Solution-treated Specimens Research Articles

Microstructures, transformation temperatures and superelastic properties of the rapidly solidified (TiZrHf)50Ni25Co10Cu15 HESMAs

In this study, the effects of the rapid solidification process on microstructures, transformation behaviors and superelastic properties of the multi-component (TiZrHf)50Ni25Co10Cu15 (at%) high-entropy shape memory alloy (HESMA) were investigated. The as-spun (TiZrHf)50Ni25Co10Cu15 fibers were prepared by a rapid solidification process. The solution-treated (TiZrHf)50Ni25Co10Cu15 alloy bulk specimen consisted of a (NiCoCu)-rich matrix, (TiZrHf)2(NiCoCu)-type phase and carbide, while the as-spun fiber specimen consisted of (TiZrHf)-rich matrix and carbide. The (TiZrHf)2(NiCoCu)-type phase is dissolved in the matrix of as-spun fibers due to the rapid solidification process. The martensitic transformation start temperature of the (TiZrHf)50Ni25Co10Cu15 alloy increased from 53.5 °C to 91.5 °C after the rapid solidification process. Both the (TiZrHf)50Ni25Co10Cu15 alloy bulk and fiber specimens showed clear superelasticity and the total superelastic recovery strain increased from 4.6 % to 5.7% after the rapid solidification process.

Tailoring microstructure and mechanical properties of 17-4PH steel fabricated by wire-arc directed energy deposition via a combination of intrinsic and post-processing heat treatments

This paper examines the effect of intrinsic heat treatment, i.e., multiple transient heating from subsequent passes, and post-processing heat treatment on the microstructure and mechanical properties of 17–4PH stainless steel fabricated by wire-arc directed energy deposition. By carefully controlling the interpass temperature to remain below the martensite start (Ms) temperature, intrinsic heat treatment effectively triggers the formation of Cu-rich clusters and the reversion of austenite. This approach results in a concurrent enhancement in the ultimate tensile strength, increasing from 1036 MPa to 1128 MPa, and elongation, increasing from 13.2% to 18.5%, for the as-built part. During post-processing heat treatment, the microstructural response to direct aging treatment hinges on the initial microstructure induced by intrinsic heat treatment. Notably, direct aging treatment significantly enhances the hardness and ultimate tensile strength of the as-built part created at an interpass temperature surpassing the Ms temperature. Conversely, its influence on the mechanical properties of the as-built component, fabricated at an interpass temperature lower than the Ms temperature, remains minimal. Additionally, the application of a solution treatment effectively eradicates the inherent microstructural characteristics, regardless of the initial fabrication conditions. Nonetheless, the grain size of the solution-treated specimen is markedly influenced by the as-built microstructure. Ultimately, a judicious amalgamation of intrinsic and post-processing heat treatments yields an exceptional combination of properties, including high hardness (465 HV), ultimate tensile strength (1437 MPa), and elongation (14.5%). These results underscore the significance of a comprehensive consideration of intrinsic and post-processing heat treatments to tailor the microstructure and mechanical properties of 17–4PH steel fabricated by wire-arc directed energy deposition.

Effect of Mn content on microstructure and transformation behavior of TiZrHfNiCoCu multi-component high-entropy shape memory alloys

Multi-component Ti16.667Zr16.667Hf16.667Ni25Co10Cu15-XMnX at.% (X = 0, 1, 3, 5 and 7) high-entropy shape memory alloys (HESMAs) were prepared by arc-melting and then microstructures, transformation temperatures, phase constituents and superelastic properties were investigated by scanning electron microscope observation, differential scanning calorimetery, X-ray diffraction and dynamic mechanical analysis in tensile mode, respectively. The microstructure of solution-treated TiZrHfNiCoCuMn HESMAs specimens consisted of (NiCoCuMn)-rich matrix, (TiZrHf)2(NiCoCuMn)-type phase and (TiZrHf)(NiCoCuMn)-type phase. The area fraction of (TiZrHf)2(NiCoCuMn)-type phase increased from 1.7 to 5.1% with increasing Mn content from 0 to 7 at.%. The area fraction of the (TiZrHf)(NiCoCuMn)-type phase increased from 1.9 to 17.2% with increasing Mn content from 3 to 7 at.%. -ΔHmix effect was dominant over ΔSmix and δ effects for phase constitution in TiZrHfNiCoCuMn HESMAs. The martensitic transformation start temperature decreased with the addition of Mn content up to 3 at.% and then increased with increasing Mn content from 3 at.% to 7 at.%. TiZrHfNiCoCuMn HESMAs showed clear superelasticity and the total recovered strain decreased with increasing Mn content.

