Irene J. Beyerlein

Statistical analyses of deformation twinning in magnesium

To extract quantitative and meaningful relationships between material microstructure and deformation twinning in magnesium, we conduct a statistical analysis on large data sets generated by electron backscattering diffraction (EBSD). The analyses show that not all grains of similar orientation and grain size form twins, and twinning does not occur exclusively in grains with high twin Schmid factors or in the relatively large grains of the sample. The number of twins per twinned grain increases with grain area, but twin thickness and the fraction of grains with at least one visible twin are independent of grain area. On the other hand, an analysis of twin pairs joined at a boundary indicates that grain boundary misorientation angle strongly influences twin nucleation and growth. These results question the use of deterministic rules for twin nucleation and Hall–Petch laws for size effects on twinning. Instead, they encourage an examination of the defect structures of grain boundaries and their role in twin nucleation and growth.

Microstructure of cryogenic treated M2 tool steel

Cryogenic treatment has been claimed to improve wear resistance of certain steels and has been implemented in cutting tools, autos, barrels etc. Although it has been confirmed that cryogenic treatment can improve the service life of tools, the underling mechanism remains unclear. In this paper, we studied the microstructure changes of M2 tool steel before and after cryogenic treatment. We found that cryogenic treatment can facilitate the formation of carbon clustering and increase the carbide density in the subsequent heat treatment, thus improving the wear resistance of steels.

Modeling texture and microstructural evolution in the equal channel angular extrusion process

Abstract In this work, we develop a modeling framework for predicting the visco-plastic deformation, microstructural evolution (distributions of grain shape and size) and texture evolution in polycrystalline materials during the equal channel angular extrusion (ECAE) process, a discontinuous process of severe shear straining. The foundation of this framework is a visco-plastic self-consistent (VPSC) scheme. We consider a 90° die angle and simulate ECAE up to four passes for four processing routes, (A, C, B A and B C , as denoted in the literature) for an FCC polycrystalline material. We assume that the FCC single crystal has a constant critical resolved shear stress (CRSS), so that hardening by dislocation activity is suppressed, and the influence of grain shape distribution and texture as well as their interaction can be isolated. Many deformation microstructural features, such as grain size and shape distribution, texture, and geometric hardening–softening, were highly dependent on processing route. Using a grain subdivision criterion based on grain shape, route A was the most effective, then route B A and route B C and lastly route C, the least effective for grain size refinement, in agreement with redundant strain theory. For producing refined equiaxed grains, route B C was more effective than routes B A and A. We show that grain–grain interactions tend to weaken texture evolution and consequently geometric hardening and softening in all routes.

Micromechanical simulation of the failure of fiber reinforced composites

Abstract The strength of unidirectionally reinforced fiber composites is simulated using the three dimensional shear lag model of Landis, C. M., McGlockton, M. A. and McMeeking, R. M. (1999) (An improved shear lag model for broken fibers in composites. J. Comp. Mat. 33, 667–680) and Weibull fiber statistics. The governing differential equations for the fiber displacements and stresses are solved exactly for any configuration of breaks using an influence superposition technique. The model predicts the tensile strength of well bonded, elastic fiber/matrix systems with fibers arranged in a square array. Length and strength scalings are used which are relevant for elastic, local load sharing composites. Several hundred Monte Carlo simulations were executed to determine the statistical strength distributions of the composite for three values of the fiber Weibull modulus, m =5, 10 and 20. Stress–strain behavior and the evolution of fiber damage are studied. Bundle sizes of 10×10, 15×15, 20×20, 25×25, 30×30 and 35×35 fibers of various lengths are investigated to determine the dependence of strength on the composite size. The validity of weakest link statistics for composite strength is examined as well.

Atomic structures of symmetric tilt grain boundaries in hexagonal close packed (hcp) crystals

