纯钼高温塑性变形及流变应力行为

采用MMS-300热模拟实验机研究纯钼在变形温度为900~1 300℃和应变速率为0.004~1 s-1条件下的高温塑性变形行为。分析了纯钼流变应力与应变速率、变形温度之间的关系,计算了纯钼高温塑性变形时的变形激活能。研究结果表明:纯钼在热变形过程中流变应力随应变速率的增加而增加,随温度的升高而降低,且其高温塑性变形行为可以用Zener-Hollomon参数的流变应力方程进行描述。该纯钼在实验条件范围内发生了明显的动态回复与动态再结晶。     

超细晶纯锆热压缩变形行为及组织演变研究

采用Gleeble-3500热模拟机对等通道转角挤压(ECAP)+旋锻复合细化工艺制备的超细晶纯锆,在温度为300～450℃、应变速率为1×10~(-3)～1×10~(-1) s~(-1)的条件下进行压缩实验,分析了超细晶纯锆在热变形过程中流变应力特点及显微组织演变规律。并使用人工神经网络建立了超细晶纯锆压缩本构模型。结果表明:变形初期流变应力迅速升高,达到峰值应力后逐渐进入稳态流变阶段。随着温度的升高和应变速率的降低,稳态流变应力降低。超细晶纯锆屈服强度显著高于粗晶纯锆;超细晶纯锆的应变速率敏感指数m值为0.028～0.132,高于粗晶纯锆,而且低应变速率和高温有利于提高超细晶纯锆的塑性;透射电镜(TEM)结果表明:随着变形温度的升高和应变速率的降低,超细晶纯锆呈现明显的动态回复与再结晶,晶内位错密度减小,晶界逐渐清晰,晶粒尺寸逐渐增大。基于人工神经网络的压缩本构模型预测结果表明:预测值与实验值的平均相对误差(AARE)为0.5385%,相关系数(R)为0.9991,模型预测精度高。     

超细晶钛铝基合金的高温压缩性能研究

采用高能球磨及真空热压烧结的方法制备超细晶/纳米晶双相γ-TiAl基合金,将名义成分为Ti-45Al-7Nb(%,原子分数)的混合粉末经40 h高能球磨后,粉末达到纳米级。球磨后的混合粉末经真空热压烧结(烧结温度1200℃,压力30 MPa,保温保压1 h)。研究该合金在温度为1000,1050和1100℃,应变速率为1×10^-4,1×10^-3和1×10^-2s^-1 3个变形速率条件下的高温压缩组织、流变行为和本构关系。研究结果表明：经过高能球磨及真空热压烧结原位合成的组织为超细晶α₂-Ti₃Al及γ-TiAl双相等轴状合金组织,晶粒尺寸小于5μm。合金为热敏感型和应变速率敏感型合金,合金压缩流变应力随应变速率的降低和温度的升高而下降。高温热压缩时,合金组织由规整等轴状被压变形为长条形,形变主要发生在γ-TiAl相中,晶界和γ相晶内可见位错及孪晶,孪晶及位错为主要的形变机制。在1000,1050和1100℃,1×10^-4,1×10     

高能球磨-放电等离子烧结法制备双晶分布钛

以平均粒径约150μm的球形钛粉为原料,采用高能球磨结合放电等离子烧结技术制备由双尺度晶粒组成的高致密纯钛块体材料,研究高能球磨过程中钛粉的形貌、尺寸及显微组织的变化,分析球磨钛粉放电等离子烧结时的致密化行为和显微组织的演变规律,测试烧结钛块体材料的室温压缩性能。结果表明:钛粉在球磨初期发生剧烈的塑性变形并相互焊合,形成层片状团聚粉末。球磨10 h时,钛粉的部分晶粒细化至40~100 nm。放电等离子烧结过程中,随烧结温度升高和烧结时间延长,烧结钛的密度逐渐增大。在烧结温度为800℃、保温时间为4 min、烧结压力为50 MPa的条件下,烧结钛的密度达到4.489 g/cm3,接近全致密,其显微组织由双尺度的等轴晶组成,细晶区晶粒尺寸为1~2μm,粗晶区晶粒尺寸为5~20μm,二者呈层状交替分布;该试样在室温压缩条件下的综合力学性能与铸锻Ti-6Al-4V合金相当。     