Influence of heat treatment processes on microstructure evolution, tensile and tribological properties of Ti6Al4V alloy

The influence of heat treatment processes on microstructure, tensile and tribological properties of Ti6Al4V alloy was investigated. The specimens were heated for 30 min at 925 °C and then cooled at various rates by water quenching, air cooling, and furnace cooling. After that, the samples were aged for four hours at 600 °C. Three phases make up the microstructure: primary α-phase (αp), secondary α-phase (αs), and retained β-phase (βr). Cooling in the air and water followed by aging (AC + Aging and WQ + Aging) resulted, αs-phase precipitating inside βr-phase. The highest hardness of 35 HRC was recorded for WQ + Aging specimen due to existence of a high amount of βr-phase and precipitation of αs-phase. On the other hand, the lowest hardness of 26 HRC was obtained for the FC specimen. AC specimen achieved the highest elongation value of 14%. However, WQ + Aging specimen exhibited the highest ultimate tensile strength of 1028 MPa. For WQ + Aging and AC + Aging specimens, the ideal balance of strength and elongation was discovered. The wear resistance of solution-treated specimens was significantly improved by the aging process and 125% improvement could be achieved in WQ compared to WQ + Aging specimens.

Age-hardening Behavior in <i>γ</i>′-phase Precipitation-hardening Ni-based Superalloy

Dislocations are often introduced in Ni-based superalloys to impart sufficient strength at both room temperature and high temperatures prior to their use in automobile exhaust gaskets. However, the interaction between the representative γ′ (Ni3(Al, Ti))-phase precipitates and dislocations in high temperature remains unclear. Therefore, this study examined the effect of cold rolling on age-hardening behavior and microstructure evolution, focusing on the formation of γ′-phase Ni3Ti during aging at 700°C for up to 400 h after 60% cold rolling of solution-treated specimens. During the early stage of aging, at 0.03 h, the hardness rapidly increased from 401 HV to 496 HV. Age-hardening continued until 3 h and reached its peak of 536 HV, followed by gradual decrease with aging time. 3D atom probe investigation revealed that the γ′-phase was confirmed after 0.3 h of aging. However, the composition-modulated structure speculated to be caused by spinodal decomposition was observed in the 0.03 h aged specimen. The change in strength with aging time was considered by calculating the contribution of each strengthening mechanism. In the initial stage of aging (0–3 h), dislocation and solid-solution strengthening dominated along with spinodal strengthening. Strengthening by spinodal decomposition in the 0.03 h aged specimen is presumptively accelerated by the introduced dislocations, which is followed by further precipitation strengthening caused by γ′-phase precipitates. In the later stage of aging (3–400 h), precipitation strengthening became dominant and reached its peak at 20 h aging, while dislocation strengthening decreased with aging time.

Precipitation transformation pathway and mechanical behavior of nanoprecipitation strengthened Fe–Mn–Al–C–Ni austenitic low-density steel

Precipitation strengthening has been widely adopted in austenitic low-density steel owing to excellent hardened effects. This approach generally employs the coherent κ′-carbides and non-coherent B2 particles. Revealing the precipitation transformation pathway is decisive for further optimizing the microstructures under specific engineering applications. Herein, the detailed precipitation sequence of Fe–28Mn–11Al–1C–5Ni (wt%) austenitic low-density steel as well as its influence on mechanical properties during aging process is systematically investigated. Our results reveal that nano-sized κ′-carbides domains (2 nm) exist in the solution-treated specimen. During aging at 500 °C for 1 h, the cuboidal κ′-carbides (15–20 nm) uniformly disperse in austenite matrix. However, after aging at 700 °C for 15 min, the coarsen κ′-carbides (30–35 nm) inhomogeneously distribute and align preferentially along the 〈1 0 0〉 directions. Further, extending the aging time to 60 min, the needle-type B2 particles replace the κ′-carbides due to the enrichment of Ni elements at the phase boundaries among the austenite and κ′-carbides. After aging at 900 °C, κ′-carbides entirely dissolve into the austenite matrix, and the intragranular B2 particles are the sole precipitates in the austenite matrix and follow the K-S orientation relationship with austenite. The work hardening capability seriously deteriorates due to the shearing of κ′-carbides by gliding dislocations. While the intragranular B2 particles preserve excellent work hardening rate by dislocations bow-out mechanism. The present work is meaningful for guiding the design of new generation dual-nano precipitation austenitic lightweight steel.

