Sharvan Kumar

Crack interaction with microstructure

Designing microstructure for damage tolerance requires a detailed understanding of how an advancing crack interacts with the microstructure (and sometimes modifies it locally) at multiple length scales. Advances in experimental techniques, such as the availability of well-controlled straining stages for optical and electron microscopes, the focused ion beam, electron backscattered diffraction, and nanoindentation, enable probing at these length scales in real time and through interrupted tests. Simultaneously, increasing computational power coupled with new computational methods, such as finite element analysis (FEA) incorporating cohesive elements at the continuum level, discrete dislocation methodology at the mesoscopic level, and coupled atomistic/continuum methods that transitions atomic level information to the mesoscopic level, have made it possible to begin addressing these complex problems. By reviewing crack growth in a variety of multiphase alloys including steels, titanium aluminides, Mo alloys, and nanocrystalline metals, we demonstrate various aspects of crack interaction with microstructure, and how these problems are being addressed through experiments and computations.

Understanding the Correlation Between Intermetallic Growth, Stress Evolution, and Sn Whisker Nucleation

Stress due to intermetallic (IMC) growth is generally accepted as the driving force for Sn whisker formation, but there are still many unanswered questions regarding the development of stress and how it relates to the growth of whiskers. We have made simultaneous measurements of the evolution of stress, IMC volume, and whisker density on samples of different thicknesses to address the underlying mechanisms of whisker formation. Finite-element simulations are used to study the stress evolution due to IMC growth with various stress relaxation mechanisms: plastic deformation coupled with grain boundary diffusion is found to explain observed stress levels, even in the absence of whisker growth. A model of whisker growth suggests that the average steady-state stress is determined primarily by relaxation processes (dislocation- and diffusion-mediated) and that whisker growth is not the primary stress relaxation mechanism. Implications of our results for whisker mitigation strategies are discussed.

Imaging Dislocations in Complex Materials

Deformation of metals and alloys by dislocations gliding between well-separated slip planes is understood but most crystal structures do not possess such simple geometric arrangements. Examples are the Laves phases with the AB2 stoichiometry that are the most common class of intermetallic compounds and exist with ordered cubic, hexagonal and rhombohedral structures. These compounds are usually brittle at low temperatures and transformation from one structure to another is slow. Based on geometric and energetic grounds, a dislocation-based mechanism consisting of two shears in different directions on adjacent atomic planes has been proposed to explain both, deformation and phase transformations in this class of materials [1,2]. This paper reports direct observations, made by Z-contrast atomic resolution microscopy, of stacking faults and dislocation cores in Cr2Hf that prove that a complex dislocation scheme does indeed operate in this ordered intermetallic material. Knowledge gained of the dislocation core structure will enable improved understanding of deformation mechanisms and phase transformation kinetics in this and other complex structures.

Dislocation dynamics in Ni{sub 3}Al: Experiments and computations

A modified electrolytic etchant has been successfully used to observe dislocation etch-pits on {l{underscore}brace}100{r{underscore}brace} surfaces of single crystal Ni{sub 3}Al containing Hf and B additions. Four-point bending tests have been used to obtain the dependence of dislocation velocity on resolved shear stress at room temperature. A comparison with earlier studies reveal that the rate of change of dislocation velocity with resolved shear stress, to a first approximation, is independent of alloying. Atomic level simulations have been performed using the embedded atom method (EAM) to study dislocation core structures and frictional stress in Ni{sub 3}Al.