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Abstract: NiTi alloy has a unique combination of mechanical properties, shape memory effects and superelastic behavior that makes it attractive for several biomedical applications. In recent years, concerns about its biocompatibility have been aroused due to the toxic or side effect of released nickel ions, which restricts its application as an implant material. Bulk ultrafine-grained Ni50.8Ti49.2 alloy (UFG NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study. A homogeneous and smooth SrO–SiO2–TiO2 sol–gel coating without cracks was fabricated on its surface by dip-coating method with the aim of increasing its corrosion resistance and cytocompatibility. Electrochemical tests in simulated body fluid (SBF) showed that the pitting corrosion potential of UFG NiTi was increased from 393mV(SCE) to 1800mV(SCE) after coated with SrO–SiO2–TiO2 film and the corrosion current density decreased from 3.41μA/cm2 to 0.629μA/cm2. Meanwhile, the sol–gel coating significantly decreased the release of nickel ions of UFG NiTi when soaked in SBF. UFG NiTi with SrO–SiO2–TiO2 sol–gel coating exhibited enhanced osteoblast-like cells attachment, spreading and proliferation compared with UFG NiTi without coating and CG NiTi. [Copyright &y& Elsevier]

Abstract: Fully dense, nanocrystalline tantalum (average grain size as small as ∼40nm) has been processed for the first time by high-pressure torsion. High-resolution transmission electron microscopy reveals non-equilibrium grain boundaries and grains decorated with high-density dislocations. Microhardness measurements and instrumented nanoindentation experiments indicate that the mechanical property is quite uniform except for the central area of the disks. Nanoindentation experiments at different strain rates suggest that the strain rate sensitivity of nanocrystalline tantalum is increased compared to the coarse- and ultrafine-grained counterparts and is accompanied by an activation energy of the order of a few ∼b 3 (b is the magnitude of the dislocation Burgers vector), implying a shift in the plastic deformation mechanism from the screw dislocation dominated regime. We thus infer the plastic deformation mechanisms of nanocrystalline body-centered cubic (bcc) and face-centered cubic metals converge. To examine the stress–strain behavior, we have used microcompression to measure the compressive stress–strain curves on microscale pillars fabricated by focused ion beam technique. Yield strength as high as 1.6GPa has been observed. High-strain rate behavior has been investigated using a miniature Kolsky bar system. We have found that at high-strain rates the nanocrystalline tantalum specimens exhibit adiabatic shear banding, a dynamic plastic deformation mode common to many ultrafine-grained and all nanocrystalline bcc metals. [Copyright &y& Elsevier]

We report a significant grain size effect on the phase transformation in a Ti–36Nb–2.2Ta–3.7Zr–0.3O alloy during severe plastic deformation. For grain sizes larger than the submicrometer range, the body-centered cubic β phase transformed to hexagonal ω phase when deformed by high-pressure torsion under a relatively low pressure of 3GPa and at room temperature. However, a reverse ω to β transformation was observed under the same deformation conditions when the grain sizes were less than 100nm. [Copyright &y& Elsevier]

Abstract: The microstructure and the mechanical properties of ultrafine grained Pd and Pd–x% Ag (x =10, 20, 40) alloys processed by high pressure torsion (HPT) were thoroughly investigated using transmission and high resolution electron microscopy, XRD analysis and tensile tests. The increased Ag content resulted in decrease of the stacking fault energy (γ SF), which made it possible to produce by means of HPT different ultrafine-grained structures in the alloys and to study their superior strength and ductility. The nature of such unique mechanical behaviour is discussed on the basis of analysis of strain hardening mechanisms and effect of stacking fault energy. [Copyright &y& Elsevier]

Abstract: An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x =5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals. [Copyright &y& Elsevier]

