Alla V. Sergueeva

Advanced Mechanical Properties of Pure Titanium with Ultrafine Grained Structure

Abstract The mechanical properties of commercially pure (CP) Ti at room temperature were investigated. The decreasing grain size in CP Ti leads to significant increases in its hardness and/or strength. A combination of severe plastic deformation by high pressure torsion at 5 GPa followed by short annealing at low temperatures allows one to obtain a high strength (more than 1200 MPa) in pure material that is comparable in strength to alloyed Ti.

The effect of annealing on tensile deformation behavior of nanostructured SPD titanium

Abstract Tensile behavior of titanium was studied at room temperature in the as-processed condition (using severe plastic deformation by high pressure torsion) and also in various annealed states. It was found that controlled annealing could lead to enhancement of both strength and ductility of this material. The origin of this phenomenon is discussed in the context of TEM/HREM and SEM observations.

Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)Tm nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.

Superplastic behaviour of ultrafine-grained Ti–6A1–4V alloys

Abstract Superplastic behavior of the ultrafine-grained (UFG) Ti–6A1–4V alloy produced by high pressure torsion (HPT) has been studied. High elongations (more than 500%) have been observed in this alloy during tensile tests at relatively low temperatures and high strain rates. At the same time, the superplastic behavior of this alloy has several specific features such as the relatively low values of strain rate sensitivity of flow stress and significant strain hardening. Moreover, it was shown that the alloy, processed by HPT, demonstrates an outstanding room temperature strength about 1500 MPa after superplastic deformation.

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.

Cooperative grain boundary sliding in nanocrystalline materials

Superplastic behaviour of microcrystalline materials is now believed to be controlled by cooperative grain boundary sliding (CGBS). An increasing role of grain boundary mediated plasticity with decreasing grain size down to the nanoscale was predicted leading to the prospect of enhanced superplasticity in nanocrystalline materials. Nevertheless, materials with nanosized grains have revealed a significant decrease in plasticity contrary to theoretical prediction. Direct evidence of CGBS in nanocrystalline Ni3Al alloy from SEM surface analysis and in-situ TEM tensile testing was detected, confirming one similarity in the rheology of deformation processes between micro- and nanomaterials. Thus, differences in deformation behaviour of materials at these two length scales are related to the probability of sliding surface formation, sliding distance and related accommodation mechanisms.

Mechanical response of Zr-based metallic glass

The new family of Zr-based multicomponent metallic glasses shows a beneficial combination of yield strength value as high as 2 GPa and microplasticity of up to 1% at room temperature and excellent glass-forming ability in a wide supercooled liquid region. Partial devitrification of glassy alloys upon heating or hot working above the glass transition temperature can lead to formation of nanocrystalline precipitates in the glassy matrix. In this case, the mechanical properties depend on the amount of crystalline precipitates. The nature of the brittle crystalline intermetallic phases is likely to dominate the mechanical behavior, leading to the observed decrease in ductility for the annealed samples since the deformation is no longer governed by the deformation mechanism of the amorphous phase. A fully crystallized alloy upon heating has revealed increasing ductility with temperature, exhibiting superplastic-like deformation with high elongation.

Strategies for developing bulk materials nanotechnology (BMN) into industrial products

Abstract Microstructural formation occurring on the nanoscale level is often challenging and complex to understand but can lead to compelling properties and superior material performance. Commercial development of bulk materials nanotechnology (BMN) is enabled through new discoveries overcoming age old problems specifically: (1) opening up of process window to enable nanoscale structure formation in industrial products and (2) utilisation of new nanoscale ductility mechanisms to achieve combinations of high strength with ductility. Strategies for using BMN as either a surface technology or as a standalone monolithic technology are dependent on the operable ductility/toughness mechanisms which are overriding factors for successful commercialisation. For each route, the pathway for microstructural formation, the targeted nanoscale structures and the process window goals are described along with mainstream examples of each technology for a multitude of real world industrial applications.

Superplasticity in Nanocrystalline Ni 3 Al and Ti Alloys

The advent of nanocrystalline materials has provided new opportunities to explore grain size dependent phenomenon. Superplasticity is such a grain size dependent phenomenon defined by the ability to attain tensile elongation of 200% or more. Superplasticity in microcrystalline materials has been well characterized. The constitutive equations that describe microcrystalline superplasticity predict enhanced properties for nanocrystalline materials. Enhanced properties in such nanocrystalline material include lower superplastic temperature at constant strain rate, higher superplastic strain rate at constant temperature, and lower flow stresses. Investigations with nanocrystalline Ni 3 Al and ultra-fine grained Ti-6Al-4V alloy have shown a reduction in the superplastic temperature. However, the flow stresses in these materials are significantly higher than expected. The high flow stresses are accompanied by strong strain hardening. Transmission electron microscopy in situ straining of nanocrystalline Ni 3 Al has shown that grain boundary sliding and grain rotation occurred during straining. The sliding and rotation decreased with strain. Dislocation activity was observed but was not extensive. There was no observable dislocation storage. The parameters of the generalized constitutive equation for superplasticity for nanocrystalline Ni 3 Al and Ti-6Al-4V are in reasonable agreement with the parameters for microcrystalline material. The rate parameters suggest that nanocrystalline superplasticity shares common features with microcrystalline superplasticity. In contrast, the observed flow stresses and strong strain hardening indicate that nanocrystalline superplasticity is not a simple extension of microcrystalline behavior scaled to finer grain size.

Tensile Superplasticity in Nanomaterials - Some Observations and Reflections

The synthesis of nanocrystalline materials has provided new opportunities to explore grain size dependent phenomenon to a much finer scale. Superplasticity is a well-established grain size dependent phenomenon. In this paper, we analyze some of the tensile superplasticity data obtained in the last few years on Ti-6Al-4V, Ni 3 Al, and 1420-Al alloy processed by severe plastic deformation (SePD). The experimental results show higher flow stresses for superplasticity in nanocrystalline materials than in microcrystalline materials. It is suggested that the conventional slip accommodated grain boundary sliding is likely to be difficult in nanomaterials.