Using architectured materials to control localized shear fracture

Abstract

A medium carbon steel was partially decarburized to create a compositionally graded material consisting of a surface layer with 0.1 wt% C and a core with 0.4 wt% C. The graded material was heat-treated to produce a martensitic microstructure which was subsequently cold-rolled in the as-quenched state. Localized shear bands developed within the core region during the rolling process. The shear bands did not, however, penetrate the outer decarburized layer. In this way, it was possible to roll 0.4 wt%C martensite, in the as-quenched state, to equivalent strains in excess of 2. The microstructure evolution of the cold-rolled martensite was followed using optical microscopy, Electron Back Scatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and nano-indentation. The development of shear bands is interpreted in terms of the collapse of martensite blocks during deformation, leading to geometric softening and shear localization within the high carbon core.

Introduction

One important mode of failure is by localized shear often at low overall strain. Localized shear fractures occur in a wide variety of materials including geological materials [1], bulk metallic Glasses [2], high strength steels [3], [4], heat treated aluminum alloys [5], [6] and magnesium alloys [7]. A variety of criteria have been developed to describe localized shear failure in terms of mechanics formulations [3], [8]. However the general formulation of the transition from general uniform plasticity to localized shear is complex because a variety of phenomena such as particle decohesion and void formation within the shear band, adiabatic heat leading to strain softening, or effects of negative rate sensitivity may arise depending on the specific material which is tested; Thus, it is difficult to delay the occurrence of localized shear fracture by changing the condition of a uniform microstructure. An alternative possibility is to limit the propagation of localized shear bands by introducing non-uniformities into the structure which prevent the growth of the localized shear bands. This is the concept of an architectured material in which a new length scale is introduced into the material in order to modify the mesoscopic non-uniformity of microstructure and properties in the material. In the current work, we have chosen to introduce this concept of an imposed architecture into a heat treated medium carbon steel which normally exhibits strain localization in its homogenous form. In the early literature, a seminal paper by Tanaka and Spretnak [3] analyzes the plastic instability in pure shear of a 4340 steel. We have chosen to perform tests on a steel of very similar composition in order to illustrate the benefits of the use of novel architectured materials to control this form of shear instability.