The study of sigma and carbide in cast austenitic stainless-steel grade HH after 24 years of high-temperature service

Cast austenitic stainless steel (CASS) has been widely used for long-term service periods in high-temperature applications. Nonetheless, the behavior of CASS after high-temperature and long-term service is insufficiently researched alongside the microstructural analysis for sigma phase and carbide. Here, intermediate pipe support made from CASS grade HH was investigated after 24 years of service at 700 °C and compared with the solution-treated specimen at 1100 °C for 2 h. The chemical composition was analyzed by optical emission spectroscopy (OES), while the microstructure was observed using optical microscopy and scanning electron microscope (SEM). The confirmation of phase composition and lattice parameters were further analyzed by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). The mechanical properties were assessed by tensile tests, hardness tests, and impact tests, whereas the corrosion property was analyzed using potentiodynamic polarization. Based on experiment and analysis, the effects of 24 years of exposure on the sigma phase are spheroidization and Cr depletion, and even partial transformation to ferrite. The sigma phase significantly increases the hardness, but Cr depletion slightly reduces the hardness to 96.86 HRB. Meanwhile, the carbide will increase in quantity and develop an irregular interface at long-term high-temperature exposure. Microstructure evolution of the sigma phase and carbide decreased tensile strength to 46%, elongation to 3%, and impact value to 1.6% of the minimum specifications, respectively, while the corrosion rate increased 10 times (about 7.35 μm per year).

Microstructures and mechanical behavior of Ti-(38+x)Ni-12Cu (at%) (x=0∼3) alloys

In this study, various aging conditions were done on (Ni,Cu)-rich Ti-(38+x)Ni-12Cu (at %) (x=0∼3) alloy sheets to improve the superelasticity and it is found that precipitation hardening is available on some alloys at proper aging conditions, leading to the realization of perfect superelasticity with small stress hysteresis. For the solution-treated specimens, their microstructures and mechanical properties are dependent on the Ni content and solution treatment temperature. In 39 at %Ni and 40 at %Ni alloys, Ti(Ni,Cu)2 particles were embedded in the matrix of as-rolled specimens and disappeared after solution treatment at 1373 K, while they remained in the 41 at %Ni alloy. The 40 at %Ni alloy containing Ti(Ni,Cu)2 particles showed a fracture strain of 30.0 % which was large compared with the alloy without the particles (∼10.2 %). Aging treatment on the specimen solution treated at 1373 K increased micro Vickers hardness and the maximum hardness of the aged 39 at %Ni and 40 at %Ni alloys were Hv 274 and Hv 377, respectively, which was ascribed to the precipitation of the C11b-type precipitates. After optimizing aging conditions, 39 at %Ni and 40 at %Ni alloys showed perfect superelasticity, which was due to the precipitation hardening effect by the C11b-type precipitates. More importantly, the properly precipitation-hardened 40 at %Ni alloy specimen aged at 723 K for 2.5 h following the solution treatment at 1373 K for 1 h showed perfect superelasticity with a small stress hysteresis of 36 MPa associated with the B2-B19 transformation, which was ascribed to the short mean free path between the C11b-type precipitates. With raising the aging temperature to 873 K, the stress hysteresis increased up to 112 MPa, which was attributed to the increase in the mean free path between the C11b-type precipitates.

