Microstructure investigations on explosive TiNi(or Ni)/TiC-composite-formation reaction during mechanical alloying

The microstructure of TiNi(or Ni)/TiC composite prepared by mechanical alloying (MA) of elemental Ti, Ni and C powders has been investigated in detail. The result shows that large agglomerates with size 3-10 mm were abruptly formed for Ti50Ni20C30, Ti40Ni40C20 and Ti30Ni50C20 powders at the milling duration of 3 h 30 min-3 h 35 min, which suggests melting of the powders and subsequent quenching has occurred during MA. Transmission electron microscopy shows that spherical TiC grains, lath twin martensite (M) and B2 phase are directly formed after 3 h 35 min MA of Ti50Ni20C30 powders. It is also found that there exists definite orientation relationships between M and B2 phases which are very close to those obtained by Otsuka et al. in equiatomic TiNi martensite. The resultant phases are predominantly TiC and M phases with a small amount of B2 phase for Ti40NI40C20; and Ni and TiC phases for Ti30Ni50C20 after 3 h 35 min of MA. The microstructure characteristics of the as-milled materials are very similar to those of melted and solidified ones, which proves that melting of the powders and subsequent quenching has really occurred during the MA process. We concluded that MA in Ti50Ni20C30, Ti40Ni40C20 and Ti30Ni50C20 is not governed by a gradual diffusional reaction, but by a self-sustained high-temperature synthesis (SHS) one. The SHS reaction is believed to be triggered by the release of heat of formation of the TiC phase and ignited by the mechanical collisions. The thermodynamic and kinetic conditions for the SHS reaction is discussed, and it shows that MA is a versatile method in inducing SHS reaction in systems with large heat of formation.