纳米氧化铝弥散强化铜的放电等离子体烧结动力学及机制

利用经典热压模型,系统研究纳米氧化铝颗粒弥散强化铜的放电等离子烧结(SPS)致密化过程与机理。结果表明,放电等离子烧结初期,氧化铝弥散强化铜的致密化过程由晶界滑移和晶界扩散共同控制。随保温时间延长,烧结机制转变为由晶界滑移所主导。烧结后期致密化主要以塑性变形的方式进行。纳米氧化铝颗粒抑制了铜的烧结致密化,导致材料的密度降低。抑制机理为氧化铝颗粒阻碍晶界和位错运动,导致晶界滑移和塑性变形的激活能提高,从而增大致密化抗力。在外力和纳米氧化铝颗粒的共同作用下,塑性变形的主要形式为孪生。     

机械合金化制备Al-10Pb纳米相复合结构的热稳定性

利用X射线衍射仪、扫描电镜和透射电镜,研究机械合金化制备的Al-10%Pb(质量分数)纳米相复合结构的热稳定性。结果表明Al-10%Pb纳米相复合结构中Pb相的长大可以用LSW理论描述。但是Pb相的长大激活能显著低于常规多晶材料中溶质原子(Pb)在溶剂基体(Al)晶格中扩散的激活能,而接近于溶剂基体(Al)的晶界自扩散激活能。这主要是由于纳米相复合结构中Pb相的长大机制与常规两相合金不同所致。在纳米相复合结构中,溶质原子的迁移以沿溶剂基体的晶界扩散为主,纳米相基体高的晶界分数可促进扩散的进行。     

镍基合金晶界贫铬区演变的动力学模拟和试验验证

利用Thermo-Calc、DICTRA软件及其数据库,建立碳化物长大-Cr原子扩散模型,进行镍基690合金晶界贫Cr区演变的动力学模拟研究,并与扫描透射电子显微分析和能谱分析技术得到的合金晶界Cr含量实测值相比较,验证模型的合理性.结果表明:600～800℃敏化温度范围内进行等温处理时,敏化初期M23C6颗粒长大速度快,尺寸迅速增大;一定时间后长大速度急剧下降,尺寸增速减缓直至M23C6颗粒达到稳定的尺寸.等温条件下晶界Cr含量变化与敏化时间密切相关,敏化初期碳化物快速长大是影响晶界附近最低Cr含量的主要原因;而随着敏化时间延长,晶内Cr原子向贫Cr区的扩散愈加充分,贫Cr区最低Cr含量逐渐提高.该模型计算得到的晶界附近最低Cr含量值接近实测值,该模型可准确预测不同敏化过程中的晶界附近最低Cr含量.     

Ti-6Al-4V钛合金表面纳米化机制研究

借助X射线衍射仪、透射电镜及显微硬度仪等先进仪器,研究了经超音速微粒轰击( SFPB)形变热处理Ti-6Al-4V合金表面自身纳米化晶粒尺寸演化及纳米化机制.研究结果表明:超音速微粒轰击使Ti-6Al-4V合金表面获得了纳米组织,并发生显著的加工硬化,表面显微硬度比基体硬度提高了1倍多;随着SFPB处理时间的延长,纳米结构层厚度不断增加,晶粒尺寸逐步细化,当SFPB处理30 min后晶粒尺寸趋于稳定,在表层形成了晶粒尺寸约为20 nm具有随机取向的纳米等轴晶.Ti-6Al-4V合金表面自身纳米化是由于位错运动、孪晶的形成及交割共同作用的结果;在多方向载荷的重复作用下,在塑性变形区产生了大量的由位错线和高密度位错缠结分割的位错胞,并在位错寨集处产生应力集中,进而形成孪晶;孪晶自身相互交割和位错的滑移相互协调,形成了细小的孪晶和胞状组织;晶胞组织转变为细小多边形亚晶;当孪晶尺寸细化到亚纳米级时,位错的滑移起主导作用,最终通过位锗的湮灭和重组形成了具有随机取向的等轴状纳米晶粒.     

