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Microstructural evolution and mechanical behavior of commercially pure Fe during severe deformation by cold caliber rolling followed by wire drawing were investigated using transmission electron microscopy and electron backscatter diffraction. Following by a drastic increment of strength in the early stage of deformation, shear banding as a softening mechanism leads to decreasing of work hardening rate and finally a steady state situation at medium strains, creating a bimodal microstructure. Increasing strain beyond 3 is associated with increasing the rate of work hardening and refinement of the material. Severely deformed Fe after cold caliber rolling to equivalent strain of 4.5 evolves from lamellar ultrafine-grained structure. Additional deformation by drawing results in more homogeneous structure and activates new mechanisms. A dynamic recovery appears at severe strains through mechanically assisted triple junction motion. It is found that suppression of triple junction motion enhances the refinement of microstructure and the strength, such that the highly deformed Fe after equivalent strain of 7 has a nano/ultrafine-grained structure combined with a high tensile strength of 1115 MPa.

In this study, a newly developed γ′ precipitation hardened high entropy superalloy (designated as HES-A1 hereafter) was produced and its microstructural features were characterized. The new alloy composition is based on the major elements of conventional IN718 alloy, however other elements were added or removed to minimize possible presence of δ (or γʺ) phase and increase the volume fraction of very fine γ′ precipitates instead. Broadening the hot working window, lower density, and avoiding the use of expensive elements were other considerations taken into account in the design of the new alloy. CALPHAD (CALculation of PHAse Diagrams) and PhaComp methods were used for the prediction of phases and their evolutions. The microstructure of the HES-A1 alloy in as-cast, homogenized, hot rolled, annealed and aged conditions were characterized using optical, scanning, and transmission electron microscopes as well as X-ray diffraction technique. The as-cast microstructure of HES-A1 contained 5.5% of Laves phase, which was reduced to less than 0.3% through the development of a homogenization treatment. Hot rolling with reductions up to 36% at 1015 °C did not produce any cracking, indicating a good forming potential for the new alloy. The application of double aging treatment, similar to the one for IN718 alloy, showed no presence of γʺ or formation of δ phase in the microstructure. HES-A1 has been designed with a higher Al/(Ti + Nb) ratio and higher proportion of Al + Ti + Nb so that it could be mainly strengthened by γ′ precipitation. The average size of monomodal γ′ precipitates was smaller than that observed in conventional alloys after similar treatments.

The nanostructural hierarchy of γ and γ′ phases is characterized in Ni-8.5Al-5.4Ti (at.%) alloy, after double aging treatment at 1000 and 780 °C for 6 and 20 h. Using electron microscopy and atom probe tomography, disordered γ phase inside ordered γ′ precipitate was detected, which underwent plate-like growth during aging treatment. This unique hierarchical structure enabled the simultaneous study of direct (γ′ precipitates/γ matrix) and inverse (γ nanoparticles/γ′ precipitates) order-disorder interfaces in 6 and 20 h double-aged samples. It was found that the compositional interface width, δ, of direct interfaces decreased during aging treatment, whereas the thickness remained almost unaffected for inverse interfaces. The difference in thickening behavior is explained based on the change of the Gibbs-Thomson pressure for γ plates embedded in γ′ and cuboidal γ′ precipitates surrounded by the γ matrix. This inference suggests the possibility of prediction and control of δ(t) variations during heat treatment processes based on the current thermodynamic approach.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process.

A newly developed precipitation-hardened high entropy superalloy was produced using double vacuum melting, and its microstructural evolutions were examined after homogenization, hot rolling, annealing, and aging. The precipitation behaviour of the samples was studied using two different heat treatment cycles (with and without solution treatment). Microstructural observations using scanning electron microscope and transmission electron microscopy showed that the direct-aged samples consisted of nano-sized γ′ precipitates in γ matrix, while for the solution-aged samples, reprecipitation of the Laves phase occurred in the interior of the grain and at the grain boundaries. The sluggish diffusion present in the proposed alloy is what mainly accounts for the smaller than 20 nm of γ′ precipitates formed after about 18 h of aging. The strengthening effects of precipitation on the mechanical properties of the annealed and aged conditions were studied using a universal testing machine under a tensile load. The room temperature tensile strength and total elongation of the direct-aged samples were as high as 1310 MPa and 32%, respectively, which represent a promising combination of strength and ductility for industrial applications. In addition, the direct-aged specimens exhibit significant enhancements of 150% and 46% for yield and tensile strengths, respectively, as compared to the annealed condition. These results confirm that the precipitation-strengthening mechanism is an effective way of increasing the strength of face-centered cubic high entropy alloys.

