Institute of Metallurgy at the Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia:

L. Yu. Udoeva, Senior Researcher, Candidate of Technical Sciences, e-mail: lyuud@yandex.ru
R. I. Gulyaeva, Senior Researcher, Candidate of Chemical Sciences, e-mail: gulroza@mail.ru
V. M. Chumarev, Principal Researcher, Doctor of Technical Sciences, e-mail: pcmlab@mail.ru
A. V. Larionov, Senior Researcher, Candidate of Technical Sciences, e-mail: a.v.larionov@ya.ru

This paper describes the results of a study that looked at the effect produced by rare earth metals (REM) – in particular, Sc and Y – on the structural and phase state of high-temperature composite (Mo)ss – Mo₃Si and its thermal stability. To obtain sample model alloys, a burden consisting of binary Mo – 15.3 Si% (at.) alloy and inoculants was subjected to vacuum arc melting. With the help of X-ray diffraction, electron microscopy and thermal analysis, the authors analysed the phase composition, microstructure and the distribution of alloying agents in the metal-silicide composites Mo – 14.8 Si – 2.8 REM% (at.). They also analysed phase stability during heat application. It is shown that the binary alloy constitutes a dual-phase in-situ composite with a Mo₃Si matrix reinforced with solid solution particles of (Mo)ss. A third structural component can be found in the samples doped with Sc or Y, which is the result of eutectic solidification and originates from Mo₃Si and silicides of REMs. Introduction of Sc and Y leads to an increased microstructural dispersion and an increased (Mo)ss/Mo₃Si volumetric ratio. An exothermic effect was identified in the range of 810–860 ^oC during dynamic heating of the Mo – 15.3 Si% (at.) alloy to 1,500 ^oC (differential scanning calorimetry), which was attributed to the processes of ordering and recrystallization, the intensity of which is less pronounced in the Sc or Y doped samples. Following 100 hours of soaking at 900 ^oC, the microstructure of the binary alloy became much coarser, and the phase composition and proportions changed. This was observed in both continuous and cyclic (10 10-hour cycles) regimes of heat treatment. It was established that, after cyclic heat application, the doped samples retain their original microstructural dispersion, while only slight changes occur in the continuous heating regime. Consequently, the introduction of around 3% (at.) of Sc or Y in the (Mo)ss – Mo3Si alloy can lead to stabilized structure and phase state of the composite and a stronger resistance to heat, which is a pre-requisite for high-temperature materials.

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