Crystal Plasticity Analysis of Stress Partitioning Mechanisms and Their Microstructural Dependence in Advanced Steels

Abstract

Two-phase advanced steels have an optimized combination o f high yield strength and large elongation strain at failure, as a result o f stress partitioning between a hard phase (martensite) and a ductile phase (ferrite or austenite). Provided with strong interfaces between the constituent phases, the failure in the brittle martensite phase will be delayed by the surrounding geometric constraints, while the rule o f mixture will dictate a targe strength of the composite. To this end, the microstructural design o f these composites is imperative especially in terms o f the stress partitioning mechanisms among the constitu ent phases. Based on the characteristic microstructures o f dual phase and multilayered steels, two polycrystalline aggregate models are constructed to simulate the microscopic lattice strain evolution o f these materials during uniaxial tensile tests. By comparing the lattice strain evolution from crystal plasticity finite element simulations with advanced in situ diffraction measurements in literature, this study investigates the correlations between the material microstructure and the micromechanical interactions on the inter granular and interphase levels. It is found that although the applied stress will be ulti mately accommodated by the hard phase and hard grain families, the sequence o f the stress partitioning on grain and phase levels can be altered by microstructural designs. Implications o f these findings on delaying localized failure are also discussed. Keywords: dual phase steel, multilayered steel, lattice strain, crystal plasticity finite element methodDemands from automotive and aerospace industries have stimulated the investigations of multiphase steels that can achieve both high strength and high ductility and thus have tremendous applications in vehicle and aircraft body structures. In advanced high strength steels (AHSS) [1] that are broadly used in these applications, two phases are introduced, including a hard phase that provides high strength and the other phase that is capable of large elongation prior to failure. As a result of complex intergra nular and interphase interactions, a good balance of strength and elongation strain can be achieved. One example is the dual phase (DP) steel, consisting of low-carbon, hard martensite phase (about 5-30 vol. %) dispersed in the ferrite matrix [2-4], An alternative design beyond AHSS is the multilayered steel as processed by rolling bonding of stacked, alternating martensite and austenite layers [1,5,6], On the so-called "banana curves" plotting the ten sile strength against fracture elongation (i.e., a ductility measure) in Ret.[1], DP steel shows a better combination of strength and ductility than conventional steels. However, the multilayered steel has the potential to even outperform than the DP steel as shown in Ref.[1] and as reviewed in Ref.[6], Provided with strong interfaces between the constituent phases, the failure strain in the brittle martensite phase will be delayed by the surrounding geometric constraints, while the...

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