High-temperature B2 Research Articles

High Temperature B2 Precipitation in Ru-Containing Refractory Multi-principal Element Alloys

High Temperature B2 Precipitation in Ru-Containing Refractory Multi-principal Element Alloys

Influence of Current Density upon Hydrogenation on the Shape Memory Effect of Binary TiNi Alloy Single Crystals

Some results concerning the hydrogen effect at electrolytic saturation at a current density of j = 1500 and 3500 A/m2 for 3 h at room temperature on the temperature dependence of the yield stress σ0.1(T) and the shape memory effect (SME) under tension of the [011]-oriented Ti-50.55%Ni (at.%) alloy single crystals are presented. It was shown that hydrogen is in a solid solution and forms particles of titanium hydride TiH2 after hydrogenation at j = 1500 and 3500 A/m2, respectively. Both hydrogen in the solid solution and TiH2 particles led to a decrease in the Ms temperature of the onset of the forward martensitic transformation (MT) upon cooling and the Md temperature (Md is the temperature at which the stresses for the onset of the stress-induced MT are equal to the stresses for the onset of plastic flow of the high-temperature B2 phase), and increased the yield stress σ0.1 of the B2 phase at the Md temperature compared to hydrogen-free crystals. It was found that the SME under stress depends on the tensile stress level and current density. The maximum SME εSME = 10 ± 0.2% at σex = 200 MPa and εSME = 10.5 ± 0.2% at σex = 300 MPa was observed in the hydrogen-free crystals and after hydrogenation at j = 1500 A/m2, respectively, which exceeded the theoretical value of lattice deformation ε0 = 8.95% for the B2-B19′ MT in [011] orientation under tension. At j = 1500 A/m2, the physical reason for the excess of the SME of the theoretical ε0 value was due to the increase in the plasticity of B19′ martensite upon hydrogenation. At j = 3500 A/m2, εSME = 8.0 ± 0.2%, and it was less than ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension. The decrease in SME after hydrogenation at j = 3500 A/m2 was associated with the interaction of two types of B19′-martensite: oriented under stress and non-oriented, formed near TiH2 particles. It was shown that the redistribution of hydrogen in the bulk of the crystals during long-term holding for 168 h at 263 K after hydrogenation at j = 1500 A/m2 increases the SME relative to crystals without long-term holding: 3.5 times at 50 MPa and 1.8 times at 100–150 MPa. After long-term holding, εSME = 9.5 ± 0.2% at 150 MPa, which exceeds the theoretical value ε0 = 8.95% for B2-B19′ MT in [011] orientation under tension.

First-principles study of Ti50Ru50-xYx (Y = Ni, Pd and Pt) with a prospect to induce SME on stable CsCl-type Ti50Ru50 phase

Most CsCl-type intermetallics composed of group IV and VIII–XI transition metals have shape memory effect (SME), a phenomenon that occurs on a certain class of materials with an ability to undergo martensitic transformation (MT) during cooling. This advanced functional materials’ property is enabled by MT from high-temperature B2 phase of high symmetry to lower symmetry phases such as L10, B19 or B19’ upon cooling. Peculiarly, Ti50Ru50 with similar ordered B2 at high temperature remains ordered and stable with no phase transition down to room temperature. In this study, first-principles calculations based on density functional theory (DFT) are used to investigate the structural, thermodynamic and electronic properties of the stable Ti50Ru50 compound by systematically substituting part of the Ru atoms with ductile group 10 metal (Ni, Pd and Pt). This is an attempt to destabilizing B2 phase at 0 K through Ti50Ru50-xYx ternary alloying to promote MT that could yield SME.Graphical

Nanostructure-dependent nanobubble drag reduction on NiTi self-adaption surface

Under the inspiration of visualized nanobubbles associated with rugged interface, molecular simulations are performed to study thermally induced phase transition in NiTi shape-memory alloys. The perfect reversibility between the cubic austenite B2 and monoclinic martensitic B19′ phases showed two types of morphological structures in the nanoscale. The nanostructures were established as the wall of the nanochannel, which was used to predict the nanobubble flow behavior in the ternary system. Results indicated that the interface with low-temperature B19′ exhibited attractiveness to trap the nanobubble, resulting in low drag coefficient. Conversely, the interface with high-temperature B2 repulsed the nanobubbles as the bulk one, resulting in high drag coefficient. This study provides foresight to invest in shape memories for nanostructure temperature response, which helps realize nanobubble drag reduction.

