Advances in niobium silicide in situ composites

Abstract

The compression behavior of a two-phase (Mo solid solution + T2) Mo–6.1Si–7.9B (at.%) alloy is examined as a function of strain rate in the 1000–1400 °C range and compared to that of commercially available powder-metallurgy-processed TZM (often referred to as MT104). A limited number of tests was also conducted on a three phase alloy with a composition of Mo–8.6Si–8.7B (at.%) that lies in the three phase field, Mo solid solution + T2 + Mo3Si. These compression studies confirmed that deformation in the temperature-strain rate space evaluated is matrix-controlled, yielding an activation energy of ∼415–445 kJ/mol. As a consequence, the response of the three-phase material overlaps that of the two-phase material. In all instances evaluated, the Mo–Si–B alloys exhibit superior flow stress relative to their TZM counterpart. Examination of the deformed microstructure illustrates that recovery and in some instances, recrystallization occurs in the Mo solid solution matrix whereas the T2 phase either cracks or deforms plastically depending on the temperature and strain rate used. Finite element analysis assuming an elastic–plastic matrix and an elastic second-phase illustrates strain localization in the matrix, the extent being more severe when the work-hardening rate in the matrix decreases (i.e., increasing temperature and decreasing strain rate), while the T2 particles are highly stressed. However, if plastic deformation is permitted in the T2 particles, strain distribution is homogenized substantially and the level of stress build up in the T2 particles diminishes by an order of magnitude. The interplay between matrix and T2 properties and their dependence on temperature and strain rate are used to explain the observed deformed microstructure.