Processing and high temperature properties of refractory alloy single crystals

Creep behavior of molybdenum-based alloy single crystals at an elevated tem perature (similar to 0.6T(m)) was examined in this study. Grain boundary sl iding is absent in solution strengthened single crystalline alloys, resulti ng in significantly reduced creep rates compared to their polycrystalline c ounterparts. Hafnium (Hf) exhibits better strengthening effects in molybden um (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 an d viscous drag are two competing processes during creep. The dominant creep mechanism depends on solute type, concentration and temperature. Results f rom this study confirm that the creep behavior in solution strengthened Mo single crystals corresponds to the class II alloys, as defined by Sherby an d Burke. In addition, the creep response of these single crystals was found to be governed by the average nearest distance between the strengthening s olute atoms in the lattice. Creep data show that the activation energy for creep contains two contributions: the normal diffusion energy and an additi onal solute-related energy which is proportional to the product (ne*c(1/3)) where n is the stress exponent, e* is the atomic misfit factor and c is th e 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. (C) 1999 Elsevier Science S.A. All rights rese rved.