微波烧结锇的工艺研究及动力学分析

采用微波烧结技术制备锇（Os）烧结体,研究了生坯压制压强和微波烧结主要工艺参数（升温速率、烧结温度和保温时间）对Os烧结体组织结构和相对密度的影响规律,分析了微波烧结致密化的机理。结果表明,1350 ℃微波烧结后Os平均晶粒尺寸约0.22 μm,与粉体颗粒尺寸差别不大;随着烧结温度增加到1500 ℃,晶粒尺寸长大到0.76 μm。1500 ℃烧结时,延长保温时间,Os烧结体的相对密度先快速增加,后缓慢增加。1500 ℃微波烧结60 min后,Os烧结体相对密度为94.3%,平均粒径小于1 μm。烧结动力学分析表明,Os的致密化过程是体积扩散和晶界扩散共同作用的结果,随着烧结温度的升高,扩散机制从晶界扩散逐渐向体积扩散转变。     

表面机械研磨工业纯钛表面纳米化研究

表面机械研磨处理可以使工业纯钛形成纳米表面层, 通过扫描电镜、透射电镜和高分辨电镜观察SMAT处理后的工业纯钛表层组织, 并研究了工业纯钛表面纳米化机制. 工业纯钛表面纳米化机制为: 孪晶的形成和孪晶的交割使得原始晶粒尺寸减小, 同时使晶格取向发生改变, 有利于位错滑移; 孪晶通过自身交割, 以及位错密度增加及其相互作用, 形成了细小的孪晶与胞状组织; 胞状组织转变为多边形亚晶; 亚晶不断吸收位错形成大角度晶界, 亚晶以及取向不同的细小孪晶逐渐转变为随机取向的纳米晶.     

粉末冶金铝合金烧结致密化过程

以纯Al粉为主要原料,添加Cu单质粉末以及Al-Mg、Al-Si中间合金粉,利用粉末冶金压制烧结方法制备出相对密度98%以上的Al-Mg-Si-Cu系铝合金.研究表明,烧结致密化过程主要分为3个阶段:初始阶段(室温~460℃),坯体内首先形成Al-Mg合金液相,液相中的Mg原子分别扩散至Al或Al-Si粉末中,与Al₂O₃反应并破除氧化膜,形成Al-Mg-O等化合物;同时,Al-Cu发生互扩散,形成Al₂Cu等金属间化合物.第二阶段(460~560℃),Al-Cu、Al-Si液相快速填充颗粒缝隙或孔洞,坯体相对密度显著提高;此阶段的致密化机制主要是毛细管力引起的颗粒重排,以及溶解析出导致的晶界平直化.第三阶段(560~600℃),随温度的升高,液相润湿性提高,晶粒快速长大,使得大尺寸孔洞填充,烧结体基本实现全致密,此阶段的致密化主要由填隙机制控制.在铝合金晶界处发现了MgAl₂O₄和MgAlCuO氧化物的存在,推测Al粉表面氧化膜的破除机制与合金成分有关.由于Al-Cu液相在Al表面的润湿速率远高于AlN的生长速率,因为在本体系中未发现AlN的存在.      

Fe--36 Ni因瓦合金的热塑性

采用Gleeble-3800热模拟试验机研究Fe-36Ni合金在900～1200℃的热塑性行为，并用FactSage软件、扫描电镜及透射电镜等研究该合金热塑性的影响因素及作用机理.结果表明:合金中主要形成Al₂O₃+Ti₃0₅＋MnS复合夹杂，夹杂物颗粒尺寸集中分布在0.5μm以下.合金热塑性在900～1050℃受晶界滑移及动态再结晶共同影响.晶界上分布的纳米级别（<200nm）夹杂物有效钉扎晶界，抑制动态再结晶发生的同时减小晶界结合力.微米级别（>200nm）夹杂物则促进显微裂纹在晶界滑移过程中的形成和扩展，损害合金热塑性.当温度高于1050℃时，较高的变形温度使再结晶驱动力大于钉扎作用力，合金发生动态再结晶，有效提高热塑性.在1100～1200℃区间内，枝晶间裂纹的形成、晶界滑移的加剧及动态再结晶晶粒尺寸增大都降低合金热塑性.      

