High Martensitic Transformation Research Articles

Electronic structure, magnetism and martensitic transformation in all-d-metal Heusler alloys Mn2PtZ (Z = Sc, Ti, V)

The atomic ordering, electronic structure, magnetic properties and martensitic transformation of three new all-d-metal Heusler alloys Mn2PtZ (Z = Sc, Ti and V) have been investigated theoretically to discover new magnetic shape memory alloys (MSMAs). Depending on the different third transition metal element Z, Mn2PtSc and Mn2PtTi tend to form XA structure while Mn2PtV forms L21 structure. This difference is related to their DOS structures. Charge density difference calculations indicate enhanced d-d hybridization in these all-d-metal Heusler alloys. Mn2PtZ are all ferrimagnets with their Mn-Mn or Mn-V local spin moments antiparallel coupled. In these alloys, Mn2PtTi is a possible new MSMA with the energy difference ΔEM between the martensite and austenite as large as −262 meV/f.u., which indicates a high martensitic transformation temperature and is related to the Jahn-Teller effect in the electronic structure. A large volume effect of 2.9% cell shrinkage occurs during the martensitic transformation. All this makes Mn2PtTi a promising candidate for MSMAs with wide applications.

An investigation on the formation and hardening mechanisms of white etching layer in hypoeutectoid rail surface

The stress distribution in a hypoeutectoid U71Mn rail surface due to the wheel-rail contact was simulated via finite element, and the microstructure and hardening of the after-service rail surface were comparatively studied with heat treated (holding at 850°C for 30min and cooling at 15°C/s) samples under atmospheric pressure (AP-850) and under hydrostatic pressure of 1GPa (HP-850), respectively. The rail surface experiences 3-dimensional stress-state of 1GPa and large shear stress. A white etching layer (WEL) was formed, showing martensite-type microstructure with spherical cementite particles and a hardness of 1101HV. The phase composition and hardening show great resemblance to those of the HP-850 sample, but differ significantly from those of AP-850 sample. High pressure martensitic transformation is proposed to interpret the formation of WEL and the hardening is analyzed according to the structural cause.

Wide tuning the carbon supersaturation within Fe-C lath martensite via high pressure martensitic transformation of Fe-0.45C alloy

The carbon supersaturation within lath martensitic lattice was tuned via thermal treatment of a Fe-0.45C alloy under hydrostatic pressure of 1–6 GPa. The content of supersaturated carbon increases linearly with high pressure till 84.4% of the nominal carbon content under 6 GPa. The strengthening of carbon supersaturation is remained proportional to the square root of its content but the contribution was underestimated.

Tempering mechanism of lath martensite induced in IF steel under high pressure

In this paper, low- and high-strength lath martensite (350 and 640 HV) was fabricated in an IF steel via high pressure martensitic transformation. The microstructure and the softening during their tempering from 200 °C to 800 °C for 1 h were systematically investigated. A carbon-irrelevant tempering process was proposed, exhibiting a three-stage structural evolution pattern depending upon the tempering degree (1-(HV-HVFP)/(HVNP-HVFP), where the HV is the instant hardness, HVNP is the non-tempered hardness and HVFP is the fully tempered hardness): (1) low tempered (<10%), removing the loose dislocations and dislocation boundaries within martensitic variants; (2) medium tempered (10%-50%), eliminating the martensitic variant laths via the migration of their terminal tips; (3) highly tempered (>50%), clearing up the remained variant laths via the migration of the triple junctions. Martensite-type microstructure is tailored by low-index lamellar variant boundaries and is thus intrinsically thermally stable, whereas the mobile terminal tips decrease the tempering resistance. The underlying mechanism for such carbon-irrelevant process was discussed and the potential effect on the tempering behavior of carbon-contained martensite was highlighted.

