What Is Liquid Phase Sintering?
Many sintering systems generate a liquid phase during the sintering process. Typically, the added elements or compounds do not react with or dissolve in the matrix, and may even become liquid during sintering. They do not wet the matrix material, such as lead in bronze or MnS in stainless steel. This liquid phase does not promote or affect the sintering process; instead, it exists in the matrix as droplets, and the entire system is sintered through solid-state diffusion.
Furthermore, when a liquid phase with limited reaction to the matrix and which wets the matrix material exists, excessive formation of this liquid phase can lead to localized collapse of the framework because the atomic diffusion rate of the liquid phase is much faster than that of the solid phase. Therefore, controlling the liquid phase content is beneficial for the sintering process, as the faster mass transfer rate leads to rapid densification of the parts. The liquid phase can exist in two forms: one is when the liquid phase is present throughout the entire sintering hold, called a continuous liquid phase; the other is when the liquid phase solidifies during the sintering hold, called a transient liquid phase.
Continuous liquid phase sintering is divided into two types: the first type involves heating a mixed powder to form a liquid. A typical example is heavy alloys, such as W-Fe-Ni alloys, which are heated to form liquid Fe-Ni, where W has limited solubility; or WC-Co alloys, where Co dissolves some WC to form a eutectic, but WC dissolves only a very small amount of Co. Figure 7.9 shows a micrograph of a 90W-7Ni-3Fe alloy, which reveals spherical tungsten grains in the Ni-Fe-W alloy matrix. During sintering, Fe-Ni melts into a liquid phase and dissolves tungsten, leading to spheroidization of the tungsten particles. Excess tungsten exceeding its solubility limit precipitates in the liquid, a typical example of dissolution-reprecipitation during liquid-phase sintering.
(The presence of circular tungsten grains in the Ni-Fe-W alloy matrix is a typical example of dissolution-reprecipitation during phase sintering.)
The second type is supersolid-line liquid phase sintering (SLPS). When pre-alloyed powder is heated above the solidus line, the grain boundaries on the surface and inside the particles melt, generating a small amount of liquid phase, which results in supersolid-line liquid phase sintering. The generation of this liquid phase facilitates rapid densification. A typical example of using SLPS is M2 type tool steel. Figure 7.10 shows a typical sintered microstructure of M2 type tool steel. The figure shows small carbide phase particles in the matrix and a larger quantity of carbide phase along certain grain boundaries.
(There are a small amount of carbide phase inside the grains and a large amount of carbide phase at some grain boundaries.)
There are two types of transient liquid phase sintering: the first is reaction sintering, which occurs when element A and element B form a compound, releasing heat and generating compound AB. NiAl is such an example. The second is when the transient liquid phase disappears due to the diffusion of a certain element to form a solid solution. For example, carbon forms a eutectic with iron and chromium. When carbon diffuses into the matrix, it forms a solid solution, and the liquid phase solidifies.