Deformation Response

behavior, or two-stage TRIP (γ→ε→α) character. Medium manganese in the range of 5 to 12 wt.% Mn steels are used in conjunction with intercritical partitioning treatments.

The γ→α TRIP behavior is more complicated in these medium Mn steels by the potential for twin induced plasticity (TWIP) prior to TRIP, and two-stage TRIP where γ-austenite first transforms to ε-martensite and subsequently transforms to α-martensite. Mechanical twins and ε-martensite are both formed by the motion of partial dislocations in the γ-austenite. The distinction between the two is often generalized by calculation of the intrinsic stacking fault energy (ISFE) [43-45]. Typically twins are formed when the

partials occur on every close packed {111}γ, ε-martensite however, is observed when partial dislocations are placed on every other {111}γ. There have been many works on the ISFE range at which the formation of ε-martensite will transition to mechanical twinning. For intrinsic stacking fault energies ≥ 20 mJ/m² alloys will mechanically twin and no ε-martensite is observed. The works by Remy and Pineu [43], Allain et al. [44]

and Lee et al. [45] showed that in the range of 12 – 17 mJ/m², using the model developed by Pisarik and Van Aken [17], both twins and ε-martensite are observed simultaneously.

These investigations also observed that for alloys formulated with calculated ISFE ≤ 12 mJ/m² the γ-austenite will transform to ε-martensite without twin formation. Grässel et al. [46] investigated high manganese (> 15 wt. pct) TWIP – TRIP steels with varying levels of silicon and aluminum. It was concluded that when the γ→α TRIP effect was activated the tensile strength increased significantly (180 MPa). The cause for this increase in strength was theorized to be due to delaying necking of the steel. A

lightweight duplex steel investigated by Song et al. [47] et al. with composition of 0.32C – 5.8Mn – 5Al (in wt. pct.) was processed by cold rolling followed by intercritical annealing and produced a fine grained γ-austenite and α-ferrite structure between the elongated δ-ferrite grains. The unstrained γ-austenite contained annealing twins, but when plastically strained deformation bands formed with α-martensite forming by TRIP within the deformation bands producing a combined TWIP – TRIP behavior. Lee et al.

[48], showed that for a TWIP – TRIP steel the α-martensite that formed during tensile loading occurred at the intersection of the twin bands. In the work by Lee et al. [49] a set of constitutive models were developed using nano-indention to determine the strength of

the γ-austenite and the annealed α-ferrite. A Hall-Petch relationship for the yield strength

A two-stage TRIP behavior [4-6, 42, 50] is observed in medium-manganese steels (7-15 wt. pct. Mn) when carbon is held below 0.2 wt. pct. Alloying or partitioning of the γ-austenite is formulated to produce an ISFE ≤ 10 mJ/m². The strain induced martensitic transformation products have been quantitatively followed using interrupted tensile testing and x-ray diffraction to characterize microstructural evolution during tensile straining [4, 50]. For total strains less than 4% (strains up to 10% if yield point

elongation is observed) the γ-austenite first transforms to ε-martensite and segments the γ-austenite into smaller volumes. Pisarik and Van Aken [12] showed using electron backscattered diffraction (EBSD) that the two-stage athermal transformation behavior (γ→ε→α) has an intrinsic grain refinement that is three times greater than the

conventional athermal transformation to α-martensite. In addition to this grain refinement aspect the formation of ε-martensite directly lowers the chemical driving force to form α-martensite.

The hcp ε-martensite is a denser close packed structure than γ-austenite. Work on the ferrous shape memory alloys have reported that there is a significant volume

expansion when ε-martensite transforms to α-martensite (2.2%) during Stage II TRIP.

[52] The microstructural refinement and significant volume expansion associated with the Stage II TRIP response is thought to be responsible for the high work hardening rates,

high tensile strengths (>1200 MPa) and elongations to failure in excess of 25 pct [4, 5, 6, 50]. De Cooman et al. [53] reports on a 7Mn steel annealed at 873K and 923K (600 °C and 650 °C) with a dual-phase α + γ ultrafine grain structure. After annealing the γ-austenite is reported to have a stacking fault energy of -5 mJ/m² and -14 mJ/m². The γ-austenite is shown through TEM analysis to contain both ε-martensite and α-martensite and they state that the α-martensite observed is always nucleated within the ε-martensite.

Shape memory Fe-Mn alloys also show a sequential martensitic transformation if strained beyond the elastic limit to produce unrecoverable strain. Both Huang et al. [15]

and Shin et al. [14] independently have shown that for strains greater than 4 pct. the recovery response in the shape memory system is significantly deteriorated. At the 4 pct.

strain the formation of α-martensite within intersecting ε-martensite bands was observed using transmission electron microscopy. Recent work with the medium-Mn two-stage TRIP steels by Field and Van Aken [50] showed that the Stage II (ε→α) martensitic reaction occurred at 4-6 pct. strains. Their results and this is in close agreement with the works on shape memory alloys. Both shape memory alloys [14, 15] and the earlier work of McGrath et al. [5] show that the transformation to ε-martensite occurs at stresses below 300MPa when the austenite grain size is 30 to 100µm. Recent developments with cold working followed by annealing of the two-stage TRIP steels at 873K (600 °C) to produce nanocrystalline grain structures have shown promise in increasing the yield-strength up to 830 MPa [42, 50].

2.3.2. Effect of Chemistry. Automotive steel is typically produced as a hot band