Yuntian Zhu

Paradox of strength and ductility in metals processed by severe plastic deformation

It is well known that plastic deformation induced by conventional forming methodssuch as rolling, drawing or extrusion can significantly increase the strength of metalsHowever, this increase is usually accompanied by a loss of ductility. For example, Fig.1 shows that with increasing plastic deformation, the yield strength of Cu and Almonotonically increases while their elongation to failure (ductility) decreases. Thesame trend is also true for other metals and alloys. Here we report an extraordinarycombination of high strength and high ductility produced in metals subject to severeplastic deformation (SPD). We believe that this unusual mechanical behavior is causedby the unique nanostructures generated by SPD processing. The combination ofultrafine grain size and high-density dislocations appears to enable deformation by newmechanisms. This work demonstrates the possibility of tailoring the microstructures ofmetals and alloys by SPD to obtain both high strength and high ductility. Materialswith such desirable mechanical properties are very attractive for advanced structuralapplications.In this work, we report on how inducing severe plasticdeformation (SPD) by equal channel angular pressing(ECAP) and high pressure torsion (HPT)

Microstructures and dislocation configurations in nanostructured Cu processed by repetitive corrugation and straightening

Abstract The microstructures and dislocation configurations in nanostructured Cu processed by a new technique, repetitive corrugation and straightening (RCS), were studied using transmission electron microcopy (TEM) and high resolution TEM. Most dislocations belong to 60° type and tend to pile up along the {111} slip planes. Microstructural features including low-angle grain boundaries (GBs), high-angle GBs, and equilibrium and non-equilibrium GBs and subgrain boundaries were observed. Dislocation structures at an intermediate deformation strain were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as isolated dislocation cell (IDC), dislocation tangle zones (DTZs), and uncondensed dislocation walls (UDWs).

Deformation behavior and plastic instabilities of ultrafine-grained titanium

Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals.

Deformation twinning in nanocrystalline copper at room temperature and low strain rate

The grain-size effect on deformation twinning in nanocrystalline copper is studied. It has been reported that deformation twinning in coarse-grained copper occurs only under high strain rate and/or low-temperature conditions. Furthermore, reducing grain sizes has been shown to suppress deformation twinning. Here, we show that twinning becomes a major deformation mechanism in nanocrystalline copper during high-pressure torsion under a very slow strain rate and at room temperature. High-resolution transmission electron microscopy investigation of the twinning morphology suggests that many twins and stacking faults in nanocrystalline copper were formed through partial dislocation emissions from grain boundaries. This mechanism differs from the pole mechanism operating in coarse-grained copper.

Observations and issues on mechanisms of grain refinement during ECAP process

The equal channel angular pressing route, defined by rotating schemes between adjacent passes, significantly affects effectiveness of grain refinement. It is of interest to study the mechanisms of grain refinement. Previous work has considered the accumulative strain and the effects of shear strain plane in the interpretation of certain experimental observations. However, they are not sufficiently general, and contradict each other in some cases. In this paper, we analyze experimental results available in the literature, and investigate the fundamental mechanisms of grain refinement. We believe that the interaction of shear plane with texture and crystal structure plays a primary role in grain refinement, while the accumulative strain plays a secondary role. Our model can explain the experimental results in the literature very well. Issues on the grain refinement are discussed and further research to solve these issues is suggested.

Deformation mechanism in nanocrystalline Al: Partial dislocation slip

We report experimental observation of a deformation mechanism in nanocrystalline face-centered-cubic Al, partial dislocation emission from grain boundaries, which consequently resulted in deformation stacking faults (SFs) and twinning. These results are surprising because (1) partial dislocation emission from grain boundaries has not been experimentally observed although it has been predicted by simulations and (2) deformation stacking faults and twinning have not been reported in Al due to its high SF energy.

Nanostructural hierarchy increases the strength of aluminium alloys

Increasing the strength of metallic alloys while maintaining formability is an interesting challenge for enabling new generations of lightweight structures and technologies. In this paper, we engineer aluminium alloys to contain a hierarchy of nanostructures and possess mechanical properties that expand known performance boundaries-an aerospace-grade 7075 alloy exhibits a yield strength and uniform elongation approaching 1 GPa and 5%, respectively. The nanostructural architecture was observed using novel high-resolution microscopy techniques and comprises a solid solution, free of precipitation, featuring (i) a high density of dislocations, (ii) subnanometre intragranular solute clusters, (iii) two geometries of nanometre-scale intergranular solute structures and (iv) grain sizes tens of nanometres in diameter. Our results demonstrate that this novel architecture offers a design pathway towards a new generation of super-strong materials with new regimes of property-performance space.

Deformation twins in nanocrystalline Al

Due to its high stacking fault energy, no deformation twin has ever been observed in coarse-grained Al. Recent molecular dynamic (MD) simulations predicted the formation of deformation twins in nanocrystalline (nc) Al. Here, we report transmission-electron-microscopic observations of two types of twins in nc Al processed by cryogenic ball milling. They were formed via mechanisms suggested by the MD simulations. We also observed curved twin boundaries caused by partial dislocations. These results indicate that deformation mechanisms not accessible to coarse-grained Al are active in nc Al. They could be responsible for some unique mechanical properties of nc materials.

Microstructure and properties of pure Ti processed by ECAP and cold extrusion

Abstract Equal channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) Ti as well as other metals and alloys. CP-Ti is usually processed at about 400°C because it lacks sufficient ductility at lower temperatures. The warm processing temperature limits the capability of the ECAP technique in improving the strength of CP-Ti. We have employed cold extrusion following warm ECAP to further refine the grains and improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route BC, with a clockwise rotation of 90° between adjacent passes. They were further processed by successive cold extrusions to an accumulative reduction in cross-section area by 47 or 75%. This paper reports the surface quality, microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold extrusion.

Electrochromatic carbon nanotube/ polydiacetylene nanocomposite fibres

Chromatic materials such as polydiacetylene change colour in response to a wide variety of environmental stimuli, including changes in temperature, pH and chemical or mechanical stress, and have been extensively explored as sensing devices. Here, we report the facile synthesis of carbon nanotube/polydiacetylene nanocomposite fibres that rapidly and reversibly respond to electrical current, with the resulting colour change being readily observable with the naked eye. These composite fibres also chromatically respond to a broad spectrum of other stimulations. For example, they exhibit rapid and reversible stress-induced chromatism with negligible elongation. These electrochromatic nanocomposite fibres could have various applications in sensing.