热力耦合作用下脉冲电流烧结初期驱动力研究

采用L–S型广义热弹性理论,建立了氧化铝粉末非等径三颗粒模型,探究脉冲电流烧结过程的热力耦合传播规律和烧结驱动力。结果表明,三颗粒系统中小颗粒的温度明显高于大颗粒,在颈部形成高温度梯度,由温度梯度形成的热扩散通量与空位扩散通量共同为脉冲电流烧结初期颈部物质迁移提供驱动力;随着烧结过程的进行,热扩散对烧结驱动力的影响越大;三颗粒系统的计算值小于两颗粒系统,这是由于两颗粒系统产生的热耗散较小。     

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。     

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。     

金纳米颗粒烧结的分子动力学模拟

采用分子动力学模拟方法研究了不同尺寸Au纳米颗粒在烧结过程中晶型转变及烧结颈长大机制.研究发现纳米颗粒的烧结颈生长主要分为两个阶段:初始烧结颈的快速形成阶段和烧结颈的稳定长大阶段.不同尺寸纳米颗粒烧结过程中烧结颈长大的主要机制不同:当颗粒尺寸为4 nm时,原子迁移主要受晶界（或位错）滑移、表面扩散和黏性流动控制;当尺寸在6nm左右时,原子迁移主要受晶界扩散、表面扩散和黏性流动控制;当颗粒尺寸为9 nm时,原子迁移主要受晶界扩散和表面扩散控制.烧结过程中Au颗粒的fcc结构会向无定形结构转变.此外,小尺寸的纳米颗粒在烧结过程中由于位错或晶界滑移、原子的黏性流动等因素会形成hcp结构.      

化学镀法制备镍包铜复合粉末及其烧结行为

用化学镀法制备了镍包铜粉,并利用固相烧结法将镍包铜粉成功地制成了块状烧结体.通过SEM、XRD与EDS分析研究了烧结过程中镍包铜粉中界面的迁移情况,同时制备了镍铜混合粉末烧结体并和镍包铜粉末烧结体进行了对比.结果表明:镍包铜粉烧结体在扩散过程中,由于镍的扩散系数比铜大,镍层扩散进入铜形成了铜镍固溶体,界面单向迁移.随烧结温度升高,颗粒内原子扩散系数越大,烧结体界面扩散越容易,形成较大的晶粒,同时镍包铜粉烧结体能克服镍铜混合粉烧结产品中的组织偏析.     

不同尺寸HDH钛粉末烧结后的显微组织与力学性能分析

研究了不同颗粒尺寸的HDH钛粉对烧结钛致密度、显微结构及力学性能的影响。结果表明:HDH钛粉颗粒尺寸越小,越有利于烧结,烧结密度越大,孔隙球形且分布均匀;随HDH钛粉颗粒尺寸增大,烧结试样中孔隙尺寸增大,连通孔隙增多,对烧结工艺的要求更高。颗粒尺寸小于38μm的HDH钛粉烧结后,相对密度达到99.5%;而颗粒尺寸为150~200μm的HDH钛粉烧结后的相对密度仅为89.1%。随着钛颗粒尺寸的不断增大,烧结后试样的维氏硬度和压缩屈服强度降低,颗粒尺寸小于38μm的HDH钛粉烧结后压缩屈服强度最高,约为1 000 MPa,其断裂强度约为2 250 MPa;颗粒尺寸为150~200μm的烧结试样的压缩屈服强度最低,约500 MPa。     

