| Résumé | Transient liquid-phase sintering (TLPS) is a unique powder metallurgy (PM) processing technique typically used to form near-net-shape parts1 and more recently, as in this study, as a means of developing liquid-rich low-temperature solders2,3 and brazing filler materials4 that exhibit variable melting point (VMP) characteristics. In TLPS, starting mixtures normally consist of a low-melting-point additive powder and a higher-melting-point base-metal powder. The transient liquid phase that forms during heat-up past the additive’s melting point aids in rapid densification of the mixture.1,5–8 This liquid alloys with the basemetal powder during sintering and can lead to complete isothermal, or diffusional, solidification and a shift in melting point for the bulk powder mixture.4 In order to achieve the maximum melting-point shift for VMP brazing applications the isothermally solidified phase should have a completely homogenized composition. The consequence of diffusional solidification and incomplete homogenization was previously studied4 via differential scanning calorimetry (DSC). DSC results for nickel and copper powder mixtures (65 w/o Cu) showed quantitatively that a hold time of 150 min at 1,140°C enabled complete isothermal solidification of the copper-rich liquid during TLPS. Upon reheating, the TLP-sintered specimens exhibited a measurable melting-point increase, and an enhanced melting range due to incomplete homogenization of the isothermally solidified microstructure. Metallographic characterization of the post-sintered DSC specimens revealed that significant compositional gradients remained between the nickel-rich particle cores and the surrounding copper-rich solid-solution regions, even after holding for 150 min at 1,140°C.4 |
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