Unstable Plastic Flow and Mechanical Properties of Substitutional Iron Alloys with Polygonized Ultrafine Grain Microstructure

Abstract

A study was made of the strength-ductility relationship in substitutional iron alloys with ca. 2at% Al, Si, Ti, Cr, Mn, Ni, or Mo at ambient and low temperatures. They were cold rolled and annealed to attain either recovered or partially recrystallized structure composed largely of Polygonized subgrains of 1μm or less. It was shown that the deformation behavior in tension of these ultrafine grained iron alloys was characterized by a very small elongation due to the occurrence of plastic instability. It was found, however, that some species of solute elements manifested a remarkable effect of increasing the elongation, and furthermore, that the effect was filly maintained in the presence of Nb, which was added to retain high strength pertaining to recovered state as a result of its well-known effect on retarding recrystallization.It was suggested that the unstable plasticity in tension was closely related to the formation of the Luders bands. Various factors which may affect the Luders strain, i.e., the mode of slip, dislocation structure, and distribution of interstitial atoms, were investigated in terms of the influence of substitutional solutes. It was shown that a high dislocation multiplication rate was the most distinctive feature of the alloys exhibiting a large elongation at low temperatures.