On the mechanism of strain-enhanced grain growth during superplastic deformation

Transition from poor ductility to room-temperature superplasticity in a nanostructured aluminum alloy

Recent developments of nanostructured materials with grain sizes in the nanometer to submicrometer range have provided ground for numerous functional properties and new applications. However, in terms of mechanical properties, bulk nanostructured materials typically show poor ductility despite their high strength, which limits their use for structural applications. The present article shows that the poor ductility of nanostructured alloys can be changed to room-temperature superplastisity by a transition in the deformation mechanism from dislocation activity to grain-boundary sliding. We report the first observation of room-temperature superplasticity (over 400% tensile elongations) in a nanostructured Al alloy by enhanced grain-boundary sliding. The room-temperature grain-boundary sliding and superplasticity was realized by engineering the Zn segregation along the Al/Al boundaries through severe plastic deformation. This work introduces a new boundary-based strategy to improve the mechanical properties of nanostructured materials for structural applications, where high deformability is a requirement.

Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 T_m) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Synthesis of monolayer graphene having a negligible amount of wrinkles by stress relaxation

For the chemical vapor deposition (CVD) of graphene, the grain growth of the catalyst metal and thereby surface roughening are unavoidable during the high temperature annealing for the graphene synthesis. Considering that nanoscale wrinkles and poor uniformity of synthesized graphene originate from the roughened metal surface, improving surface flatness of metal thin films is one of the key factors to synthesize high quality graphene. Here, we introduce a new method for graphene synthesis for fewer wrinkle formation on a catalyst metal. The method utilizes a reduced graphene oxide (rGO) interfacial layer between the metal film and the wafer substrate. The rGO interlayer releases the residual stress of the metal thin film and thereby suppresses stress-induced metal grain growth. This technique makes it possible to use much thinner nickel films, leading to a dramatic suppression of RMS roughness (~3 nm) of the metal surface even after high temperature annealing. It also endows excellent control of the graphene thickness due to the reduced amount of total carbon in the thin nickel film. The synthesized graphene layer having negligible amount of wrinkles exhibits excellent thickness uniformity (91% coverage of monolayer) and very high carrier mobility of ~15,000 cm(2)/V·s.

Instability of Superplastic Aluminium Alloys

The influence of the strain hardening coefficient,γ,and the strain rate sensitivity exponent, m, on the onset and growth of necking has been pointed out in many models about plastic instability controlled by the mechanical properties of materials. In this paper, the instability parameter I (=(l-γ-m)/m) proposed by Hart is used to relate the values of m (=∂lnσ/∂lnέ) and γ (=∂σ/(σ·∂ε)) for some superplastic aluminium alloys. These values of m and γ were calculated from the load-elongation curve at various strain-rates in constant strain-rate tests with periodic m determinations by strain-rate departures of small magnitude. It will be shown that the strain hardening due to grain growth contributes to an increase in the stability of plastic deformation during superplastic flow. However the decrease of m due to dynamic grain growth with strain causes superplastic deformation to be unstable near the onset of fracture. The transition of I values from negative to positive coincides with a decrease in m value and flow stress.

It is concluded that the development of necks can be characterized by an instability parameter I which include the effect of strain hardening caused by strain induced grain growth.

Mantle superplasticity and its self-made demise

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 μm) rocks to coarse-grained (1-10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

Dynamic Growth of Ultra-Fine Austenite Grains Deformed above Ad3 Temperature in a Nb-V-Ti Steel

Generally, three typical behaviors are recognized in hot-deformation of austenite. However, considering that those austenite grains involved in single-pass deformation are mostly on the scale of dozens of micrometers or even much larger than that, it is meaningful to investigate hot-deformation behaviors of austenite grains smaller than 10μm. In the current paper, austenite grains of different sizes were prepared through repetitive treatment of rapid reheating and quenching with changing the holding temperature and/or holding time. Kinds of true stress-true strain curves at 900oC and 950oC indicate that austenitic deformation can be gradually coordinated by grain boundary behaviors, such as grain boundary sliding and/or diffusion. Simultaneously, the macroscopic deformation is more likely to be dominated through co-operation grain boundary sliding (CGBS).