Order-disorder coherent interfaces determine the microstructure and mechanical properties of precipitation-hardened high-temperature alloys. The characteristics of these interfaces can be defined by a compositional width, δ, and structural width, δ′. The latter, which can be considered as the width of the ordered part of the interface, can play an important role in high-temperature mechanical behavior of precipitation-hardened alloys. This is due to the fact that diffusion in the ordered part of the interface is generally much slower than diffusion in the disordered phase, thus hindering the solid-state diffusion-based phenomena. Here, we investigate the order-disorder interface in a Ni-19Al (at.%) alloy as a model alloy for Ni-based superalloys using atomic-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. Then, we employ thermodynamic modeling to describe the interplay between the structural and compositional interface widths in binary Ni-Al and in ternary Ni-Al-Cr and Co-Al-W systems. We introduce the δ′/δ ratio as a critical parameter that varies significantly in different alloys. Our findings offer a general pathway to control the δ′/δ ratio of interfaces, which in turn affect the high-temperature properties of precipitation-hardened alloys.

A commercially pure titanium (CP-Ti) of grade 1 as a hard-to-deform material was processed successfully by ECAP processing up to four passes at room temperature via the core–sheath method using a die with an internal channel angle of 90°. The simulation and analytical calculations demonstrated that imposed back pressure on the core was increased at each pass due to strain hardening of sheath metal (AISI 1015 steel) during deformation which prevented damage accumulation and crack initiation at a high number of passes. The scanning electron microscopy and transmission electron microscopy observations of ECAP-processed Ti revealed a severely deformed microstructure which consisted of a high dislocation density and an average grain size of ∼250 nm. Mechanical properties of four-pass ECAP-processed CP-Ti showed a substantial enhancement of ultimate tensile strength up to 890 MPa associated with a reasonable elongation to failure of 15.3%.

A CoCrFeNiMnTi0.1 high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) followed by post-deformation annealing (PDA) at 200–900 °C. Microstructural evaluations revealed that the initial and HPT-processed microstructures consisted of a single fcc phase and there was no evidence for decomposition during severe plastic deformation. However, PDA at temperatures below 900 °C promoted the formation of a multi-phase microstructure containing new precipitates and significant grain coarsening occurred after PDA at > 800 °C due to a dissolution of the precipitates. PDA at 800 °C for 60 min led to very good mechanical properties with an ultimate tensile strength (UTS) and elongation to failure of > 1000 MPa and ~ 40%, respectively. The results demonstrate that the minor addition of Ti to the CoCrFeNiMn alloy has no direct effect on the strengthening mechanisms but nevertheless this addition significantly increases the thermal stability of the precipitates and these precipitates are effective in minimizing grain coarsening. Therefore, the Ti addition plays an important role in strengthening the HEA.

A CoCrFeNiMn high-entropy alloy (HEA) with an addition of 2 at% Ti was processed by high-pressure torsion to produce a grain size of ~30 nm and then tested in tension at elevated temperatures from 873 to 1073 K using strain rates from 1.0 × 10−3 to 1.0 × 10−1 s−1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of 830% at a temperature of 973 K. It is shown that the Ti addition contributes to the formation of precipitates and, combined with the sluggish diffusion in the HEA, grain growth is inhibited to provide a reasonable stability in the fine-grained structure at elevated temperatures. By comparison with the conventional CoCrFeNiMn HEA, the results demonstrate that the addition of a minor amount of Ti produces a smaller grain size, a higher volume fraction of precipitates and a significant improvement in the superplastic properties.