扩散CuSn10粉末烧结膨胀行为及影响因素的研究

采用雾化Cu粉和Sn粉为原料,通过调整原料粒度,利用扩散工艺制备了4组CuSn10粉末,将粉末分别压制成轴承生坯进行烧结,研究了轴承的烧结膨胀行为,并分析了原料粒度变化对烧结膨胀行为的影响。结果表明,4组轴承样品烧结后都发生了尺寸膨胀,原因主要有:雾化粉末呈近球形,压制后总孔隙率少;富Sn区熔化后产生液相,由于Cu、Sn互溶,液相Sn扩散渗入Cu粉内部,造成Cu的晶格变大,使Cu粉颗粒体积增大,同时在Sn颗粒原有位置形成空位;液相渗入Cu粉颗粒之间,使其颗粒间距增加;压坯的闭孔中存在气体,随着烧结温度的提高,闭孔内部气压增大,造成体积膨胀。CuSn10烧结膨胀率随原料粒度的增大而增大,Sn粉粒度对CuSn10烧结试样膨胀率的影响约是Cu粉的3倍。     

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。     

复烧工艺对Fe-Cu粉末烧结件尺寸变化的影响

研究了复压复烧工艺中复烧温度和复烧时间对Ｆｅ－Ｃｕ粉末烧结件尺寸变化的影响。运用扩散方程对Ｆｅ－Ｃｕ粉末烧结合金化程度与时间的关系作了估算。结果表明，在研究条件下，复烧温度超高，烧结件经向膨胀量越大；复烧时间越长，烧结件经向膨胀量越小。     

热压金属粉末变形机构探讨

本工作为研究金属粉末固相热压和热等静压致密化初期烧结颈部的物质迁移机构,建立了高温下受压金属半球对的模型,用来模拟研究金属粉末受压烧结接触颈部的物质迁移机构。文中应用Nabarro,Herring,Weertman等人对于蠕变过程中蠕变速率ε和应力σ间的关系:ε=Kσ~n来判断蠕变变形机构的成果。导出半球对的绝对收缩量(h h_0)与瞬间     

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.     

Thermodynamic and kinetic study of diffusion paths in the system Cu-Fe-Ni

An understanding and modeling of diffusion paths in ternary systems requires a combined thermodynamic and diffusion kinetic approach. The driving force for the intrinsic diffusion of each component is the gradient of the chemical potential (or activity) which can be calculated from the concentration profile if the thermodynamic properties of the system are known. For studying the diffusion behavior in ternary metallic systems, the Cu-Fe-Ni-system was chosen because of its experimental and thermodynamic simplicity. Concentration profiles and diffusion paths in single-phase areas and across an α/β interface were studied experimentally at 1000 °C using the diffusion couple technique. Coefficients for interdiffusion and tracer diffusion have been calculated at the intersection points of two independent diffusion paths with a common composition. A concentration dependence for the tracer diffusion coefficients for each component was calculated and found to be consistent with the literature data in the binary Cu-Ni and Fe-Ni systems. The calculated vacancy flux in the couples was consistent with the experimentally observed marker shift.     

Mass transport at interfaces in single component systems

Mass transport at interfaces is induced by a gradient of chemical potential along the interface; typically, at surfaces, this is caused by a gradient in curvature and, at grain boundaries, by a gradient of normal stress. In addition, interface mass transport in metallic conductors is induced by strong electric fields/currents. On a sufficiently small scale, depending on the temperature, this interface transport dominates bulk diffusion. Continuum equations that specify the interface fluxes in terms of the preceding driving forces and continuity equations that describe the consequences of a divergence of these fluxes are presented; the chemical potential whose gradient is used as a driving force is that in local equilibrium with an element of interface. The equations are subject to boundary conditions at interface junctions that require the total emerging flux to vanish and that require, at junctions that pass flux freely, the chemical potential to be continuous. With the use of several approximations, solutions of the equations are given to describe, in a unified way, basic models of surface morphological evolution, Coble creep and diffusion-based models of sintering, and electromigration. Some of the approximations, not necessarily made simultaneously, are (1) isotropy of interface properties, both within the interface and with regard to the interface orientation; (2) surface slopes everywhere small compared to a reference plane; and (3) steady-state stress in grain boundaries. Limitations and possible extensions of the framework are discussed.     

