Volume 98, Issue 4

The Chinese philosopher Lao Tzu wrote that “a journey of a thousand li begins with a single step”. † My first step on the road to ultrafine-grained materials occurred on 5 May, 1989, when I boarded a plane in Los Angeles on the way to Ufa via Copenhagen and Moscow. This review provides a very brief summary of the historical development of ultrafine-grained materials and my personal journey through this important area of materials research.

Transmission electron microscopy methods were used to study the nanocrystalline structure achieved by severe plastic deformation of L1 2 long-range ordered Ni 3 Al. The deformation by high-pressure torsion (up to 40 000 %) leads to a duplex structure consisting of a disordered nanocrystalline and an ordered coarse-grained structure carrying different shear strains. The volume fraction of the nanocrystalline structure increases with strain but varies locally. During the deformation micrometer sized coarse-grained volumes embedded in the nanocrystalline structure are retained; they show a predominance of orientations of high symmetry. This leads to the conclusion that the stability of the coarse grains results from their homogeneous slip caused by a homogeneous activation of the slip systems.

The strain rate sensitivity of nanocrystalline nickel was studied at different temperatures in tensile tests and with a nanoindenter in order to examine the effect of grain size on the different deformation mechanisms of nanocrystalline materials. The experiments yielded, depending on temperature and strain rate, the strain rate sensitivity, the activation volume and the creep exponents as a function of stress and grain size. From the creep experiments the transition between grain boundary sliding and dislocation climb as a function of temperature was obtained. The strain rate jump tests gave extremely small activation volumes, nearly a factor of 100 smaller than in conventional nickel as a function of grain size. To help in understanding this behaviour the strain rate sensitivity of single grains was tested with a nanoindenter. The results clearly showed that the primary interaction of dislocations with grain boundaries is the reason for the strong rate effects and small activation volumes observed.

Samples of pure Cu and Cu with 0.17 wt.% Zr were processed by equal-channel angular pressing up to 12 passes with a 90° die angle using route B c . Microstructure evolution after deformation and subsequent heat treatment of this severely deformed material was investigated. The deformed and annealed states were characterized by electron back scatter diffraction, transmission electron microscopy, optical microscopy and microhardness tests. The microstructural changes were compared to the behavior of specimens produced by conventional cold rolling. The annealing phenomena were analyzed with regard to recrystallization and grain growth mechanisms. It is shown that the microstructural change during annealing of equal-channel angular pressing deformed specimens has to be associated with discontinuous recrystallization. CuZr samples showed a significantly higher thermal stability against recrystallization than pure Cu samples. This is demonstrated to be primarily due to impurity drag.

Torsion at elevated temperatures and pressures was used to severely deform the intermetallic compound NiAl. The microstructure and texture as a function of shear strain were investigated by orientation imaging microscopy and diffraction of synchrotron radiation, respectively. The results show that at high shear strains a steady state grain structure and texture develops by continuous dynamic recrystallisation. The lowest grain size achieved is in the micron range, the main texture component is {110}<100>. It is concluded that high-strain torsion may open new possibilities in terms of grain refinement, texture formation and ductilisation of NiAl.

Shear samples with special geometry were produced from high purity nickel and copper in order to study the deformation mechanism in the saturation region of severely plastically deformed metals. The shear samples, pre-deformed by high pressure torsion, were tested in situ at room temperature. Electron back scatter diffraction and back scatter images were used to characterise the change in micro-texture and microstructure after the shear deformation. The experiment revealed that almost all grains were sheared simultaneously, while at higher shear strains the formation of micro shear bands were observed.

The paper reports on non-microscopical methods which allow for close investigation of ultrafine-grained metals especially of those processed by different techniques of Severe Plastic Deformation. These are as follows: (i) X-ray Bragg profile analysis, (ii) electrical residual resistivity, (iii) annealing calorimetry, and (iv) positron annihilation spectroscopy. The advantage of these methods lies in their integration/averaging over a certain sample volume so that they are often much closer to macroscopic physical properties than methods of microscopy; furthermore, the values of quantities obtained are more reliable because of strictly objective rules of determination. Typical quantities being relevant for ultra-fine grained materials are (a) grain size and grain size distribution, (b) long range internal stresses, (c) dislocation densities, and (d) concentration of vacancies and/or vacancy agglomerates. While quantities (a) and (b) are important for all classes of ultra-fine grained materials, those of (c) and (d) particularly occur with nanomaterials processed by severe plastic deformation also being responsible for their specific features such as ductility and the presence of non-equilibrium phases.