First-principles study on the effect of Ni addition on the stability of B2 Ti50Ru50 – a supercell approach

Shape memory alloys (SMA's) are a special kind of metal alloys with unique structural properties, such as shape memory effect (SME) and pseudo-elasticity (SE) that were well-observed in TiNi alloy, which commercially trades as Nitinol. This polymorphic crystal behaviour arose from the reversible phase transition between an ordered high-temperature B2 and low-temperature martensite phase (L10/B19/B19′). Due to the high ductility of Ru, this work employed a supercell approach to track the effect of nickel addition on highly stable B2-Ti50Ru50 with the intent to stimulate SME. Thermodynamic, mechanical, electronic structure and lattice dynamic characteristics were estimated using first-principles calculations. The phonons spectra of B2 Ti50Ru50 consisted of only positive frequencies, but the addition of Ni above 18.75 at. % resulted in positive and negative frequencies, indicative of a possible martensitic transformation, which is a strong character for SME.

CHANGES IN THE STRUCTURAL-PHASE STATE AND DISLOCATION DENSITY OF TI49.8NI502 ALLOY DEPENDING ON THE ISOCHRONOUS ANNEALING TEMPERATURE AFTER SEVERE PLASTIC DEFORMATION BY ABC PRESSING AT 573 K

X-ray diffraction studies were conducted to examine changes in the structural-phase state and dislocation density of Ti498Ni50.2 alloy depending on the isochronous annealing temperature after severe plastic deformation by abc pressing at 573 K. The total true strain achieved in the alloy samples during abc pressing was e = 9.55. Isochronous annealing was carried out for 1 hour at 573, 673, 773, 873 and 973 K. Room-temperature analysis of all studied samples revealed the coexistence of R and B19' phases, whose relative fractions varied with annealing temperature. The high-temperature B2 phase was not detected. It was found that the most rapid decrease in the dislocation density, which was measured at 393 K (in the B2 state), occurs after annealing the samples at 673 and 773 K. After annealing at 773 K, the dislocation density reaches a minimum, and its value decreases by more than an order of magnitude compared to the value immediately after abc pressing. In the same temperature range there is a significant decrease in the root-mean-square B2 lattice microdistortions (e2)1/2 and a slight increase in the average size of coherent scattering regions. After abc pressing and after isochronous annealing, the main contribution to the physical peak broadening is made by the B2 lattice microdistortions, while the contribution from coherent scattering regions is insignificant. The obtained results show that intense recrystallization in Ti49 8Ni50.2 alloy after abc pressing at 573 K begins at T > 773 K.

Phase Composition, Microstructure, Multiple Shape Memory Effect of TiNi50−xVx (x = 1; 2; 4 at.%) System Alloys

The phase composition, microstructure, and multiple shape memory effect of TiNi50-xVx alloys were studied in this work. The phase composition of the TiNi50-xVx system is the TiNi matrix, spherical particles of TiNiV, the secondary phase Ti2Ni(V). Doping of TiNi alloys with vanadium atoms leads to an increase in the stability of high-temperature B2 and rhombohedral R-phases. An increase in the atomic volume with an increase in the concentration of the alloying element V from 1 to 4 at.% was established. Vanadium doping of the Ti-Ni-V system alloys leads to an increase in the temperature interval for the manifestation of the multiple shape memory effect. It has been established that the value of the reversible deformation of the multiple shape memory effect both during heating and during cooling increases linearly from 2 to 4% with an increase in the vanadium concentration.

Microstructural evolution and effect of heat treatment on the precipitation and mechanical behavior of Al0.7CoCrFeNi alloy

The microstructural features and promising properties exhibited by Al0.7CoCrFeNi have led to extensive studies on the alloy. The current work discusses the microstructural evolution, thermal stability and mechanical behavior of the alloy. CALPHAD studies predicated the alloy to be near eutectic with pro-eutectic BCC and eutectic FCC and BCC/B2. However, experimental studies displayed the FCC phase to evolve from a high-temperature B2. Interfacial energy, chemical inhomogeneity and cooling rate were attributed to play a key role in the microstructural evolution and phase stability. Heat treatment resulted in the dissolution of BCC and the formation of Al-Ni-rich B2 phase. During solidification and heat treatment, Widmanstätten FCC formed and coarsened via superledge mechanism. Further, thermal exposure resulted in lamellar instability and localized globularization of the B2 phase. The heat-treated alloy displayed excellent resistance to thermal softening when compared to the cast alloy. Further, the thermally exposed alloy exhibited a superior work hardening rate.