Process-microstructure-properties of CuAlNi shape memory alloys fabricated by laser powder bed fusion

Cu -based shape memory alloys (SMAs) show great potential in the application of high temperature, but their performance improvement combining with additive manufacturing is still challenging. In this paper, nearly defect-free (99.7% relative density) CuAlNi SMAs with two types of thermal-induced martensites, 18R and 2H, were successfully fabricated by laser powder bed fusion (LPBF). The remelting printing strategy with systematic optimization framework including densification and printability analysis was used to get optimal process window. The twin-related self-accommodation structure with long period stacking order substructure was formed during additive manufacturing process. The as-build samples show excellent mechanical properties (ultimate compressive strength of 1593 ± 10 MPa and large fracture strain of 23.0% ± 0.1%) with double-yielding, and outstanding shape memory effect, with maximum recoverable strain of 3.39% and nearly 100% recovery rate under compressive strain not exceeding 6%. Furthermore, the microstructural evolution in LPBF involving remelting, martensitic transformation, and 2H martensite was discussed. This result not only builds the microstructure framework involving high reflectivity alloys and martensitic transformation but also provides insight into the applicability of LPBF process in producing Cu-based SMAs.

Crystal structure and site preference, magnetic and elastic properties, and martensitic transformation of Ni- and Fe-doped Co2VGa alloys: A first-principles study

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co2−xMx)VGa (M1x), Co2(V1−xMx)Ga (M2x), and Co2V(Ga1−xMx) (M3x, M = Ni and Fe, 0≤x≤1.0). It is shown that instead of the L21 and XA structures, the fcc one is energetically preferred in the cubic M3x (x≥0.8) alloys. In L21-Ni2x (x≤0.6) and fcc-Ni3x (x=0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x&amp;lt;0.8), and Fe and Fe atoms in Fe3x (x≥0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x≥0.4 as well as Ni3x with x≥0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co2VGa. A lower shear modulus (C′=(C11−C12)/2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C′&amp;lt;36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x=0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be.

Order degree and self-accommodation of γ phases in Ni-Mn-Ga alloys

High temperature shape memory alloys Ni-Mn-Ga have high martensitic transformation temperature, good thermal stability and moderate shape memory performance. The second phase γ significantly affects the alloys’ macroscopic performances, but its atomic structure and microstructural feature still remain almost unknown. Microstructure and degree of atomic ordering of γ precipitate are uncovered in Ni54Mn30+xGa16–x (x = 0, 2, 5, 9, 11) and Ni57+yMn25Ga18–y (y = 0, 1, 2, 3, 4) alloys. For as-cast Mn-rich alloys, a hierarchical “nano-lamellae within micro-lamellae” microstructure is found. Nano-lamellae are paired to form a variant, two variants constitute a micro-lamella, a pair of micro-lamellae makes up single group, and there are six groups in a grain. Single nano-lamella is a partially ordered phase with tetragonal crystal structure. All nano-lamellae are of nearly identical crystallographic orientation. This arrangement of the 24 variants is self-accommodated. In annealed state, the lamellae in large γ precipitates still exist due to the constraint from martensites, but in small grains partly or completely disappear and further evolve into nanotwins. For as-cast Ni-rich alloys, the paired nano-lamellae are disordered and another partially ordered phases, respectively. After annealing, they become single crystal with ordered structure. Models describing atomic positions of γ phases with different orderings are built. For Mn-rich alloys, the martensitic transformation temperature reaches about 600 °C, shape memory strain is up to 5%, and compressive stress and strain are higher than 2100 MPa, and 26%, respectively. Relative to Ni-rich samples, Mn-rich alloys have very high martensitic transformation temperature, moderately enhanced ductility and reasonably good shape memory strain. They are closely related to martensite’s electron concentration and γ’s atomic ordering, crystal structure, and lamellar microstructure.

The effect of systematic substitution of Nb with Ti on a potential high-temperature shape memory alloy: B2 NbRu

NbRu has been considered as a potential high-temperature shape memory alloy due to its high martensitic transformation temperature, however, it has poor superelasticity and other shape memory properties, which limits its applications. In this paper, the effect of systematic substitution of Nb with Ti on the structural, thermodynamic, and elastic properties of supercell 2x2x2 B2 Ru50Nb50-xTix spanning the entire composition range (x = 0, 6.25, 12.50, 18.75, 25.00, 31.25, 37.50, 43.75, 50 at. %) is investigated using ab initio calculations. Elasticity results showed that the mechanical stability of the B2 phase improved with increasing Ti content, which could be associated with a decrease in transformation temperature. However, a suitable composition range within which Cʹ~0, resembling that of well-known TiNi, is identified. This predicts Ti addition to be effective in improving shape memory properties of B2 NbRu alloy.