Deformation processing of initially polycrystalline material can lead to ultrafine-grained or even to nanocrystalline material, depending on the applied deformation route. In addition to different grain sizes, materials processed by different deformation methods also display different thermal stabilities. In order to analyze the impact of defects – particularly dislocations – near the grain boundaries, pure Pd and pure Ni have been processed sequentially by different methods. Transmission electron microscopy investigations at high resolution were conducted to characterize the microstructures of the materials. In addition, geometrical phase analysis was used to locate defects and visualize their strain fields. The results indicate that the presence of a high dislocation density at the grain boundaries increases the stability of the nanocrystalline structure against coarsening.

A study is presented on the microstructural evolution and mechanical behaviour of three ultrafine grained steels produced by severe plastic deformation. Two Nb – V microalloyed steel grades differing only in their Nb content and a C – Mn steel were extensively refined at 773 K by caliber rolling to a reduction of section of the order of 98 %. Grain stability and evolution of mechanical properties during annealing at 873 K were then investigated. It was demonstrated that tailored annealing treatments lead to a modification of the tensile behaviour of the ultrafine steels, with a more satisfactory combination of strength and uniform elongation. Nb – V microalloying contributed to increasing the stability of the grain structure and allowed achieving slightly improved properties with respect to the C – Mn grade.

Recent studies have revealed that bulk nanostructured metals and alloys produced by severe plastic deformation can demonstrate extraordinary superplasticity at low temperatures and/or high strain rates. This work presents new results on superplasticity in several nanostructured Al and Ti alloys focusing on microstructural evolution and strain hardening, as well as the challenges of their application. Grain refinement in these alloys was accomplished using severe plastic deformation techniques. Subsequent superplastic deformation allowed not only to attain their efficient forming, but also to improve the ultrafine-grained structure and to obtain enhanced mechanical properties in the articles produced. The results demonstrate the possibilities of new applications of superplastic forming using bulk nanostructured materials.

The technologically relevant aluminium alloy AA6016 was successfully processed using accumulative roll bonding to 8 cycles (∊ von Mises = 6.4) in order to obtain an ultrafine-grained microstructure with an average grain size of approximately 200 nm. With the aim of optimising the accumulative roll bonding process detailed investigations on the robustness, the type of rolling mill, the influence of temperature, the microstructural evolution and the mechanical properties have been carried out. Processing at 230 °C provided a good compromise between thermal stability and interlamellar bonding. Samples strained up to 6 cycles (∊ von Mises = 4.8) showed an increase in the yield strength by a factor of 3 in comparison to the as-received material. The ductility of the roll bonded samples was slightly sacrificed, although an increase in ductility can be achieved by increasing the number of accumulative roll bonding cycles. The mechanical properties depend on the strain rate, as has also been found for many other ultrafine-grained materials. Annealing of ultrafine-grained samples revealed a stability limit of approximately 200 °C.

Equal Channel Angular Pressing is a well-known method of severe plastic deformation used to obtain ultrafine grained materials with significantly improved mechanical properties. To the present, Equal Channel Angular Pressing has essentially been restricted to the treatment of bulk billets of square or circular cross-section of limited size. A machine developed at Monash University allows the Equal Channel Angular Pressing of sheet material. In the present work, improvement in biaxial formability, associated textures and mechanical properties after one Equal Channel Angular Pressing pass followed by annealing for aluminium alloy 5005 were investigated. An optimal annealing treatment to obtain a good balance of properties was defined.

The microstructural evolution, tensile response and wear properties of a two-phase Zn – Al alloy (Zn-8 wt.% Al, ZA-8) have been studied after severe plastic deformation by equal channel angular extrusion (ECAE). The experimental results reveal that the strength levels of the ECAE processed samples were considerably improved regardless of the processing route. More importantly, the elongation at fracture was dramatically increased after ECAE. The optimum tensile properties (high strength and high ductility) were reached after eight ECAE passes following route B A . It was also found that the wear rate of severe plastically deformed ZA-8 is considerably lower than that of the as-cast alloy, especially under high applied pressures, demonstrating improved wear resistance of ZA-8. Moderate strength and high ductility along with improved wear resistance in the ECAE processed ZA-8 samples in comparison with the brittle as-cast alloy makes these alloys attractive for wear-sensitive structural applications.

Original data of the authors and literature data are presented describing the mechanical behavior of nanostructured metals and alloys at cryogenic temperatures. Bulk ultrafine grained metals and alloys processed by severe plastic deformation through equal channel angular pressing are considered. The mechanical behavior of a number of nanostructured metals and alloys (Cu, Ni, Ti, VT6 and others) were investigated under uniaxial compression and tension at cryogenic temperatures in a wide range of strain-rates. The most important low temperature peculiarities of the nanostructured materials mechanical behavior have been indicated.