30CrMnSiA钢超塑流变中的力学行为和显微组织的研究

经两次调质热处理细化的30CrMnSiA 钢在770℃以■=2.78×10~(-4)·S~(-1)的应变速率拉伸下呈现了良好的超塑性:δ=867%,σ=34.3MN/m~2,m=0.41。在拉伸时,经预处理的非平衡组织会经碎化而变成微细等轴的晶粒组织,并具有两相体积分数近似相等的α+v 双相组织。这种组织在超塑性流变过程中具有很高的稳定性。此钢对空洞的敏感性比较低,但在变形后期,由于空洞的形成、长大和连接而导致试样呈空洞型沿晶断裂。     

20Cr2Ni4A钢低温回火脆性探讨

用光学金相及扫描电镜分析方法研究了２０Ｃｒ２ｎＩ４Ａ钢的低温回火脆性机理。研究结果表明，钢在２５０－３５０℃区间回火时出现低温回火脆性，其原因与碳化物沿奥氏体－马氏体相界面板出及杂质元素偏聚于原奥氏体晶界有关，得易导致沿晶断裂。     

铝硅合金超塑性研究

本文对 Al—13Si 共晶合金在超塑性变形时产生早期断裂的原因进行了研究。对影响Al—Si 共晶合金断裂的因素——第二相 Si 粒子形状、Si 粒子长大以及 Si 相与α相高温硬度差进行了详细分析。文章揭示出 Al—13Si 共晶合金的超塑性拉伸断裂是外部无颈缩的空洞型断裂。Si 粒子周围产生空洞是由于晶内位错堆积在 Si 粒子周围,造成应力集中,以及 Si 相与α相高温硬度相差悬殊,不能协调变形引起的。第二相 Si 粒子为带有尖角的短棒状,使得空洞沿尖角指向不均匀扩展,导致该合金发生早期断裂。     

铸造和轧制的Hy-130钢板氢致开裂敏感性的比较

采用改进的植入试验比较了铸造和热轧的HY-130钢的氢致开裂(HAC)敏感性。使用了2ppm和19ppm两种不同氧含量的焊缝。氢含量为2ppm时,所有材料在所有应力水平下的主要断裂方式是显微空洞的聚集。氢含量为19ppm时,所有材料在接近下临界应力(LCS)时的主要断裂方式是晶间断裂,而在高应力下,显微空洞的聚集仍是主要断裂方式。采用LCS和脆化指数来评定轧制的和铸造的材料对HAC的敏感性。除了铸造材料的熔炼炉次在氢含量为2ppm时最抗HAC、在氢含量为19ppm时最不抗HAC外,所有材料是类似的。氢含量为2ppm时,沿焊缝界面和靠近焊缝界面的热影响区部分的区域,是所有材料的断裂位置。氢含量为19ppm时,这一区域仍然是铸造材料的断裂位置,但是在这种情况下轧制板材的断裂位置却变成在焊缝金属内。将结果与以前发表的HY-130钢的HAC结果作了对比,发现在断裂方式和断裂位置方面有差异。根据依赖于氢浓度的断裂方式解释了这些差异。发现HY-130钢的氢致开裂系因显微空洞的聚集、准解理断裂和晶间断裂所致,这就进一步支持了由比彻姆提出的HAC模型[12]。     

Al-6Cu-0.5Zr合金超塑变形后微观结构的研究

相分析表明该合金由Al基固溶体和CuAl_2、ZrAl_3两种粒子所组成。合金超塑性变形后,利用标记线法测定了晶界滑移量对总变形量的贡献为60％左右。在δ≈30％的试样上观察到晶界变宽,在晶界上呈现折皱区,并在遇到第二相时改变方向。透射电镜分析表明,晶界滑移时出现晶界位错,在三晶交界处或晶界坎处向晶内激发位错,晶界是位错源与壑,激活的晶内位错通过滑移和攀移会形成位错亚晶界,晶内位错的激活与运动是晶界滑移的重要协调机制,晶界滑移与晶界位错运动有关。合金超塑性变形时,在晶界和CuAl_2相界处有空洞形成,研究了空洞面积分数与面缩率的关系。靠近断口处,空洞数和面积分数急剧增加,说明空洞的增殖、扩展和连接导致断裂。     

提高高强度钢使用寿命的科技进展

系统介绍了近5年在国家973重大基础研究规划支持下,由一支多专业结合的团队,开展了"提高钢铁质量和使用寿命的冶金学基础研究".为提高工程结构用钢的使用寿命,研究了高强度廉价耐候钢,特别是Mn-Cu-细晶,P-RE-超细晶的协同作用和超低碳贝氏体组织对钢耐蚀性的影响;为提高机械制造用钢的使用寿命,研究了高强度钢的超高周疲劳断裂机制,特别是夹杂物尺寸对钢疲劳性能的影响;为提高合金结构钢的质量研究了微米级夹杂物的形成理论和控制技术.     