Bc路径等径角挤压SiC_p/Al基粉末材料的显微组织及力学性能

以SiCp/Al基复合粉末材料为研究对象,在250℃下采用粉末包套-等径角挤压工艺沿Bc路径成功将粉末颗粒直接固结成高致密度的块体细晶材料。结果表明:复合粉末材料成分分布均匀性和致密度在等径角挤压强烈的剪切细化作用下效果显著。初始SiC平均粒径为13.69μm,复合粉末初始相对密度为0.75,经过3个道次等径角挤压后,得到相对密度达0.97接近完全致密,SiC颗粒得到一定程度细化且分布均匀的细晶组织,平均显微硬度高达75HV,约为工业致密纯铝的2.2倍,初始SiC颗粒的尖锐棱角特征也得到明显改善。压缩性能测试表明,挤压后SiCp/Al基复合材料表现出明显优于工业纯铝的变形行为特征。     

高应变下挤压态Mg-8Zn-2.5Nd-0.8Y镁合金的力学性能和组织演变分析

采用分离式霍普金森压杆,测试挤压态Mg-8Zn-2.5Nd-0.8Y镁合金在不同应变速率下的真应力-真应变曲线,并分析其高应变压缩时的组织演变。结果表明:横向压缩曲线未形成明显屈服,表现出连续屈服的特点,当应变速率增大,最大流变应力随之增大。纵向压缩曲线具有连续屈服的特点,表现出明显的正应变速率强化特征。横向与纵向屈服强度都随应变速率的增大而增大。挤压态镁合金基体中生成了明显的再结晶组织,晶粒平均尺寸约为25μm。试样进行横向压缩后,基体中生成了很多孪晶组织,孪晶之间相互平行。纵向压缩变形组织中生成了更多细小的孪晶组织。     

纯钼的多向锻造数值模拟及实验研究

采用DEFORM–3D有限元模拟软件对纯钼坯体多向锻造大塑性变形过程进行数值模拟,结合锻造实验,研究了变形温度、锻造压下量及锻造工步等对锻件等效应变及其均匀性分布的影响,优选出了反复拔长–镦粗的锻造工艺。研究发现,随着锻造的进行,等效应变分布趋于均匀,在第三次拔长过后,锻件心部等效应变值可达到3.75以上,锻件整体相对密度接近于100%。初始平均晶粒尺寸约55 μm的纯钼烧结坯经多向锻造后,烧结孔洞明显减少,相对密度增加,晶粒尺寸减小至2～3 μm。     

超细晶工业纯钛热变形流动应力特征研究

在室温下以等径弯曲通道变形(ECAP)技术制备超细晶工业纯钛,采用Gleeble-1500热模拟实验机对粗晶和超细晶工业纯钛在热变形条件下的流动应力特征进行了研究。结果表明:超细晶工业纯钛在热压缩过程中,热压缩条件不同流动应力变化规律会有所差异。在较低应变速率条件下,随变形程度的增加,流动应力增加到峰值后开始下降,呈现明显的动态再结晶特征;而在较高应变速率下,呈现稳态流变特征。此外,与粗晶工业纯钛相比,超细晶工业纯钛的屈服应力显著增强。     

添加剂Nb和烧结温度对ZrN粉末材料性能的影响

在ZrN粉末中添加Nb粉，经热压烧结得到Nb-ZrN粉末材料，研究添加剂Nb与烧结温度对Nb-ZrN材料烧结和力学性能的影响。结果表明:提高烧结温度有利于加快材料的致密化，Nb-ZrN1480粉末出现了1个致密化速率峰，其它粉末材料还生成了另外2个小的致密化速率峰，烧结Nb-ZrN1680粉末的相对密度达到98.6%。各粉末材料的X射线衍射谱图中都存在ZrO₂衍射峰，添加剂Nb在烧结阶段全部溶入ZrN内。添加Nb后，ZrN晶格常数减小，随烧结温度上升，ZrN晶格常数基本保持稳定。纯ZrN材料表现为沿晶断裂，添加Nb后，粉末材料发生穿晶断裂，气孔数明显降低。     

