Microstructure and Strain Hardening in Tensile-Tested Fe-Mn-Al-Si Steels

Abstract

The exceptional combination of strength, ductility and strain hardening of high-Mn transformation-and twinning-induced plasticity (TRIP/TWIP) steels makes them appealing for automotive applications (e.g. vehicle weight reductions through down-gauging and room-temperature (RT) forming of complex shaped parts). The present study uses three Fe-22/25/28Mn-3Al-3Si alloys to investigate the effect of changes in stacking-fault energy (SFE) on the evolution of microstructure and mechanical properties during RT tensile deformation. The SFEs were previously measured by analysis of partial-dislocation separations using weak-beam dark-field TEM [1-4] that ultimately [1] incorporated single-crystal elastic constants measured on polycrystalline specimens by a novel nano-indentation method [5,6]. The RT SFEs of the Fe-22/25/28Mn-3Al-3Si alloys are 15±3, 21±3, and 39±5 mJm -2 , respectively. Details of alloy and specimen preparation, tensile testing (see Figure 1), and specimen preparation for transmission electron microscopy (TEM) have been described elsewhere [1][2][3][4]. Microstructural characterization included optical microscopy, X-ray diffraction and TEM (performed at 200 kV with a Philips CM20T).The following important conclusions were drawn from this work: (i) A SFE of 15 mJm -2 (Fe-22Mn-3Al-3Si) resulted in a deformation microstructure dominated by highly planar slip, suppression of dislocation cross-slip, and α bcc /ε hcp -martensite transformation as the dominant secondary deformation mechanism (see Figure 2). The onset of grain refinement due to the formation of multiple variants of ε hcp -martensite within any given grain occurs from the beginning of plastic deformation and provides superior work hardening at low and intermediate strains (0-0.34 true strain), and the highest strength (687±7 MPa) but lowest elongation (85±3%) of the three alloys. (ii) A SFE of 21 mJm -2 (Fe-25Mn-3Al-3Si) resulted in a dislocation structure that exhibits both planar and wavy characteristics. The formation of both ε hcpmartensite and mechanical twinning (see Figure 3) results in excellent strain hardening in the initial, intermediate and final stages of deformation, along with the largest elongation (91±1%) of the three alloys, albeit with intermediate strength (642±7 MPa). (iii) At low strains (0 to 0.1 true strain), a SFE of 39 mJm -2 (Fe-28Mn-3Al-3Si) facilitates greater dislocation cross slip and mobility resulting in the formation of a dislocation cell structure (see Figure 4a) and reduced strain hardening compared to that of lower SFE alloys. Formation of ε hcp -martensite is completely suppressed, but mechanical twinning (see Figure 4b) enhances the strain hardening from ~0.1 true strain to failure, resulting in excellent ductility (87±2%) but the lowest strength (631±5 MPa) of the three alloys. (iv) The range of SFE from 15 to 39 mJm -2 results in an excellent product of strength and elongation (55-58 GPa%) with only small variations in strength and ductility, despite the transitioning of the steels from TRIP-to TWIPdominated...