Martensite Stabilization Research Articles

First-principles study of Co- and Cu-dopedNi2MnGaalong the tetragonal deformation path

The influence of Co and Cu doping on Ni-Mn-Ga Heusler alloy is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. Single-element doping and simultaneous doping by both elements are investigated in Ni${}_{50\ensuremath{-}x}$Co${}_{x}$Mn${}_{25\ensuremath{-}y}$Ga${}_{25\ensuremath{-}z}$Cu${}_{y+z}$ alloys, with dopant concentrations $x,\phantom{\rule{0.28em}{0ex}}y$, and $z$ up to 7.5 at. %. Doping with Co in the Ni sublattice decreases the ${(c/a)}_{\text{NM}}$ ratio of the nonmodulated (NM) martensite, but it simultaneously increases the cubic phase stability with respect to the NM phase. Doping with Cu in the Mn or in Ga sublattices does not change the ${(c/a)}_{\text{NM}}$ ratio significantly and it decreases the cubic phase stability. For simultaneous doping by Co in the Ni sublattice and Cu in the Mn or Ga sublattices, the effects of the individual dopants are independent and about the same as for the single-element doping. Thus, the ${(c/a)}_{\text{NM}}$ ratio can be adjusted by Co doping while the phase stability can be balanced by Cu doping, resulting in stable martensite with a reduced ${(c/a)}_{\text{NM}}$. The local stability of the cubic phase with respect to the tetragonal deformation can be understood on the basis of a density-of-states analysis.

Stress induced martensitic transformations and phases stability in Cu–Al–Be shape-memory single crystals

The stress induced martensitic transformations between different metastable phases in Cu–Al–Be shape-memory alloy single crystals were assessed experimentally. In particular, the successive stress induced transformations from the high temperature austenitic DO3 structure (β1) to the 18R and to the 6R martensites were considered. Several distinct features which characterize the stress induced transformations in Cu–Al–Be and distinguish this system from other Cu-based shape-memory alloys are reported in this work. It was found that the 6R phase forms from a distorted 18R and that this stress induced transformation is highly reversible with no plastic deformation accompanying the process. The stress induced transformation from the distorted 18R to the 6R structure exhibits wide hysteresis, comparable with values observed in the NiTi system. A slightly negative temperature dependence of the critical stress to induce the 6R martensite was determined. Finally, a stress–temperature phase transformation diagram involving the metastable β1, 18R and 6R phases is proposed.

A thermodynamic potential for Ni45Co5Mn36.7In13.3 single crystal

A thermodynamic potential as a function of the ferromagnetic, antiferromagnetic, and martensite order parameters is developed using existing experimental data. It successfully reproduces the single domain properties of Ni45Co5Mn36.7In13.3 bulk crystal, including the Curie temperature, the transition temperature from ferromagnetic austenite phase to antiferromagnetic martensite phase as well as its response to an externally applied magnetic field, the saturated magnetization, the martensite strain, the entropy change, and the magnetic permeability. It can be applied to explore the stress-strain and magnetic field-strain behaviors and implemented in phase field simulations to study the ferromagnetic, antiferromagnetic, and martensite domain stability and evolution.

Grinding-induced martensite stabilization in Mn50Ni33.5Sn8Co8.5 alloy

We found that the martensitic transformation in Mn50Ni33.5Sn8Co8.5 alloy is very sensitive to the surface stress produced during grinding. The grinding-induced martensite in a Mn50Ni33.5Sn8Co8.5 alloy is found to be more thermally stable than the thermally-induced martensite, as indicated by a large increase in the reverse transformation temperature and its latent heat. Annealing the stress-induced martensite was found effective in restoring the austenite structure. The stress-induced martensite was also found to exhibit much enlarged coercive force of 3kOe, compared to 330Oe for the thermally-induced undeformed martensite. This was attributed to the pinning of domain walls by internal stress. These observations were noted in contrast to that observed in most NiMn-based ferromagnetic martensitic alloys.

Site preference and phase stability of Ti doping Ni–Mn–Ga shape memory alloys from first-principles calculations

The effects of Ti content on martensitic transformation and phase stability of Ni50Mn25Ga25−xTix shape memory alloys were investigated from first-principles calculations based on density functional theory. The formation energy results indicate that the added Ti preferentially occupies the Ga sites in Ni2MnGa alloy due to the lowest formation energy. The total energy difference between austenite and martensite increases with Ti alloying, being relevant to the experimentally reported changes in martensitic transformation temperature. The phase stability of Ni50Mn25Ga25−xTix austenite decreases with increasing Ti content, which results from the reduced Ni 3d–Mn 3d hybridization when Ga is replaced by Ti.

