Mechanisms of growth and loss of coherency of HfN particles in molybdenum

Abstract

Internal nitriding of a Mo-1 at.% Hf solid-solution alloy produces a homogeneous dispersion of coherent plate-like HfN precipitates oriented on {001} planes of the Mo matrix. During nitriding growth of the HfN particles occurs principally at the advancing nitriding front by Ostwald ripening, which produces an increase in average particle size with increasing depth below the external surface. The high nitrogen activity in the alloy behind the nitriding front effectively inhibits dissociation and growth of the precipitates. The plate-like HfN particles grow in diameter in a coherent manner. Thickening of the particles occurs by the nucleation and growth of coherent ledges on the planar surfaces. This thickening process produces an increase in the coherency strain which is relaxed by the formation of intrinsic dislocation loops within the particles by the coalescense of vacancies. The large HfN particles developed by this growth mechanism retain substantial coherency strain. A severe particlecoarsening mechanism is operative at high temperatures in regions of the nitrided alloy containing a high density of small fully coherent precipitates. The coarsening occurs by the migration of grain boundaries which sweep up the small fully coherent precipitates, the components of which then feed large semicoherent particles growing behind the boundary.