Hanry Yang

On the microstructural aspects of the nonhomogeneity of superplastic deformation at the level of grain groups

Abstract The surface relief of a superplasticity deformed magensium alloy (Mg-1.5% Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching. Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 × 10−3 Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35–60° to the tensile axis. Spacing between the localized deformation bands is 6–8 grain diameters after a tensile elongation of 20% and 3–4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.

Interaction of high-temperature deformation mechanisms

High-temperature tensile tests have been conducted on a magnesium alloy (Mg-1.5 pct Mn-0.3 pct Ce) with randomly mixed fine and coarse grains. The microstructural examinations clearly show that different mechanisms operate in the regions of coarse and fine grains. The coarse grains deform by dislocation slip, while grain boundary sliding occurs in the fine grains. The influence of these mechanisms on each other has also been observed in terms of dislocation density, intragranular slip lines, and grain boundary sliding. The analytical equations describing the interaction of two deformation mechanisms operative in materials with regions of fine and coarse grains were derived. The analysis is applicable for determining the controlling mechanism of two interacting mechanisms. It is predicted that at a critical volume fraction of fine grains of approximately 40 pct, a transition from superplastic to nonsuperplastic behavior occurs.

Superplastic behavior of regular α2 and super α2 titanium aluminides

Superplasticity in two titanium aluminides, regular α2 and super α2 were studied at temperatures ranging from 920 to 1040 °C and strain rates from 2 × 10−5 to 5 × 10−3 s−1. Results show that superplastic elongations of over 500% can be obtained in this group of intermetallic materials. Dynamic grain growth and changes in volume fraction of the phases were observed in both alloys. The regular α2 alloy failed mainly by intergranular separation whereas transgranular fracture with extensive ductility occurred in the super α2 alloy. Possible mechanisms of superplastic deformation of these intermetallic alloys are also proposed.

Strain induced morphological changes of α2 and β phases in Ti3Al alloys during superplastic deformation

Results of a study of morphological changes of alpha2 and beta phases during superplastic deformation of Ti3Al are presented. A possible mechanism of the superplastic deformation of two-phase material is proposed on the basis of cooperative grain boundary sliding of two intersecting systems. Morphological studies of superplastically deformed titanium aluminides revealed the formation of two systems of stringers of alpha2 and beta phases. The orientation of the stringers or alpha2 and beta phases is at 30-60 deg to the tensile axis. Surface irregularities were found to be related to the intersection of stringers with the surface. The initially lathlike alpha2 phase in the super alpha-two alloy broke up into a more equiaxed morphology. 15 refs.

Mechanical properties and microstructural characterization of a superplastic TiAl alloy

Superplastic deformation in a γ-TiAl which had a duplex microstructure, equiaxed primary γ and lamellar α2 + γ, was investigated with respect to the effect of temperature, strain rate, and environment. A maximum elongation of 540% was obtained at 1280 °C and a strain rate of 8 × 10−5 s−1 in argon. The specimens tested in argon yield substantially higher ductility than those tested in vacuum. An argon environment is also more favorable than vacuum in terms of desirable microstructure because of the retardation of aluminum depletion at the testing temperature. Microstructural examination revealed significant grain growth and cavitation in a Ti-rich layer near the surface. The α phase interspersed between the γ phase accommodated grain boundary sliding and retarded grain growth because of the chemical dissimilarity between the α and γ phases. Possible high temperature deformation mechanisms of this alloy are also discussed.

Two-Step SPD Processing of a Trimodal Al-Based Nano-Composite

An ultrafine-grained (UFG) aluminum nano-composite was fabricated using two severe plastic deformation steps: cryomilling of powders (and subsequent consolidation of blended powders by forging) followed by high-pressure torsion (HPT). The forged bulk composite featured a trimodal structure comprised of UFG, coarse grain (CG) regions, and ceramic particles. The additional HPT processing introduced finer grain sizes and altered the morphology and spatial distribution of the ductile CG regions. As a result, both strength and ductility increased substantially compared to those of the Al nano-composite prior to HPT. The increases were attributed to the more optimal shape and spacing of the CG regions which promoted uniform elongation and yielding during tensile loading. Microstructural changes were characterized at each processing step to establish the evolution of microstructure and to elucidate structure-property relationships. The toughening effect of the CG regions was documented via fracture analysis, providing a potential strategy for designing microstructures with enhanced strength and toughness.

Investigation of the deformation behavior of samples with superplastic layers

Abstract The tensile deformation behavior of samples made of magnesium and titanium alloys with superplastic layer(s) was investigated. It was observed that deformation took place by means of layer-by-layer shear in the superplastic region. Traces of such shear were visible to the unaided eye on the surface of the deformed specimens. The spacing of these shear surfaces was about 6–8 grains. It is suggested that superplastic deformation proceeds by cooperative grain shear along two intersecting grain boundary systems oriented at approximately 45°–60° to the tensile axis. Using optical microscopy and back-scattered electron imaging on a scanning electron microscope, traces of shear systems were also observed on the prepolished surfaces of deformed magnesium alloy samples and the etched surfaces of deformed titanium alloy samples.

Nanoscratch-induced deformation behaviour in B4C particle reinforced ultrafine grained Al alloy composites: a novel diagnostic approach

We report on the novel application of nanoscratch characterization to provide insight into the plasticity mechanisms responsible for the behaviour of composites. Accordingly, we conduct deformation characterization with nanoscratch testing (DCNT) to study the deformation behaviour of two B4C reinforced ultrafine grained Al alloy tri-modal composites with average B4C particle sizes of ~1–6 μm and ~500 nm, respectively. To highlight the type of mechanistic information revealed in a DCNT study of composites, we concentrate on the influence of B4C particle size on deformation mechanisms.

High temperature deformation of a magnesium alloy with controlled grain structures

Abstract A special technique has been used to produce tensile specimens of a magnesium alloy (Mg-1.5%Mn-0.3%Ce) with regions of fine and coarse grains connected either in parallel or in series, or with a randomly mixed structure of fine and coarse grains. The high temperature deformation behavior of such samples was investigated. It was found that the mechanical properties were dependent on the volume fractions of fine or coarse grains, the nature of the connection ( i.e. in parallel or in series) and the interaction of the deformation processes operative within the fine- and coarse-grain regions. The analytical equations describing the deformation behavior of materials with regions of fine and coarse grains connected either in parallel or in series were derived and the theoretical predictions were compared with the experimental data.

Novel fabrication of bulk Al with gradient grain size distributions via powder metallurgy

We describe a novel approach to synthesize gradient microstructures, defined hereafter as containing a broad but continuous distribution of grain sizes. These microstructures extend the concept of a bimodal grain size distribution and the ability to design with multiple length scales. We demonstrate the proposed approach via experiments involving cryogenic ball milling of Al–4.5Mg–0.4Mn–0.05Fe and Al–50Mg powder followed by subsequent consolidation. Our results reveal that the grains in the consolidated powder present a gradient size distribution ranging from <100 nm to >3 μm. Moreover, phase composition analysis revealed a unique “interfingered” structure where the two starting phases were intermixed in a complex three-dimensional mesh. Hardness studies of this gradient microstructure show average Vickers hardness values of 200 ± 2.6, 204 ± 4.3 and 266 ± 50 for macrohardness, microhardness and nanoindentation, respectively. The standard deviation values highlight that the gradient microstructure is disordered locally, but homogenous macroscopically.