Processing and high temperature properties of refractory alloy single crystals

Abstract

Abstract Creep behavior of molybdenum-based alloy single crystals at an elevated temperature (∼0.6Tm) was examined in this study. Grain boundary sliding is absent in solution strengthened single crystalline alloys, resulting in significantly reduced creep rates compared to their polycrystalline counterparts. Hafnium (Hf) exhibits better strengthening effects in molybdenum (Mo) relative to niobium (Nb), as a result of the larger solute–solvent atomic size misfit in Mo–Hf compared to than in Mo–Nb. Dislocation climb and viscous drag are two competing processes during creep. The dominant creep mechanism depends on solute type, concentration and temperature. Results from this study confirm that the creep behavior in solution strengthened Mo single crystals corresponds to the class II alloys, as defined by Sherby and Burke. In addition, the creep response of these single crystals was found to be governed by the average nearest distance between the strengthening solute atoms in the lattice. Creep data show that the activation energy for creep contains two contributions: the normal diffusion energy and an additional solute-related energy which is proportional to the product (ne*c1/3) where n is the stress exponent, e* is the atomic misfit factor and c is the atomic solute concentration. Results also indicate that for Mo–Nb, there is a transition from class II to class I behavior at an Nb concentration of about three atomic percent.