Abstract: The high-cycle fatigue behavior of smooth and notched samples of ultrafine-grained titanium prepared by severe plastic deformation is compared with the corresponding properties of conventional titanium. It is shown that the combination of high strength and enhanced ductility of ultrafine-grained titanium lead to an increase of the fatigue endurance limit. Using a combination of equal-channel angular pressing and subsequent thermal and mechanical treatment, it was possible to increase the fatigue endurance limit of commercial-purity titanium by a factor of 1.5. Furthermore, it is shown that post-deformation annealing can additionally enhance the ductility of the ultrafine-grained Ti and lower fatigue-notch sensitivity particularly in comparison with Ti-6Al-4V. [Copyright &y& Elsevier]

Abstract: Microstructure evolution in severely deformed Cu has been investigated using high-energy synchrotron light during in situ high-pressure torsion (HPT) at room temperature. Relative changes in broadening of Bragg peaks and crystal lattice expansion were studied in the loading–unloading regime of torsion straining. Experimental results revealed fast relaxation (on the order of hundred of seconds) that occurred due to annihilation of HPT-induced crystal lattice defects, which were generated directly during deformation. The kinetics of relaxation is probably diffusion-controlled; therefore, the enhanced diffusivity can be explained by extremely high excess vacancy concentration, which is usually achieved at thermal equilibrium near the melting point. [Copyright &y& Elsevier]

Abstract: It was shown that introducing an ultrafine-grained (UFG) microstructure in pure metals as well as some alloys leads to strongly enhanced fatigue properties. The cyclic deformation behavior of UFG Ti-6Al-4V ELI (extra low interstitials) alloy is studied by both strain and stress controlled fatigue tests using plastic strain amplitudes between 3×10−4 and 5×10−3 and stress amplitudes ranging from 550 to 670MPa. The UFG microstructures were obtained by equal channel angular pressing (ECAP) with different number of passes followed by a subsequent thermomechanical treatment (TMT). When compared to the conventional grain (CG) size counterpart, the UFG alloy exhibited a pronounced enhancement in the fatigue life in the S–N (Wöhler) diagram. It was also shown that additional UFG processing prior to TMT did not result in any further improvement of the fatigue resistance. Furthermore, microstructural investigations revealed a high cyclic stability of the UFG microstructure. [Copyright &y& Elsevier]

• Physical-vapor deposited (PVD) coating was applied to Ultra-fine grained (UFG) Ti alloy. • PVD coating improves protective properties and provides higher endurance of UFG Ti alloy. • PVD coating on UFG Ti alloy has large increase in adhesive strength vs. when on coarse-grained alloy. • PVD-coated UFG Ti alloy exhibits high erosion resistance under gas-abrasive wear. The paper presents the results of a study of the service properties of an ion-plasma protective coating deposited on a titanium substrate with an ultrafine-grained structure in the process of Physical vapor deposition (PVD). Data on the microhardness, adhesion strength, and erosion resistance of the ion-plasma coating on the Ti-6%Al-4%V alloy with the initial Coarse-grained (CG) structure and Ultrafine-grained (UFG) state produced by Severe plastic deformation (SPD) are presented. Data were also obtained on the long-term strength of cylindrical specimens in the UFG state with and without coating at the operating temperatures of this alloy. A significant increase in the service properties of the coating on the titanium alloy with an UFG structure was established in comparison with the properties of this coating on the original coarse-grained alloy. The physical nature of the increase in the properties of the coating on the UFG alloy is discussed and a hypothesis about the influence of the UFG state of the substrate on the fine microstructure and properties of the protective coating is considered. [ABSTRACT FROM AUTHOR]

Abstract: The influence of high-pressure torsion (HPT) at various temperatures on microstructure and mechanical properties of commercially pure (CP) titanium (Grade 4) has been studied using transmission electron microscopy and tensile tests. The temperature range of 70–450°C for isothermal HPT processing has been used to fabricate nanocrystalline samples with diameter 20mm under a pressure of 6GPa. Using the new installation, we demonstrated that the ultimate tensile strength of CP Ti (Grade 4) after HPT processing achieves 1600MPa as a result of significant grain refinement. At the same time, ductility of the HPT samples has been affected strongly by processing and annealing temperature. [Copyright &y& Elsevier]

