Kashyap, Bhagwati P.

Superplasticity is the ability of certain polycrystalline materials to exhibit exceptionally large elongations by grain boundary sliding and its accommodation by diffusion, dislocation slip, or both. One of the critical requirements for this phenomenon to occur is the fine equiaxed grains, which should be stable during deformation at elevated temperature. Under this condition, there exists a unique relationship between stress and strain rate as a function of temperature and grain size, with the high values of strain rate sensitivity index m of greater than 0.3. However, there is ample evidence of the variation in flow stress with strain along with concurrent microstructural evolution. Often the microstructures are known not to be equiaxed yet exhibit superplasticity by rapid change in morphology and size during early stage of deformation. This leads to flow softening and hardening with strain as a result of the dominance of one type of change over the other. Mechanistically, accommodation of grain boundary sliding by grain boundary migration leads to grain growth, resulting in flow hardening, whereas the absence of the same may cause the formation of cavities because of stress concentration at triple points and at two-phase or particle-matrix interphase boundaries. In view of this, the constitutive relationship needs to be modified by incorporating the dependence of stress on strain and concurrent microstructure changes. The present work critically examines the nature of stress-strain curves and microstructure evolution in an attempt to account for the non-steady-state flow and attain a generalized form of constitutive relationship for superplastic deformation. An attempt is made to quantify the variations in flow stress and microstructures interdependently as well as with strain by exploring the suitable trend lines. These equations can numerically and physically help in correlating the microscopic and macroscopic properties to the mechanisms for superplastic deformation.

Material processing plays a vital role in fabrication of appliances and components of specific properties by selecting a right kind of material. In today's context, the widely used structural steels must have properties of high strength and environmental corrosion resistance to overcome the limitations of iron-carbon plane steels, which show pitting resistance of zero equivalent number (PREN). Therefore, towards overcoming the limitations and enhancing the strength, various notch wavy rolling techniques were developed for environmental friendly austenitic stainless steel (ASS) sheets, especially, for thin sections to have light weight. The ASS material shows good corrosion resistance (PREN > 18 for AISI 304 SS) but it also shows low yield strength and high ductility after solution annealing treatment. Furthermore, it has a drawback of undergoing phase transformation from austenite (fcc-crystalline structure) to martensite (bcc/bct) when subjected to large plastic deformation at ambient temperature. By applying the notch wavy rolling techniques, the yield strength of such 304 ASS material can be increased from 300 MPa to 750 MPa, along with the optimized ductility ranging from 52% to 25%, respectively. These are considered to be very important and favorable properties for advanced structural applications. Strain hardening ratio after the process was also determined. The microstructural changes responsible for such improvements in properties of the processed material were studied using optical microscopy and AFM techniques.

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS.

This research work is an attempt to understand the effect of stored strain energy heterogeneity over macro-scale, induced during interrupted annealing treatment, on the micro structural evolution in a heavily cold rolled AISI 304L austenitic stainless steel. The cold rolled AISI 304L stainless steel (SS) samples were subjected to different annealing treatments. The microstructures obtained after each annealing treatment were characterized using optical and electron microscopy techniques. Further, an attempt was made to estimate the stored strain energy by a novel approach of micro hardness measurement both over small scale (area 3600 μm2) and large scale (area ~0.25 mm2). The stored strain energy was determined using hardness values. Using these data, spatial stored strain energy distribution contour maps were created. They were, in turn, used to characterize the local softening behavior both at micro and macro-scale levels after each interrupted annealing treatment. The results showed that, interrupted annealing treatment induces increased stored strain energy heterogeneity after each annealing interruptions. Also, it was possible to determine recrystallization kinetics of the material using stored strain energy map, which helps in designing annealing cycles for achieving ultrafine grained microstructure.

The microstructures and mechanical properties of friction stir processed 2507 super duplex stainless steel were examined. Experimental results revealed that there is an optimum traverse speed for a given rotational speed that gives minimum grain size. The individual and synergistic effects of FSP parameters such as heat input, strain rate and strain on the grain size of the material were evaluated. The results indicate that counteracting effect of heat input and the combined effect of strain rate and strain results in achieving minimum grain size at an intermediate traverse to rotation speed ratio. The twin boundaries (particularly Σ3 CSL boundaries) in the stir zone of friction stir processed material reduced considerably compared to that in the base material. Both the base material and the friction stir processed material with the smallest grain size achieved were subjected to tensile testing at ambient and elevated temperatures under different strain rates. The results obtained are presented and discussed here.

