.     

Effect of current pulses on fracture morphology in superplastic deformation of 2091 Al Li alloy

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初始晶粒度对GH720Li合金等温热变形组织演变的影响

对3种不同初始晶粒度的锻态GH720Li合金的等温热变形组织演变行为进行研究。结果表明,细于ASTM 6.5级的晶粒度是最终获得均匀细小热变形晶粒组织的关键临界初始组织条件,而初始ASTM 10级以上晶粒度合金具有较宽的热加工窗口。初始晶粒度对流变行为的影响非常明显,晶粒组织越粗大,合金的流变抗力越大。初始ASTM3级合金热变形时不连续和连续动态再结晶都发挥重要作用,初始ASTM 6.5级合金热变形时以不连续动态再结晶为主,初始ASTM 10级合金热变形时除了动态再结晶,超塑性变形机制也起重要作用。进一步确定了初始ASTM 6.5和ASTM 10级晶粒度的合金获得均匀细晶组织的热加工区间。