TiC Core Research Articles

Formation Conditions of Presolar TiC Core–Graphite Mantle Spherules in the Murchison Meteorite

Formation of TiC and graphite grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars is investigated to reveal the formation conditions of presolar TiC core-graphite mantle spherules extracted from the Murchison meteorite. We employ the nonequilibrium condensation theory involving chemical reactions in nucleation and grain growth to derive the formation conditions not only from the condensation sequence for realizing the core-mantle structure but also from the sizes of observed TiC cores and graphite mantles. The results of calculations show that the sizes of TiC cores constrain the mass-loss rate and the gas outflow velocity v at the formation sites to satisfy the relation 2.5×10−3<v2L½4/5<0.185, where L4 is the stellar luminosity in units of 104 L☉, v is in km s-1, and 5 is in 10−5 M☉ yr−1. The constraint on the condensation sequence limits the C/O abundance ratio to less than 1.26-1.48 for =10−6 to 10−4 M☉ yr−1. The more stringent constraint on and v at the formation sites expressed by 2.5×10−3<v2L½4/5<1.35×10−2 with <1.26-1.48 is imposed by the conditions necessary for reproducing the range of the observed sizes of TiC cores and graphite mantles simultaneously. The derived total gas pressure in the range of 2×10−3<P<0.1 dyn cm−2 and gas outflow velocity in the range 0.015<v<0.4 km s−1 at the formation sites of TiC cores seem to be consistent with the picture of steady state dust-driven winds around carbon-rich AGB stars. The discrepancy between the observed and the predicted size ratios of graphite mantles to TiC cores would be closely related to the detailed formation process of graphite mantles on TiC cores.

Mechanical properties and microstructures of submicron cermets

The microstructures of submicron cermets were investigated in order to improve their mechanical properties. Cermets of the system TiC-TiN-WCTaC-Mo 2C-Ni-Co were sintered using very fine TiC powders. HIPed specimens showed higher transverse rupture strength and higher hardness than conventional ones. These cermets showed different microstructures compared to conventional cermets. The SEM image of cermet made from submicron TiC showed the decrease in the number of grains with a TiC core. The SEM and TEM images of cermets made from 0.3 μm TiC showed many grains without cores. Its microstructure was much finer and more homogeneous than the others.

Effects of Additional Titanium Nitride on the Strength of TiC-Mo2C-Ni Alloy

The transverse-rupture strength (σm) and the strength (σ0) of the sound matrix free from structural defects of TiC-(0, 20)%Mo2C-(0-30)%TiN-20%Ni alloys were investigated at room temperature as a function of the amount of additional titanium nitride (TiN).The results obtained were as follows: (1) The effects of TiN addition on the carbide grain size, σm and σ0, were found to be negligible in TiC-20%Ni alloy, while remarkable in TiC-20%Mo2C-20%Ni alloy. (2) In TiC-Mo2C-TiN-Ni alloys, additional TiN generally formed carbonitride with TiC and Mo2C around TiC particles, and decreased the grain size of carbonitride including TiC core with increasing TiN contents up to about 15%TiN. The σ0 value increased with increasing TiN contents and attained to a maximum at this TiN content. The σm, value, however, decreased with increasing TiN contents, owing to the increased dimension of defects which acted as a fracture source. From the results, it was estimated that σm, for example, of TiC-20%Mo2C-15%TiN-20%Ni alloy would exceed about 290 kg/mm2 (according to JIS) when the dimension of defects is decreased to values of about 5μ in average. Brief consideration was also given as to the results mentioned above.

TiC-Mo&lt;SUB&gt;2&lt;/SUB&gt;C-Ni合金における周辺組織形成機構

The purpose of this paper is to make clear the mechanisms and the processes of formation of surrounding structure (s.s.) around TiC grains in sintered TiC-Mo2C-Ni alloy. The partial 1300°C isothermal section of the quasi-ternary high carbon alloy was studied. Referring to this diagram, metallographic studies mainly on the coarse-grained alloys having various compositions and sintered under various conditions were made in detail.Results obtained were as follows: (1) The fromation of s.s.occurred through two or three steps, according to the alloy composition, sintering temperature and time, sintering method, etc. For instance, when the alloys with comparatively high contents of Mo2C and Ni were sintered as usual, there could be observed three steps in the following order: the step of precipitation of Mo2C-25%TiC solid solution (accompanied by the dissolution in particular of Mo2C-2%TiC solid solution) on the surface of TiC grains; the step of solution-reprecipitation of s.s.and TiC core (bared as a result of dissolution of s.s.); and the step of precipitation during cooling. (2) Interdiffusions in or through the carbide phase during sintering seemed in general not to associate with the growth of s.s.and its composition changes. (3) Therefore, the s.s.was generally consisted of two or three layers of different thickness and compositions.