Growth of preexisting abnormal grains in molybdenum under static and dynamic conditions

Abstract

This investigation compares the growth rates of preexisting abnormal grains under both static and dynamic conditions. Abnormal grains several millimeters in length were produced in two commercial-purity molybdenum (Mo) materials by tensile straining at temperatures from 1923 to 2073 K (1650 to 1800 • C). This process is termed dynamic abnormal grain growth (DAGG) because it produces abnormal grains during concurrent plastic straining. DAGG creates abnormal grains at much lower temperatures than does static abnormal grain growth (SAGG). Abnormal grains created through DAGG were characterized with their surrounding microstructures and were then subjected to annealing treatments. Only one-third of the preexisting abnormal grains subsequently grew by SAGG. Among these, SAGG occurred only in those specimens that required the largest strains to initiate DAGG when creating the abnormal grain(s). The rates of boundary migration observed for SAGG were approximately two orders of magnitude slower than those for DAGG. When DAGG in one specimen was interrupted by extended static annealing, it did not recur when straining resumed. The dislocation substructure developed during hot deformation, which includes subgrains typical of five-power creep, is critically important to both DAGG and SAGG of preexisting abnormal grains under the conditions examined.

Identifying the microstructural features associated with void nucleation during elevated‐temperature deformation of copper

“…In this study, several cases of grain growth consuming areas of elevated dislocation density were observed; see Figure 7. Many prior studies of grain growth indicate that grains with low dislocation densities consume grains with relatively higher dislocation densities, 67–71 though the mechanism controlling such instances of grain growth remains controversial 72,73 . Regardless of mechanism, if grain growth prevents cavitation, it may improve material resilience during high‐temperature deformation.…”

“…Many prior studies of grain growth indicate that grains with low dislocation densities consume grains with relatively higher dislocation densities, [67][68][69][70][71] though the mechanism controlling such instances of grain growth remains controversial. 72,73 Regardless of mechanism, if grain growth prevents cavitation, it may improve material resilience during high-temperature deformation. While not examined in the present study, we would likewise expect dynamic recovery and recrystallization to impede void nucleation, as recently observed by the authors in high-purity aluminum during room temperature deformation.…”

Creep Deformation and Dynamic Grain Growth in an Interstitial-Free Steel

“…Okayasu, Fukutomi and coworkers investigated dynamic grain growth in Al-Mg and Al-Cu binary solid-solution alloys [13,14] and in Fe-3 pct Si. [15] Another is recent work on dynamic abnormal grain growth in the BCC refractory metals Mo [16,[18][19][20][21][22] and Ta. [17] Investigations of superplasticity are particularly interested in dynamic grain growth because the grain boundary-sliding deformation mechanism responsible for the majority of superplastic behavior is very sensitive to grain size.…”

“…However, the literature does contain information from a number of studies concerned with dynamic normal grain growth [9][10][11][12][13][14][15] and dynamic abnormal grain growth. [16][17][18][19][20][21][22] This study investigates the nature of grain growth in an IF steel for both static and dynamic conditions.…”