The Zr55Cu30Al10Ni5 BMG as a base alloy was alloyed with Hf and Ti to develop a high entropy bulk metallic glass. The aim of the present work was to investigate the superplastic formability of HE-BMG, which can be used for micro-forming applications. Therefore, thermal stability and crystallization kinetics of the newly developed HE-BMG was first studied and then the results were used for the designed hot deformation experiments. It was demonstrated that HE-BMG has higher thermal stability than the base BMG, supposedly due to the sluggish diffusion and high entropy effect. Hot deformation behavior of the developed HE-BMG was investigated and it was found that at temperature of 753 K and the strain rate of 2 × 10–5 s-1, the alloy shows a good superplastic deformation with a plastic strain of at least 50% at compression stress around 500 MPa that is good enough for near net shape forming.

CoCrFeNiMnTix (x = 0.1 and 0.3) high entropy alloys (HEAs) were processed by thermomechanical treatment including heavy cold rolling followed by post deformation annealing (PDA) at 773 to 1173 K for up to 60 min. Microstructural evolutions by adding titanium and thermomechanical treatment reveal significant enhancement in the mechanical properties with combination of high strength and ductility in CoCrFeNiMn HEA. The initial microstructures consisted of a single fcc phase while the PDA led to formation of complicated microstructure due to precipitation and recrystallization. The results indicated that adding titanium promoted thermal stability of precipitates and postponed recrystallization and grain growth by solute drag and Zener pinning. The CoCrFeNiMnTi0.3 HEA showed high strength even after PDA at 1173 K due to stability of precipitates at this temperature. However, similar procedure resulted in dissolution of precipitates which facilitated coarsening and reduce the strength significantly in the CoCrFeNiMnTi0.1 HEA. The latter alloy showed remarkable mechanical properties with an ultimate tensile strength (UTS) and elongation to failure of ~900 MPa and ~ 30%, respectively, after PDA at 973 K.

CoCrFeNiMnTix (x = 0, 0.1 and 0.3) high-entropy alloys were subjected to a large plastic deformation followed by annealing. Annealing at 700 and 800 °C led to decomposition of single-phase HEAs and formation of new phases including Cr-rich bcc, Cr-rich sigma and ɳ-Ni3Ti which the latter only detected in the CoCrFeNiMnTi0.3 HEA. The microstructural evolutions revealed the significant of Ti on thermal stability and variety of formed precipitates in the microstructure of CoCrFeNiMn HEA.

This study investigates the structure, martensitic phase transformation behavior, and shape memory properties of the quaternary Ni44.8Cu5Ti45.2Hf5 and quinary Ni44.8Cu5Ti40.2Hf5Zr5 and Ni45.5Cu5Ti39.5Hf5Zr5 shape memory alloys (SMAs) in the as-cast state. The alloys exhibited a dual-phase microstructure composed of matrix phase (austenite or martensite depending on the chemical composition) and Ti2Ni-type phase. The experimental results revealed that the alloys underwent a one-step B2↔B19′ transformation. The Ni44.8Cu5Ti45.2Hf5 alloy showed a maximum shape memory strain of 6.2% with a recovery ratio of 81% upon thermal cycling under a tensile stress of 300 MPa, while in the Ni44.8Cu5Ti40.2Hf5Zr5 alloy, a maximum shape memory strain of about 5% was obtained under 500 MPa with a recovery ratio of 86%. The as-cast Ni44.8Cu5Ti45.2Hf5 alloy showed superelastic response with 0.3% maximum transformation strain at 125 ℃ . On the other hand, the Ni44.8Cu5Ti45.2Hf5 alloy showed the best superelastic response at 65 ℃ with a recoverable strain of about 1.9%. Two-way shape memory strains of 0.8% and 3.3% were obtained in the Ni44.8Cu5Ti45.2Hf5 and Ni44.8Cu5Ti40.2Hf5Zr5 alloys, respectively. The Ni45.5Cu5Ti39.5Hf5Zr5 alloy, however, did not reveal the shape memory effect. Results obtained from low-temperature X-ray diffraction (XRD) analysis indicated that the B2 → B 19 ′ transformation was considerably suppressed in this alloy and the volume fraction of B 19 ′ martensite did not exceed 8% even at an ultra-low temperature of − 150 °C.

