Abstract
Microstructure and high temperature deformation behavior of Ti-Si/sub 3/N/sub 4/ mechanically alloyed (MA) powder compacts were investigated. Powders of the elements Ti and Si/sub 3/N4 whose composition was Ti-20 mass% Si/sub 3/N/sub 4/ were blended for MA. A planetary ball mill was used for milling under an Ar gas atmosphere. The MA powder milled for 720 ks was heated at various temperatures. The MA powder was consolidated by vacuum hot pressing (VHP) at 200 MPa for 10.8 ks at 803 K. The specimens were provided for compression tests at 913 K-1073 K at various initial strain rates. The MA process of Ti and Si/sub 3/N/sub 4/ powders for 720 ks resulted in the formation of an amorphous and /spl alpha/-Ti phases. These phases changed to TiN, Ti/sub 2/N and Ti/sub 5/Si/sub 3/ phases after the heat treatment at elevated temperatures. A (TiN+Ti/sub 5/Si/sub 3/) ultrafine microduplex structure was obtained after the heat treatment at 1473 K for 3.6 ks. The compression tests revealed that the 803 K-VHP specimen with non-equilibrium phases show the lowest flow stress at 993 K at initial strain rate of 4.2/spl times/10/sup -4/ s/sup -1/ in the three VHP specimens. Furthermore, the 803 K VHP specimen indicated lower flow stress at 993 K at an initial strain rate of 4.2/spl times/10/sup -4/ s/sup -1/ rather than that at 2.1/spl times/10/sup -4/ s/sup -1/. Such a reverse of the flow stress between two different initial strain rates was attributed to the phase transformation during the deformation. The slower strain rate test produced larger amount of harder phases such as TiN, Ti/sub 2/N and Ti/sub 5/Si/sub 3/. The specimen compressed to 25% (/spl epsi/=0.28) at 993 K at an initial strain rate of 4.2/spl times/10/sup -4/ s/sup -1/ consisted of an (/spl alpha/-Ti+Ti/sub 2/N+Ti/sub 5/Si/sub 3/) microduplex structure with an average grain size of approximately 40 nm. Therefore, there exists an appropriate condition for a low temperature and high strain rate forming process. A (TiN+Ti/sub 5/Si/sub 3/) microduplex structure with an average grain size of approximately 250 nm was also obtained in the specimen compressed to 25% after annealing at 1473 K for 3.6 ks.
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