(TiB+La2O3)/Ti-V-Al lightweight high temperature shape memory composites with high strength fabricated by reaction hot pressing and hot rolling

In the present study, lightweight high-strength Ti-V-Al based high temperature shape memory composites were fabricated to broaden the application of shape memory alloys. In-situ TiB whiskers and La2O3 particles were introduced and the distribution of reinforcements was optimized by the combination of low-energy milling, hot-press sintering and subsequent hot rolling. The in-situ reinforcements formed a network structure, which evolved from an equiaxed shape to an ellipsoidal shape. Furthermore, the direction of the reinforcement was inclined to the rolling direction. In the reinforcement-lean area, the phase constitution evolved from the β parent phase to the α″ martensite phase, and the width of the α″ martensite gradually widened with increasing distance to the in-situ reinforcements. The composites maintained high martensitic transformation temperature characteristics. Excellent mechanical properties with a high tensile strength of 885 MPa and a large elongation of 13.1% are obtained in the composite with a 1.0 wt% LaB6 addition and 70% deformation ratio. Meanwhile, the composite realized 3.49% recoverable strain under 6% pre-strain conditions.

Low-cost (ZrCu)50-xTax high temperature shape memory alloys showing excellent shape memory effect

ZrCu-based alloys, as one of the most potential high-temperature shape memory alloys, show quite high martensitic transformation temperature, relatively low price for the raw materials, and good casting fluidity. In this work, the martensitic transformation (MT) and shape memory effect of (ZrCu)50-xTax high-temperature shape memory alloys were systematically investigated. Both X ray diffraction and SEM backscattered electron image demonstrated the coexistence of the main phase monoclinic martensite phase and the second phase Ta2Cu. SEM results also revealed that increasing Ta content resulted in a larger volume fraction of Ta2Cu phase. Differential scanning calorimetry results showed that the MT temperatures remarkably increased with Ta addition. The ductility of the (ZrCu)50-xTax alloys was reduced to some degree with increasing Ta content. The shape memory effect was improved significantly, in which (ZrCu)98Ta2 alloy showed the 6.19% maximum recovery strain during 8% pre-strain under compression at 25 ​°C.

Transient Nucleate Boiling Process Used for Obtaining Super Strong Carbon Steels and Irons

Based on self–regulated thermal process, in the paper four types of thermomechanical treatments are considered. The first is a high temperature thermomechanical treatment (HTTMT) followed by complete martensitic transformation. The second is a low temperature thermomechanical treatment (LTTMT) plus martensitic transformation. The third is the high and low temperature thermomechanical treatment (HTTMT and LTTMT) plus martensitic transformation. And the last includes HTTMT and LTTMT plus bainitic transformation to obtain super strong and ductile materials. It is shown in the paper that listed technologies are enough intensive to obtain very strong and ductile materials using plain high carbon steels. A detailed consideration of all processes in the paper will motivate engineers to perform mentioned technologies in forging shops to receive super strong and ductile materials without costly alloying that saves energy and alloying elements. The paper discusses the opportunity of preventing martensite transformation to receive fine and nano–bainitic microstructure during intensive quenching. A hypothesis is forwarded that explains possible technology used in 8th and 9th centuries in the Middle East to manufacture Damascus steel. The secret of Damascus steel could be the duration of transient nucleate boiling process needed for preventing martensite transformation during forging of steel.

