Abstract
The strain hardening and damage behavior of Al-added twinning induced plasticity (TWIP) steels were investigated. The study was focused on comparing two different alloying concepts by varying C and Mn contents with stacking fault energy (SFE) values of 24 mJ/m 2 and 29 mJ/m 2 . The evolution of microstructure, deformation mechanisms and micro-cracks development with increasing deformation was analyzed. Al-addition has led to the decrease of C diffusivity and reduction in tendency for Mn-C short-range ordering resulting in the suppression of serrated flow caused due to dynamic strain aging (DSA) in an alloy with 0.3 wt.% C at room temperature and quasi-static testing, while DSA was delayed in an alloy with 0.6 wt.% C. However, an alloy with 0.6 wt.% C showing DSA effect exhibited enhanced strain hardening and ductility compared to an alloy with 0.3 wt.% C without DSA effect. Twinning was identified as the most predominant deformation mode in both the alloys, which occurred along with dislocation glide. Al-addition has increased SFE thereby delaying the nucleation of deformation twins and prolonged saturation of twinning, which resulted in micro-cracks initiation only just prior to necking or failure. The increased stress concentration caused by the interception of deformation twins or slip bands at grain boundaries (GB) has led to the development of micro-cracks mainly at GB and triple junctions. Deformation twins and slip bands played a vital role in assisting inter-granular crack initiation and propagation. Micro-cracks that developed at manganese sulfide and aluminum nitride inclusions showed no tendency for growth even after large deformation indicating the minimal detrimental effect on the tensile properties.
Highlights
Through many years of development and application, advanced high strength steels (AHSS), aluminum, magnesium and titanium alloys have proved themselves to be versatile and effective materials for automotive parts [1,2,3,4,5,6]
Al-addition has increased stacking fault energy (SFE) thereby delaying the nucleation of deformation twins and prolonged saturation of twinning, which resulted in micro-cracks initiation only just prior to necking or failure
The mechanism of dynamic strain aging (DSA) in C containing high Mn twinning induced plasticity (TWIP) steels can be attributed to the formation of Mn-C short-range order or short-range cluster (SRO/SRC) [11,22,24]
Summary
Through many years of development and application, advanced high strength steels (AHSS), aluminum, magnesium and titanium alloys have proved themselves to be versatile and effective materials for automotive parts [1,2,3,4,5,6]. It is necessary to develop new materials focusing on the special requirements. Through careful control of chemistry and processing, materials can be tailored to provide optimum performance, keeping an aim on specific applications. By choosing different alloying elements and careful processing, the required microstructural constituents with unique mechanical behavior can be obtained resulting in many types of materials for automotive applications [8]. Microstructure, and deformation mechanisms are crucial aspects in the design and performance of metallic materials. The current work reveals the Metals 2019, 9, 367; doi:10.3390/met9030367 www.mdpi.com/journal/metals
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