烧结温度与混粉比例对双尺度钛合金组织与性能的影响

采用高能球磨-放电等离子烧结技术制备具有双尺度显微组织的Ti-6Al-4V合金,并研究烧结温度和加入未球磨粉末的比例对烧结试样显微组织及力学性能的影响。实验结果表明:当球磨时间为6 h时,由于球磨粉末内部的应变量不均匀,球磨粉末烧结试样的显微组织均由粗晶网篮组织和细晶等轴组织组成。随烧结温度升高,烧结试样的密度逐渐提高,细晶区的晶粒尺寸逐渐增大,材料强度呈先上升后下降的趋势。在球磨粉末中加入一定比例的未球磨粉末可显著提高烧结材料的塑性,并且保持较高的强度。当球磨粉末与未球磨粉末按质量比4:1混合、SPS烧结温度为850℃、保温时间为4min时,烧结材料相对密度达到99.0%,细晶区晶粒尺寸为1~2μm,烧结试样获得最佳强度-塑性组合,其拉伸屈服强度为1012 MPa,断裂强度为1056 MPa,伸长率为10%。     

初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。     

热挤压和旋锻粉末冶金纯钛的组织和力学性能

采用粉末冶金技术结合热挤压和旋锻工艺制备纯钛棒,利用万能试验机、维氏显微硬度仪、金相显微镜、高精度多功能密度计等设备测试纯钛棒的屈服强度、维氏硬度、显微组织和相对密度,研究了纯钛棒的制备工艺及其微观组织结构对材料力学性能的影响。研究表明,利用粉末冶金技术结合热挤压和旋锻工艺制备的纯钛棒屈服强度是880 MPa,均匀延伸率是4.06%,在拉伸变形过程中发生韧性断裂。纯钛棒显微组织为等轴状的细晶粒组织,平均晶粒尺寸约1 μm,组织分布均匀,无明显裂纹和缺陷,有较高的相对密度。     

Microstructural instabilities during superplastic forging of a nickel-base superalloy compact

The high temperature flow behavior of a nickel-base superalloy powder compact, prepared by hot isostatic pressing has been examined by means of uniaxial compression testing in terms of the microstructures developed during plastic flow. The tests were done isothermally at 1050 and 1100 °C and at constant true strain rates between 10^-5 s^-1 and 1 s^-1. The fine grained compact exhibits some degree of superplasticity which always increases with compressive flow as the grain structure is refined. The faster the rate of deformation, the finer is the grain size produced at high strains, when steady state conditions of flow appear to develop. By deforming to different strains at a given strain rate or into the steady state regimes at various strain rates, grain sizes in the range 1 to 5 pun were produced. By unloading and restraining the test pieces in situ, the effect of grain size on the onset of plastic flow has been examined and the yield stress observed to increase with grain size. It is shown that, in this material, hardening or softening occurs during flow depending on the size of the initial grains. The changes in microstructure and flow stress observed during deformation are analyzed and the potential offered for control of the microstructure during isothermal forging is discussed.     

不同碳含量粉末烧结钢冷压流变致密化行为研究

在材料万能试验机上对碳含量（质量分数）为0%、0.3%、0.6%、0.9%、1.2%的粉末烧结钢进行冷压变形试验,利用Hollomon方程对粉末烧结钢流变应力数据进行非线性拟合,结合显微组织形貌分析烧结钢冷压流变致密化行为和孔隙、晶粒变形机制。结果表明:烧结钢冷压应变硬化行为符合Hollomon方程,随着碳含量的增加,极限断裂应变量逐渐减小;在相同应变下,随着碳含量的增加,烧结钢应变硬化率逐渐上升,致密化效果先增强后减弱,在碳质量分数为0.9%时最佳;随着应变增加,烧结钢孔隙闭合,晶粒由等轴状变为片状;烧结钢的流变致密化过程在低应变下以致密化和致密化硬化为主,在高应变下以变形和基体加工硬化为主。     

The effect of grain size on the high temperature plastic deformation of nb3sn

A study of high temperature plastic deformation has been undertaken on 10, 20, and 60 μm grain size Nb₃Sn. The materials were produced by the hot isostatic pressing of powder blends. The 20 and 60 μm grain size material involved a stoichiometric blend of Nb and Sn powder, whereas the 10 μm grain size material involved a blend of 30.2 wt pct Sn powder and 69.8 wt pct Nb-1 Zr powder. The ZrO₂ formed during processing limits grain size and NbO formation. Through compression testing and load relaxation testing, deformation has been studied over a strain rate range from 10^-6 to 10^-2 per second and a temperature range from 1150 to 1650 °C. “Power law creep” was generally observed, although stress exponent reduction at the higher temperatures and lower strain rates suggests substructural coarsening. Analysis of stress-strain rate-temperature data projected an activation energy for creep of 400 to 500 kJ/mol. Grain size refinement clearly strengthened the polycrystals. Assuming a Hall-Petch relationship, “lattice friction stresses” and “unpinning constants” were calculated, both increasing with decreased temperature and increased strain rate. Grain size refinement from 60 to 10 μm lowered the ductile-to-brittle transition temperature for simple compression by the order of 125 °C.     