Intrinsic origin of rubber-like behavior in thermoelastic alloys

The evolution of polytwinned martensitic variants, which accompanies changes in various energies under applied stress, is investigated using a 3-dimensional phase-field simulation. The intrinsic origin of the rubber-like behavior (RLB) in a thermoelastic alloy is revealed as the disappearance of one variant during loading and its reappearance during unloading, whereas long “stress aging” enhances martensite stabilization. The applied stress and elastic strain energies drive the respective microstructural evolution and its reversal. This intrinsic origin of the RLB cannot be excluded in thermoelastic alloys for which the RLB can be explained by “symmetry conforming-short range order” principle.

Nanocrystallization and amorphization of NiTi shape memory alloy under severe plastic deformation based on local canning compression

As a new method for severe plastic deformation (SPD), local canning compression was applied in order to fabricate amorphous and nanocrystalline nickel–titanium shape memory alloys (NiTi SMA) at room temperature. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were used for understanding microstructural evolution as well as deformation mechanism of NiTi SMA under local canning compression. Deformation mechanism of NiTi alloy under local canning compression involves stress-induced martensite transformation, deformation twinning, dislocation slip, nanocrystallization and amorphization. The (001) martensite compound twins, the stacking faults, primary deformation bands, secondary deformation bands and amorphous bands are observed in the NiTi sample subjected to SPD. The (001) martensite compound twins belong to the deformation twins and thus lead to mechanical stabilization of stress-induced martensite. Deformation twinning and dislocation slip are of great importance in amorphization and nanocrystallization of NiTi SMA subjected to SPD based on local canning compression.

Microstructure, thermomechanical and shape memory properties of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with B addition

Microstructure, martensitic transformation, thermomechanical stability and shape memory effect of two-phase Ni–Mn–Fe–Ga high-temperature shape memory alloys with small B addition, were investigated by thermomechanical cycling. It is found that the high degree of martensite stabilization is observed in all studied alloys due to the compressive pre-strain. The results show that B addition and thermomechanical cycling facilitate the precipitation of the ordered γ' phase. The precipitation alters the chemical compositions of martensite and results in the decrease of martensitic transformation temperature of the alloys. As a result, the formation of the ordered γ' precipitate has negative effect on the thermomechanical stability of the alloys. Results further demonstrate that the martensite reorientation proceed becomes easier with small B addition due to the decrease of the yield strength, which can effectively improve shape memory effect of the alloys. However, with increasing B concentration and number of cycles, the shape memory effect gradually decreases due to the combined effects of the roles of γ phase and γ' precipitate, and thermomechanical cycling process.

Martensitic Transformation in Ni-Al-Pt High Temperature Shape Memory Alloys

In alloys Ni–39(41)Al–xPt (x = 5,10,15,20 at.%) the alloying by Pt strongly increases the Ms point. There is no direct proportionality between the Pt content and the Ms point increasing. No traces of Ni3Al were found in the alloys. The precipitation of Ni5Al3was observed in Ni–39Al–15Pt alloy after cycling through the temperature range between the room temperature and 800°C. The effect of Pt alloying on the martensitic transformation and high temperature martensitic transformation stability is governed by the competition between the martensitic transformation and formation of the Ni5Al3phase upon the cooling. Pt addition, instead of Ni, can resolve the problem of decomposition processes because the alloy composition is shifted out of the Ni3Al domain on the phase diagram and it reduces the influence of the Ni5Al3phase on the degradation of martensitic transformation.

Aging and deaging effects in shape memory alloys

Physical properties in shape memory alloys are known to change as a result of aging over time. However, undesired aging effects in the martensite phase can be eliminated once the aged martensite is brought into the parent phase, often referred to as deaging. We propose a Landau free-energy model to study the aging and deaging of martensite. These two effects can be modeled via the increase and decrease of an internal field which adopts the same ``symmetry'' as the crystal symmetry of the host lattice. Time-dependent simulations based on our model successfully reproduce many of the observed martensite aging effects, such as martensite stabilization, rubberlike behavior, the domain memory effect, and aging of the elastic modulus, as well as deaging effects in the parent phase, including elimination of martensite aging and parent phase stabilization. Furthermore, we predict the time-dependent change of the elastic modulus and the stress-strain response in the parent phase, which need to be verified experimentally.

