The influence of high-temperature tensile deformation on microstructure evolution in select BCC metals




Noell, Philip James

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High-temperature plastic deformation can enhance the rate of grain boundary migration in some metals. In the body-centered-cubic (BCC) refractory metals molybdenum and tantalum, this increased rate of grain boundary migration produces abnormal grain growth at temperatures significantly lower than is possible by static annealing. This phenomenon is termed dynamic abnormal grain growth (DAGG). The influence of microstructure on DAGG is studied by examining the morphology of DAGG grains produced in two Mo sheet materials. DAGG grain propagation in these materials is not uniform throughout the sheet thickness. Variations through the sheet thickness in texture and grain size are explored as causes of these behaviors. DAGG grains in both materials preferentially grow into the finest-grained polycrystalline regions of the sheet. Direct effects of local crystallographic texture variation are not evident in microstructures containing DAGG grains. The initiation of abnormal grains in Mo materials by plastic straining at elevated temperatures is investigated. The minimum strain necessary to initiate DAGG, termed the critical strain, decreases approximately linearly with increasing temperature. The variation in critical strain values observed at a single temperature and strain rate is well described by a normal distribution. An increased fraction of grains aligned with the <110> along the tensile axis, a preferred orientation for DAGG grains, appears to decrease the critical strain for DAGG initiation. Uniaxial tension tests at temperatures between 1650 and 1950 °C were conducted with commercial-purity Mo rods to determine if DAGG can be used to produce large single crystals of Mo. Necking in these tensile rod specimens during the production of DAGG grains presents a potential issue with translation to commercial application. High-temperature tensile tests were also performed on another BCC metal, an interstitial-free steel. Grain growth in this interstitial-free steel during high-temperature plastic deformation is significantly accelerated above that of the static case. Grains generally oriented between {112}<110> and {111}<113> grow more readily than other grains during dynamic grain growth in this material.


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