Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic M
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I.
INTRODUCTION
A complete constitutive description of material behavior requires information about a material's response as a function of strain, stress (or strain) state, strain rate, and temperature. Of these variables strain state and strain rate have been studied least. In this two-part series we examine the evolution of microstructure in 304 stainless steel as a function of strain, strain state, and strain rate. We consider the understanding of microstructural evolution to hold the key for an eventual understanding of the complete flow and failure behavior. We chose 304 stainless steel for this study because of its obvious practical importance and because of the multitude of microstructural phenomena that occur during plastic deformation. We studied only the evolution of microstructure with strain and not the stress-strain response because the test techniques used to generate multiaxial loading and high strain-rate deformation did not permit meaningful load measurements. The microstructures produced under all loading conditions were studied principally by transmission electron microscopy (TEM), and the results of this study constitute Part II. We found that at moderate-to-large plastic deformation, strain-induced martensite was the most significant microstructural change. The progress of the martensitic transformation was followed by magnetic techniques. These results and the mechanical test results are discussed in Part I. When 304 stainless steel is deformed at room temperature it transforms martensitically from fcc austenite (7) to bcc martensite (a '). This transformation has been studied extensively in uniaxial tension at low strain rates. 1-tl Early work S. S. HECKER, Deputy Leader, Chemistry-Materials Science Division, M. G. STOUT, K. E STAUDHAMMER, and J. L. SMITH, StaffMembers, are all with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted September 15, 1980. METALLURGICAL TRANSACTIONS A
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by Mathieu, n Form and Baldwin, ~3 and Powell et al ~ at moderately high strain rates showed that the amount of martensite decreased with increasing rate. Powell et al 5 and later Bressanelli and Moskowtiz 9 and Neff et a114 concluded that the increased stability of the austenite resulted from adiabatic heating at high rates. An understanding of the total elongation achieved in austenitic stainless steels is complicated by the strain-induced transformation 6'~4'15 which, in turn, depends on alloy chemistry, temperature, and strain rate. The influence of stress (strain) state on transformation has received very little attention. Patel and Cohen 16 showed that both the shear stress and the normal stress components are important. In all cases the shear stress aids the transformation. The normal stress can either aid or hinder the transformation, depending on whether or not it is in the direction of the volume change of the transformation. Powell et al 5 showed that uniaxial tensile stresses favored the 3'-'a' transformation more than uniaxial compression, although a ' formed in both cases
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