Y2O3含量对钼合金组织和性能的影响

采用粉末冶金方法制备含Y2O3的稀土钼合金,利用金相显微镜、扫描电子显微镜(SEM)、X射线衍射(XRD)、能谱分析(EDS)等手段对钼合金的断裂特征和组织结构进行对比分析,研究稀土氧化物Y2O3含量对钼合金组织和性能的影响.研究表明:添加Y2O3能细化晶粒、改善钼合金的晶粒均匀性和致密度、提高钼合金的性能:拉伸强度和屈服强度随Y2O3含量的增加呈现先增高后降低的趋势,在Y2O3含量为1％时,抗拉强度达511.43MPa,屈服强度456.99MPa,分别是纯钼材料的1.31倍和1.57倍,综合力学性能最佳；在烧结坯中,Y2O3颗粒分布均匀,主要以球形和等轴状形式存在于晶界上.     

Studies on hot deformation of sintered cobalt

The hot deformation behavior of sintered cobalt powder was studied. The Co powder prepared by thermal decomposition of cobalt oxalate was subsequently compacted by cold isostatic pressing (CIP) and sintered at 1300 °C under H₂ atmosphere. Cobalt rods of 95 pct theoretical density were obtained. Strain rate change tests in compression were conducted in the temperature range of 900 °C to 1300 °C by changing strain rates from 0.001 to 3.2 s⁻¹. Uninterrupted hot compression tests at constant strain rates and selective temperatures were also conducted. Microcracks as well as surface cracks were observed in the samples tested below 1200 °C. It was observed that the strain rate sensitivity (SRS) increased with increasing temperature and decreasing strain rate, with the maximum SRS of 0.3 being obtained at 1285 °C and strain rate of 10⁻³ s⁻¹. Despite the higher SRS at low strain rates, the hot workability of sintered cobalt was found to be poor. Extensive grain boundary microcracking was observed, with the density being lowered after deformation. However, the samples tested at higher strain rates showed less microcracking and an increase in density. On the basis of the results, it was concluded that ease of grain boundary sliding at lower strain rates and higher temperatures was responsible for the poor workability at these conditions.     

On the Phenomenon of Stress Drop During Hot Deformation of ZrTiAlV Alloy

Hot deformation behaviors of 47Zr-45Ti-5Al-3V alloys with different grain sizes were investigated by compression tests. The flow curves exhibited a pronounced stress drop at the very beginning of deformation. The magnitude of the stress drop increased with the decreasing deformation temperature and the increasing strain rate. The sudden stress drop may be associated with the disappearance of mobile dislocations in the grain boundaries. Larger initial grain size increased the flow stress, promoted the stress drop, and enhanced the activation energy of deformation.     

Dynamic and Static Softening of Sintered MgO‐PSZ / TRIP‐matrix Composites with up to 10 vol.‐% ZrO2

A metal matrix composite (MMC) consisting of AISI 304 austenitic stainless steel with up to 10 vol.‐% MgO‐PSZ was produced by a powder metallurgic process through sintering at 1300 °C and 1390 °C. The hot working of sintered samples was conducted between 900 °C and 1100 °C. The behaviour of softening kinetics was investigated using flow curve recording methods (dynamic softening) and the double‐hit method (static softening). The influence of the deformation parameters such as temperature, strain rate, inter‐pass time and relative density of the samples was determined. The microstructure development of the sintered composite after hot forming was determined by optical microscopy and SEM and was interpreted with the help of qualitative microstructure analysis. The results show a general acceleration of softening processes with increasing temperature and strain rate, with the addition of ZrO₂ particles and a decrease in the density of composite materials. A mathematical‐physical model was developed to predict the softening behaviour and optimize the forming processes of the composite in the light of these results.