Shear localization-martensitic transformation interactions in Fe-Cr-Ni monocrystal
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8/10/04
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Shear Localization–Martensitic Transformation Interactions in Fe-Cr-Ni Monocrystal M.A. MEYERS, B.Y. CAO, V.F. NESTERENKO, D.J. BENSON, and Y.B. XU A Fe-15 wt pct Cr-15 wt pct Ni alloy monocrystal was deformed dynamically (strain rate 104 s1) by the collapse of an explosively driven thick-walled cylinder under prescribed initial temperature and strain conditions. The experiments were carried out under the following conditions: (a) alloy in austenitic state, temperature above transformation temperature; (b) alloy in transformed state; and (c) alloy at temperature slightly above Ms, propitiating concurrent shear-band propagation and martensitic transformation. The alloy exhibited profuse shear-band formation, which was a sensitive function of the deformation condition. Stress-assisted and strain-induced martensitic transformation competes with shear localization. The alloy deformed at a temperature slightly above Ms shows a significantly reduced number of shear bands. The anisotropy of plastic deformation determines the evolution of strains and distribution of shear bands. The different conditions showed significant differences that are interpreted in terms of the microstructural anisotropy. Calculated shear-band spacings based on the Grady–Kipp (GK) and Wright–Ockendon (WO) theories are compared with the observed values. The microstructure within the shear bands was characterized by transmission electron microscopy. Regions of sub-micron grain sizes exhibiting evidence of recrystallization were observed, as well as amorphous regions possibly resulting from melting and rapid resolidification.
I. INTRODUCTION
METALS deform plastically by dislocation glide (slip), mechanical twinning, or martensitic transformations. This plastic deformation exhibits in many cases inhomogeneities due to microstructural effects. Shear localization is an extreme case of inhomogeneous deformation when plastic deformation localizes on a thin region of a specimen. Shear localization is very important because it is often a precursor to failure. In most cases, shear localization is associated with a local softening of the structure. This softening can be due to thermal or geometrical reasons. In geometrical softening, the structure orients itself to a direction that is easier (i.e., requires less stress) for glide. The rotation of crystallographic slip planes in response to straining in ductile crystalline solids is an example.[1] However, shear localization is not restricted to crystalline solids; metallic glasses and partially crystalline polymers are prone to shear localization. Granular and fragmented ceramics also undergo localization.[2–6] For these cases, thermal softening is replaced by structural softening such as particle breakdown. The brittle material may undergo a comminution process in a narrow region, leading to softening. In special cases, localization has been predicted to occur even during hardening (Rudnicki and Rice[7]).
M.A. MEYERS, V.F. NESTERENKO, and D.J. BENSON, Professors, and B.Y. C
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