Abstract
The strain-rate-dependent deformation behavior of an intercritically annealed X6MnAl12-3 medium-manganese steel was analyzed with respect to the mechanical properties, activation of deformation-induced martensitic phase transformation, and strain localization behavior. Intercritical annealing at 675 °C for 2 h led to an ultrafine-grained multi-phase microstructure with 45% of mostly equiaxed, recrystallized austenite and 55% ferrite or recovered, lamellar martensite. In-situ digital image correlation methods during tensile tests revealed strain localization behavior during the discontinuous elastic-plastic transition, which was due to the localization of strain in the softer austenite in the early stages of plastic deformation. The dependence of the macroscopic mechanical properties on the strain rate is due to the strain-rate sensitivity of the microscopic deformation behavior. On the one hand, the deformation-induced phase transformation of austenite to martensite showed a clear strain-rate dependency and was partially suppressed at very low and very high strain rates. On the other hand, the strain-rate-dependent relative strength of ferrite and martensite compared to austenite influenced the strain partitioning during plastic deformation, and subsequently, the work-hardening rate. As a result, the tested X6MnAl12-3 medium-manganese steel showed a negative strain-rate sensitivity at very low to medium strain rates and a positive strain-rate sensitivity at medium to high strain rates.
Highlights
Increasing demands for fuel-efficient vehicles led to the development of Advanced High-StrengthSteels (AHSS)
First generation Advanced High-StrengthSteels (AHSS) usually possess a ferritic matrix and employs phase fractions of harder phases like martensite, bainite, and metastable retained austenite to increase the product of ultimate tensile strength (UTS) and elongation to fracture εf < 20 GPa% [1,2]
The medium-manganese steels (MMnS) was homogenization annealed at 1100 ◦ C for 2 h, followed by water quenching and an additional austenitization annealing at 850 ◦ C
Summary
Increasing demands for fuel-efficient vehicles led to the development of Advanced High-StrengthSteels (AHSS). Increasing demands for fuel-efficient vehicles led to the development of Advanced High-Strength. First generation AHSS usually possess a ferritic matrix and employs phase fractions of harder phases like martensite, bainite, and metastable retained austenite to increase the product of ultimate tensile strength (UTS) and elongation to fracture εf < 20 GPa% [1,2]. In search of further improvement of this so-called ECO-Index, the second generation of AHSS with fully austenitic microstructures was developed. Reaching an ECO-Index of >50 GPa%, the remarkable mechanical properties result from very high dislocation densities in combination with the activation of additional deformation mechanisms in addition to dislocation slip which increase the work-hardening rate (WHR) [3,4]. Deformation-induced phase transformation (TRansformation-Induced Plasticity–TRIP), is the material’s stacking fault energy (SFE), which is primarily a function of the chemical composition [5,6,7,8].
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