Formation of Fine Holes and Protrusions on Ti-6Al-4V Alloy by Argon-Ion Sputter-Etching

Fabrication of fine surface textures is one of the most effective technologies to provide new functions for materials. In this research, argon-ion sputter-etching was applied to an α+β type Ti-6Al-4V alloy placed on a SUS304 steel disk at a radio frequency power of 200 W, 250 W and 300 W for 0.9 ks to 21.6 ks. For the 1013 K annealed specimens, fine holes smaller than 500 nm diameters were densely formed on the surfaces when the sputter power was 250 W and sputter time was 0.9 ks. However, when the sputter time increased to more than 1.8 ks, the holes disappeared and the groups of grain-shaped protrusions were formed in flat areas. For the 1323 K solution-treated specimens, the fine holes were also formed at the sputter power of 250 W and at a sputter time shorter than 1.8 ks. The holes connected to ridge-shaped protrusions with increasing sputter time and finally changed to independent and uniformly distributed protrusions. Energy dispersive X-ray analyses revealed an increase in the V and Fe contents on the sputtered surface. This corresponds that the Fe particles arrived from the SUS304 disk dissolved into the alloy and strongly combined with V atoms, which resulted in the selective sputtering of Al and Ti. It is considered that the argon ion radiation has induced the decomposition of surface region to fine areas with different V and Fe contents, and the fine holes are formed by preferential sputtering of the lower V and Fe contents areas.

Tensile Properties of Thermal Cycled Titanium Alloy (Ti–6Al–4V)

The effect of thermal cycling on tensile properties and microstructural characterization of Titanium (Ti–6Al–4V) alloy which were subjected to various types of thermomechanical treatments especially stress relief annealing and solution treated aging were studied and results presented. The specimens were thermally treated at a temperature of 427°C for up to 1500 cycles. The test setup used for thermal treatment of the specimens was a specially designed experimental setup. After thermal treatment the specimens, were prepared for a tensile test to get the desired tensile properties. It was noticed that the maximum tensile strength of the specimen was observed when reached at 250 cycles and thereafter marginally increasing up to 1000 cycles. After 1000 cycles the tensile strength of the specimen decreases. Similarly, the hardness of the specimen increases up to 1000 cycles and then decreases after 1000 cycles, while elongation increases marginally when the cycle increases. Solution-treated aged specimen attains greater strength and hardness when compared to stress relief annealed specimen. With the increased number of cycles, the strength of the specimen when treated with stress relief annealing and solution-treated aging specimen increases. It was observed that the hardness and strength values decrease after 1000 cycles. With an increasing number of cycles, the tensile elongation increased. When compared with both the treatment processes the solution-treated aged specimen shows better results after thermal cycling.

On the Possible Cause of Sudden Storage Modulus Increase during the Heating of PM FeMnSiCrNi SMAs.

A sudden increase in storage modulus (ΔE′) was repeatedly recorded during the heating of powder metallurgy (PM) 66Fe-14Mn-6Si-9Cr-5Ni (mass. %) shape memory alloy specimens subjected to dynamic mechanical analysis (DMA), under constant applied strain amplitude and frequency. This instability, exceeding 12 GPa, was associated with the reverse martensitic transformation of α′-body centered cubic (bcc) martensite to γ-face centered cubic (fcc) austenite, overlapped on a magnetic transition. This transition, observed by thermomagnetic measurements (T-MAG), was associated with the temporary spontaneous alignment of magnetic spins, which lasted until thermal movement became prevalent, during heating. ΔE′ was located around 250 °C on DMA thermograms and this temperature had the tendency to increase with the solution treatment temperature. On T-MAG diagrams, magnetization saturation temperature decreased from 405 °C to 52 °C with the increase in applied magnetic field from 20 Oe to 1 kOe and the increase in mechanically alloyed powder volume from 20% to 40%. On scanning electron micrographs, the presence of thermally induced α’-bcc martensite was emphasized together with the sub-bands that impede its stress-induced formation during DMA solicitation. On X-ray diffraction patterns of the solution-treated specimens, the presence of 22–82% α′-bcc martensite was identified, together with 8–55% retained austenite. It was assumed that the pre-existence of austenite together with α’-bcc martensite, in the microstructure of the solution-treated specimens, favored the magnetic transition, which destabilized the material and caused the storage modulus increase. The specimen comprising the largest amounts of austenite experienced the largest ΔE′.

