S. G. Srinivasan

Deformation twinning in nanocrystalline copper at room temperature and low strain rate

The grain-size effect on deformation twinning in nanocrystalline copper is studied. It has been reported that deformation twinning in coarse-grained copper occurs only under high strain rate and/or low-temperature conditions. Furthermore, reducing grain sizes has been shown to suppress deformation twinning. Here, we show that twinning becomes a major deformation mechanism in nanocrystalline copper during high-pressure torsion under a very slow strain rate and at room temperature. High-resolution transmission electron microscopy investigation of the twinning morphology suggests that many twins and stacking faults in nanocrystalline copper were formed through partial dislocation emissions from grain boundaries. This mechanism differs from the pole mechanism operating in coarse-grained copper.

Deformation mechanism in nanocrystalline Al: Partial dislocation slip

We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

Nucleation and growth of deformation twins in nanocrystalline aluminum

Deformation twins (DTs) in nanocrystalline (nc) Al were both predicted by atomic simulations, and observed experimentally. However, despite encouraging preliminary results, their formation mechanism remains poorly understood. Here we present an analytical model, based on classical dislocation theory, to explain the nucleation and growth of DTs in nc Al. A 60° dislocation system consisting of a 90° leading partial and a 30° trailing partial is found to most readily nucleate and grow a DT. The model suggests that the stress for twin growth is much smaller than that for its nucleation. It also predicts an optimal grain size for twin nucleation. The model successfully explains DTs observed experimentally in nc Al and is also applicable to other nc metals.

Formation mechanism of wide stacking faults in nanocrystalline Al

A full dislocation often dissociates into two partial dislocations enclosing a stacking fault (SF) ribbon. The SF width significantly affects the mechanical behavior of metals. Al has very high stacking fault energy and, consequently, very narrow SF width in its coarse-grained state. We have found that some SFs in nanocrystalline Al are surprisingly 1.4–6.8 nm wide, which is 1.5–11 times higher than the reported experimental value in single crystal Al. Our analytical model shows that such wide SFs are formed due to the small grain size and possibly also to the interaction of SF ribbons with high density of dislocations.

Impurities block the α to ω martensitic transformation in titanium

Impurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys1 to steel2 to planetary cores3. Experiments and empirical databases are still central to tuning the impurity effects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound2. Pure titanium transforms from ductile α to brittle ω at 9 GPa, creating serious technological problems for β-stabilized titanium alloys. Impurities in the titanium alloys A-70 and Ti–6Al–4V (wt%) suppress the transformation up to at least 35 GPa, increasing their technological utility as lightweight materials in aerospace applications. These and other empirical discoveries in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: the effect on the relative phase stability, and the energy barrier of the transformation. Ab initio methods4,5 calculate both changes due to impurities. We show that interstitial oxygen, nitrogen and carbon retard the transformation whereas substitutional aluminium and vanadium influence the transformation by changing the d-electron concentration6. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti–6Al–4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.

Nucleation of deformation twins in nanocrystalline face-centered-cubic metals processed by severe plastic deformation

Nanocrystalline (nc) materials are known to deform via mechanisms not accessible to their coarse-grained counterparts. For example, deformation twins and partial dislocations emitted from grain boundaries have been observed in nc Al and Cu synthesized by severe plastic deformation (SPD). This paper further develops an earlier dislocation-based model on the nucleation of deformation twins in nc face-centered-cubic (fcc) metals. It is found that there exists an optimum grain-size range in which deformation twins nucleate most readily. The critical twinning stress is found determined primarily by the stacking fault energy while the optimum grain size is largely determined by ratio of shear modulus to stacking fault energy. This model formulated herein is applicable to fcc nanomaterials synthesized by SPD techniques and provide a lower bound to the critical twining stress.

Structural, elastic, and electronic properties of Fe3C from first principles

Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010] and [001] directions. The three methods agree well with each other, the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

Excess energy of grain-boundary trijunctions: an atomistic simulation study

Abstract Atomic-scale computer simulation was used to study grain-boundary trijunctions, which are defined as the intersection of three grain boundaries. The simulation system consisted of a three-dimensional periodic array of columnar f.c.c. grains having three different orientations with a common [001] direction, and in which all grains are rotated 30° from their neighbors. The inter-atomic interactions were described by the Lennard–Jones potential. Each simulation cell contained six trijunctions plus the nine associated symmetric tilt grain boundaries. The energy of systems of differing sizes was monitored during annealing and after quenching to obtain quantitative estimates of the excess energy of the grain boundaries and trijunctions. For this system, the total excess energy contributed by the trijunctions was found to be negative. This result is consistent with recent calorimetry experiments on high-purity nanocrystalline cobalt conducted elsewhere.

Atomistic simulations of shock induced microstructural evolution and spallation in single crystal nickel

Spallation in single crystalline nickel was studied using molecular dynamics simulations. The shock waves—incident waves, the waves reflected from sample free surfaces, and interference between reflected waves—create and destroy many microstructural features. These features, though unimportant in determining the spall strength, control the spall nucleation site. Spall occurs by cavitation at a grain boundary junction in cold, defective, tensile regions of the sample. Atomistic calculations and experiments, though separated by six orders of magnitude in strain rates, follow a universal strain rate behavior.

Unexpected strain-stiffening in crystalline solids

Strain-stiffening—an increase in material stiffness at large strains—is a vital mechanism by which many soft biological materials thwart excessive deformation to protect tissue integrity. Understanding the fundamental science of strain-stiffening and incorporating this concept into the design of metals and ceramics for advanced applications is an attractive prospect. Using cementite (Fe3C) and aluminium borocarbide (Al3BC3) as prototypes, here we show via quantum-mechanical calculations that strain-stiffening also occurs, surprisingly, in simple inorganic crystalline solids and confers exceptionally high strengths to these two solids, which have anomalously low resistance to deformation near equilibrium. For Fe3C and Al3BC3, their ideal shear strength to shear modulus ratios attain remarkably high values of 1.14 and 1.34 along the (010)[001] and slip systems, respectively. These values are more than seven times larger than the original Frenkel value of 1/2π (refs 4, 5) and are the highest yet reported for crystalline solids. The extraordinary stiffening of Fe3C arises from the strain-induced reversible ‘cross-linking’ between weakly coupled edge- and corner-sharing Fe6C slabs. This new bond formation creates a strong, three-dimensional covalent bond network that resists large shear deformation. Unlike Fe3C, no new bond forms in Al3BC3 but stiffening still occurs because strong repulsion between Al and B in a compressed Al–B bond unsettles the existing covalent bond network. These discoveries challenge the conventional wisdom that large shear modulus is a reliable predictor of hardness and strength of materials, and provide new lessons for materials selection and design.