Abstract
Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were used to examine microstructural changes of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels during cold rolling. As the strain increased, deformation mechanisms, such as stacking faults, slip, mechanical twinning, and shear banding were activated in both steels cold rolled up to strain of 0.69. Only slip was noted in these steels at low strain (e=0.11) and slip dominance was detected in the 8Al steel at higher strains. Shear banding became active at higher strain (e~0.7) in these materials. An inhomogeneous microstructure formed in both alloys at such strain level. More extensive mechanical twinning in the 2Al alloy than that in the 8Al alloy was observed. Fish bone-like structure patterns were revealed in the 8Al steel and a river-like structure in the 2Al steel. Detailed microstructure features as elongated and fragmented grains along the rolling direction (RD) were found for both steels, as already observed in other high-Mn steels. These deformed structures are composed by lamellar packets which can contain mechanical twins or slip lines and shear bands.
Highlights
Austenitic high manganese lightweight steels have an excellent combination of strength and ductility at room temperature, due to the occurrence of different hardening mechanisms in their deformed microstructures, which were here characterized during cold rolling
Cold rolling microstructures are shown on the Kikuchi pattern image quality (IQ) and Kernel average misorientation (KAM) maps in Figure 1, which revealed that the hot-rolled materials is fully austenitic, i.e. fcc phase, containing equiaxed grains with grain size of 148 μm (8Al) and 102 μm (2Al)
It can be observed that the grains start to elongate and to fragment, resulting in a structure of grains elongated along the rolling direction (RD) with a fish bone-like structure patterns revealed in the 8Al steel, and a river-like structure in the 2Al steel, at the higher strain level, as already found in some highMn steels (VERCAMMEN et al, 2004; LÜ et al, 2011)
Summary
Austenitic high manganese lightweight steels have an excellent combination of strength and ductility (ultimate tensile strength: 1.0-1.5 GPa and elongation: 30-80%) at room temperature, due to the occurrence of different hardening mechanisms in their deformed microstructures, which were here characterized during cold rolling. These steels are good candidates for structural application purposes in the automobile industry (GUTIERREZ-URRUTIA and RAABE, 2012, 2013). Dislocation substructure ening in the Fe-30.5Mn-8.0Al-1.2C alrefinement and subsequent activation loy is attributed to precipitation of of deformation twinning (Twinning κ–carbides and their role on the planar Induced Plasticity, TWIP effect) play a dislocation substructure development significant role in the strain-hardening (HUANG et al, 1994; GUTIERREZof the Fe-30.5Mn-2.1Al-1.2C (wt.%) URRUTIA and RAABE, 2012; PARK, alloy. To avoid intergranular precipitanucleated deformation twins gradu- tion, alloying addition should be limited ally continue to emerge, increasing to within about 5.5 wt.% Al and 0.67
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