Sigma Phase in Tungsten-Rhenium Alloys. I

Abstract

Abstract In developing tungsten alloys for service both above the recrystallization temperature and below room temperature the main task is to make an alloy, which in addition to having a low ductile to brittle transition temperature in the recrystallized condition can readily be manufactured into various configurations. Tungsten-rhenium alloys have been found to be useful for such applications, since they have a combination of high strength and good ductility. The best ductility is reached in alloys with a rhenium content close to the solubility limit in tungsten. During manufacturing of such alloys the forming properties are adversely affected by the precipitation of the intermetallic compound W2Re3 (σ-phase). The purpose of this work is to clarify the influence of sigma phase on the formability and product quality after high temperature densification and thermomechanical treatment (TMT) of powder metallurgy manufactured W-25%Re alloys using different processing parameters. The study consists of two parts. Part I deals with theoretical and practical aspects on the formation of sigma phase particles and their influence on metal flow and product quality. Different ways of interpreting the W-Re phase are discussed and the characteristics of the σ-phase are presented. Practical recommendations to control the negative influence of the σ-phase are also given. A more complete understanding of the effect of various processing parameters will be the subject of Part II of this paper.