The effect of vanadium and nickel on the microstructure and transformation temperatures of Ti50Pt50 alloy

Significant research has been done to produce shape memory alloys that have good shape memory properties and high martensitic transformation temperatures. The Ti50Pt50 alloys have been found to have high transformation temperature of around 1050℃ however, they exhibit negligible shape memory properties. The solid solution strengthening, and improved shape memory properties could be enhanced by ternary alloying. Therefore, this work investigates the effect of varying V and Ni contents, in the range of 6.25 to 12.5at%, on the austenitic and martensitic transformation temperatures, and hardness of the equi-atomic Ti50Pt50 alloy. Arc melting followed by casting and solution heat treatment was carried out to produce the alloys. As-produced alloys were characterized by using scanning electron microscopy, differential scanning calorimetry and hardness testing. The microstructures showed high volume fraction of second phases formed in the TiPtV alloy compared with Ti50Pt50 and TiPtNi alloys. The multiple phases formed in the TiPtV alloys could be the cause of high hardness values observed in these alloys as compared withTi50Pt50 and TiPtNi alloys. Thermal transformation studies revealed that TiPtV alloys exhibit transformation temperature close to Ti50Pt50 alloy, in contrast with TiPtNi alloys. TiPtNi alloys thermal behaviour was improved by solution heat treatment.

Brittle-to-ductile transition in Ti–Pt intermetallic compounds

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

Enhancing fatigue life by ductile-transformable multicomponent B2 precipitates in a high-entropy alloy

Catastrophic accidents caused by fatigue failures often occur in engineering structures. Thus, a fundamental understanding of cyclic-deformation and fatigue-failure mechanisms is critical for the development of fatigue-resistant structural materials. Here we report a high-entropy alloy with enhanced fatigue life by ductile-transformable multicomponent B2 precipitates. Its cyclic-deformation mechanisms are revealed by real-time in-situ neutron diffraction, transmission-electron microscopy, crystal-plasticity modeling, and Monte-Carlo simulations. Multiple cyclic-deformation mechanisms, including dislocation slips, precipitation strengthening, deformation twinning, and reversible martensitic phase transformation, are observed in the studied high-entropy alloy. Its improved fatigue performance at low strain amplitudes, i.e., the high fatigue-crack-initiation resistance, is attributed to the high elasticity, plastic deformability, and martensitic transformation of the B2-strengthening phase. This study shows that fatigue-resistant alloys can be developed by incorporating strengthening ductile-transformable multicomponent intermetallic phases.

Martensitic Transformation of High-Entropy and Medium-Entropy Shape Memory Alloys

As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti45Zr5Pd25Pt20Au5, and Ti45Zr5Pd25Pt20Co5 had the martensitic transformation. The perfect recovery was obtained in Ti45Zr5Pd25Pt20Co5 during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti45Zr5Pd25Pt20Au5 during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti45Zr5Pd25Pt20Co5 is the one possible HT-SMA working between 342 and 450 °C.

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel. Based on the microstructure analysis, the texture evolution and martensite transformation behavior in the strain hardening process were studied. The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the continuous hardening stage. As the strain increases, the texture of retained austenite evolves towards the F orientation {111}<112>, which is not conducive to martensite transformation. After the strain of 5%, the number of austenite grains with high Schmid factor orientations is gradually increased, and then significantly reduced when the strain is over 10 % due to the occurrence of martensitic transformation, which results in a high martensitic transformation rate. However, the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage. Moreover, because of the coordination deformation of austenite grains, strain preferentially spreads between adjacent austenite grains. Consequently, the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite, leading to a differential contribution to each strain hardening stage.

A study on the cold workability and shape memory effect of NiTiHf-Nb eutectic high-temperature shape memory alloy

In this work, we report a (Ni50.1Ti34.9Hf15)85Nb15 alloy that shows excellent cold workability and good high-temperature shape memory effect. The as-cast sample shows eutectic microstructure consisting of β-Nb and the NiTiHf(Nb) matrix. The homogenized (Ni50.1Ti34.9Hf15)85Nb15 alloy shows high compressive fracture strain of 40% and excellent cold-rolling reduction ratio of 80%. It is suggested that the formation of (semi-) coherent β-Nb is the main reason on improving the ductility of NiTiHf alloy. After solution treatment, the material shows a high martensitic transformation start temperature of 146 °C, and a maximum shape memory effect strain of 2.3%. This work demonstrates that (Ni50.1Ti34.9Hf15)85Nb15 alloy with good cold workability and high-temperature shape memory effect may provide new opportunities for applications.

