K. A. Padmanabhan

Plastic deformation of nanocrystalline materials

A model for the deformation of nanocrystalline materials based on grain boundary sliding and formation of mesoscopic glide planes is presented. The experimental result of decreasing hardness with decreasing grain size (inverse Hall-Petch relationship) found in metals and intermetallics with nanometer grain sizes can be described by this model. For metallic systems a transition from dislocation dominated plastic deformation to grain boundary sliding occurs at a critical grain size when the stress required for dislocation motion/formation becomes larger than for grain boundary sliding.

A model for the deformation of nanocrystalline materials

Abstract A model based on grain-boundary sliding control of the flow process of nano-crystalline materials is proposed. It is demonstrated that the agreement between the theory and the experimental results concerning both nanocrystalline metals and ceramics is quite satisfactory.

Microstructure evolution during rolling of inert-gas condensed palladium

During cold-rolling of nanocrystalline Pd consolidated from clusters we observed a strong increase in stacking fault density, conclusive evidence for lattice dislocation activity. However, the absence of texture and the retention of an equiaxed grain shape even after large deformation suggested grain boundary sliding and grain rotation as concurring processes. The rate of tensile creep at 313 K and at low stress is in agreement with predictions for Coble creep.

Mechanical properties of nanostructured materials

It is pointed out that many microstructural features like grain size and shape, their distribution, pores and their distribution, other flaws/defects and their distribution, surface condition, impurity level, second phases/dopants, stress, duration of its application and temperature affect the mechanical properties of nanocrystalline (n-) materials. Detailed studies to identify the effects of each of these variables are not available. Unequivocal as well as conflicting experimental results in this regard are summarised. The present level of theoretical understanding of the mechanical behaviour of n-materials is touched upon.

Mechanical response of nanostructured materials

The present knowledge concerning the mechanical response of nanocrystalline materials as a function of temperature, microstructural features and modes of deformation is critically examined.

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite-pearlite and quenched and tempered microstructures

In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.

Effect of prior cold work on the room-temperature low-cycle fatigue behaviour of AISI 304LN stainless steel

Room-temperature total strain-controlled low-cycle fatigue tests were carried out on AISI 304LN austenitic stainless steel specimens that were cold worked by swaging to different levels (10, 20 and 30% reduction in area) prior to testing. Cyclic softening was mostly noticed. A crossover in the total strain-life plots for the material with different degrees of prior cold work is explained in terms of the differences in strength and ductility. Prior cold work increased the total strain fatigue resistance at total strain amplitudes less than about 0.50%. A reduction in the transition fatigue life with an increase in the percentage of prior cold work was observed. Masing behavior was observed only in the 30% prior cold worked material.

A theory of structural superplasticity

A summary of the experimental facts which a theory of structural superplasticity should account for, is provided. A stress-aided, diffusion controlled, viscous boundary approach is adopted which predicts the correct kinetics and inter-dependence of variables. The activation energy requirement of the model is reconciled with experimental data and the eventual loss of superplasticity is explained. A detailed discussion reveals that the proposed mechanism can explain most of the observations concerning the phenomenon of structural superplasticity.

Model for grain boundary sliding and its relevance to optimal structural superplasticity Part 1—Theory

An assessment of the experimental findings leads to the conclusion that optimal structural superplasticity results from grain/interphase boundary sliding—diffusion coupled flow. An analysis of the boundary sliding process is presented first. By suggesting that both regions I and IIa (lower stress range of region II) of superplastic flow result from sliding—diffusion coupled flow, and—treating mesoscopic (cooperative) boundary sliding as the rate controlling mechanism for optimal superplasticity, the stress, temperature, and grain size dependences of the strain rate of deformation are predicted. The above equation is then related to the stress exponent n (the inverse of the strain rate sensitivity index m). An analysis for determining the true activation energy for the rate controlling process is presented. Expressions for the distribution of internal stresses arising from sliding and the boundary viscosity are derived, and the presence of an initial unsteady region, predicted in an earlier analysis, is sh...

A comparison of the room-temperature behaviour of AISI 304LN stainless steel and Nimonic 90 under strain cycling

Abstract The influence of room-temperature low-cycle fatigue (LCF) deformation on the microstructure and the consequent modification of the LCF behaviour were examined in the case of AISI 304LN stainless steel and the superalloy Nimonic 90. Secondary hardening due to martensite formation in AISI 304LN enhanced its resistance to plastic flow. On the other hand, in Nimonic 90 shearing of γ′ particles led to cyclic softening. A change in the number of operating slip systems as well as the fracture mode was responsible for the observed two-slope behaviour in the Coffin-Manson, the cyclic stress-strain and the energy-life plots in Nimonic 90. While Nimonic 90 resisted the applied strain elastically on the basis of its strength, AISI 304LN resisted the strain plastically on the basis of its ductility. Nimonic 90 had a much higher plastic strain energy absorption capacity than AISI 304LN.