Bing Q. Han

Modeling the constitutive response of bimodal metals

The mechanical response of metals with a bimodal grain-size distribution is modeled using the secant Mori-Tanaka (M-T) mean-field approach. The actual microstructure of bimodal metals involves a grain size distribution in the ultrafine and coarse regimes; the model approximates this in terms of two phases with distinct grain sizes and with specific volume fractions. The model is applied to two bimodal materials: the Al-5083 alloys of Laverniaet al. and the Cu of Wanget al. In both the materials, the predictions agree well with the experiments. In the bimodal Al alloy, the effect of extrusion on the anisotropy in yield strength and flow behavior is also addressed. Finally, based on the model predictions, an empirical expression of the Voce form is proposed to describe the overall flow behavior of both bimodal metals.

Tensile behavior of bulk nanostructured and ultrafine grained aluminum alloys

In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and low strain hardening were observed in bulk nanostructured and ultrafine-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an influence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation hardening and solid-solution hardening. The large and sudden stress drops in the stress-strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids.

Mechanical behavior of ultrafine-grained cryomilled Al 5083 at elevated temperature

The mechanical behavior of cryomilled and consolidated Al 5083 with a duplex ultrafine-grained microstructure is described for compression testing at different temperatures and strain rates. The relationship among flow stress, strain rate, and testing temperature provides an initial hotworking guide for finegrained cryomilled aluminum alloys. The ultrafine-grained material exhibits similar deformation characteristics, such as strain-rate sensitivity, which would be expected in a conventional aluminum alloy. These findings are discussed in the context of recent empirical and theoretical models for the deformation of materials with grain sizes between 100 and 1000 nm.

Mechanical behaviour of an Al–matrix composite reinforced with nanocrystalline Al-coated B4C particulates

In metal–matrix composites (MMCs), interfacial bonding between the metal matrix and the ceramic reinforcement plays a crucial role in their mechanical performance. In the present study, B4C particles were cryomilled with an Al alloy to produce a composite powder, in which the B4C was uniformly distributed in nanocrystalline Al. The cryomilling developed a strong bond between the B4C and the Al, allowing the nanocrystalline Al to act as a coating with a strong ceramic–metal interface. This cryomilled composite powder was then introduced, as a reinforcement, into a conventional Al alloy to strengthen the material. After consolidation, the result was a bulk Al–matrix composite reinforced with B4C particles encapsulated in nanocrystalline Al. This composite exhibits greatly improved strength and stiffness.

Factors contributing to creep strengthening in discontinuously-reinforced materials

Abstract There have been several experimental investigations of the creep behavior of materials with discontinuous reinforcement. For these materials, logarithmic plots of the steady-state or minimum strain rate against the applied stress usually reveal significant curvature such that the stress exponent, determined from the slope of the line, increases with decreasing stress. Plots of this nature are usually interpreted by invoking a threshold stress, σo and replacing the applied stress, σ, with an effective stress, defined as (σ−σo). This paper examines the implications of this approach using published creep data for several aluminum-based materials. It is shown that the introduction of an effective stress leads to a stress exponent that is similar to that observed in the unreinforced matrix material but, nevertheless, the creep rates in the reinforced materials are often significantly slower than in the matrix. This difference is examined with reference to the occurrence of load transfer and substructure strengthening.

Creep behaviour and thermal stability of cryomilled Al alloy

The creep behaviour of a cryomilled 5083 Al alloy has been investigated at two temperatures, namely 573 K and 623 K. The creep resistance of the cryomilled sample is found to exceed that of conventional 5083 Al alloy, an observation that is attributed to the presence of nanoscale precipitates and impurities. The creep behaviour is analogous to that of dispersion-strengthened aluminium alloys processed via mechanical alloying and powder metallurgical techniques, as manifested by three creep regions, i.e. a low-stress region with a stress exponent of 1.1, an intermediate-stress region, characterized by a much higher stress exponent, and a high-stress region with a stress exponent of 9. The present work reveals that the microstructure is extremely stable during elevated temperature exposure. After a long-time exposure at temperatures of 573 and 623 K there is only slight grain growth, associated with the appearance of extremely fine precipitates approximately 20–50 nm. The existence of second phases (nanoscale aluminium oxide, nitride, carbide, or precipitates), in combination with grain-boundary segregation of solute and/or impurity elements is considered to play a significant role in stabilizing the microstructure.

On the behavior of microstructures with multiple length scales

The current study aims to provide fundamental insight into the behavior of microstructures containing grain sizes that span multiple length scales. A commercial 5083 Al alloy was selected as the material of interest to facilitate comparison with recently published data. The materials studied here were prepared via the thermal consolidation of powders that were cryomilled for different times (i.e., 0, 2, 4, and 8 hours). Following consolidation, the resultant microstructure was characterized by an equiaxed grain morphology with a size distribution centered around 200∼300 nm. Dispersed among the 200- to 300-nm grains were coarse-grained regions or ligaments with a grain size ranging from 600 nm to 2 µm. The occurrence of coarse-grained regions is rationalized on the basis of recrystallization or subgrain coarsening, whereas the occurrence of equiaxed fine regions is proposed to be a result of continuous grain growth. Two types of microstructures were selected for study, containing coarse-grained volumes of approximately 28 pct and 43 pct that corresponded to an ultimate tensile strength (UTS) of 566 MPa and 535 MPa, and a fracture strain of 3.2 pct and 3.5 pct, respectively. The observed ductility and the relevant toughening mechanisms were discussed in light of the presence of multiple length scales.

Enhanced tensile ductility in a nanostructured Al-7?5%Mg alloy

Abstract This paper reports work on the enhanced tensile ductility in a nanostructured Al–7·5%Mg alloy with a mean grain size of 90 nm processed via consolidation of cryomilled Al–Mg powders. An annealing treatment at a temperature of 773 K for 2·5 h modified the extruded microstructure slightly without causing significant grain growth, as revealed by TEM and XRD patterns. The annealing treatment significantly improved the ductility, with a remarkably small loss in strength. The observed high thermal stability of the cryomilled Al alloy was attributed to the existence of impurity elements introduced during cryomilling and the presence of a supersaturated solid solution. The reported phenomenon of enhanced tensile ductility was attributed to a mechanism involving dislocation activity in submicron grains during plastic deformation.

Tensile Deformation and Fracture in a Bulk Nanostructured Al-5083/SiCp Composite at Elevated Temperatures

An ultrafine-grained Al-5083 alloy reinforced with 5 vol.% nano-sized β-SiC particles was fabricated with a powder cryomilling and consolidation technique. Tensile tests were conducted at temperatures from 298 to 773 K for this composite. The mechanisms for deformation and fracture of this nanostructured composite at various temperatures are discussed.