Research
Phase-reversion Induced Nanostructured Materials: Deformation and Fracture
Nanograined (NG) materials exhibiting "high strength-high ductility combination" are excellent vehicles to obtain an unambiguous understanding of deformation mechanisms. Toward this end, we have developed the concept of phase reversion-induced NG structure that enables high strength-high ductility combination to be obtained has been developed. In the phase reversion approach, deformation (~60-80%) of austenite at room temperature leads to transformation of face-centered cubic austenite to body-centered cubic martensite (α'). Upon annealing, martensites reverts to austenite via diffusional or shear mechanism, depending on the chemistry of the alloy. The uniqueness of the concept is that it enables us to obtain a spectrum of grain size from NG to the coarse-grain (CG) regime through a single set of parameters (extent of cold deformation and temperature-time annealing sequence). Utilizing this novel concept, the present objective is to fundamentally understand grain size dependence on deformation mechanisms from NG to CG regime in austenitic alloy. The objective is being accomplished by combining nanoscale deformation experiments and electron microscopy.
There is a clear distinction and fundamental transition in the deformation behavior of NG and CG Fe-17Cr-7Ni austenitic alloy such that "deformation twinning" contributes to the excellent ductility of "high strength" NG alloy, while in "low strength" CG alloy, ductility is also good but because of strain-induced martensite nucleation at shear bands. The hypothesis is that in a given austenitic alloy, the decrease in grain size from CG to NG regime increases the stability of austenite such that there is a change in the deformation mechanism from strain-induced martensite in the CG structure to nanoscale twinning in the NG structure. An accompanying hypothesis is that intense mechanical twinning is an active deformation mechanism contributing to the excellent ductility of NG FCC alloy, which is radically different from its CG counterpart. The hypothesis is being confirmed by addressing the following issues: (a) dependence of grain size (from CG to NG regime) on deformation mechanisms, (b) the mechanistic contribution of twinning on strain hardening response and strain rate-sensitivity as a function of grain size, and (c) the effectiveness of twinning in enhancing ductility of NG materials.
The experimental understanding is being complemented with the analysis of transformation plasticity using finite element method coupled with a phase field model.
Furthermore, given that austenitic stainless is used for hip-implants and other biomedical devices, the research is significant in developing implants with nanograined (NG) structure. Toward this end, it is proven that NG structure significantly modulates cell-substrate interactions and enhances osteoblasts (bone forming) functions. Simultaneously, the high strength of NG biomedical device provides the required wear resistance and is in addition to thinner and reduced mass (high strength/weight ratio) for long term stability.
