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Advancing Materials Research
the acting stress will favor the formation of those martensitic crystallographic orientations that have optimal displacive components for “yielding” to the stress. The resulting transformation-induced plasticity (TRIP) reflects a novel interplay between the kinetics of a structural change and macroscopic stress-strain behavior. Indeed, there are now cases in which constitutive relations involving transformation plasticity have been derived to predict flow stress as a function of strain, strain rate, temperature, and stress state.95 In austenitic steels having appropriate deformation-induced transformation characteristics, the uniform ductility in a tensile test has been increased about fivefold beyond that of the untransformed parent phase.96
Transformation plasticity offers an attractive mechanism for enhancing fracture toughness at high-strength levels by coming into play in the vicinity of an advancing crack, particularly in alloys that undergo shear instability before rupture. In such instances rather small amounts of mechanically induced transformation occurring in the plastic zone of a crack tip can delay the impending strain localization and thereby increase the fracture toughness substantially. Two examples are shown in Figure 52 for high-strength austenitic steels as a function of normalized test temperature.96 The maximum toughness values correspond to about KIc=250 MPa m1/2 at—70°C, which is very tough for these 1300-MPa yield-strength steels. The beneficial effect on sharp-crack toughness arises not only from the transformation plasticity as such but also from a reduction of the triaxial stress state because of the volume expansion that attends the phase change.
The decrease in fracture toughness at lower test temperatures in Figure 52 is caused by the formation of too much martensite, which in itself is less tough than the parent phase. Hence, the eventual use of transformation toughening will require compositional modifications to decrease the temperature sensitivity of the transformation—e.g., by decreasing the entropy change of the transformation in order to reduce the temperature dependence of the thermodynamic driving force. Manganese is known to affect the transformational thermodynamics in that manner.
It is conceivable that the greatest potential for transformation toughening lies in its applicability to the retained austenite in martensitic steels. This is an unexplored field that warrants intense study, inasmuch as the technological use of ultrahigh-strength steels could be materially advanced even by modest increases in toughness.
One can imagine a potential martensitic transformation under conditions where the driving force is not large enough for nucleation or interfacial motion to ensue. But if some compositional partitioning is permitted at the temperature in question, the driving force may then be sufficient for the phase