Microstructure and martensitic transformation behavior of Ni–Mn–Fe–Ga–B high-temperature shape memory melt-spun ribbons

The microstructure and martensitic transformation behavior of (Ni56Mn17Fe8Ga19)99.8B0.2 and (Ni56Mn16Fe9Ga19)99.8B0.2 high-temperature shape memory melt-spun ribbons were studied. The results show that the rapidly melt-spun process is an effective method to refine the grain size of NiMnGa-based alloys. It is also found that the rapidly melt-spun process restrains the formation of γ phase in the as-spun ribbons. However, when the as-spun ribbons are annealed at 600°C, the γ phase preferentially forms and distributes along the grain boundaries between martensite variants. This result is different from that of the as-quenched buttons, where the γ phase randomly distributes within the martensite. These obtained results may provide useful information for improving the ductility and shape memory effect of NiMnGa-based alloys. Results further show that the martensitic transformation temperatures of the as-spun ribbons and the as-annealed ribbons are obviously higher than those of the as-quenched buttons due to an increase in the electron concentration of martensite and the martensite stabilization induced by the rapidly melt-spun process.

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile test. The results show that Si can promote the transformation of austenite (γ) to ferrite (α), enlarge the (α+γ) region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite (M/A) constituents, as well as the decomposition of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Effect of annealing on martensite stabilization due to deformation via cooling under stress in cold-worked NiTi thin wire

Most practical applications of NiTi shape memory alloys involve self-motivated shape recovery of the alloys associated with the reverse transformation. An increase of the reverse transformation temperature (martensite stabilization) implies a retardation, or complete avoidance, of the shape recovery, thus failure of the function. Therefore, understanding the mechanism of martensite stabilization in NiTi alloy is extremely important and still needs in-depth studies for its successful use. In the present paper the influence of annealing on martensite stabilization due to deformation via cooling under constant tensile stresses in cold-worked NiTi thin wires has been investigated. The annealed samples exhibit different degrees of martensite stabilization. The stabilization effect was much smaller in 550°C and 650°C annealed samples than that in 350°C and 450°C annealed ones. It increases with increasing annealing temperature up to 450°C and decreases when annealed above 450°C. The result implies that other mechanisms are involved apart from plastic deformation under such deformation mode.

Investigating the effect of the quench environment on the final microstructure and wear behavior of 1.2080 tool steel after deep cryogenic heat treatment

Deep cryogenic heat treatment is an add-on heat treatment which has been added to the conventional heat treatment to improve the wear behavior of cold worked tool steels in recent years. In this study, the effect of the different quench environments with different quench severities, including water, oil, air, −30°C ethanol and −195°C liquid nitrogen upon the final microstructure and wear behavior of the 1.2080 tool steel was investigated. Results showed that increasing the quench severity decreases the retained austenite before the deep cryogenic heat treatment, and the final microstructure shows a more homogenous carbide distribution with higher carbide percentages. Despite the low quench severity of liquid nitrogen, the samples quenched in this environment show the highest wear resistance and hardness after the ethanol-quenched samples. This behavior is a function of a very low quenching temperature and a long incubation time for the nucleation of other phases except the martensite. The wear rate and hardness of the ethanol-quenched samples shows the highest values due to the low temperature, higher thermal conductivity (as compared with the liquid nitrogen) and a less stable martensite structure. The formation of nano-sized carbide also shows an important role in the improving mechanical properties. The predominant wear mechanism is adhesive wear.

Superelasticity and compression behavior of porous TiNi alloys produced using Mg spacers

In the scope of the present study, Ni-rich TiNi (Ti-50.6 at %Ni) foams with porosities in the range 38–59% were produced by space holder technique using spherical magnesium powders as space formers. Single phase porous TiNi alloys produced with spherical pores were subjected to loading-unloading cycles in compression up to 250MPa stress levels at different temperatures in as-processed and aged conditions. It has been observed that strength, elastic modulus and critical stress for inducing martensite decrease with increasing porosity. Partial superelasticity was observed for all porosity levels at different test temperatures and conditions employed. Irrecoverable strain was found to decrease with pre-straining and with increasing test temperature. Unlike in bulk TiNi alloys a constant stress plateau has not been observed during the compression testing of porous TiNi alloys. Instead linear superelasticity with a quite steep slope allowing 5% applied strain to be recovered after pre-straining or aging was observed. Even at test temperatures higher than austenite finish temperature in as-sintered and aged condition, strain applied could not be recovered fully due to martensite stabilization resulting from heavy deformation of macro-pore walls and sintering necks. TiNi foams produced with porosities in the range of 38–51% meet the main requirements of biomaterials in terms of mechanical properties for use as bone implant.

Transformation behaviors of Ti48.5Ni48Fe2Nb1.5 dependence of annealing and thermomechanical cycling

The effect of annealing treatments and thermomechanical cycling on the transformation behaviors and shape memory effect of Ti48.5Ni48Fe2Nb1.5 shape memory alloys were investigated using electrical resistivity measurement and tensile testing. It is found that the transformation behaviors are influenced considerably by the annealing treatments. Both Ms and As increase with increasing annealing temperature and cooling rate. Martensite stabilization occurs during thermomechanical cycles, thus resulting in lower Ms and recovery ratio. Moreover, thermomechanical cycling leads to pseudo-elasticity. The yield point of stress-induced martensite (σSIM) increases with increasing cycles and pre-formation.