Hybrid aging and nitriding treatment of nickel aluminum bronze by plasma-based ion implantation

Nickel-aluminum bronzes (NABs) comprise a versatile group of copper-based age hardenable quaternary alloys. Their composition usually comprises aluminum, nickel, iron, and manganese, besides copper. These alloys have a large range of applications, from architectural use to naval propellers. In the present work, a case study of a specific NAB alloy of technical importance was performed aiming to investigate the difference in wear resistance between samples that were aged and samples that received a novel hybrid aging + nitriding treatment carried out on a plasma immersion ion implantation (PIII) apparatus. Both regular aging and the hybrid treatment were carried out for solution-treated coin-shaped specimens, with martensitic matrix, in a PIII chamber under a nitrogen-based atmosphere. Three different voltage biases, 8, 10, and 12 kV, resulted in three distinct treatment temperatures, 370, 400 and 470 °C, for treatment times of 180 and 90 min, being the latter (90 min) used together with the smaller voltage (370 °C) for comparison purposes. From the coin-shaped sample, it was possible to study the two distinct treatments: i) the flat-surface-side exposed to plasma was subjected to the hybrid treatment; ii) the other flat-side (directly supported on the sample holder) was only aging-treated (hereafter termed ‘aged only’). Such methodology allowed to infer the bulk effects on the surface modification process. The XRD patterns of the hybrid-mode treated (hereafter termed ‘aged + nitrided’) surfaces showed formation of nitrogen-expanded phases, these surfaces showing hardness gains varying on the 11%–27% range when compared to the respective surfaces aged only. Scratch tests, in association with hardness and elastic modulus measurements, showed the role of the aging on the elasto-plastic response of both aged only and aged + nitrided surfaces. The nitriding resulted in significant gains of wear resistance for PIII carried out at the lowest temperatures: 43% and 41% less volume loss than the respective aged only surfaces for the treatments at 370 °C for 90 min and 180 min, respectively. Differently, the treatments at the highest temperature (450 °C) resulted in the worst wear performances, since the bulk over-aging surpassed any strength delivered by nitriding.

Splitting of γ′ Precipitates in the Context of Phase Equilibrium

The splitting of γ′ (Ni3Al and Ni3Si) precipitates in five binary Ni-Al alloys and one Ni-Si alloy is reviewed in the context of phase equilibrium. Two mechanisms are considered: Purely Elastic (PE) splitting, driven solely by competition between elastic and interfacial free energies; Thermodynamically Driven (TD) splitting, involving precipitation of the Ni-Al or Ni-Si solid solution γ phases within supersaturated γ′ particles. The main assertion is that TD splitting is responsible for all the observations, with the possible exception of dendritic growth of Ni3Si precipitates; dendritic morphologies can mimic split configurations. In three of the six investigations splitting was reported for alloy compositions lying within the single-phase γ regions of the binary Ni-Al and Ni-Si phase diagrams wherein the γ′ phases are unstable. For the three others the aging temperatures were at or barely below the solvus temperatures, suggesting that five of the alloys were compositionally heterogeneous, “solution treatment” having failed to dissolve pre-existing γ′ particles. TD splitting was thus a byproduct of slow cooling to the aging temperatures, as in the formation of hierarchical microstructures. The nature of secondary γ′ precipitation in some of the alloys indicates that their compositions exceeded the authors’ quoted values, the enrichment enabling precipitation of γ′ during solution treatment followed by TD splitting on slow cooling. PE splitting is the only possible mechanism in solution-treated specimens that are quenched and subsequently isothermally aged. Splitting under such conditions has never been reported, lending further support to the viability of the TD mechanism.