Pt?Au?Cu 三元核壳结构纳米线的制备与结构表征

通过水溶液还原法在80 ℃合成Cu纳米线，再利用液相还原法在低温水溶液中将Au负载于其表面，最后通过暴露的Cu纳米线与Pt前驱体盐发生Galvanic置换反应，将Pt负载在Au?Cu纳米线表面，构成Pt?Au?Cu三元核壳结构纳米线。根据对样品形貌、结构的表征和分析，探讨了Pt?Au?Cu纳米线的合成机理。结果表明:合成纳米线物相组成为单质Cu，平均直径约为83 nm；负载Au后的Au?Cu纳米线平均直径约为90 nm，表面附着的小颗粒为单质Au颗粒，构成了核壳结构；负载Pt后得到Pt?Au?Cu三元核壳结构纳米线，平均直径约为120 nm。Cu纳米线表面Au颗粒的形成依赖于异相形核与长大机制，并遵循先层状后岛状生长的混合生长模式。负载Pt过程中存在Pt、Cu互扩散，使得最终纳米线表面多为Pt颗粒而整体则形成CuPt 合金相。      

超低碳钢等温时效过程中析出物的电镜观察

采用扫描电子显微镜研究了超低碳钢在650 ℃经100 s、1000s、100 h、300 h等温时效过程的组织变化及铜的析出颗粒,探讨了时效时间与铜原子扩散对富铜析出颗粒的影响.研究结果表明,等温时效后的组织为多边形铁素体晶粒,富铜析出颗粒优先在铁素体晶界处析出,随着时效时间的延长,在铁素体晶粒内部也出现了富铜析出颗粒.计算表明在晶粒内部的铜原子完全可以借助位错和晶界的高扩散率通道,逐渐富集、长大并粗化.     

Sintering study of silver particles by in situ electron microscopy

Micron size silver spherical particles were sintered in the hot stage of an electron microscope between 370 and 720°C. In the 370 to 520°C temperature range, sintering can be explained either by grain-boundary diffusion or dislocation slip. At higher temperatures, the controlling mechanism changes from surface or grain-boundary diffusion to volume diffusion from 570 to 720°C. These conclusions are based on observations of neck growth and shrinkage and on analyses of the shapes of the sintering curves.     

Kinetics of neck growth during loose stack sintering

The kinetics of loose stack sintering of spherical iron powder were monitored by measuring some global microstructural attributes associated with the interparticle contact necks. The neck size and the number of interparticle contacts per unit volume were calculated from these data. A direct comparison of the experimental and theoretically predicted neck sizes has been carried out. The neck sizes predicted by the model for surface diffusion controlled sintering are muchhigher than the corre-sponding experimental values. The number of interparticle contacts per particle does not change with isothermal sintering time or temperature, and it is predominantly determined by the initial stacking of particles in the powder mass. The change of atmosphere from dry hydrogen to argon does not affect the neck growth kinetics significantly.     

EFFECTS OF PARTICLE SIZE ON THE SINTERING KINETICS IN TUNGSTEN POWDER

Abstract

An experimental study has been made of the effects of initial particle size on sintering kinetics in tungsten powder within the temperature range 1100–1500°C. Particle size, compacting pressure, sintering time and temperature all influence the rate of sintering. Isothermal changes in density and volume have been measured. The results indicate grain-boundary diffusion as the mechanism principally responsible for material transport in the case of particle sizes <4 μ Surface diffusion appears to bethe mechanism of material transport in compacts with particle sizes of 14– 16 μ The temperature-dependence of the rate of sintering is characterized by activation energies of 101± 2 and 72± 2 kcal/mole for fine particles (< 4 μ) and coarse particles (14–16 μ), respectively.     

烧结理论进展——2.烧结初期多种扩散机制耦合作用的烧结模型

在恒体积条件下给出了多种扩散机制耦合作用两球单元烧结模型的推导过程,编制了相应的计算软件,计算了铜在不同的粒度、温度、时间条件下的颈长方程和对心收缩方程。结果表明:在相同条件下,多机制综合作用颈长方程略高于表面扩散机制单独起作用的颈长方程;多机制综合作用的对心收缩小于只考虑晶界扩散和体积扩散的对心收缩。颈长方程的时间指数随时间的增加而变小。在不同烧结条件下各种机制对颈部物质的贡献存在很大差别。加热速率影响颗粒的对心收缩,在相同烧结时间内,加热速率越低,烧结收缩量越大。压密度越高,在烧结初期颗粒的颈长速率和收缩速率越低。     

Spark-Plasma Sintering of W-5.6Ni-1.4Fe Heavy Alloys: Densification and Grain Growth

W-5.6Ni-1.4Fe heavy alloys were prepared by the method of spark-plasma sintering, and the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature and dwell duration. It is found that the local temperature gradient at the vicinity of the pores can cause the matrix phase melting or softening, resulting in a viscous layer coating the W particles and an improved solubility of W into the matrix phase. In the initial stage, particle rearrangement and neck formation and growth take place, and γ-(Ni, Fe) matrix phase has formed. Dissolution-precipitation and Ni-enhanced W grain boundary diffusion together with viscous process contribute to the simultaneous densification and grain growth in the intermediate stage. During the final stage, fast grain growth, controlled by both gas-phase diffusion and dissolution-precipitation mechanisms, dominates over the densification.     

