A microstructure-based model for shape distortion during liquid phase sintering

Tight dimensional control is a major concern in consolidation of alloys via liquid phase sintering. This research demonstrates the role of microstructure in controlling the bulk dimensional changes that occur during liquid phase sintering. The dimensional changes were measured using a coordinate measuring machine and also on a real-time basis using in situ video imaging. To quantify compact distortion, a distortion parameter is formulated which takes into consideration the compact distortion in radial as well as axial directions.; The microstructural attributes considered in this study are as follows: solid content, dihedral angle, grain size, grain contiguity and connectivity, and solid-solubility. Sintering experiments were conducted with the W-Ni-Cu, W-Ni-Fe, Mo-Ni-Cu, and Fe-Cu systems. The alloy systems and the compositions were selected to give a range of microstructures during liquid phase sintering. The results show that distortion correlates with the measured microstructural attributes. Systems containing a high solid content, high grain coordination number and contiguity, and large dihedral angle have more structural rigidity. The results show that a minimum two-dimensional grain coordination number of 3.0 is necessary for shape preservation. Based on the experimental observations, a model is derived that relates the critical solid content required for maintaining structural rigidity to the dihedral angle. The critical solid content decreases with an increasing dihedral angle. Consequently, W-Cu alloys, which have a dihedral angle of about 95{dollar}spcirc,{dollar} can be consolidated without gross distortion with as little as 20 vol.% solid.; To comprehensively understand the gravitational effects in the evolution of both the microstructure and the macrostructure during liquid phase sintering, W-Ni-Fe alloys with W content varying from 78 to 93 wt.% were sintered in microgravity. Compositions that slump during ground-based sintering also distort when sintered under microgravity. In ground-based sintering, low solid content alloys distort with a typical elephant-foot profile, while in microgravity, the compacts tend to spheroidize. This study shows that microstructural segregation occurs in both ground-based as well as microgravity sintering. In ground-based experiments, because of the density difference between the solid and the liquid phase, the solid content increases from top to the bottom of the sample. In microgravity, the solid content increases from periphery to the center of the samples. A model is derived to show that grain agglomeration and segregation are energetically favored events and will therefore be inherent to the system, even in the absence of gravity.; Real time distortion measurement in alloys having appreciable solid-solubility in the liquid phase, such as W-Ni-Fe and Fe-Cu, show that the bulk of distortion occur within the first 5 min of melt formation. Distortion in such systems can be minimized by presaturating the matrix with the solid phase.