Transition of Crack from Type IV to Type II Resulting from Improved Utilization of Boron in the Modified 9Cr-1Mo Steel Weldment

The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423 K (1150 °C) normalized base metal, was found to be much higher than that prepared from 1323 K (1050 °C) normalized base metal because of the stabilization of lath martensite by fine M23C6 precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423 K (1150 °C) normalized base metal compared with that of the weldment prepared from 1323 K (1050 °C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature.

Martensite stabilization and thermal cycling stability of two-phase NiMnGa-based high-temperature shape memory alloys

The martensite stabilization and thermal cycling stability of four types of two-phase NiMnGa-based high-temperature shape memory alloy, including Ni56+xMn25Ga19−x (x=0,1,2,3,4), Ni56Mn25−yFeyGa19 (y=4,8,9,12,16), Ni56Mn25−zCozGa19 (z=4,6,8) and Ni56Mn25−wCuwGa19 (w=2,4,8) alloys, were investigated. It is found that the martensite stabilization is closely related to the strength of the alloy and the volume fraction of γ phase; and increases as the alloy strength decreases. It is also found that in Ni56Mn25−yFeyGa19 alloys, with increasing Fe content to 12 and 16at.%, the volume fraction of γ phase increases and the martensite stabilization decreases. The thermal cycling stability differs among different alloy systems and is related to the microstructural changes during thermal cycling and to the strength of the γ phase. Poor thermal cycling stability is observed in Ni56+xMn25Ga19−x (x>0), Ni56Mn25−zCozGa19 and Ni56Mn25−wCuwGa19 alloys due to the formation of the ordered γ′ phase and the high strength of the γ phase. Results further show that Fe addition to Ni56Mn25Ga19 alloy can broaden the (bcc+γ) two-phase region and shift it to the Ni–Ga and Ni–Mn sides, hence stabilizing the two-phase region to lower temperatures. These effects can retard the formation of the ordered γ′ phase in the Ni56Mn25−yFeyGa19 system during thermal cycling, thus leading to good thermal cycling stability.

Surface alloy depletion and martensite formation during glass to metal joining of austenitic stainless steels

Preoxidised and glass to metal (GtM) sealed austenitic stainless steels displayed a ferritic (bcc) layer near the metal/oxide interface, as determined by electron backscatter diffraction and X-ray diffraction. Through electron probe microanalysis, it was determined that this layer was depleted of alloying elements due to the oxidation and sealing processes. Characterisation of the layer morphology suggested that it formed through the martensite transformation mechanism. Thermochemical modelling with ThermoCalc also supported a martensitic transformation as opposed to diffusional ferrite formation. The composition gradient through the layer was correlated to the Eichelman and Hull empirical relationship for martensite start (Ms) temperatures. Because of Cr, Mn and Si depletion during preoxidation and glass sealing, Ms temperatures near ambient are possible in this surface region. The martensite layer was non-uniform, however, with laths extending deeper into the alloy due to stabilised growth in the material above its Ms temperature. This behaviour was characterised by image analysis techniques and discussed in terms of martensite stability and microstructural effects. Possible negative aspects of bcc phase formation on GtM seal properties are discussed, and analyses of alternative alloys 21-6-9 (tradename Nitronic 40; Armco Holding Corp., West Chester, OH, USA) and 22-13-5 (Nitronic 50) showed reduction or elimination of martensite after GtM joining.

Origin of magnetic interactions and their influence on the structural properties of Ni2MnGa and related compounds

In this work, we perform first-principles DFT calculations to investigate the interplay between magnetic and structural properties in Ni2MnGa. We demonstrate that the relative stability of austenite (cubic) and non-modulated martensite (tetragonal) phases depends critically on the magnetic interactions between Mn atoms. While standard approximate DFT functionals stabilize the latter phase, a more accurate treatment of electronic localization and magnetism, obtained with DFT+U, suppresses the non-modulated tetragonal structure for the stoichiometric compound, in better agreement with experiments. We show that the Anderson impurity model, with Mn atoms treated as magnetic impurities, can explain this observation and that the fine balance between super-exchange RKKY type interactions mediated by Ni d and Ga p orbitals determines the equilibrium structure of the crystal. The Anderson model is also demonstrated to capture the effect of the number of valence electrons per unit cell on the structural properties, often used as an empirical parameter to tune the behavior of Ni2MnGa based alloys. Finally, we show that off-stoichiometric compositions with excess Mn promote transitions to a non-modulated tetragonal structure, in agreement with experiments.