An in-situ study of static recrystallization in Mg using high temperature EBSD

It has been a common method to improve the mechanical properties of metals by manipulating their microstructures via static recrystallization,i.e.,through heat treatment.Therefore,the knowledge of recrystallization and grain growth is critical to the success of the technique.In the present work,by using in-situ high temperature EBSD,the mechanisms that control recrystallization and grain growth of an extruded pure Mg were studied.The experimental results revealed that the grains of priority for dynamic recrystallization exhibit fading competitiveness under static recrystallization.It is also found that grain boundary movement or grain growth is likely to show an inverse energy gradient effect,i.e.,low energy grains tend to swallow or grow into high energy grains,and grain boundaries of close to 30°exhibit superior growth advantage to others.Another finding is that{10-12}tensile twin boundaries are sites of hardly observed for recrystallization,and are finally swallowed by adjacent recrystallized grains.The above findings may give comprehensive insights of static recrystallization and grain growth of Mg,and may guide the design of advanced materials processing in microstructural engineering.

铝基碳纳米管增强复合材料的动态力学行为及应变率效应

利用电子万能实验机及Hopkinson拉压杆对碳纳米管质量分数为1%的铝基碳纳米管增强复合材料(CNTs/Al)进行了动静态拉压力学试验研究,以揭示碳纳米管对铝基材料的应变率效应及力学行为的影响。实验结果表明,与纯铝试样相比,CNTs/Al因掺杂CNTs而引起的晶粒细化、位错强化和载荷转移等强化机制作用,强度具有明显提升;通过动静态加载下的单轴拉压实验表明,与纯铝相比,掺杂CNTs增加了铝基复合材料在拉伸加载下的应变率强化效应,与压缩加载下略有不同。另一方面,纯铝与CNTs/Al在高应变率下的应变硬化能力均显著提高,且动态拉伸下的拉伸断裂应变均高于静态值,分析认为与高应变率下位错动态回复受到抑制有关。

自由锻Ti-Ni形状记忆合金的显微组织和超弹性（英文）

采用放电等离子烧结(SPS)法将元素钛粉和镍粉制备成Ti-51%Ni形状记忆合金(SMAs)。研究目的是采用自由锻二次加工以提高SPS合金的性能。对自由锻前后合金的显微组织、相变温度和超弹性进行比较。结果表明,自由锻可以显著提高Ti-Ni形状记忆合金的拉伸强度和形状记忆性能,自由锻后合金的韧性从6.8%提高到了9.2%,超弹性的应变范围从5%增加到7.5%,应变恢复速率从72%提高到92%。Ti-51%Ni合金显微组织中含有立方奥氏体相(B2)基体、单斜马氏体相(B19′)、第二相(Ti_3Ni_4,Ti_2Ni和TiNi_3)和氧化物相(Ti_4Ni_2O,Ti_3O_5)。自由锻后再经500°C时效处理的最终样品的马氏体相变温度向高温方向偏移,这是由于Ti_3Ni_4相(透射电镜下可以观察到)的析出而导致基体中Ni含量的减少。总之,自由锻可以提高Ti-51%Ni形状记忆合金的超弹性和力学性能。

A thermal activation based constitutive model for the dynamic deformation of AA5083 processed by large-scale equal-channel angular pressing

Aluminum alloy 5083(AA5083)processed by large-scale Equal-channel angular pressing(ECAP)is an excellent engineering material with great prospects for industrial applications.An accurate assessment of the underlying constitutive relationships with easily determined material constants is critical for the predictive design and informed processing of such structural materials.To develop such a design framework,uniaxial dynamic compressive tests over a wide range of temperatures(293-573 K)were carried out for an ECAP-processed AA5083 alloy.Additionally,the microstructure before and after dynamic loading was characterized by SEM and TEM.Based on the experimental results,a new dynamic constitutive model,based on thermal activation theory,was established to describe the plastic flow behavior of the AA5083 alloy that incorporates the effects of plastic strain,temperature,and strain rate.The input parameters of the new model were determined using a particle swarm optimization(PSO)method.The model predictions show excellent agreement with experimental results,which suggests that the current predictive constitutive model is highly effective in reproducing the dynamic deformation behavior of the large-scale ECAP-processed AA5083.