Study on microstructure and properties of TiBw/Ti-V-Al light weight high temperature shape memory composite

This study investigated a novel light weight high temperature shape memory composite with a quasi-continuous network microstructure, which greatly promoted to the application of shape memory alloy in aerospace. In-situ TiB whisker was introduced and mainly distributed along the large Ti-V-Al matrix, which contributed to the breakthrough of the mechanical properties. No obvious influence on the martensite structure, and the spear-like martensite variants still exhibited the typical self-accommodation morphology with {011} type I twin relationship. The XRD results showed that the phase constitution gradually evolved from sole α″ phase to the coexistence of the α″ phase and β phase with the increasing of TiB2 contents. The reverse martensitic transformation peak temperature increased firstly and then decreased with the increasing of TiB2 contents, owing to the influence of composition variation and stress field. It was noteworthy that the TiBw/Ti-V-Al composite with 2.0 wt% TiB2 addition exhibited a higher strength, accompanied by the superior elongation (15.7%) and high martensitic transformation temperature. The yield strength and fracture strength were 692 MPa and 932 MPa, respectively. In addition, the shape memory effect increased firstly and then decreased with the increasing of the TiB2 contents. The maximum recoverable strain of 4.4% with 6% pre-strain can be gained in 0.3 wt%TiB2 added Ti-V-Al composite.

Superelastic Behavior of Ti-Nb Alloys Obtained by the Laser Engineered Net Shaping (LENS) Technique.

The effect of Nb content on microstructure, mechanical properties and superelasticity was investigated for a series of Ti-xNb alloys, fabricated by the laser engineered net shaping method, using elemental Ti and Nb powders. The microstructure of as-deposited materials consisted of columnar β-phase grains, elongated in the built direction. However, due to the presence of undissolved Nb particles during the deposition process, an additional heat treatment was necessary. The observed changes in mechanical properties were explained in relation to the phase constituents and deformation mechanisms. Due to the elevated oxygen content in the investigated materials (2 at.%), the specific deformation mechanisms were observed at lower Nb content in comparison to the conventionally fabricated materials. This made it possible to conclude that oxygen increases the stability of the β phase in β–Ti alloys. For the first time, superelasticity was observed in Ti–Nb-based alloys fabricated by the additive manufacturing method. The highest recoverable strain of 3% was observed in Ti–19Nb alloy as a result of high elasticity and reverse martensitic transformation stress-induced during the loading.

Investigation of thermal property of spark plasma sintered Ti-Zr-Ta shape memory alloys

There is increasing demand for the performance improvement of novel shape memory alloys (SMAs), such as the Ti-Zr-based systems. The effects of fabrication parameters and material compositions are critical to the thermal properties of the alloys. This study is designed to investigate the effects of ternary Ta additions on the thermal properties of spark plasma sintered (SPSed) novel Ti-Zr based SMAs for high temperature applications. The SPSed Ti-Zr/Ta specimens were characterized for their microstructural and crystallographic features using scanning electron microscopy (SEM) incorporated with Energy Dispersive X-ray Spectroscopy (EDS), and X-ray powder diffractometry (XRD). Thermal property measurements were conducted using the Laser Flash technique to obtain the thermal diffusivity behaviour, while the phase transformation properties were determined by the differential scanning calorimetry (DSC). The presence of both alpha martensite, and beta structures were revealed by the SEM/EDX and XRD respectively. High thermal diffusivity values were obtained, and they show an increasing trend with temperature. The DSC measurement revealed high reversible martensitic transformation (above 200 deg Celcius) for all the alloys, a two-stage phase transformation in the binary alloy, and a single-phase transformation event in the ternary alloys. A downward parabolic gradient of the specific heat and thermal conductivity with respect to temperature was recorded in the alloys, thus revealing an interaction between phonon-mediated and electron-mediated thermal transport processes. Two-stage phase transformations, noticed in the binary alloy indicate the reversible transformation events of alpha-Ti and alpha-Zr phases at different temperatures to-and-fro the austenite phase, whereas, in the ternary phase the single transformation event indicates the presence of only alpha-Ti phase. These results further strengthen the understanding of material atomic compositions effect on the thermal behaviour of the as-sintered alloys. The high thermal diffusivity and high martensitic transformation behaviour of the Ti-Zr/Ta alloys suggest they can be suitable candidates for high temperature applications in the aerospace technologies.