The deformation of metals under small stresses during phase transformations

Abstract

Stresses are developed internally in metals when a change in density and strength arises from a phase transformation. It is shown that plastic flow, generally confined to the weaker phase, results from the accommodation of strain due to the transformation front. The form of the plastic flow is considered in terms of the extreme cases of yield and creep behaviour. It is deduced that, for a complete cycle both ways through the transformation temperature, the resultant deformation varies linearly with the applied stress (provided this is small), the fractional volume change on transformation and inversely as the flow stress of the weaker phase. The deformation is not zero in the absence of external stress except where the phase transformation front has random orientation and movement. The theoretically deduced relations are examined experimentally by observing the deformation of specimens with attached weights, giving small tensile stresses, while their temperature was cycled through a transformation point. Phase transformations were examined in a number of metals involving a variety of crystal structures: reasonable agreement between theory and experiment was obtained in all cases.

Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

“…Transformational Plasticity [20]). It is further noted that the phase transformation described in Equation (3) …”

A constitutive model for analyzing martensite formation in austenitic steels deforming at high strain rates

“…Additionally, the martensitic transforma tion causes the Transformation Induced Plasticity (TRIP) effect, and therefore steels showing SIMT are usually known as TRIP steels (Iwamoto et al, 1998). Two mechanisms are commonly proposed to explain transformation induced plasticity: Green wood and Johnson (1965) and Magee (1966). The first accounts for the instantaneous volume expansion and shape change produced by the martensitic transformation.…”

A Comparative Analysis of Austenite-to-Martensite and Austenite-to-Bainite Phase Transformation Kinetics in Steels

“…The following lath, however; tends to grow perpendicular to the formerly grown one (Figure 2(a)). This happens despite the superimposed stress due to the large amount of distortion within the microstructure associated with the formation of the first martensitic lath, such that a volume difference between the parent and the product phases prevails and the variant selection is affected to a lesser extent by the superimposed stress leading to lower absolute anisotropic strains as compared with the bainitic phase transformation [5,24]. Furthermore, a careful inspection of the microstructure (Figure 2(a)) reveals that no cementite or other carbides precipitated during or after the phase transformation as only one phase (ferritic distorted) can be identified.…”

“…Furthermore, production processes involving bainitic phase transformations are often accompanied by stress and temperature gradients yielding undesired shape changes in the final work pieces, i.e. non-isotropic strains evolving when stresses are superimposed during the phase transformation [4][5][6]. Thus, the work pieces often need postprocessing by milling or turning, resulting in additional production costs.…”