Materials with submicron to nanometer-sized grains by virtue of their high grain boundary area to grain size ratio provide valuable tools for studying deformation behavior in ultrafine-grained structures. In this regard, the well-known strain-induced martensite transformation and its reversal to the parent austenite phase were used to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The results of the electron microscopy study of phase-reversion-induced microstructure and deformation behavior of nanograined/ultrafine-grained (NG/UFG) austenitic stainless steel during tensile straining are described here. The phase-reversion-induced structure was observed to depend on the cold rolling reduction and temperature-time annealing cycle. The optimized structure consisted of nanocrystalline (d < 100 nm), ultrafine (d % 100 to 500 nm), and submicron (d % 500 to 1000 nm) grains and was characterized by a high yield strength (800 to 1000 MPa)-high ductility (30 to 40 pct) combination. Austenite nucleation during phase-reversion annealing occurred in the form of thin plates or as equiaxed grains along the martensite laths. Twinning and dislocation glide were identified as the primary deformation mechanisms, where twinning had a varied character. However, the high elongation seems to be associated with the gradual transformation of metastable austenite, with twinning having only a minor contribution.

In vivo modular taper corrosion in orthopedic total joint replacements has been documented to occur for head-neck tapers, modular-body tapers, and neck-stem tapers. While the fretting corrosion mechanism by which this corrosion occurs has been described in the literature, this report shows new and as yet unreported mechanisms at play. A retrieved Ti-6Al-4V/Ti-6Al-4V neck-stem taper interface, implanted for 6 years is subjected to failure analysis to document taper corrosion processes that lead to oxide driven crack formation on the medial side of the taper. Metallurgical sectioning techniques and scanning electron microscopy analysis are used to document the taper corrosion processes. The results show large penetrating pitting attack of both sides of the taper interface where corrosion selectively attacks the beta phase of the microstructure and eventually consumes the alpha phase. The pitting attack evolves into plunging pits that ultimately develop into cracks where the crack propagation process is one of corrosion resulting in oxide formation and subsequent reorganization. This process drives open the crack and advances the front by a combination of oxide-driven crack opening stresses and corrosion attack at the tip. The oxide that forms has a complex evolving structure including a network of transport channels that provide access of fluid to the crack tip. This emergent behavior does not appear to require continued fretting corrosion to propagate the pitting and cracking. This new mechanism is similar to stress corrosion cracking where the crack tip stresses arise from the oxide formation in the crack and not externally applied tensile stresses.

There are serious concerns on the toxicity of vanadium in Ti-6Al-4V alloy. In this regard, we describe the biological footprint of Ti-6Al-4V and compare with a viable alternate Ti-6Al-7Nb alloy, in terms of novel experimentation pertaining to cellular activity that include qualitative and quantitative analysis of Feret's diameter of cells, area, and perimeter, and proteins-actin, vinculin, and fibronectin. Interestingly, Ti-6Al-7Nb was characterized by superior cell attachment, proliferation, viability, morphology, and spread, which were significantly different from Ti-6Al-4V alloy. Additionally, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions in Ti-6Al-7Nb alloy. These striking observations suggest enhanced cell-substrate interaction and activity on the surface of niobium-containing titanium alloy. The significant differences in the cellular response between the two alloys clearly point to the determining role of alloying element (Nb versus V) in a conclusive manner. Based on this study, next generation of titanium alloys is proposed to focus on niobium-containing alloy.

We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d<100 nm), ultrafine (d % 100 to 500 nm), and submicron (d % 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)-high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocationcell-type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell-type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell-type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperaturetime annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell-type structure in lieu of lath-type facilitates NG/UFG structure with a high strength-high ductility combination. Interestingly, the yield strength follows the Hall-Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.In the context of obtaining high strength-high ductility combination, we recently described a novel processing route of developing nanograined/ultrafinegrained (NG/UFG) structure in a metastable austenitic stainless steel (AISI 301LN) involving controlled phase reversion annealing of the cold-deformed austenite. [1][2][3][4] In this approach, severe deformation of adequately metastable austenite at room temperature leads to strain-induced transformation of austenite (fcc c) to martensite (bcc a¢). On annealing, the severely deformed strain-induced martensite reverts back to austenite [1][2][3][4] via diffusional or shear reversion mechanism. [5,6] The optimal phase reversion annealing sequence resulted in the structure that was characterized by a combination of high yield strength and excellent elongation of 800 to 1000 MPa and 30 to 40 pct, respectively. [1] In a diffusional reversion mechanism, the phase reversion process and the ''final'' microstructure are not only a function of diffusion rate in bcc (martensite) and fcc (austenite) phase, but also depend on the martensite structure and density of defects. These two characteristics may accelerate the transformation characteristics or provide an increased number of nucleation sites. Thus, in sequel to our earlier work, there is a need to...

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