As-cast binary Al–Si alloys containing ∼2 to 30 wt.% Si were deformed by differential strain rate and differential temperature test techniques at 600–840 K and strain rates 10−4–10−2 s−1 to find the parameters of the constitutive relationships for high-temperature deformation. The maximum strain rate sensitivity index (m) of flow stress was found to be 0.28 in the eutectic Al-12Si alloy. The Arrhenius plot for activation energy for deformation (Q) exhibits bilinear behaviour, with Q being greater at higher test temperatures than at lower temperatures. The magnitude of Q varies as a function of Si content: 95.7 ± 9.8 kJ/mol for Al-2Si to 311.4 ± 98.1 kJ/mol for Al-12Si alloy. Also, the constant initial strain rate tests performed at selected strain rates and temperatures revealed flow hardening at small strains followed by flow softening. Deformation of Al–Si alloys is suggested to be controlled by the dislocation climb mechanism through the participation of both the grain interior and grain boundaries by consideration of effective stress instead of applied stress.

As-cast binary Al–Si alloys containing ∼8, 12, 20, and 30 wt.% Si, representing the hypoeutectic, eutectic (12 wt.% Si), and hypereutectic compositions, were subjected to severe plastic deformation by friction stir processing (FSP) for grain refinement to 2.3–2.7 μm by dynamic recrystallization. Tensile tests conducted by differential strain rate test technique over the strain rates of 10−4–10−2 s−1 and test temperatures in the range of ∼840–530 K led to strain rate sensitivity index (m) varying from ∼0.04 to 0.40 depending on temperature, strain rate, and alloy composition. The constant initial strain rate tests conducted at 10−4 s−1 and 840 K exhibited a maximum elongation of 250% in the hypereutectic Al–20Si alloy, but the same increased linearly with m irrespective of alloy composition. Generally, with increasing Si content, the activation energy for deformation increased from 104.7 ± 14.4 kJ/mol for eutectic Al–12Si to 310.4 ± 32.3 kJ/mol for hypereutectic Al–30Si, which increased further to 572 ± 148 kJ/mol over the higher temperature range of 840–800 K. Analysis of observed deformation and microstructure behaviour supports the occurrence of superplasticity, whereby the accommodation of grain boundary sliding by grain boundary migration led to enhanced grain growth or else the local high-stress concentration at the particle-matrix interface led to cavity formation. There was no evidence of dynamic recrystallization during high-temperature tensile deformation but the flow softening observed is ascribed to the occurrence of concurrent cavitation.

Deformation of metallic materials at elevated temperatures occurs by the movement of atoms either individually or in a group by diffusion, grain boundary sliding, and the slip processes as facilitated by vacancy, dislocations, and grain boundaries. The constitutive relationship for deformation is developed based on the contributions of external test conditions like temperature and strain rate or stress applied and the internal conditions like chemical composition, phases present, and the length scale of microstructures. Deformation can occur coherently by synergisms of all participating mechanisms, failing to which becomes a source for initiating the damage in the material. The same mechanisms that were initially responsible for deformation to occur then become the facilitators for void nucleation and its growth, which then go through cavity coalescence, and crack formation on the way to cause ultimate fracture of materials. The constitutive relationships for damage mechanisms then take the form that promotes the removal of atoms from the damaged part to a farther site, which further enhances the act of material damage. In the present work, the micro-mechanisms responsible for deformation and failure will be examined to evolve a relationship towards establishing connectivity between these two processes by integrating the effects of internal and external variables for a range of materials.

Al-Si alloys of hypoeutectic, near-eutectic, and hypereutectic compositions, prepared from commercial pure aluminum and silicon, were investigated to study their tensile and compressive properties. The microstructures in as-cast stage were found to be refined after subsequent remelting and casting for two, five, and ten cycles. The proportion of eutectic phase and silicon particles increased with silicon content and influenced the nature of stress–strain curves. Both tensile and compressive stress–strain curves exhibited increase in flow stress with increasing strain, which could be expressed by Hollomon type relationships, but the tensile deformation occurs at lower flow stress than that in compression. While tensile specimens showed limited elongation to failure, the compression ones exhibited larger strains, up to 30% without failure. The tension–compression flow asymmetry was noted to vary as a function of the number of remelting cycles and of the silicon content in the alloy. The work hardening rate showed only stage III behavior in tension but both stage III and IV behavior in compression. The flow stress, including the yield strength, in compression was found to increase with decrease in grain size. However, in tensile deformation, instead of grain size effect, the yield strength was noted to increase with the decrease in inter-particle spacing. In both these cases, the Hall–Petch type relationships were found to be obeyed.