Optimal microstructures for martensitic steels

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Optimal microstructures for martensitic steels P. Shanthraj and M.A. Zikrya) Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina 27695-7910 (Received 22 December 2011; accepted 17 April 2012)

A dislocation–density-based model for slip transmission at variant boundaries and a microstructural failure criterion accounting for variant cleavage planes have been developed to determine optimal variant distributions for significantly improved ductility, through increased slip transmission, and fracture toughness, through increased resistance to crack propagation, in martensitic steels with refined blocks and packets. A crystal plasticity framework, accounting for variant morphologies and orientation relationships that are uniquely inherent to lath martensite, and specialized finite-element methodologies using overlapping elements to represent evolving fracture surfaces are used for a detailed analysis of fracture nucleation and intergranular and transgranular crack growth. The results indicate that the block sizes, variant orientations, and distributions are the key microstructural characteristics for toughening mechanisms, such as crack arrest and deflection, and for desired ductility, delayed crack nucleation, and greater fracture toughness. This approach can be the basis for validated design guidelines for the desired optimal behavior of high-strength and toughness steels.

I. INTRODUCTION

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.127

treatments have been used to achieve high strengths and toughness.16,17 However, the relative effects of block and packet boundaries on dislocation transmission and the coherence length on the slip and cleavage planes are not clearly understood, where the size of cleavage facets in transgranular fracture modes have been related to both the packet18,19 and block sizes.20 The presence of block and packet boundaries is also known to significantly influence deformation in martensitic microstructures.21,22 Complex interactions between dislocations and boundaries can lead to different interfacial behavior that can have a significant effect on the nucleation and propagation of transgranular and intergranular cracks.23,24 The high angle misorientations at variant interfaces act as strong barriers to dislocation movement, resulting in dislocation absorption and dislocation pileups, and can consequently affect crack directions, such as rendering transgranular cracks into intergranular cracks.25–27 These investigations clearly indicate that the morphology and distribution of the blocks and packets have a significant influence on the ductility and toughness of martensitic microstructures, through the complex interactions of the packet and block boundaries with the evolving dislocation microstructure and propagating cracks. However, a systematic investigation of the relationship between refined lath microstructures and material behavior is lacking, and design guidelines for the tailoring of variant distribut