Effect of process parameters and heat treatments on delta-phase precipitation in directed energy deposited alloy 718

This article outlines a detailed study of solution treatments and delta precipitation treatments carried out on laser-directed energy deposited (DED) alloy 718 specimens. Two different sets of DED process parameters were used in high and low energy conditions that yield different microstructural features to study the effect of process parameters on delta precipitation. These two conditions were subjected to solution treatment at 1010 °C and 1050 °C each for 1 h, which improved homogeneity and altered grain texture with introduction of annealing twins. The as-built and solution-treated specimens served as the initial reference condition for subsequent delta processing treatments (DPT) performed at three temperatures of 850 °C, 900 °C, and 950 °C to study the effect of short- and long-term exposures ranging from 1 to 48 h. When as-built specimens were subjected to DPT, interdendritic delta precipitates were observed at Nb-rich regions. In contrast, solution-treated specimens under short-term exposure to DPT resulted in intergranular delta phase precipitates whereas under long-term exposures to DPT yielded predominantly intragranular delta precipitates, which grew denser and longer with increased time of treatment. For longer exposure times of 24 and 48 h, a continuous film of intergranular delta phase was noticed. The morphology, location, and volume fraction of delta phase precipitates studied in this research are imperative for designing the performance of alloy 718 built by DED process.

Microstructure, transformation behavior and superelasticity of an aged Ti-40Ni-12Cu (at%) shape memory alloy

In this study, microstructure, aging behavior and mechanical properties of a (Ni,Cu)-rich Ti–40Ni–12Cu (at%) shape memory alloy were investigated by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), micro Vickers hardness tests, transmission electron microscopy (TEM) and tensile tests. Ti(Ni,Cu)2 phase remained after solution treatment at 1123 K, while it was almost dissolved into matrix after solution treatment at 1373 K. Aging at 723 K increased hardness of the 1373 K solution treated specimen, which was ascribed to the formation of the C11b-type precipitate that has tetragonality of 1.05 and coherent interface with matrix. The micro Vickers hardness increased from Hv 269 to the maximum of Hv 366 as aging time increased from 0 to 2.5 h and then decreased with further prolonging aging time. Transformation temperatures of aged specimens were dependent on aging time, displaying an increase from 274 K to 285 K with increasing aging time from 1.5 h to 2 h and did not change significantly with prolonging aging time to 2.5 h and then again increased with further increasing aging, which was explained by composition effect, strain field around the precipitates and mean free path between precipitates. Compared with the 1373 K solution-treated specimen, the specimen aged at 723 K for 2.5 h showed more excellent superelasticity with a recovery ratio of 98.3%, higher yield stress of 445 MPa and smaller stress hysteresis of 36 MPa, which was ascribed to precipitation hardening caused by metastable C11b-type precipitates. Also, small transformation hysteresis of about 8 K was obtained in the 723 K aged specimens. Aging at 873 K for 45 h of the 1373 K solution-treated specimen induced the precipitation of stable Ti(Ni,Cu)2 phase with tetragonality of 2.55, thus the C11b-type precipitate was considered to be transient metastable precipitate.

Outstanding enhancement in critical resolved shear stress of basal slip with precipitation in Mg-10Dy-0.7La alloy: A micro-pillar compression study

The strengthening effect of precipitates on the operation of basal < a > slip in Mg-10Dy-0.7La has been studied. Micropillar compression was performed on grains of solution-treated and peak-aged specimens selected to favour basal < a > slip. The CRSS of basal < a > slip was calculated from the stress–strain curves. The peak-aged alloy showed a remarkable increase in the CRSS for basal < a > slip as compared to that of solution-treated specimen: the highest reported increase for any Mg-RE alloy. Increased CRSS can be rationalised by considering network of precipitates.

Effect of Cold Rolling and Hot Rolling on Recrystallization Texture and Superelastic Properties of Ti–30Zr–10Nb–2Sn Shape Memory Alloy

A comparative study on the microstructure, texture evolution, and superelasticity of cold-rolled and hot-rolled Ti–30Zr–10Nb–2Sn (at.%) alloys was performed using electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and tensile test. The cold-rolled sheet was solution-treated at 1173 K after a 98.0% cold rolling at room temperature. Hot rolling was carried out at 1273 K. Both cold-rolled and hot-rolled sheets were solution-treated at 1173 K for 1.8 ks. Both solution-treated alloy specimens consisted of a single β-phase at room temperature. A very high (200)β peak was observed for the cold-rolled specimen, and a high (110)β peak was observed for the hot-rolled specimen after the solution treatment at 1173 K for 1.8 ks. The {001}β 〈110〉 β recrystallization texture was obtained in the 1173 K solution-treated alloy specimen owing to grain growth after recrystallization. The {001}β 〈110〉 β recrystallization texture was not observed in the hot-rolled specimen; instead, a (221)β[¯1¯14]β recrystallization texture was observed. The 1173 K solution-treated alloy specimen exhibited superelasticity with complete recovery resulting from the {001}β 〈110〉 β recrystallization texture. Superelasticity was also observed for the hot-rolled specimen, but was not complete, probably because of the low critical stress for slip.