Abstract: We have used two general methodologies for the production of ultrafine grained (UFG) and nanocrystalline (NC) tungsten (W) metal samples: top-down and bottom-up. In the first, Equal channel angular extrusion (ECAE), coupled with warm rolling has been used to fabricate UFG W, and high pressure torsion (HPT) was used to fabricate NC W. We demonstrate an abrupt shift in the deformation mechanism, particularly under dynamic compressive loading, in UFG and NC W. This novel deformation mechanism, a dramatic transition from a uniform deformation mode to that of localized shearing, is shared by other UFG and NC body-centerd cubic (BCC) metals. We have also conducted a series of bottom-up experiments to consolidate powdered UFG W precursors into solid bodies. The bottom-up approach relies on rapid, high-temperature consolidation, specifically designed for UFG and NC W powders. The mechanical property results from the top-down UFG and NC W were used as minimum property benchmarks to guide and design the experimental protocols and parameters for use in the bottom-up procedures. Preliminary results, showing rapid grain growth during the consolidation cycle, did not achieve full density in the W samples. Further development of high-purity W nanopowders and appropriate grain-growth inhibitors (e.g., Zener pinning) will be required to successfully produce bulk-sized UFG and NC W samples. [Copyright &y& Elsevier]

Abstract: Fully dense nanocrystalline tungsten (nc-W) with extremely high strength (∼3.0GPa under quasi-static compression and ∼4.0GPa under dynamic compression) has been obtained by high-pressure torsion (HPT) at low temperature (500°C). The nanocrystalline microstructure is revealed by transmission electron microscopy (TEM). The grain boundaries (GBs) are mostly of the large-angle type. High-resolution TEM (lattice images) suggests that the GBs are clean and well defined (atomically sharp). GBs are non-equilibrium and of a high-energy nature. Edge dislocations are present within the grains. The authors hypothesize that these edge dislocations, combined with a depleted impurity concentrations along pre-existing GBs, contribute to enhance the ductility of nc-W. Under dynamic compression, the specimens exhibit localized shearing followed by cracking and subsequent failure, similar to their ultrafine-grain (UFG) counterparts processed by equal-channel angular pressing plus cold rolling, and to many other body-centered cubic metals with UFG/nanocrystalline microstructures. The shear band width in the HPT-processed nc-W is much smaller (shear band width <5μm) than that observed in the UFG counterparts (shear band width ∼40μm). [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The objectives of the present study were to assess and to optimize the fatigue behaviour in strain-controlled tests of different ultrafine-grained (UFG) metals and alloys (Cu, Al, AA6061, α-brass) prepared by equal channel angular pressing (ECAP). Quite generally, the UFG materials investigated exhibited shorter fatigue lives in the low-cycle fatigue (LCF) regime than corresponding conventional grain (CG) size specimens. In all cases, the fatigue lives could be enhanced appreciably by a suitable heat treatment following ECAP without, however, paralleling the fatigue lives of the CG specimens. The microstructural reasons for this behaviour are discussed, and a microstructure-based fatigue life model for UFG materials is presented. [Copyright &y& Elsevier]

Abstract: High-resolution transmission electron microscopy and three-dimensional atom probe observations show clearly that a reverse transformation of body-centred cubic ferrite to face-centred cubic austenite occurs during severe plastic deformation of a pearlitic steel resulting in a nanocrystalline structure, something that never occurs in conventional deformation of coarse-grained iron and steels. The driving force and the mechanisms of this reverse transformation are discussed. It is shown that nanostructure and shear stresses are essential for this process, and the results confirm molecular dynamics predictions of such transformations in nanocrystalline iron. [Copyright &y& Elsevier]

Abstract: Microstructure development during severe plastic deformation by simple shear is modeled using a disclination model for substructure formation incorporated into the viscoplastic self-consistent (VPSC) polycrystal model. The strain incompatibilities between a homogeneous effective medium and a grain calculated by the VPSC model are assumed to result in an accumulation of disclinations at junctions of the grain. These disclinations are then relaxed by the growth of low-angle dislocation boundaries from the junctions. These boundaries split the grain into smaller, misoriented volumes which with further deformation can lead to its subdivision into new grains. The evolution of misorientation angles between the newly formed grains and the parent with applied straining agrees reasonably well with experimental misorientation angle distributions for geometrically necessary boundaries. Accounting for subgrain boundaries and grain subdivision results in textures after large strains that are more diffuse and lower in intensity than those predicted using standard VPSC modeling. [Copyright &y& Elsevier]