Effects of tungsten particle size and copper content on densification of liquid-phase-sintered W-Cu

Tungsten and copper exhibit negligible solubility for each other, so densification during liquid-phase sintering of W-Cu is limited to rearrangement of the W particles and solid-state sintering of the W skeleton. Experiments are conducted to evaluate the effects of Cu volume fraction and particle size on these densification mechanisms. Dilatometry shows that particle rearrangement is limited for low Cu volume fractions and large W particle sizes. After an initial rearrangement stage, densification rates at low Cu volume fractions decrease to those of solid-state sintering of W. Higher Cu volume fractions enable high sintered densities through rearrangement. The dominance of rearrangement or solid-state sintering is determined by the balance of capillary and bonding forces, which depends on the particle size and the ratio of the neck diameter to the particle diameter.     

A reanalysis of the kinetics of neck growth during liquid phase sintering

During liquid phase sintering, solid particles make contact and can subsequently coalesce into one particle. This coalescence phenomena can affect the type of microstructure formed and its subsequent coarsening behavior during liquid phase sintering. The mechanism of particle coalescence is assumed to be the liquid state analog of the evaporation-condensation mechanism of sintering. In this work, a detailed study of the geometry appropriate for analysis of the coalescence phenomena during liquid phase sintering is made. It is found that in the early stages of particle coalescence, the neck between the particles acts as a geometrical barrier to diffusion and the neck between the particles grows approximately ast ^1/5,i.e., the same kinetics appropriate for solid state sintering are obtained. At longer times, the neck area no longer restricts diffusive flow and at ^1/6 dependency of neck growth is obtained. The use of numerical techniques also allows the analysis to be carried out with fewer geometrical restrictions than in the original analysis of the evaporation-condensation mechanism.     

Structural changes during sintering of powder of the metastable alloy Al-5% Cu prepared by centrifugal atomization of melt

The formation of liquid-phase interlayers between particles of rapidly cooled powder of the alloy Al-5% Cu during sintering was studied by the methods of complex thermal analysis and electron-scanning metallography. It was found that fluctuations of the chemical potential which arise as a result of the decomposition of the solid solution and excess grain-boundary energy drive the sintering process. Contact bridges between the particles are formed as a result of extrusion of melt to the particle surface during grain growth.     

广义热弹性扩散下非等径颗粒的烧结驱动力分析

在广义热弹性扩散理论框架下建立非等径两颗粒系统三维有限元模型，研究颗粒系统温度场和浓度场的分布规律，分析场分布对脉冲电流烧结初期迁移驱动力的影响。结果表明，颗粒颈部空位浓度梯度、温度梯度、由温度场和应力场产生的浓度梯度是颗粒颈部物质迁移的共同驱动力。烧结颈部的温度会产生两次突变，烧结过程中小颗粒一直保持高温状态；温度变化会引起浓度改变，使得颈部浓度高于边缘浓度；热扩散占总扩散通量的2/3，浓度扩散占1/3，因此烧结颈部的热扩散驱动力和浓度扩散驱动力是脉冲电流烧结过程的主导驱动力，提高热扩散能力和浓度扩散通量可显著提高烧结过程驱动力。非等径颗粒的烧结驱动力远远大于等径颗粒，为非等径颗粒的烧结比等径颗粒更为迅速提供了理论依据。     

Some Factors Influencing the Sintering Behaviour of Austenitic Stainless Steel Powders

Abstract

Densification studies of sintered stainless steel powder blends have shown that the introduction of delta-ferrite into the micro structure increases substantially the sintering rate of the compact. Model experiments, in which spherical particles of a 316 powder were sintered on several different base plates including iron, 18Cr–12Ni stainless steel, iron–12% nickel, have been carried out to provide more detailed information. Quantitative observations have been made on the rate of neck growth and on the compositional gradients which develop between particle and plate. It has been found that the sintering rates were very sensitive to the composition of the base plate and to the presence of austenite/ferrite interfaces. Micro-examination and the use of electron probe microanalysis indicate that interdiffusion of elements between particles and plates is by both surface and volume diffusion.