Pressure Assisted Reactive Sintering

Even though many of the examples discussed above can be reactively sintered to densities greater than 97% theoretical, this small amount of porosity can be detrimental to the mechanical properties--specifically tensile and fracture behavior--of these compounds. Commonly, a postreactive sintering routine, such as hot isostatic pressing (HIP), is used to fully densify these materials.

Alternatively, the powders can be sintered under pressure by RHP or RHIP. Greater densities can be achieved in the final product if the reaction is initiated under pressure (i.e., RHP), as opposed to applying pressure subsequent to the reaction (i.e., RS + HP), under identical hot pressing conditions. This is illustrated in Fig. 12(a) (Ref 28) for a variety of RHP in situ TiAl-boride composites. The composites that were reacted under pressure were considerably more dense than similar composites in which the pressure was applied subsequent to the reaction (e.g., after the hold temperature was reached). During RHP, compaction of the powders (as indicated by ram travel, Fig. 12b) occurs as the hot-press temperature nears the reaction initiation temperature and the transient liquid phase forms (Ref 34). The applied pressure forces the transient liquid phase into the voids between the solid powder particles, and as a consequence the compact densifies. Further, the heat liberated by the reaction raises the temperature of powder compact. At this temperature, the product may be soft and deform under the applied pressure, resulting in densification (Ref 35).

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Fig. 12 Influence of the application of pressure on reactive sintering. (a) Densities of TiAl-boride in situ composites fabricated by reactive hot pressing (RHP) and reactive sintering followed by hot pressing (RS + HP). Source: Ref 28. (b) Hot press ram travel during RHP of Ni3Al. Source: Ref 34

While the simultaneous application of pressure to the reacting powders promotes densification, it can result in inhomogeneous microstructures. This is particularly true for RHP aluminides. Alman and coworkers (Ref 25, 27, 28) noted that microstructures of RHIP-NiAl and RHP-TiAl contained "hot" spots of aluminum-rich intermetallic phases. However, the microstructure of similar aluminides, produced by RS + HIP or RS + HP, were homogeneous. The microstructural inhomogeneities that developed during RHP were attributed to the same mechanism for the enhancement in density. The transient liquid phase is forced into the voids between the solid particles (presumably nickel or titanium).

The liquid is presumably aluminum rich and, upon solidification, results in the microstructural inhomogeneity. Liu and coworkers (Ref 21, 36) found that the application of pressure has a negative impact on the density and microstructure of reactive sintered Ni3Al. In their studies, single-phase and dense (98%) Ni3Al was produced by RS + HP. However, porous (70% dense) and multiphase (Al3Ni, Al3Ni2, and nickel) microstructures resulted when the nickel and aluminum powders were RHP under stresses of 5 and 50 MPa. The low densities and inhomogeneity were attributed to the heat transfer associated with the contact of the ram and the powder compact, quenching the reaction. This was further supported by a size effect of the powder compact: the thinner the powder compact hot pressed, the more inhomogeneous was the resultant microstructure.

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