超高强度钢低倍粗晶与高倍组织的关系

对一种飞机起落架用超高强度钢的低倍粗晶与高倍组织进行了对应研究。结果表明，低倍粗晶不仅是粗大奥氏体晶粒的宏观表现，还与一种特殊上贝氏体组织的存在有关。     

IF钢压薄滚焊接头断裂机理

研究了ＩＦ钢压薄滚焊接头断裂的机理，结果发现，裂纹首先在熔合底部的熔核区以微孔聚合的方式形成，并向粗晶区扩展，然后裂纹在粗晶区内一沿一定的解理面扩展，最后在部分再结昌区以滑移面分离的方式断裂。     

形变温度对高碳铬硅钢超塑性能的影响

通过超塑拉仲试验(初始应变速率5×10~(-4)s~(-1))和对形变试样徽观形貌的观察,研究了高碳铬硅钢在650—800℃温度范围内的超塑性能.经过对拉伸数据的分析,发现该钢在所试验的温度范围都可超塑成形,其最佳超塑温度为973K。微观组织的分析表明,该钢在较低温度形变时的破坏为延性断裂,而在1073K 形变时的断裂则是空洞扩展造成的脆性断裂。文中还讨论了形变造成钢中渗碳体和晶粒粗化加速的机理。     

X100管线钢在含SRB的盐碱土壤溶液中的SCC行为

为证实SRB对X100管线钢在土壤中应力腐蚀行为的影响,采用慢应变速率拉神(SSRT)实验和SEM研究了X100管线钢在含有SRB的海滨模拟盐碱土壤溶液的应力腐蚀开裂行为.结果表明:X100钢母材和焊缝在无菌的海滨模拟盐碱土壤溶液的断裂模式为穿晶+沿晶SCC混合断裂,而在有菌的海滨模拟盐碱土壤溶液的断裂模式为穿晶SCC断裂;且X100钢母材和焊缝在无菌的海滨模拟盐碱土壤溶液中的SCC敏感性高于有菌时的,说明SRB的存在抑制了X100钢的脆变,导致X100钢的SCC敏感性降低.     

等温锻造TiAl合金超塑拉伸断裂机理的研究

通过对等温锻造ＴｉＡｌ合金的超塑拉伸断口和组织的扫描电镜观察，研究了其超塑拉伸断裂的机理。发现等温锻造ＴｉＡｌ合金在超塑拉伸过程中，不同形态的孔洞的产生，长大和连是导致试样断裂的主要机制。     

Mechanisms of cavity growth in a fine-grained 7475 Al superplastic alloy

Under creep conditions, cavity growth may be controlled by vacancy diffusion or power-law creep. The two growth mechanisms are examined with reference to a superplastic 7475 Al alloy as a function of test temperature, superimposed strain rate and starting grain size. It is found that power-law cavity growth by plastic deformation of the matrix surrounding the cavities dominates at all test conditions. In addition, growth and interlinkage of cavities is an important parameter in controlling the ease and type of fracture and is enhanced by the ease of superplastic deformation.     

Superplasticity and cavitation in AZ31 Mg alloy at elevated temperatures

Superplasticity of AZ31 alloy at elevated temperatures was examined by uniaxial tension test. Optical microscopy (OM) and scanning electronic microscopy (SEM) were employed to observe the cavities and topography during the superplastic deformation of AZ31 alloy, respectively. It is indicated that the intergranular cavities are formed by the continuous fracture of filaments along grain boundaries and the filaments are not formed by liquid phase but by severe elongation of O-shaped cavities under tensile stress. Finally, a filament-induced cavitation model was proposed.     

定向凝固NiAl合金的拉伸行为研究

研究了定向凝固NiAl-Mo(Hf)和NiAl-Fe(Nb)合金的拉伸行为和显微组织变化.结果表明,两种合金在一定的拉伸条件下均具有反常的屈服行为和中温脆性.反常屈服和中温脆性行为与合金中含有的Ni3Al相有关.两种合金在高温时还表现出高应变速率的超塑性变形行为.超塑性变形的主要机理是位错滑移和攀移产生的应变硬化与动态回复和动态再结晶的应变软化作用相平衡.超塑性变形试样的断口呈韧性特征,在断裂区有孔洞产生.     

