Modelling the influences of solid-state interdiffusion and dissolution on transient liquid phase sintering kinetics in a
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ENT liquid phase sintering (TLPS) has been used as a novel powder metallurgy (PM) process for forming bulk materials, ceramics, and composites.[1,2] Powder mixtures consisting primarily of a high melting point base metal powder (considered as constituent B) can be more easily sintered when in the presence of a low melting point additive powder (hereafter referred to as constituent A, the melting point depressant or solute phase).[1,2] Both constituents are typically in prealloyed or elemental powder form. A simple example of a TLPS mixture is based on the schematic binary isomorphous phase diagram in Figure 1. The typical temperature segments used in TLPS are shown as well. The TLPS of powder mixtures depends on the formation of a liquid phase at the processing temperature (TP) below the melting point of the base metal powder yet above that of the additive phase (TA). The presence of the liquid enhances mass transport rates and densification (versus solid-state sintering) by exerting a capillary pull to rearrange powder particles.[1–3] This liquid isothermally solidifies over time by solute diffusion into the base metal particles as the TP is held above TA. Figure 2 shows the different stages that have been identified in the TLPS process.[2,4] A TLPS mixture of pure A and B powders initially mixed to have a solute content does the following: (1) solid state sinters during the heating stage, leading to interdiffusion of additive and based metal atoms; (2) forms a liquid phase through the melting of phase A and (if T . TA) through the dissolution of the B phase that spreads through the powder mixture and causes densification; (3) undergoes gradual isothermal solidification of the liquid at TP by solute diffusion into the base metal particles to form a single solid phase eventually; and D.M. TURRIFF, Graduate Student, and S.F. CORBIN, Associate Professor, are with the Department of Mechanical Engineering, University of Waterloo, 200 University Avenue, Waterloo, Ontario, Canada N2L 3G1. Contact e-mail: [email protected] Manuscript submitted September 6, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
(4) homogenizes to eliminate compositional gradients. Base metal particle size (aB), solute diffusivity (D) within the base metal powder, TP, heating rate (RH), and bulk solute content of the mixture (CO) have been found to influence the amount of liquid formed significantly as well as the amount of time required for it to isothermally solidify.[4–11] The advantages that TLPS offers over conventional solid-state sintering include rapid densification during sintering because of the capillary action and enhanced mass transport created by the liquid phase, lower sintering temperatures, and reduced microstructural coarsening attributable to shorter and lower temperature sintering. In addition, the advantage that TLPS offers over conventional liquid phase sintering, where the liquid is persistent, is that the liquid is ‘‘removed’’ from the microstructure before cooling, leading to a more homogeneous and higher strength compac
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