Abstract
New-generation multi-phase martensitic steels derive their high strength from the body-centered cubic (BCC) phase and high toughness from transformation of the metastable face-centered cubic (FCC) austenite that transforms into martensite upon loading. In spite of its critical importance, the in-situ transformation strain (or “shape deformation” tensor), which controls ductility and toughness, has never been measured in any alloy where the BCC lath martensite forms and has never been connected to underlying material properties. Here, we measure the in-situ transformation strain in a classic Fe-Ni-Mn alloy using high-resolution digital image correlation (HR-DIC). The experimentally obtained results can only be interpreted using a recent theory of lath martensite crystallography. The predicted in-situ transformation strain agrees with the measurements, simultaneously demonstrating the method and validating the theory. Theory then predicts that increasing the FCC to BCC lattice parameter ratio substantially increases the in-situ transformation strain magnitude. This new correlation is demonstrated using data on existing steels. These results thus establish a new additional basic design principle for ductile and tough alloys: control of the lattice parameter ratio by alloying. This provides a new path for development of even tougher advanced high-strength steels.
Highlights
The urgent need for energy efficiency and reduced emissions is driving the development of new lightweight, high strength, high-toughness, affordable structural materials
The newlygrown martensite grows crystallographically matched with the pre-existing bulk martensite and so the in-situ transformation strain is not affected by the surface measurement
High Resolution Digital Image Correlation (HRDIC) measurements between 0 and 5.2% strain yield the spatial distribution of in-plane Green-Lagrange strains Exx, Eyy (Fig. 2) and Exy
Summary
The urgent need for energy efficiency and reduced emissions is driving the development of new lightweight, high strength, high-toughness, affordable structural materials. Among the most promising are multiphase martensitic steels (such as those investigated in [1]) that derive high strength from the body centered cubic (BCC) “lath martensite” phase [2,3,4] and high toughness from transformation of the metastable face-centered (FCC) austenite that transforms into martensite upon loading. The in-situ transformation strain is a tensorial quantity and is called the shape deformation, and is usually indicated in the martensite crystallography literature [8,9] as P(1) and in much of the micromechanics modelling literature [10] as Ftr. The in-situ transformation strain is a tensorial quantity and is called the shape deformation, and is usually indicated in the martensite crystallography literature [8,9] as P(1) and in much of the micromechanics modelling literature [10] as Ftr This transformation strain is accommodated by plasticity and dissipates energy as well as provides the strain that increases ductility [11,12]. This connection between micromechanical behaviour and macroscopic response has been the subject of a number of investigations, e.g. [13,14,15,16,17] but see [10] for a general review
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