7B04铝合金超塑变形过程中空洞的演变和能量耗散

对平均晶粒尺寸分别为10和20 μm的7B04铝合金板材在530℃/3×10^-4 s^-1变形条件下开展了不同变形量的超塑拉伸实验。结果表明,随着变形量的增大空洞形态的变化为：空洞形核→球形空洞弥散分布→非球形空洞沿拉伸方向伸长→空洞沿拉伸方向聚合→大尺寸空洞的非拉伸方向聚合。在拉伸断裂前的变形阶段,合金组织中出现尺寸大于260 μm的聚合空洞。在空洞聚合的初期,沿拉伸方向的空洞聚合不会使材料断裂。大尺寸空洞沿非拉伸方向聚合,是判断材料失稳的依据。根据实验数据计算空洞长大的公式并绘制了空洞的演变机理图,包括空洞的形核、扩散长大、塑性长大和聚合长大的公式,据此可判断空洞的形态和材料失稳。根据组织演变建立了空洞扩散、塑性长大的物理模型,可用于计算超塑变形过程中空洞演变所需的能量耗散和绘制能量耗散图。     

Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

Studies of cavitation in Y-TZP during superplastic flow have been made for both tensile and compressive deformation conditions. It was observed that the morphologies of cavities near the fracture faces of tensile specimens varied markedly with testing conditions and in most cases differed from those near the gauge heads. Two quite different forms of cavitation behaviour were observed leading to high and low strains to failure, respectively. For optimum conditions of superplastic flow, of high temperature/low strain rate (low stress), when large elongations were observed, cavities were either spherical or elongated parallel to the tensile axis. Those near the fracture face interlinked in a plastic (necking) mode to give transverse cavities and subsequent failure. At high strain rate/low temperature (high stress), transverse intergranular cracking played a dominant role in failure at low elongations. For intermediate conditions of temperature/strain rate, elongated cavities developed parallel to the tensile axis, but near the fracture face these usually interlinked by transverse cracking. These conditions were associated with intermediate elongations to failure. For the assessment of cavity growth mechanisms, artificial pores were introduced into fine grain Y-TZP specimens and changes in their shape and size during tensile or compressive deformation were investigated. Results show that the change of pore volume, in the superplastic regime, is controlled by plastic deformation of the matrix and can be described by the relationship of dR/d = ;R, where is the true strain, the cavity growth rate parameter and R is the radius of the pore.     

Cavitation and fracture in the superplastic Al-33% Cu eutectic alloy

Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on the- interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys     

Cavitation during diffusion creep of a magnesium alloy

Cavities associated with grain-boundary triple edges have been found in a Mg-0.55 Zr alloy after diffusion creep and the angular distribution of cavitated boundaries measured; cavities occur predominantly on boundaries normal to the stress axis and are rounded in form. Nucleation of cavities can, therefore, be associated with grain-boundary sliding and their growth with diffusion. It is suggested that similar cavities found in some alloys after superplastic deformation may also result from significant diffusion creep but this is not typical of all superplastic deformation.     

Effect of thermomechanical treatment of 0.45 wt.% C steel in the superplastic mode on plastic zone parameters after impact fracture in the ductile-brittle transition temperature range

Transmission electron microscopy was used to study the structure of plastic zones (PZ) resulting from crack propagation during impact bending tests of a steel containing 0.45 wt.% C at +20, −40, −110 °C in the as-received state (d_g ≈ 45 μm) and after super-plastic thermomechanical treatment at intercritical temperatures (d_g ≈ 2–5 μm). Scanning electron microscopy was used to study the sample fracture surfaces. Superplastic deformation of steel was found to be responsible for the 2–6-fold increase in dislocation density in PZ induced by low-temperature plastic deformation and also the crack branching and secondary cracking as compared with the as-received state. The cold formability of the steel subjected to superplastic thermomechanical treatment can be expected to improve.     

Fracture of a tin-lead eutectic alloy in Region III of superplastic flow

With the aid of scanning electron microscopy, cavitation and fracture behaviour in the Sn-Pb eutectic alloy, whose reduction in area of cross-section before failure is close to 100%, has been investigated in Region III of superplastic flow (where both the elongationto-fracture and the strain-rate sensitivity index decrease with increasing strain rate). It has been demonstrated that, although it decreases, grain-boundary sliding persists in this range as the strain rate is increased. At all strain rates the final failure was due to tearing by plastic flow of the inter-cavity ligaments, but the interlinkage of cavities along the graininterphase boundaries decreased with increasing strain rate. The features of cavitation and fracture did not differ much from an earlier study on a pseudo-single phase copper alloy, although copper alloys usually fail non-ideally, i.e., a large area of cross-section is present at fracture.