Migration of liquid film and grain boundary in Mo-Ni induced by temperature change

Migration of liquid films and grain boundaries in liquid phase sintered 95Mo-5Ni (wt%) alloy occurs if the sintered specimens are heat-treated at temperatures above or below those of the initial sintering treatment. Behind the migrating boundaries, solid solutions in equilibrium at the heat-treatment temperature are deposited on the parent grains and the process is analogous to discontinuous precipitation and diffusion induced grain boundary migration (DIGM). The migration rate is varied by changing the sintering temperature while keeping the heat-treatment temperature constant; it increases parabolically with the expected composition difference between the initial and the final solid solutions. This result agrees with Hillert's proposal that the coherency strain energy in a diffusion layer in the retreating grain is the driving force. The observed migration rate and retardation effect due to the boundary curvature also agree in an order of magnitude with the coherency strain energy as the driving force. The results show that chemically induced migration of liquid films between the grains can be readily controlled experimentally and analyzed theoretically in terms of well known thermodynamic and kinetic laws.     

Effect of a temperature gradient on bubble growth in tungsten

Bubble distributions in doped tungsten wires have been examined following high temperature anneals (2500° to 3000°C) both in the presence and absence of a thermal gradient. Electron and optical microscopy show that much more bubble growth occurs during gradient anneals, and the results suggest that the gradient is a driving force for bubble growth. This process takes place predominantly on the grain boundaries. Semiquantitative estimates of the bubble flux were made by measuring the change in aggregate bubble surface areas found at various positions in the gradient. Stress induced bubble growth was shown not to be a factor.     

The effect of small plastic deformation and annealing on the properties of polycrystals: Part II. Theoretical model for the transformation of nonequilibrium grain boundaries

The nonequilibrium grain boundary state which has a high energy state is the result of absorption of a certain density of extrinsic grain boundary dislocations (EGBD’s). The equilibrium of such a boundary occurs by annealing at higher temperatures. A model has been proposed in this paper which assumes that the equilibrium of a nonequilibrium grain boundary involves the annihilation of EGBD’s by climbvia lattice diffusion of vacancies at the triple points. Due to the stress field of the EGBD’s, there is a vacancy concentration gradient around the triple points. The profile of the vacancy concentration gradient is derived by assuming a steady state flux of vacancies. Using this vacancy concentration profile, the expressions for the rate of climb of EGBD’s are derived. The proposed model predicts that the time required for the equilibration of nonequilibrium grain boundaries is dependent not only on the annealing temperature but also on the initial density of EGBD’s and the boundary length (which is related to the grain size). It has also been shown that the equilibrium behavior predicted by our model is in good agreement with the experimental results obtained for 316L stainless steel.     

The suppression of chemically induced liquid film migration in CoCu at high temperature

When a Co-20 wt% Cu alloy prepared by liquid phase sintering at 1150°C is heat-treated at 1300°C, some intergranular liquid films initially migrate and reverse their directions to return to their initial locations, while others do not migrate at all. Previously, extensive liquid film migration (LFM) has been observed when the sintering and the heat-treatment temperatures have been reversed. When the same alloy sintered at 1150°C is heat-treated at 1150°C in contact with a Cu-10Co-10Sn (wt%) powder mixture, extensive LFM occurs, producing Cu-depleted solid behind the migrating liquid films. When the same experiment is carried out at 1300°C, very little LFM is observed and the grain composition is equilibrated by lattice diffusion. Upon increasing the Sn content in the powder mixture to 15 and 20 wt%, extensive LFM occurs at both 1150 and 1300°C. These observations are consistent with the prediction of the coherency strain theory that there are high temperature limit and minimum critical driving force for LFM. At high temperatures and low driving forces, the ratio of the solute lattice diffusivity to the migration velocity is so large that the coherency in the frontal diffusion zone is expected to be broken, eliminating the driving force for LFM. The same principle should apply to the grain boundary migration.