TRIP-Assisted Steels? H. K. D. H. BHADESHIA University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK. E-mail: hkdb@cus.cam.ac.uk (Received on April 30, 2002; accepted in final form on May 29, 2002 ) 1. Introduction The weight of an automobile can be reduced by using high-strength steels as long as they have the ductility essen- tial in metal-forming operations. 1–3) One class of suitable alloys is known as the “TRIP-assisted steels”, in which the microstructure is a mixture of allotriomorphic ferrite and bainite; the term TRIP stands for transformation-induced plasticity. The typical chemical composition is Fe–0.15C– 1.5Si–1.5Mn wt%; the high silicon concentration ensures that cementite is not precipitated during the growth of upper bainite. The carbon that is partitioned from bainitic ferrite stabilises the residual austenite, enabling it to be re- tained at ambient temperature. The final microstructure typ- ically contains 20 % bainitic ferrite, 10 % retained austenite with the remainder being allotriomorphic ferrite. There have been many publications on the microstructure and property relationships of TRIP-assisted steels. Most of these studies highlight the presence of the retained austenite and many imply that the observed high uniform elongation is a consequence of the martensitic transformation of the retained austenite under the influence of an applied stress or strain. The purpose of the present work was to investigate theoretically, the magnitude of the contribution that trans- formation plasticity can make to the total elongation. 2. Method Like all displacive transformations in steels, the growth of martensite is associated with a shape deformation which is characterised as an invariant-plane strain. The invariant- plane is the habit plane of the martensite. The deformation is a combination of a large shear (s 0.26) parallel to the invariant-plane and a dilatation (d 0.03) normal to the plane (Fig. 1). The deformation can be written in matrix form using the basis symbol Z, as follows 4,5) : The effect of the invariant-plane strain on a vector u is to change it to another vector v which is in general distorted and rotated relative to u. Consider a tensile test on a sample which is fully austenitic and polycrystalline. An applied stress can stimu- late martensitic transformation, but there are 24 possible crystallographic variants of martensite that can form. A ten- sile stress has a maximum interaction with the shape defor- mation when the tensile axis u, habit plane normal z 3 and shear direction z 1 lie in the same plane and when the axis is at an angle q to the plate normal such that tan{2q }=s/d . 6) With s=0.26 and d =0.03, q =42°; it follows that the ten- sile axis, represented as a unit vector with respect to the basis Z, is given by [Z ; u]=[sin q 0 cos q ] [Z ; v]=(Z P Z)[Z ; u] so that [Z ; v]=[(sin q +s cos q ) 0 (1+d ) cos q ] In a tensile test the effect of constraint due to the grips is to cause the sample to rotate [e.g. Ref. 5)] making v  u so that the net strain along the tensile axis is given by 1- v  /  u =0.15 This means that when an austenitic tensile test specimen transforms completely into martensite, the maximum ten- sile elongation due to phase transformation is about 15 %. However, the TRIP-assisted steels which are the subject of this paper typically contain 5–15% retained austenite. Therefore the contribution to elongation from transforma- tion plasticity is the calculated level for a fully austenitic sample multiplied by the volume fraction of austenite, 7) i.e. in the range 0.75–2.25 %. This assumes the formation of the most favoured variant and complete transformation into that variant. If the variants form at random then the total strain must be even smaller; the shear strains cancel com- pletely in the limit of random transformation. 3. Summary TRIP-assisted steels of the type discussed here typically exhibit uniform tensile strains of about 15–30 %; of this, only about 2% may be a consequence of transformation plasticity. It is possible that the role of TRIP has been exag- gerated in explaining the good mechanical properties of (Z P Z) = + 1 0 0 1 0 0 0 1 s δ           ISIJ International, Vol. 42 (2002), No. 9, pp. 1059–1060 1059 © 2002 ISIJ Note Fig. 1. An invariant–plane strain with a shear s and dilatation d . The coordinates z i represent an orthonormal set in which z 3 is normal to the invariant-plane and z 1 is parallel to the shear direction. (Z P Z) is the deformation matrix describ- ing the strain.