In this research, the gradient structure and mechanical properties of an austenitic/martensitic (Ni50.8Ti/Ni45TiCu5 (at.%)) bi-layer thin film was investigated. The bi-layer was deposited on a Si substrate using DC magnetron sputtering. After crystallization of the film at 773 K for 60 min, the microstructure and mechanical properties of the bi-layer thin film were characterized using X-ray diffraction (XRD), transmission electron microscopy (STEM and HRTEM) and nanoindentation, respectively. The diffraction pattern illustrated that the crystallized bi-layer was combined of martensitic and austenitic layers while secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy analysis demonstrated the existence of a compositional gradient through the thickness and residual strain in the interface of the bi-layer, respectively. Furthermore, the compositional gradient in the bi-layer led to gradual variations in the structure, hardness and Young's modulus through the thickness of the bi-layer. The intrinsic two-way shape memory effect due to its functionally graded structure and the induced stress was confirmed by experimental test and finite element simulation.

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase (α + γ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e., controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.

Nowadays, industrial demands on materials with high strength and superior physical properties have made ultra-fine grain metallic materials one of the hot topics for materials science studies. In the present study, the newly developed technique based on severe plastic deformation so-called Repetitive Corrugation and Straightening by Rolling (RCSR) was applied to study its effect on the microstructure and mechanical behavior of commercially pure aluminum. The microstructure and correlated mechanical behavior of specimens after RCSR was systematically studied. A polymodal microstructure as revealed by the microstructural studies, and high elongation with relative good strength after RCSR indicate promising results. Our studies show that the heterogeneous mode of deformation in RCSR could be responsible for the poly-modal microstructure that along with the proper evolution of texture components gives the material both high ductility and reasonable strength. In fact, the combination of RCSR performance and the physicomechanical properties of aluminum provide the opportunity to get outstanding tensile properties without any subsequent annealing process.

Shape memory alloys sustaining stable functional properties for millions of cycles have been reported in NiTiCuCo thin films. The mechanism behind such ultralow functional fatigue was proposed to originate from the epitaxy strains surrounding coherent Ti2Cu precipitates. Despite having a well-understood microstructural origin, such fatigue-resistant property has never been achieved in bulk NiTiCuCo counterparts. In this study, we first show that the semi-coherent Ti2Cu precipitates help satisfy closely the compatibility criteria under thermal phase transformation. Assisted by aging at 600 °C for 1 h, an unprecedented thermal stability is reported in bulk coarse-grained Ti54Ni31.7Cu12.3Co2, where the transformation temperatures migrate by ∼ 0.1 °C for 200 cycles. To achieve stable superelastic response, a thermomechanical processing route is proposed that results in a uniform nanocrystalline microstructure with embedded Ti2Cu precipitates. Such a microstructure exhibits much improved superelastic cyclic stability as evidenced by ∼ 22 MPa shakedown of plateau stress for 200 superelastic cycles.

Bainitic ferrite is a key microstructural constituent in High-Strength Low-Alloy (HSLA) steels, influenced by the prior austenite grain size and heat treatment conditions. This study investigates the decomposition of coarse- and fine-grained austenite under rapid step quenching heat treatments around the martensite-start ( M S ) temperature in an ultra-low-carbon HSLA steel. An isothermal-to-nonisothermal transition occurred in the austenite phase transformation behavior after the grain size refinement from ~69 μm to ~10 μm resulting in the formation of a complex combination of various ferritic products consisting of quasi-polygonal ferrite, granular bainite, bainitic ferrite, and martensite in the microstructure of the below- M S -treated samples. Fine bainitic ferrite laths with an average thickness of ~150 nm formed, which were about 70% thinner than those of the above- M S -treated samples, contributing in part to an ultimate tensile strength of 791 MPa along with a uniform elongation of 6.4% for the below- M S -treated samples. The results can give insight into the methodologies for transferring the concept of nanoscale bainite to ultra-low-carbon steels.