Molecular dynamics simulation and interface defect theory are used to determine the relaxed equilibrium atomic structures of symmetric tilt grain boundaries (STGBs) in hexagonal close-packed (hcp) crystals with a \( [0\bar{1}10] \) tilt axis. STGBs of all possible rotation angles θ from 0 deg to 90 deg are found to have an ordered atomic structure. They correspond either to a coherent, defect-free boundary or to a tilt wall containing an array of distinct and discrete intrinsic grain boundary dislocations (GBDs). The STGBs adopt one of six base structures, \( P_{B}^{(i)} \), i = 1, …, 6, and the Burgers vector of the GBDs is related to the interplanar spacing of the base structure on which it lies. The base structures correspond to the basal plane (θ = 0 deg, \( P_{B}^{(1)} \)); one of four minimum-energy, coherent boundaries, \( (\bar{2}111),\;(\bar{2}112),\;(\bar{2}114) \), and \( (\bar{2}116)\;\left( {P_{B}^{(2)} - P_{B}^{(5)} } \right) \); and the \( \left( {11\bar{2}0} \right) \) plane (θ = 90 deg, \( P_{B}^{(6)} \)). Based on these features, STGBs can be classified into one of six possible structural sets, wherein STGBs belonging to the same set i contain the same base boundary structure \( P_{B}^{(i)} \) and an array of GBDs with the same Burgers vector \( b_{\text{GB}}^{(i)} \), which vary only in spacing and sign with θ. This classification is shown to apply to both Mg and Ti, two metals with different c/a ratios and employing different interatomic potentials in simulation. We use a simple model to forecast the misorientation range of each set for hcp crystals of general c/a ratio, the predictions of which are shown to agree well with the molecular dynamics (MD) simulations for Mg and Ti.

Anomalous Basal Slip Activity in Zirconium under High-strain Deformation

In this letter, we reveal anomalous basal slip activity in zirconium under high strains. The frequently reported classical rolling texture of Zr is shown to develop as a result of substantial amounts of basal slip. The reason is not that physical barriers to basal slip have become easier but that over a large straining period, easy prismaticslip has significantly strain-hardened and crystallographic texture has evolved to be more favorable for basal slip. Basal slip is, therefore, an important deformation mechanism in Zr at room temperature under high to severe strain-deformation conditions.

Double twinning mechanisms in magnesium alloys via dissociation of lattice dislocations

In this work, we propose dislocation mechanisms for the formation of double twin structures in hexagonal close packed (HCP) crystals through the nucleation of secondary twins within primary twin domains. The model considers that secondary twins associated with the most commonly observed double twin variants (i.e. type 1 and type 2) nucleate and thicken by a sequence of three distinct dissociation reactions of mixed basal dislocations. Provided that the less frequently observed double twin variants (i.e. type 3 and type 4) also form by a dislocation-based mechanism, we show that their development must proceed by a separate set of dissociation reactions involving pyramidal ⟨c+a⟩ slip dislocations. Mechanistic, crystallographic and energetic considerations indicate that the type 1 variant should be the most prevalent. The mechanisms proposed here would also apply to the analysis of compound twins and HCP metals other than Mg that exhibit double twinning, such as titanium.

Pure-Shuffle Nucleation of Deformation Twins in Hexagonal-Close-Packed Metals

The propagation of deformation twins in hexagonal-close-packed metals is commonly described by a conventional glide-shuffle mechanism. The widely accepted convention is that this process is also responsible for twin nucleation, but lacks direct confirmation. Using atomistic simulations, we identify an unconventional pure-shuffle mechanism for the nucleation of (1¯012) twins, which then grow through the conventional glide-shuffle mechanism entailing the glide of twinning disconnections. The pure-shuffle nucleation of twins at grain boundaries can be ascribed to a high-stress concentration and pre-existing grain boundary dislocations.

Atomic-level study of twin nucleation from face-centered-cubic/body-centered-cubic interfaces in nanolamellar composites

We report deformation twinning in Cu within accumulative roll-bonded Cu-Nb nanolamellar composites. Twins appear connected to the Nb{112}//Cu{112} interface with the Kurdjumov-Sachs orientation relationship, which we show to be ordered and faceted. The interface adopts a different faceted structure after twinning. Our analysis suggests that deformation twinning involves facet dissociation and slip-transfer from the Nb layer to the Cu layer due to a geometrically favorable slip transmission pathway.

Role of twinning on texture evolution of silver during equal channel angular extrusion

The role of deformation twinning in the texture evolution of pure polycrystalline silver subjected to equal channel angular extrusion (Route A, three passes) has been examined. Microstructural characterization using electron backscattering diffraction and transmission electron microscopy revealed high twinning activity in every pass, as well as significant grain refinement. Polycrystal modelling combined with experimental analysis shows that texture evolution is a result of slip and deformation twinning that occurs in every pass. It is shown that the primary consequence of twinning is the reorientation of the A1 ideal component into the A2 orientation. This process results in a weak A1 and a strong A2 component. This twinning mechanism is repeated in each pass aided by the strain path changes associated with Route A and an apparent regeneration of the microstructure. As a result, with each pass the A1 and C ideal shear components weaken, whereas the components strengthen, tendencies that are distinct from those of high stacking fault fcc metals like Al, Cu and Ni.