Computer study of tweed as a precursor to a martensitic transformation of a Bcc lattice

Using molecular dynamics computer simulations we have studied a bcc crystal with an interatomic potential having a second derivative which passes through zero near second neighbor distances. This causes an inherent tetragonal instability which manifests itself as random dynamic tetragonal distortions at high temperature (“dynamic tweed”), quasi-static and somewhat spatially correlated tetragonal distortions at intermediate temperatures (“static tweed”), and an apparent martensitic transformation to a close-packed structure at low temperatures. The effect of simple defects (vacancies and interstitials) on the development and spatial localization of the pretransformation structure (tweed) and the subsequent martensitic transformation will also be discussed. This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformation in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.