Effect of Sn on Microstructures and Transformation Temperatures of (TiZrHf)50Ni25Co10Cu14 Multi-Component High Entropy Shape Memory Alloys

Multi-component (TiZrHf)50Ni24.5Co10Cu14.5Sn1, (TiZrHf)50Ni24Co10Cu14Sn2, and (TiZrHf)50Ni23.5Co10Cu13.5Sn3 high entropy shape memory alloys (HESMAs) were prepared in this study, and the microstructures, transformation temperatures (TTs), and superelasticity were investigated through scanning electron microscopy, differential scanning calorimetry, and dynamic mechanical analysis in the tensile mode, respectively. The microstructure of the solution-treated TiZrHfNiCoCuSn HESMA specimens consisted of the matrix, (TiZrHf)2(NiCoCuSn)-type and (TiZrHf)5(NiCoCuSn)3-type second phases. The TTs of the solution-treated specimens increased with the addition of a 1 at.% of Sn content and then decreased with an additional increase in the Sn content. The (TiZrHf)50Ni24Co10Cu14Sn2 HESMAs exhibited clear superelasticity, and their total recovery strain was 2.2%.

Influence of microstructure-driven hydrogen distribution on environmental hydrogen embrittlement of an Al–Cu–Mg alloy

The hydrogen trap sites and corresponding hydrogen binding energies in an Al–Cu–Mg alloy with the different microstructures were investigated to unravel the environmental hydrogen embrittlement (HE) behavior of the alloy. The results showed that hydrogen can reside at interstitial lattices, dislocations, S′-phase, and vacancies. In the aged specimen with the highest hydrogen content, it was firstly reported that hydrogen resided at S′-phase particles with relatively high binding energy, which is a determinant factor on HE resistance of the alloy. In the cold-rolled specimen, high content of hydrogen trapped at dislocations with a reversible nature leads to intergranular hydrogen-assisted cracking. In the solution-treated specimen, hydrogen migration to the surface due to low trap density results in low hydrogen content and prevents the GBs from reaching critical hydrogen concentration. The obtained results clearly reveal that trap site density, and the nature of trap sites can determine environmental HE susceptibility of the alloy.

Effect of post-deformation annealing on the microstructure and mechanical behavior of an Fe–Ni–Mn steel processed by high-pressure torsion

Research was conducted to investigate the effects of high-pressure torsion (HPT) and post-deformation annealing (PDA) on the microstructure evolution and mechanical behavior of an Fe-9.6Ni-7.1Mn (at.%) steel with an initial lath martensitic microstructure. The experimental results showed that HPT processing led to the formation of an ultrafine grain martensitic microstructure accompanied by small amounts of strain-induced austenite. Phase analysis and microstructural examination confirmed that during PDA at 600 °C a large fraction of fine and coaxial austenite grains was introduced in the microstructure by diffusionless shear mechanism whereas its volume fraction at ambient temperature was drastically decreased by increasing the annealing time. Also, the grain size was reduced from a value of about 5.2 μm in the solution-treated specimen to ultrafine values of about 570 and 280 nm for the martensite and austenitic phases, respectively, after PDA for 7.2 ks. PDA yielded an outstandingly good combination of an ultimate tensile strength (∼1340 MPa) and fracture strain (∼11.9%) in comparison to the solution-annealed condition which can be attributed to the finer grain size and the presence of shear-formed austenite in the microstructure. In addition, the fracture mode changed from a fully ductile nature in the solution-treated specimen to a combination of ductile and brittle nature after applying the HPT and then returned again to a ductile behavior after PDA.