Abstract: We have systematically investigated the quasi-static and dynamic mechanical behavior (especially dynamic failure) of ultra-fine grained (UFG) tungsten (W) under uniaxial compression. The starting material is of commercial purity and large grain size. We utilized severe plastic deformation to achieve the ultrafine microstructure characterized by grains and subgrains with sizes of ∼500nm, as identified by transmission electron microscopy. Results of quasi-static compression show that the UFG W behaves in an elastic–nearly perfect plastic manner (i.e., vanishing strain hardening), with its flow stress approaching 2 GPa, close to twice that of conventional coarse grain W. Post-mortem examinations of the quasi-statically loaded samples show no evidence of cracking, in sharp contrast to the behavior of conventional W (where axial cracking is usually observed). Under uniaxial dynamic compression (strain rate ∼103 s−1), the true stress–true strain curves of the UFG W exhibit significant flow softening, and the peak stress is ∼3GPa. Furthermore, the strain rate sensitivity of the UFG W is reduced to half the value of the conventional W. Both in situ high-speed photography and post-mortem examinations reveal shear localization and as a consequence, cracking of the UFG W under dynamic uniaxial compression. These observations are consistent with recent observations on other body-centered cubic metals with nanocrystalline or ultrafine microstructures. The experimental results are discussed using existing models for adiabatic shear localization in metals. [Copyright &y& Elsevier]

The paper reports on the formation of ultrafine-grained structure in the Ti–6Al–4V alloy by equal-channel angular pressing (ECAP). ECAP processing with various regimes on die-sets with angles of channels intersection equal to 135° and 120° has been studied. Special attention is paid to the features of the processed microstructures and properties of the ECAP samples. It is shown that low-temperature deformation applied after ECAP contributes to additional strengthening of the alloy preserving sufficient ductility. [Copyright &y& Elsevier]

Two kinds of shear bands (SBs) which are persistent slip bands (PSB)-like SBs and step-like SBs were found in cyclically deformed ultrafine grained copper processed by ECAP. The SBs and adjacent microstructures were investigated using electron channeling contrast in scanning electron microscope (SEM-ECC). The possible formation mechanisms of SBs are discussed based on the cooperative grain boundary sliding. [Copyright &y& Elsevier]

Abstract: Ultrafine-grained (UFG) Ti rods were produced via cold rolling UFG and coarse-grained (CG) Ti stocks. The initial UFG stock was produced via equal channel angular pressing. It was found that the initial UFG structure had beneficial influence on the mechanical properties of the cold-rolled Ti rods. Compared with Ti rods with initial CG microstructure, the Ti rods with the initial UFG microstructure have both higher strength and higher ductility after being cold rolled to varying strains. Transmission electron microscopy revealed that the Ti rods with the initial UFG microstructure had finer, more homogeneous microstructures after cold rolling. This study demonstrates the merit of UFG Ti processed by ECAP for further shaping and forming into structural components with superior mechanical properties. [Copyright &y& Elsevier]

Microstructures and mechanical properties of equal-channel angular pressing (ECAP) processed and naturally aged ultrafine grained (UFG) and coarse grained (CG) 7075 Al alloys as well as their evolutions during annealing were investigated. After the same natural aging, the tensile yield strength, ultimate strength, and microhardness of the UFG samples were 103%, 35%, and 48% higher, respectively, than those of the CG samples, because of higher densities of Guinier–Preston (G–P) zones and dislocations in the UFG sample. Upon annealing, the microhardness of the UFG sample decreased gradually, while a hardening peak appeared for the CG sample. The peak was caused by the precipitation hardening of the metastable η′ phase. For the UFG sample, the precipitation hardening was overcompensated by the significant decrease of microstrain (dislocation density) upon annealing, resulting in a lack of precipitation hardening peak. Differential scanning calorimetry indicates that the ECAP process only accelerated the phase precipitations, but did not change the sequence of phase precipitation. This study shows that severe plastic deformation has the potential to significantly improve the mechanical properties of age-hardening Al alloys. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