Behavior of Inelastic and Plastic Strains in Coarse-Grained Ti49.3Ni50.7(at%) Alloy Deformed in B2 States

The regularities of the change in inelastic strain in coarse-grained samples of the Ti49.3Ni50.7 (at%) alloy are studied when the samples are given torsional strain in the state of the high-temperature B2 phase. During cooling and heating, the investigated samples underwent the B2–B19′ martensite transformation (MT); the temperature of the end of the reverse MT was Af = 273 K. It was found that at the temperature of isothermal cycles “loading-unloading” Af + 8 K, when the specimen is assigned a strain of 4%, the effect of superelasticity is observed. With an increase in the torsional strain, the shape memory effect is clearly manifested. It is assumed that the stabilization of the B19′ phase in unloaded samples is due to the appearance of dislocations during deformation due to high internal stresses at the interphase boundaries of the B2 phase and the martensite phase during MT. The appearance of dislocations during the loading of samples near the temperatures of forward and reverse MT can also be facilitated by the “softening” of the elastic moduli of the alloy in this temperature range. At a test temperature above Af + 26 K, the superelasticity effect dominates in the studied samples.

Low-Temperature Mechanical Instabilities Govern High-Temperature Thermodynamics in the Austenite Phase of Shape Memory Alloy Constituents: Ab Initio Simulations of NiTi, NiZr, NiHf, PdTi, and PtTi

Recent studies on the constituent binaries of common NiTi-based shape memory alloys (NiTi, NiZr, NiHf, PdTi, PtTi) have led to questions regarding the accuracy of current phase diagrams, including the existence of theoretical phases, and the atomistic nature of stability in observed martensitic and austenitic phases. These issues are investigated here with ab initio molecular dynamics. Theoretically identified ground state structures are found to become mechanically unstable well below room temperature, while the experimentally determined martensite structures are found to be stabilized by anharmonic effects at elevated temperatures. The martensite phases of all binaries are found to transition to a common austenite structure (cubic B2), with transition temperatures in agreement with available experiments. Interestingly, the B2 structures are unstable at low temperature but are stabilized by large anharmonic effects that also govern the martensitic transition. An observed signature of this anharmonicity is the transitory symmetry breaking of B2 to a distorted, glassy structure known as B2’. The energy of the observed B2’ structure is quantified at low temperatures and, across the binaries, is found to be linearly correlated to the anharmonic energy and entropy of transition from the martensite phases to high-temperature B2. The implication of this finding is the properties of the low-temperature B2’ structure govern anharmonicity and ultimately the high-temperature transitions to B2. Means of utilizing these results to simplify the characterization of the austenite thermodynamics and associated transitions important to the shape memory effect are discussed. A reduced order model is presented that captures the competition between crystal lattice energy differences (B2 vs martensite) and the degree of anharmonicity characterized by the B2-B2’ energy differences for these sytems.

Stacking faults and alternate crystal structures for the shape-memory alloy NiTi

We use ab initio calculations to study the role of stacking faults in connecting the high-temperature B2 and the theoretically predicted low-temperature B33 NiTi phases. In contrast with prior work, we describe the $\mathrm{B}2\ensuremath{\rightarrow}\mathrm{B}33$ phase transformation in terms of alternate bilayer shifts by $\frac{1}{2}$ [100] on the ${(011)}_{\mathrm{B}2}$ plane, obtaining a viable pathway; the same mechanism could also work with the B19 parent phase. We then examine B33-like structures built from alternate stacking sequences of B19 bilayers, constructed to have monoclinic tilt angles close to the experimentally reported NiTi B19\ensuremath{'} martensite, and find four low-energy stacking-fault variants with energies 5.8--8.5 meV/atom above the calculated B19\ensuremath{'} martensite structure, suggesting that such structures might appear as a part of the NiTi martensite phase at low temperatures. Investigating further the occurrence of specific coordinated planar shifts in NiTi systems, we report a dynamically stable NiTi B27 phase and find that it is only 1.2 meV/atom above the calculated B33 ground-state structure, thus having a potential to also play a role in NiTi martensitic phase transformation.