The present research was conducted on the bcc Cr-rich precipitation kinetics in a heavily deformed CoCrFeNiMn HEA. Activation energy calculation of ~ 226 kJ.mol−1 suggests that precipitation is controlled by grain boundary diffusion of Ni during post deformation annealing using Johnson–Mehl–Avrami–Kolmogorov equation. This value is smaller than the activation energy for the diffusion of Co, Cr, Fe, Ni and Mn in homogenized CoCrFeNiMn HEA. It can be due to the large number of defects such as dislocations and non-equilibrium grain boundaries which serve both as fast diffusion pathways and also as preferential nucleation sites for the formation of new phases.

An equiatomic CoCrFeNiMn high entropy alloy was subjected to high-pressure torsion under a pressure of 10 GPa at room and cryogenic temperatures. Increasing straining and decreasing the deformation temperature led to deformation-induced FCC to HCP and BCC martensitic transformations, which played a significant role in the strengthening of the alloy.

Experiments were conducted to examine the effects of prior cold rolling and subsequent heat treatment on the microstructure, phase evolution and mechanical properties of an Fe–10Ni–7Mn martensitic steel. The results show that 70% cold rolling leads to deformation-induced austenite formation in the microstructure and reduces the size of the martensite blocks. Prior cold rolling increases the reversed austenite volume fraction under post-deformation intercritical annealing (PDIA) at 600 °C compared with the solution annealed condition. Electron back scattering diffraction and dilatometric studies showed that under PDIA there is a reverse transformation of martensite to austenite through a sequential combination of martensitic and diffusional mechanisms. There is also a precipitation of θ-NiMn particles in the PDIA specimen after subsequent ageing leading to an increase in the ultimate tensile strength in tensile testing. Cyclic tensile testing revealed pseudoelastic behavior in the cold-rolled specimen but this disappeared after PDIA and appeared again after subsequent ageing.

Experiments were conducted on an Fe–10Ni–7Mn (wt. %) martensitic steel processed by high-pressure torsion (HPT) after intercritical annealing in the dual phase (α+γ) region at 600 °C for 2 h. Microstructural examination and phase analysis using electron backscatter diffraction (EBSD) confirmed that post-HPT processing led to a phase transformation of austenite to α′-martensite (γ→α′ and γ→e→α′) such that after 20 turns of HPT the microstructure included multi-phases of α′-martensite, austenite and e-martensite. The HPT processing also produced a significant reduction in the α′-martensite and austenite grain sizes from initial values of ~3.8 and ~3.9 μm to ~225 and ~60 nm after 20 HPT turns, respectively, while the e-martensite final grain size was ~44 nm. Micro-hardness measurements and tensile testing showed a significant increase in hardness and in the ultimate tensile strength to about 680 Hv and 2235 MPa after 20 turns, respectively. Finally, the fracture surfaces of samples after HPT processing showed a combination of dimples, cracks and shear bands which confirmed a transition in fracture behavior from ductile to a mixture of a ductile and brittle mode.

NiTi thin films are usually sputtered on silicon wafers by magnetron sputtering. But the systems composed of thin film on flexible polymeric substrate are used in many applications such as micro electro-mechanical systems (MEMS). Investigation on mechanical properties of thin films has attracted much attention due to their widespread applications. In this paper, the mechanical properties of 1 µm-thick crystallized Ni-49.2 at%Ti thin film alloy deposited by DC magnetron sputtering on Kapton substrate are investigated by using tensile test. The as-deposited thin films are in amorphous state, then for crystallization, the thin film was annealed at 450 °C for 30 min. Formation of the austenite phase after annealing was confirmed by X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). The surface morphology of as deposited and crystallized thin films were examined by scanning electron microscopy (SEM). Stress-strain curves of the NiTi alloy thin film were obtained by subtracting of the stress-strain Kapton curves from the corresponding curves of the NiTi/Kapton composite. The XRD results revealed that the NiTi thin film deposited on the Kapton is austenitic and presents super-elastic effect at room temperature. This pseudo elastic effect leads to more recoverable strain in NiTi/Kapton composite film compared with Kapton foils on loading/unloading test. Furthermore, it was concluded that nanostructure of the NiTi thin film is responsible for remarkable improvement of ultimate tensile strength (1.4 GPa) at a strain of 30% compared with the bulk material.