We investigated the corrosion behavior of commercially pure (CP) Ti with both ultrafine-grained (UFG) and coarse-grained (CG) microstructures. It was found that the UFG Ti is more resistant to corrosion than its CG counterpart. The superior corrosion resistance of UFG Ti is believed to result from rapid passivation of UFG Ti and the impurity segregation to grain boundaries in CG Ti. [Copyright &y& Elsevier]

Friction coefficient is an important material property that affects the performance and service life of structural components that slide against each other. The frictional force consists of an adhesion force and a deformation force, producing an adhesion component and a deformation component of the friction coefficient. In this study, we compare the adhesion components of ultrafine-grained (UFG) Ti and coarse-grained (CG) Ti at temperatures from 25 to 800 °C and under a range of pressures. The UFG Ti samples were in two processing states: equal channel angular pressing (ECAP) for eight passes and ECAP+further cold rolling for 75% rolling strain. The experimental data indicate that ultrafine-grained structure results in lower adhesion component and consequently lower friction coefficient, which should in turn improve the wear property. [Copyright &y& Elsevier]

We have used severe plastic deformation (high-pressure torsion straining) to achieve cold consolidation of gas-atomised Al90Fe5Nd5 powders into bulk nanostructured disks with a dispersion of fcc-aluminium crystallites within the glassy matrix with full density and hardness in excess of 3 GPa. The micro- and nanostructure of the resulting disks is presented. [Copyright &y& Elsevier]

Zr 52.5 Cu 17.9 Al 10 Ni 14.6 Ti 5 (Vit105) Bulk metallic glass is processed by High-Pressure Torsion up to 30 rotations at room temperature, showing simultaneous enhancement of both strength (2023 MPa) and ductility (0.45%). Nano-crystal precipitates with an average size of 45 nm inside the amorphous matrix are observed after 30 turns HPT. Free volume increases from ~0.9030 Å3 (as cast) to ~0.9275Å3 after 30 turn HPT alloy with 2.83% (by volume) of nanocrystal precipitates. Strengthening due to precipitation after 30 turns HPT comply with the phase mixture model. The synergetic effect of free volume and nanocrystal precipitation on the ductility of amorphous-crystallite composite is quantitatively approached through modelling. By implementing the model, a ductilization window between bottom limit and top limit of the inter-crystallite spacing w.r.t the free volume is established, elucidating the necessary conditions required to obtain plasticity in amorphous-crystalline composites. • Both ductility and strength of BMGs are enhanced by HPT processing. • The nanocrystals precipitation during HPT increases the strength. • Free volume increase during HPT enhances ductility and reduces yield stress. • Synergetic effect of free volume and nanocrystals on ductility was observed. • Mechanism based-modelling correctly predict the synergetic effect on ductility. [ABSTRACT FROM AUTHOR]

Abstract: In this study the cyclic deformation behavior and fatigue lives of Ti–6Al–4V ELI (extra low interstitials) alloy with conventional grain sized and ultrafine-grained (UFG) microstructure are investigated. The UFG condition was obtained by a combination of equal channel angular pressing followed by thermomechanical treatment. Fatigue experiments were carried out under strain as well as under stress control using plastic strain amplitudes ranging from 3×10−4 to 5×10−3 and stress amplitudes ranging from 550 to 670MPa, respectively. Based on the results obtained, it can be concluded that the UFG microstructure results in a significant enhancement of the fatigue properties of the alloy. [Copyright &y& Elsevier]

Abstract: Deformation-induced nanocrystallization has been investigated in metallic Al88Y7Fe5 glasses. In situ TEM tensile tests at room temperature give new evidence to the atomic mobility enhancement as the underlying mechanism of nanocrystallization in shear-bands that form during deformation. High pressure torsion straining allows to control the microstructure in producing homogeneous bulk nanostructures at an extremely high number density of nanocrystals. [Copyright &y& Elsevier]