Glass-forming ability, phase formation and mechanical properties of glass-forming Cu-Hf-Zr alloys

The influence of Hf additions on the glass-forming ability (GFA), phase formation and mechanical properties of Cu50HfxZr50-x (x = 2,5,10,20 at.%) alloys has been systematically investigated. We report on a distinct correlation between phase formation and GFA of Cu50Zr50-based alloys. Increasing additions of Hf reduce the thermal stability of the high-temperature B2 Cu(Hf,Zr) phase, while the thermal stability of the corresponding undercooled melt is enhanced. The GFA of these alloy series gradually raises up to 10 at.% Hf, whereas at 20 at.%Hf, the GFA is drastically lowered, since the B2 Cu(Hf,Zr) phase becomes unstable and the precipitation of the low-temperature equilibrium phases is favoured. This interrelation determines the microstructure and results in the formation of Cu-Hf-Zr-based bulk metallic glass composites. These composites not only show appreciable macroscopic plastic strain, but also high yield strength.

Control of microstructural characteristics and martensitic transformation behavior of Ti–Ni–Cu alloys by Pt doping

The effect of the substitution of Pt for Cu on the microstructure and martensitic transformation behaviors of Ti–Ni–Cu alloys was investigated systematically. The solution-treated Ti–Ni–Cu–Pt alloys were found to consist of the B19 martensite and Ti2(Ni,Cu,Pt) phases at room temperature. The (011) compound twin was more frequently observed than the (111) type I twin in the Ti–Ni–Cu–Pt alloys owing to the lower shear strain and shear angle of the (011) compound twin during the transformation process. During heating, the lower temperature single B19 martensite transformed in one stage into high-temperature B2 austenite. Conversely, the B2 austenite was transformed directly into B19 martensite during cooling. As the Pt content increased from 1.0 at.% to 3.0 at.%, the transformation temperatures first decreased and then increased. In addition, a narrower thermal hysteresis and perfect thermal cycling stability can be obtained by tailoring the Pt content. The excellent thermodynamic characteristics can be attributed to the lower frictional work and good geometric compatibility.

Influence of substitution of Fe by Co on structural and magneto-mechanical properties of Fe-27Ga alloy

This paper investigates the influence of substitution of 18 at.% of Fe by Co in Fe-27Ga alloy as an effort to improve magneto-mechanical coupling. Study of phase transitions by neutron diffraction reveals that the formation of close packed phases is suppressed in the as-cast Fe-18Co-27Ga alloy. The degree of D03 ordering in the as-cast state for the Fe-18Co-27Ga alloy is lower than that for the Fe-27Ga alloy as estimated by intensities of super-lattice reflections of neutron diffraction spectra in these alloys. Investigation of magnetic properties by vibrating sample magnetometry and magnetic thermal gravimetry proved that the high-temperature B2 phase in the Fe-18Co-27Ga is ferromagnetic, whereas that of the Fe-27Ga is non-ferromagnetic. The Curie temperature (TC) here and later in the text in slowly-cooled Fe-18Co-27Ga is 726 °C, which is higher than that (650 °C) of Fe-27Ga. The mechanical and magneto-mechanical properties samples were studied through impulse excitation of vibration (IEV) method by the Resonance Frequency and Damping Analysis (RFDA) device. The results demonstrate that substitution of 18 at.%Fe by Co in Fe-27Ga alloy could enhance both the elastic, E0, and shear moduli, G0, at zero filed and the total (flexure mode) damping capacity, (ΔW/W)F0, under the measuring conditions.

Anomalous re-ordering of Fe3Al disordered by high pressure torsion deformation

Severe plastic deformation of bulk DO3 ordered Fe-30 at.%Al was carried out using high pressure torsion to make a chemically disordered nanocrystalline alloy. To achieve re-ordering the alloy was heated to 250, 370 and 500 °C. Transmission electron microscopy investigations reveal that in contrast to the phase diagram B2 ordering occurs at low temperatures. Upon further heating, after the B2 order is already well restored, nanosized DO3 ordered domains grow within large B2 ordered regions. It is proposed that the anomalous re-ordering is triggered by deformation-induced vacancies and other defects leading to the formation of the high-temperature B2 ordered phase.

Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ↔ B19’ martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19’ phases are measured, and the microstructure of the B19’ martensite is analyzed.

Ab initiostudy of point defects in NiTi-based alloys

Changes in temperature or stress state may induce reversible B2$\leftrightarrow$(R)$\leftrightarrow$ B19' martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich aging atmosphere on the subsequent B2 austenite $\leftrightarrow$ B19' martensite transformation path. In this paper, we employ Density Functional Theory to study properties of Ar, He, and H interstitials in B2 austenite and B19' martensite phases. We show that H interstitials exhibit negative formation energies, while Ar and He interstitials yield positive values. Our theoretical analysis of slightly Ni-rich Ni--Ti alloys with the austenite B2 structure shows that a slight over-stoichiometry towards Ni-rich compositions in a range 51--$52\,\text{at.%}$ is energetically favorable. The same conclusion holds for H-doped NiTi with the H content up to $\approx6\,\text{at.%}$. In agreement with experimental data we predict H atoms to have a strong impact on the martensitic phase transformation in NiTi by altering the mutual thermodynamic stability of the high-temperature cubic B2 and the low-temperature monoclinic B19' phase of NiTi. Hydrogen atoms are predicted to form stable interstitial defects. As this is not the case for He and Ar, mixtures of hydrogen and the two inert gases can be used in annealing experiments to control H partial pressure when studying the martensitic transformations in NiTi in various atmospheres.

Controlling grain boundary character distribution of high-temperature B2 phase in Ti–Ni–Fe alloy

A method is proposed for controlling the grain boundary character distribution of high-temperature B2 phase in Ti–Ni–Fe alloys. The method utilizes the lattice correspondence between the deformation twins in B19′ martensite and the residual planar defects in the B2 parent phase. The optimal conditions for recrystallization and additional annealing of the tensile-deformed specimen with residual strain of about 20% were determined to be 873 K for 0.9 ks and 773 K for 3.6 ks, respectively. In the specimen annealed under these optimal conditions, over 90% of grain boundaries were coincidence boundaries with Σ ≤ 27. In particular, 33% of the grain boundaries were low-angle boundaries, that is, Σ1 boundaries. The atomic structures of the Σ9 coincidence grain boundaries and the Σ1 low-angle boundaries were observed by high-angle annular dark field scanning transmission electron microscopy. The atomic structure of these boundaries was explained well by theoretical predictions.

Strategy for pinpointing the formation of B2 CuZr in metastable CuZr-based shape memory alloys

In the present work we systematically investigated the formation of high-temperature B2 CuZr intermetallics in 36 metastable CuZr-based alloys. The results demonstrate that a B2 CuZr solid-phase transformation exists at high temperature. Based on continuous heating transformation diagrams the CuZr-based alloys can be classified into three different types, related to the precipitation and stability of the B2 phase. A new prediction strategy for the formation of different sized CuZr-based shape memory bulk metallic glass composites (BMGC) is proposed by considering the competition between vitrification, the precipitation of metastable B2 CuZr and room-temperature equilibrium phases. These insights are condensed into a new parameter, K = T f/ T L, where T f and T L are the final temperature of the B2 CuZr phase transformation during heating and the liquidus temperature of the alloy, respectively, which is suggested to predict and select a compositional region of different sized BMGC. This new approach could be of great significance for the development of metastable CuZr-based shape memory alloys with enhanced deformability.

Microstructural Studies of Multiphase (Zr,Ti)(V,Cr,Mn,Co,Ni)2 Alloys for NiMH Negative Electrodes

ABSTRACTThe solidification microstructures of six Laves-based (Zr,Ti)(TM,Ni)2 alloys (TM= V,Cr, Mn,Co) intended for use as novel negative electrodes in Ni-metal hydride batteries were studied here; these alloys often have their best electrochemical properties when in the cast state. Solidification occurs by dendritic growth of a hexagonal C14 Laves phase followed by peritectic solidification of a cubic C15 Laves phase and formation of a cubic B2 phase in interdendritic regions. The observed sequence of Laves phase C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. The paper also examines the complex internal structure of the interdendritic grains formed by solid-state transformation, which plays an important role in the electrochemical charge/discharge characteristics. By studying one alloy it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-type and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni-type intermetallics, and consequently lead to a multivariant structure. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.