Pure Fe was severely deformed by a combination of shaped cold rolling and cold drawing. X-ray diffraction profiles analysis was applied in accordance with the Williamson-Hall (WH) and modified Williamson-Hall (MWH) methods to identify crystallite sizes of the deformed specimens. It was found that some differences exist between the results of WH and MWH procedures using the hkl dependent Young’s modulus or considering the average dislocation contrast factor. The latter method is more accurate and enables the determination of the character of dislocations in plastically deformed Fe. It was shown that by increasing deformation strain, the screw dislocations dominated. The enhancement of hardness occurs in the deformed Fe due to grain refinement, dislocation accumulation and deformation-induced vacancies.

In this research, the reverse transformation of martensite to austenite and pseudoelastic behavior in Fe–10Ni–7Mn (wt%) lath martensitic steel were investigated under different amounts of cold rolling. The results indicated that the transformation of stress-induced austenite to epsilon martensite is responsible for pseudoelastic behavior in 80% cold rolled specimen.

Experiments were conducted to examine the effect of high-pressure torsion (HPT) processing on the microstructure and pseudoelastic behavior of a ternary Fe–10Ni–7Mn (wt.%) shape memory alloy in both the solution-annealed and intercritically-annealed conditions. X-ray diffraction (XRD) patterns and electron backscatter diffraction (EBSD) analyses showed that the initial microstructure of the alloy in the solution-annealed condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite (α′→γ) by HPT processing. Also, the austenite formed in the dual phase (α′+γ) specimens after intercritical annealing treatment at 600 °C for 7.2 ks underwent a γ→e→α′ transformation during subsequent HPT processing such that a multi-phase microstructure was formed consisting of α′-martensite, austenite and e-martensite. The HPT processing led to a significant increase in the microhardness value to ~690 Hv due to a high density of dislocations and the associated grain refinement of the microstructure. Cyclic loading-unloading tensile tests at room temperature revealed a strain hysteresis and pseudoelastic behavior in the HPT-processed specimens with different initial microstructures. Outstanding pseudoelasticity values of about 67% and 75% were obtained at the fourteenth loading-unloading cycle after 20 HPT turns in the solution-annealed and intercritically-annealed specimens, respectively.

A CoCrFeNiMn high-entropy alloy (HEA) was subjected to shape rolling through equivalent strain of 0.5, 1.1, 1.4, 2.5, 3.7 and 4.2 at room temperature to study the microstructural evolution and formation of excess free volume during severe plastic deformation. The microstructure of the processed sample through the equivalent strain of 4.2 revealed a single fcc phase with an average grain size of ∼50 nm and an excess free volume of ∼0.96% due to formation of nano voids and defects after severe shape rolling. Microhardness measurements showed a saturation level but no nano voids were observed in the processed samples with equivalent strains of lower than 4.2. The results show precipitation and then dissolution of precipitates and also annihilation of defects due to recrystallization during heating between 700 and 900 °C. Density of processed samples reached to the initial condition after heating up to 1000 °C except the processed sample at the equivalent strain of 4.2 due to existence of stable nano voids in this sample.

A CoCrFeNiMn high entropy alloy (HEA) was processed by Severe Cold Shape Rolling (SCSR) to equivalent strain of 3.7 at room temperature. The processed alloy shows a single-phase CoCrFeNiMn HEA with an average grain size of ~50 nm and dislocation density of 2.1 × 1015 m−2. Heat treatment at 723–1223 K for up to 30 min was conducted to study microstructural evolution during short term post-deformation annealing (PDA). The results showed the maximum hardness of 527 Hv after PDA at 773 K due to formation of Cr-rich precipitates. PDA at 1223 K led to significant drop of the hardness value due to dissolution of precipitates, recrystallization and grain growth in the microstructure. The results suggested that formation of precipitates increased strength but reduced ductility significantly. Remarkable mechanical properties achieved after short-term PDA at 1023 K with an ultimate tensile strength of 1000 MPa, yield stress of 920 MPa and an elongation to failure of 20%.