• "Accumulative HPT" – to achieve a high strain in hard materials, including BMGs. • Accumulative HPT-effective method of processing for difficult-to-deform materials. • The structure of Zr-BMG during accumulative HPT transforms much more significantly than in the HPT. Studies show that with the HPT of bulk metallic glass, the actual achievable strain is significantly lower than the one predicted by the equation γ = 2 π n R / h. Such a discrepancy can be explained by the effect of "slippage" during the HPT. The authors proposed a new method − "accumulative HPT" − to achieve a high strain in hard materials, including BMGs. In the accumulative HPT procedure, a sample undergoes several cycles: "HPT n = 1 revolution → cutting a sample into pieces → pressing of stacked parts on the anvils and subsequent HPT n = 1". At the last stage, the stacked segments are subjected to HPT with many revolutions (n ≥ 3) whereby the sample is consolidated into a monolithic disk. As a result, the material receives a significant total strain by pressing and torsion. The article presents the first results of the "accumulative HPT" processing of a Zr-based BMG and a crystallized Zr-based intermetallic. [ABSTRACT FROM AUTHOR]

• Ultra-fine grain size in Mg-1Ca alloy increases its biodegradation time from days to months. • Continuous nanoprecipitation of Mg 2 Ca improves the corrosion resistance of the UFG alloy. • EIS inductive loop with negative time constant describes this increase in corrosion resistance. • Ultra-fine grained Mg-1Ca alloy is promising for biodegradable implant applications. Biodegradation studies of Mg-1Ca alloy with different microstructures in Ringer's solution reveal corrosion mechanism based on the balance between the anodic dissolution of the secondary phase and hydroxide precipitation, controlled by the secondary phase size and distribution uniformity. Alloy with ultrafine-grain structure shows at least 5-fold improvement in corrosion resistance compared to the coarse-grained alloy due to the separation of the particles of the secondary phase and its continuous intragranular nano-precipitation. This promotes an unusual low-frequency impedance behaviour described by an inductive loop with the negative time constant, which leads to the increased impedance modulus signifying the improved corrosion resistance. [ABSTRACT FROM AUTHOR]

Ti–6Al–7Nb containing harmless for tissues niobium can be a good choice replacing Ti–6Al–4V for orthopedic implants application. Formation of ultrafine-grained (UFG) structure in metals and alloys by severe plastic deformation (SPD) techniques allows for achieving unique mechanical properties. Using equal channel angular pressing (ECAP) UFG structure in Ti–6Al–7Nb alloy with an average size of grains/subgrains of 200 nm was obtained. This UFG Ti–6Al–7Nb alloy has high mechanical (ultimate tensile strength 1470 MPa) and fatigue properties, suitable for practical application. Additionally, surface modifications of titanium alloys aim induce specific responses on osteoblastic cells after implantation. Chemical surface treatments are simple methods to obtain a bioactive for apatite precipitation surface. Phosphoric acid etching combined or not with alkaline treatment presented bioactivity after seven days soaked in simulated body fluid (SBF) solution. [ABSTRACT FROM AUTHOR]

Abstract: A slightly Ni-rich nano-NiTi alloy, deformed by high-pressure torsion (HPT) was investigated. By HPT, almost complete amorphization of initial B2 NiTi is obtained. Crystallization and microstructure evolution during isothermal treatment were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy. The DSC signals observed during continuous heating experiments indicate an unusually large separation between the crystallization and the growth stages. A detailed analysis of the evolution of the enthalpy release upon annealing reveals reproducibly non-monotonous trends with annealing time that cannot be explained solely by growth of crystalline volume fractions. The size of nanocrystals increases dramatically after annealing for 5h. The effective activation enthalpies for stress relaxation (along with crystallization) and grain growth were estimated at 115 and 289kJ/mol, respectively. The results are discussed with respect to the intricate interdependencies between synthesis and thermal processing pathways and the evolution of the nanoscale microstructure. [Copyright &y& Elsevier]