We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD

Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) exp...

An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design

DAMASK – The Düsseldorf Advanced Material Simulation Kit for modeling multi-physics crystal plasticity, thermal, and damage phenomena from the single crystal up to the component scale

Integrated experimental-simulation analysis of stress and strain partitioning in multiphase alloys

An experimental study has been conducted to quantify the effects of martensite plasticity on the mechanical properties of a commercial low-carbon (0.06 wt pct) dual-phase steel. The volume fraction and morphology (banded and more equiaxed) of the martensite second phase were systematically varied by control of the intercritical annealing temperature and the heating rate to this temperature. It was observed that the yield and tensile strengths were dependent on the volume fraction of martensite but not on the morphology. In contrast, the true uniform strain, fracture strain, and fracture stress were found to have a significant dependence on martensite morphology. These results were rationalized by considering an Eshelby-based model, which allowed for the calculation of the stress in the martensite islands for different morphologies and volume fractions. By comparing the stress in the martensite with an estimate of its yield stress, it was possible to rationalize the conditions under which martensite plasticity occurs. The implications of martensite plasticity affect the work hardening of the steels but most importantly the fracture properties. For conditions where martensite codeforms with the ferrite matrix, void nucleation is suppressed and the final fracture properties are dramatically improved.

U. Liedl

Papers

An unexpected feature of the stress–strain diagram of dual-phase steel

A note on the diffusion of carbon atoms to dislocations

Simulation of martensitic transformations in TRIP-steel and Fe-based shape memory alloy