An Investigation of Short Range Residual Stress Fields in Ferrous Lath Martensite

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aim of this work was to quantify the length scale of local residual stresses in quenched ferrous martensite and to relate this to the constituent elements in the microstructure. Martensite is replete with internal stresses. Long range or Type I stresses originate from temperature gradients or chemical segregations and are responsible for undesirable macroscopic distortions in many cases. They persist over long distances and may also affect the yielding and fracture behavior of the material when it is subject to loading. A second type of residual stress exists as ‘micro’ or Type II stresses that are a related to the microstructure, varying from region to region. These have their origins in the local distortions that accompany the phase transformation from austenite to martensite during cooling. Such stresses can reach high levels, being only limited by the plastic flow stress of the martensite, which is large, and are approximately bounded by von Mises conditions with regard to stress state and magnitude. The importance of Type II stresses for controlling the yielding of martensite during loading was appreciated by

BEVIS HUTCHINSON and JOHANNES BRASK are with the SWERIM AB, Box 7047, SE-164 07, Kista, Sweden. Contact e-mail: [email protected] Manuscript submitted July 11, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

researchers of some 50 years ago[1–3] but they have not featured in more recent publication such as.[4–13] Although these later papers give a number of interpretations of the yielding behavior of martensite, it is remarkably that none of them makes any mention of these residual stresses. Nevertheless, we have demonstrated using various approaches including advanced synchrotron diffraction measurements together with crystal plasticity finite element modeling (CPFEM) that almost all aspects of the initial plastic flow of martensite are dominated by the overcoming of residual elastic stresses.[14,15] A relevant question in this context is what is the wavelength or characteristic length dimension over which the Type II stresses act? This is expected to be connected with the microstructure but that is a complex matter when considering martensites. It is now generally acknowledged, e.g., References 16 through 19, that lath martensite has a hierarchical structure commencing within grains of the prior austenite (PAG). The specific crystallographic relationship between the two phases means that growing martensite can seldom penetrate from one PAG to the next. Within a PAG the martensite grows in packets, usually up to four different types, with aligned structures that are approximately parallel in each, and which can be recognized in optical or scanning microscopy. Each packet is composed of blocks, often as repeating pairs of thin elongated crystals with well-defined orientations. Three different variants of block can constitute the microstructure in each packet. The individual crystals in the blocks are referred to as sub-blocks and are surrounded by high angle grain boundaries. Low angle lath boundaries a