The paper studies the kinetics of the diffusion redistribution of phases in the MoSi 2 -W system when tungsten samples with molybdenum silicide coatings are heated in air at 1500-1800°C. It is established that the (Mo x ,W y ) 5 Si 3 phase, which represents a molecular solid solution of lower molybdenum and tungsten silicides, forms in an exchange reaction between molybdenum and tungsten at the MoSi 2 -W 5 Si 3 interface. The MoSi 2 -W system is much more stable than the WSi 2 -W and MoSi 2 -Mo systems.It is of current importance to develop structural materials for components and mechanisms designed to operate in oxidizing and reducing media above 1500°C used in, for example, aerospace and electrothermal engineering. Refractory metals and alloys with protective silicide coatings are used as such materials. Protective coatings based on boron silicides have proved to be especially effective. The effectiveness of coatings depends on how stable the metal-coating system is. At high temperatures, the coating material actively interacts with the base metal to form intermediate compounds. In particular, the formation of lower silicide phases leads to substantial changes in the specific volume of interacting materials. The resulting stresses and worse oxidation resistance of lower silicides deteriorate the performance of the coatings. Therefore, to increase the stability of the system, the rate at which the coating interacts with the base needs to be decreased.The diffusion of silicon in silicides of refractory metals and the stability of silicide coatings at high temperatures was examined in [1][2][3][4][5][6]. It was established there that the diffusion mobility of silicon decreases in the following sequence: Mo 5 Si 3 , W 5 Si 3 , Nb 5 Si 3 , Ta 5 Si 3 . It was also pointed out that the diffusion mobility of silicon was especially low in mixed lower phases because of the exchange diffusion of metals when metal Me 1 was coated with a silicide of another metal Me 2 .It was established [2,3] in examining the WSi 2 -W system that excess silicon that was mainly concentrated in the near-surface layer of WSi 2 coating on tungsten greatly influenced the kinetics of phase redistribution in the system, thus increasing the stability and oxidation resistance of the coating. To increase the concentration of silicon in the near-surface layer, a complex coating was examined, which was produced by siliconizing a molybdenum layer preliminary gas-sprayed onto tungsten [3]. As compared with the WSi 2 -W system, the rate at which the higher
We have investigated the effect of silicide coatings on the ultimate strength of NT-50 alloy over the temperature range −196…+1100°C. From 196 to 800°C the ultimate strength of silicidated specimens was lower than that of the initial and annealed alloy whereas from 900°C it was slightly higher. We have also examined the effect of silicide coatings on the relative elongation of NT-50 alloy samples. Over the temperature range −196…+1100°C, the relative elongation of specimens annealed in a vacuum of 6.7 ⋅ 10 −3 Pa at 1250°C for 4 h was found to be lower than that of the silicidated specimens. This is attributed to the dissolution of the residual gases (O 2 , N 2 , H 2 , CO, and CO 2 ) under vacuum annealing.
The paper examines the effect of silicide coatings and testing media on the strength and plasticity of alloy 5VMTs between 25 and 1100°C. It is established that uncoated samples tested in argon have higher strength and plasticity over the entire temperature range of interest than coated samples tested in argon and air. The major cause of decrease in the mechanical properties of coated samples is the silicide layer whose properties depend on testing temperature.
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