Solid-solution hardening of the ternary system Nb-Hf-W

Abstract

Abstract Measurements of the yield stress as a function of the solute content are reported for a series of ultra-high vacuum degassed Nb-Hf-W single crystals tested at 900°K. Tests of alloys containing up to 10 a/o (Hf + W) show that the yield stress increases with solute content in a nearly linear manner and that the rate of hardening per solute addition is the highest for the most concentrated alloys. Although second-phase precipitates are not expected and are not observed by transmission electron microscopy techniques, the yield stress of the concentrated Nb-Hf-W alloys is sensitive to heat treatment. A phenomenological analysis of the solid-solution hardening of ternary alloys in terms of the binary alloy hardening increments is presented for the case in which the solute atoms have different strengths as obstacles to dislocation motion. The analysis predicts the observed strengthening at total solute concentrations of less than about 5 a/o, but the observed rate of hardening exceeds that predicted at total solute concentrations greater than approximately 5 a/o. The results are discussed in terms of a non-random solute distribution, sensitive to heat treatment, which is important in determining the yield stress at larger solute concentrations.