Efect of CNTs on Activation Volume and Mobile Dislocation Exhaustion Rate of CNTs/Al under Compression Loading

In this study,stress relaxation compression tests were performed to investigate the strain rate sensitivity,activation volume and mobile dislocations in carbon nanotubes/aluminum(CNTs/Al)composites.The results reveal that,with the addition of CNTs,the strain rate sensitivity of CNTs/Al increased.Meanwhile,a smaller V*of CNTs/Al compared with pure Al was attributed mainly to the CNT-Al interfaces and partly to the increased forest dislocations cutting activities in grain interior,which was related to the tendency of short ranges order formation during plastic deformation.The incorporation of CNTs also improved the dislocation storage capability and reduced the dislocation velocity,leading to a lower mobile dislocation exhaustion rate.

过饱和固溶合金元素在粉末冶金石墨黄铜中的析出强化

用水雾化制备了含Cr、Fe、Sn、Ti多元过饱和固溶合金元素的Cu40Zn黄铜合金粉末,将其与石墨颗粒预混合后在适当的温度烧结,热挤压后得到棒材。观测挤压棒材中石墨黄铜的微观组织并测量其切削性能和拉伸性能,研究了合金元素和石墨颗粒对切削性能和力学性能的影响。结果表明:黄铜粉末中的过饱和固溶合金元素Cr、Fe、Sn、Ti在后续的热加工过程中以微/纳米尺寸微粒析出,表现出优良的强化效果;合金元素在石墨/黄铜界面的富集和反应促进了石墨/黄铜的界面结合,提高了黄铜的切削性能但是不降低其力学性能。

以稻壳为强化原料制备低成本钛合金的摩擦学性能

以稻壳热解产物为SiO2源,通过粉末冶金法制备了Si和O固溶强化的低成本钛合金,并从磨损率、磨损表面形貌和磨屑成分等角度研究了其摩擦学性能和磨损机理。结果表明,随着SiO2加入量的增加,Ti合金的体积磨损率显著降低,与硬度的变化规律吻合良好,同时磨损表面有三处变化,即犁沟的深度变浅、塑性变形程度降低和磨屑的尺寸变小且数量减少。SiO2的加入使磨损机制从纯Ti的粘着磨损和磨料磨损转变为磨料磨损和氧化磨损,而磨屑尺寸的减小可归因于GCr15球和钛基体反复摩擦,导致磨屑的脆化。本研究提供了一种使用农业废稻壳生产具有优良综合性能的钛合金方法,在降低钛合金成本的同时也有益于生态环境的保护。

Microstructure,Phase Transformation,Precipitation Behavior and Mechanical Properties of P/M Cu40Zn-1.0 wt%Ti Brass Alloy via Spark Plasma Sintering and Hot Extrusion

The effect of titanium addition on the microstructure and mechanical properties of brass Cu4OZn has been studied via the powder metallurgy （P/M） route. The water-atomized Cu4OZn-1.0 wt% Ti alloy powder was consolidated at different temperatures in the range of 400-600℃ using spark plasma sintering （SPS） and hot extrusion subsequently. Results show that the super-saturated solid solution titanium element in rapidly cooled brass Cu4OZn powder created high chemical potential for a precipitate reaction, showing significant grain refinement effects on the consolidated Cu4OZn matrix. Consequently, excellent mechanical properties were obtained by precipitation hardening and work hardening after sintering and extrusion, with yield strength of 390 MPa, ultimate tensile strength of 617 MPa, and Vickers micro-hardness of 192 HV, which are 28.7%, 23.4%, and 23.9% higher values than those of extruded Cu4OZn brass, respectively.