A martensitic TiNi shape memory alloy was processed by high-pressure torsion (HPT) for 1.5, 10 and 20 turns followed by post-deformation annealing (PDA) at 673 and 773 K for various times in order to study the microstructural evolution during annealing and the shape memory effect (SME). Processing by HPT followed by the optimum PDA leads to an appropriate microstructure for the occurrence of a superior SME which is attributed to the strengthening of the martensitic matrix and grain refinement. A fully martensitic structure (B19' phase) with a very small grain size is ideal for the optimum SME. The results indicate that the nanocrystalline microstructures after PDA contain a martensitic B19' phase together with an R-phase and this latter phase diminishes the SME. Applying a higher annealing temperature or longer annealing time may remove the R-phase but also reduce the SME due to grain growth and the consequent decrease in the strength of the material. The results show the optimum procedure is a short-term anneal for 10 min at 673 K or only 1.5 min at 773 K after 1.5 turns of HPT processing to produce a maximum recovered strain of ~8.4% which shows more than 50% improvement compared with the solution-annealed condition.

Processing by severe cold shape rolling of commercially pure Cu up to equivalent strain of 4.5 led to bimodality of structure with significant grain refinement to nano-scale. Microstructure of the processed copper was characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) indicating the ultrafine structure including dislocation tangles, dislocation cells, subgrains, micrograins and nano/ultrafine grains with sharp grain boundaries. The bimodal structure with unequal distribution of grain sizes consisted of nanograins as well as larger grains, formed probably due to an inhomogeneity of deformation. The measurements of the mechanical properties showed an enhancement of microhardness to 114 HV and a high strength of 524 MPa with relatively good ductility that is proposed as a consequence of the bimodal structure.

Ni–Ti thin film shape memory alloys (SMAs) have become a promising material for micro-actuators because of their excellent mechanical properties and biocompatibility. Films with composition gradient have the added feature of an intrinsic two-way shape memory effect. In this study two kinds of the Ni–Ti bi-layer thin films (Ni 50.8 Ti/Ni 45 TiCu 5 and Ni 50.8 Ti/Ni 49 Ti) were developed. The thin films were deposited onto Si (1 1 1) substrates by RF magnetron sputter deposition from separate alloy targets. The three targets with atomic compositions of Ni 45 Ti, Ni 47 Ti and Ni 42 TiCu 5 were prepared by vacuum arc re-melting (VAR). For crystallization, the as-deposited amorphous thin films were annealed at 773 K for 1 h. The annealed bi-layer thin films formed compositional gradients at the interface of the films through diffusion. The effect of the composition on film structure, surface morphology, transformation temperatures and mechanical properties was studied using grazing incidence X-ray diffraction, transmission electron microscopy, atomic force microscopy, electrical resistivity and nano-indentation. The chemical and stress gradient in the bi-layers affects the structure, phase transformation and shape memory effect. Thin films exhibit multi-step phase transformation. Also, the bi-layer presented pseudo elastic and shape memory effect, simultaneously. The depth recovery ratio due to pseudo elastic in the Ni 50.8 Ti/Ni 45 TiCu 5 compared with the Ni 50.8 Ti/Ni 49 Ti bi-layer is higher while the recovery ratio due to shape memory in the Ni 50.8 Ti/Ni 49 Ti bi-layer is higher. The combination of pseudo elastic with shape memory effect produces a two-way shape memory effect with a reduced hysteresis in the bi-layers.

The crystallization of Ni-rich/NiTiCu bi-layer thin film deposited by magnetron sputtering from two separate alloy targets was investigated. To achieve the shape memory effect, the NiTi thin films deposited at room temperature with amorphous structure were annealed at 773 K for 15, 30, and 60 min for crystallization. Characterization of the films was carried out by differential scanning calorimetry to indicate the crystallization temperature, grazing incidence X-ray diffraction to identify the phase structures, atomic force microscopy to evaluate surface morphology, scanning transmission electron microscopy to study the cross section of the thin films. The results show that the structure of the annealed thin films strongly depends on the temperature and time of the annealing. Crystalline grains nucleated first at the surface and then grew inward to form columnar grains. Furthermore, the crystallization behavior was markedly affected by composition variations.