Deformation Mechanisms

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–
Si alloy at under-aged (UA), peak-aged (PA) and over-aged
(OA) conditions has been examined to understand the
micromechanism of fatigue and the associated dynamic
structural changes in this alloy. The LCF behaviour of the
differently aged AA6063 alloys has been studied at strain
amplitudes ranging between 0.2 and 1.0% under strain control
mode. The UA state exhibits pronounced cyclic hardening
unlike the PA and the OA states at strain amplitudes greater
than 0.4%. The PA and the OA states show hardening only for a
few cycles followed by prolonged softening. Characterizations
of the micro- and the sub-structural alterations due to LCF
establish that the phenomenon of dynamic precipitation
results in cyclic hardening the UA alloy. The softening of PA
alloy occurs due to shearing of precipitates and that in the OA
alloy takes place owing to reversibility of slip by the formation
and annihilation of the Orowan loops around the β (Mg2Si)
precipitates. Analyses of the hysteresis loops reveal Masing,
nearly-Masing and non-Masing behaviour in the UA, OA and
PA states, respectively. Analyses of the asymmetry factor of
the hysteresis loops assist to infer that the Masing behaviour
in the UA alloy is due to dislocation–dislocation interactions,
whereas the nearly-Masing behaviour in the OA alloy and the
non-Masing behaviour in the PA alloy are the consequence
of varying degrees of dislocation–precipitate interactions
associated with inhomogeneous deformation.

Deformation twinning plays a critical role on improving metals or alloys ductility, especially for hexagonal close-packed materials with low symmetry crystal structure. A rolled Mg alloy was selected as a model system to investigate the extension twinning behaviors and characteristics of parent-twin interactions by nondestructive in situ 3D synchrotron X-ray microbeam diffraction. Besides twinning-detwinning process, the " twinning-like " lattice reorientation process was captured within an individual grain inside a bulk material during the strain reversal. The distributions of parent, twin, and reor-ientated grains and sub-micron level strain variation across the twin boundary are revealed. A theoretical calculation of the lattice strain confirms that the internal strain distribution in parent and twinned grains correlates with the experimental setup, grain orientation of parent, twin, and surrounding grains, as well as the strain path changes. The study suggests a novel deformation mechanism within the hexagonal close-packed structure that cannot be determined from surface-based characterization methods.

Indentation scratch-induced wear in Mg-8 wt% Al-0.5 wt% Zn (AZ80) alloy is investigated in this study for solution-treated and peak-aged conditions. Aged alloy exhibited higher wear resistance, with 12–14 % reduction in wear volume. This is attributed to enhanced microstructural resistance to scratch wear due to precipitate phase. Ploughing was found to be the dominating material removal mechanism for both solution-treated and aged specimens. Microcutting was also active in solution-treated alloy, but not in aged alloy due to the presence of brittle Mg 17 Al 12 intermetallic particles in the microstructure. The localized stresses induced by indenter tip were found to exceed theoretical shear strength for magnesium crystal, resulting in intergranular as well as transgranular fracture phenomena. Cracks were more prominent in aged alloy, which is ascribed to the suppression of twinning activity in aged alloy, resulting in non-fulfilment of von Mises criterion of plastic deformation. Scratch-induced plastic flow was more pronounced in solution-treated alloy, with twinning as the major plasticity mechanism.

Copolypropylene/organoclay nanocomposites are prepared by melt intercalation method in this research. Two different routes for addition of compatibilizer are examined, i.e. addition in the twin-screw extruder along with the polymer and the clay powder simultaneously and premixing the compatibilizer with the reinforcement in a batch mixer before addition to the polypropylene (PP) matrix. Morphology, tensile and impact properties and deformation mechanisms of the samples made via two procedures are studied and compared with those of the noncompatibilized system. To study the structure of nanocomposites, x-ray diffraction and transmission electron microscopy techniques are utilized. The deformation mechanisms of different samples are examined via reflected and transmitted optical microscopy. The results reveal that introduction of compatibilizer and also the procedure in which the compatibilizer is added to the compound, affect structure and mechanical properties of nanocomposite. The elastic modulus of PP-clay nanocomposite has increased 11.5% with incorporation of compatibilizer. Also, introduction of organoclay without compatibilizer facilitates crazing at the notch tip of PP in 3PB testing. Incorporation of compatibilizer, however, makes difficulties in initiation and growth of crazes at the notch tip. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and streng...

A finite element method was recently designed to model the mechanisms that cause superplastic deformation (A.F. Bower and E. Wininger, A Two-Dimensional Finite Element Method for Simulating the Constitutive Response and Microstructure of Polycrystals during High-Temperature Plastic Deformation, J. Mech. Phys. Solids, 2004, 52, p 1289-1317). The computations idealize the solid as a collection of two-dimensional grains, separated by sharp grain boundaries. The grains may deform plastically by thermally activated dislocation motion, which is modeled using a conventional crystal plasticity law. The solid may also deform by sliding on the grain boundaries, or by stress-driven diffusion of atoms along grain boundaries. The governing equations are solved using a finite element method, which includes a front-tracking procedure to monitor the evolution of the grain boundaries and surfaces in the solid. The goal of this article is to validate these computations by systematically comparing numerical predictions to experimental measurements of the elevated-temperature response of aluminum alloy AA5083 (M.-A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, and T.R. McNelley, Deformation Mechanisms in Superplastic AA5083 materials. Metall. Mater. Trans. A, 2005, 36(5), p 1249-1261). The experimental work revealed that a transition occurs from grain-boundary sliding to dislocation (solute-drag) creep at approximately 0.001/s for temperatures between 425 and 500 °C. In addition, increasing the grain size from 7 to 10 μm decreased the transition to significantly lower strain rates. Predictions from the finite element method accurately predict the effect of grain size on the transition in deformation mechanisms.

Polypropylene/montmorillonite nanocomposite was prepared by melt intercalation method using a twin-screw extruder with starve feeding system in this paper. The effects of compatibilizer, extruder rotor speed and feeding rate on properties of nanocomposite were investigated. Structure, tensile, and impact properties and deformation mechanism of the compounds were studied. For investigation of structure and deformation mechanisms, X-ray diffraction (XRD) and transmission optical microscopy (TOM) techniques were utilized, respectively. The results illustrate that introduction of the compatibilizer and also variation of the processing conditions affect structure and mechanical properties of nanocomposite.

The mechanisms underlying the toughening in rigid natural composites exhibited by the concentric cylindrical composites of spicules of hexactinellid sponges, and by the nacre (brick-and-mortar) structure of mollusks such as Haliotis rufescens (red abalone), as well as the crossed-lamellar structure of Strombus gigas (queen conch) show commonalities in the manner in which toughening takes place. It is proposed that crack diversion, a new kind of crack bridging, resulting in retardation of delamination, creation of new surface areas, and other energy-dissipating mechanisms occur in both natural systems. However, these are generally different from the toughening mechanisms that are utilized for other classes of structural materials. Complementary to those mechanisms found in rigid natural ceramic/organic composites, special architectures and thin viscoelastic organic layers have been found to play controlling roles in energy dissipation in these structures.