Search

The formation of ω-phase under high-pressure torsion (HPT) has been studied in Ti–Fe alloys. Seven alloys with Fe concentration from 0 to 10 wt % have been annealed between 600 and 950 °C, quenched and HPT-treated at 7 GPa, 1 rpm, 5 and 0.1 anvil rotations (equivalent strain eeq = 156 and = 3.1, respectively). The strain after 0.1 rot. corresponds to the transient state of HPT, and that after 5 rot. corresponds to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. A defect-rich high-pressure ω-phase forms after HPT and persists in the samples also after the pressure release. The amount of retained ω-phase after HPT depends on the iron concentration. It increases from 40% in pure titanium, reaches maximum of 95% at 4 wt % Fe and then decreases again to 10% at 10 wt % Fe. It is because the addition of iron influences the lattice parameters in β and ω-phases in a different manner. The minimal lattice mismatch between β- and ω-phases is reached at 4 wt % Fe. A good conformity between the lattices of the β- and ω-phases enhances the probability of the martensitic (diffusionless) β→ω transformation. Based on the XRD and TEM observations, the crystallography and mechanisms of α→ω and β→ω phase transformations (which can be diffusionless as well as controlled by mass transfer) under the influence of pure shear by HPT are discussed.

Compositionally complex alloys (CCAs) in a nanocrystalline state often involve complex and poorly understood phase transformations which can consequently result in grain growth even at low temperatures. A detailed study of the microstructure and phase stability in CCAs is challenging due to the presence of multiple principal components. In view of these challenges the objective of the present study is to establish a systematic understanding of the phase evolution in a face centered cubic non equiatomic nanocrystalline CCA (CoCuFeMnNi). To accomplish this objective, we employed in-situ transmission electron microscope heating in combination with automated crystal orientation mapping (ACOM) and energy filtered transmission electron microscopy (EFTEM) to elucidate the sequence of phase decomposition of the high-pressure torsion (HPT) processed CoCuFeMnNi. Our analysis reveals a complex succession of grain boundary segregation and depletion steps leading to the formation of a FeCo-rich secondary phase. Our results show that prior to the formation of the secondary phase, Cu, Ni and Co segregate and Fe and Mn deplete at the grain boundaries. After the FeCo precipitation is triggered, Mn segregates to the grain boundaries along with Ni and Cu, whereas Fe and Co are depleted. The FeCo precipitates have a B2 crystal structure and typically exhibit a Kurdjumov-Sachs (K-S) and/or Nishyama-Wasserman (N-W) orientation relationships with adjacent fcc grains. Ex-situ heat treated CoCuFeMnNi analyzed by atom probe tomography (APT) revealed a highly heterogeneous segregation of the different elements to different grain boundaries. The FeCo-rich precipitates contain trace amounts of Ni, whereas Cu is rejected leading to the formation of a separate Cu rich phase. This complex segregation phenomenon is assisted by the high fraction of grain boundaries and triple junctions in the nanocrystalline material, which are critical for the phase evolution in this alloy, which is not frequently observed in the corresponding coarse-grained material.

Severe plastic deformation (SPD) always leads to the strong grain refinement in the materials. Logically, in two- and multiphase alloys SPD has to cause the fragmentation and dissolution of precipitates in a matrix. However, it has been observed recently, that, contrary to this generally accepted viewpoint, SPD can lead also to the decomposition of supersaturated solid solution. In this work we analyze for the first time (both experimentally and theoretically) the competition of these simultaneous processes can take place, namely (1) the dissolution of precipitates and (2) decomposition of supersaturated solid solution with precipitation of a second phase. As a result, a dynamic equilibrium between these two processes appears, and a certain steady-state concentration in a solid solution is reached. In this work we study the high pressure torsion (HPT) of a two-phase Cu–3.9 at. % Ag alloy in two different states: (i) as-cast consisting of a (Cu) solid solution with diluted 1.9 at.% Ag and another 2 at.% Ag as fine silver precipitates and (ii) an almost homogeneous solid solution with diluted 3.9 at.% Ag obtained by homogenization at T = 780 °C, 900 h and subsequent water quenching. HPT at room temperature causes the partial dissolution of precipitates in the as-cast samples and partial decomposition of the solid solution in homogenized samples. After HPT, the solute concentration in the matrix is the same in both samples (about 2.9–3.0 at.% Ag). Thus, it does not depend on the initial state and is higher than the equilibrium solubility limit at the HPT temperature. This concentration is equal to solubility limit at the effective temperature of Teff ≈ 680 °C. We also proposed the model describing the dynamic equilibrium between dissolution and precipitation in HPT. Assuming that HPT fixes the composition at matrix-precipitate interfaces, we show that HPT-enhanced diffusive transport of species is the process likely controlling the observed steady-state composition in the matrix and precipitate average diameter.

Nanograined (grain size 10 nm) ZnO films with various Co contents (0–52 at.% Co) were synthesized by a novel liquid ceramics method. The solubility limit for Co was determined at 550 °C. The lattice parameter c of the ZnO-based solid solution with wurzite structure ceases to grow at 33 at.% Co. The peaks of the second phase (Co 2 O 3 or with cubic lattice) become visible in the X-ray diffraction spectra at 40 at.% Co. The same second phase appears in the bulk ZnO already at 2 at.% Co [Bates CH, White WB, Roy R. J Inorg Nucl Chem 1966;28:397]. A few years ago it was predicted theoretically that ZnO could become ferromagnetic at room temperature and above by doping with Co and other transition metals. Recently published papers on the structure and magnetic behaviour of Co-doped ZnO allowed us to obtain the size dependence of Co solubility in ZnO for the polycrystals and small single-crystalline particles. The overall Co solubility drastically increases with decreasing grain size. The quantitative estimation leads to the conclusion that, close to the bulk solubility limit, the thickness of a Co-enriched layer is several monolayers in grain boundaries and at least two monolayers in the free surfaces.

The microstructure and hardness of binary Al–Zn and Al–Mg alloys were studied both in the as-cast state and after high-pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second-phase precipitates decreases drastically after HPT and reaches the nanometre range. During HPT, the Zn-rich supersaturated (Al) solid solution decomposes and closely approaches the equilibrium state corresponding to room temperature. The decomposition of Mg-rich supersaturated (Al) solid solution is less pronounced. In the as-cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to solid-solution hardening. However, after HPT the work hardening and Hall–Petch hardening due to the decreasing grain size compete with softening due to the decomposition of a supersaturated solid solution. As a net effect, severe plastic deformation results in softening of the binary Al–Zn and Al–Mg alloys. Softening is more pronounced in the Al–Zn alloys where HPT leads to almost full decomposition of the supersaturated solid solution.

Structure and phase composition of five binary Al–Zn and Al–Mg alloys were studied before and after high pressure torsion (HPT) with a shear strain 300. The grain size of (Al) solid solution and crystals of reinforcing second phases decreases drastically after HPT reaching nanometer range. As a result of HPT, the Zn-rich (Al) supersaturated solid solution decomposes completely and reaches the equilibrium state corresponding to room temperature. The decomposition is less pronounced for Al–Mg alloys. We conclude that the severe plastic deformation of supersaturated solid solutions can be considered as a balance between deformation-induced disordering and deformation-accelerated diffusion towards the equilibrium state.