BCC-FCC Phase Transformation Research Articles

High-pressure phase transition in 3-D printed nanolamellar high-entropy alloy by imaging and simulation insights

We report on the high-resolution imaging and molecular dynamics simulations of a 3D-printed eutectic high-entropy alloy (EHEA) Ni40Co20Fe10Cr10Al18W2 consisting of nanolamellar BCC and FCC phases. The direct lattice imaging of 3D-printed samples shows the Kurdjumov–Sachs (K–S) orientation relation {111} FCC parallel to {110} BCC planes in the dual-phase lamellae. Unlike traditional iron and steels, this alloy shows an irreversible BCC-to-FCC phase transformation under high pressures. The nanolamellar morphology is maintained after pressure cycling to 30 GPa, and nano-diffraction studies show both layers to be in the FCC phase. The chemical compositions of the dual-phase lamellae after pressure recovery remain unchanged, suggesting a diffusion-less BCC–FCC transformation in this EHEA. The lattice imaging of the pressure-recovered sample does not show any specific orientation relation between the two resulting FCC phases, indicating that many grain orientations are produced during the BCC–FCC phase transformation. Molecular dynamics simulations on phase transformation in a nanolamellar BCC/FCC in K–S orientation show that phase transformation from BCC to FCC is completed under high pressures, and the FCC phase is retained on decompression aided by the stable interfaces. Our work elucidates the irreversible phase transformation under static compression, providing an understanding of the orientation relationships in 3-D printed EHEA under high pressures.

Probing phase transformation and dislocation evolution in high-entropy alloy under cyclic loadings

Stress-induced FCC-BCC phase transformation plays a crucial role in the mechanical behaviors of high-entropy alloys (HEAs). While there has been extensive research on this transformation during monotonic deformation, studies on fatigue behavior are extremely limited. Here, we use molecular dynamics simulations to investigate phase transformation and dislocation evolution in HEAs under strain-controlled symmetric tension–compression cycles. Our results show that cyclic deformation behavior is sensitive to strain amplitude, revealing three distinct cyclic responses. Notably, a progressive FCC-BCC phase transformation process occurs at a high strain amplitude of 4.8%. Grain boundaries and their triple junctions are identified as preferred sites for phase transformation under cyclic loading conditions. These findings provide valuable atomic-scale insights for understanding fatigue deformation in HEAs with transformation-induced plasticity.

Insights into orientation-dependent plasticity deformation of HfNbTaTiZr refractory high entropy alloy: An atomistic investigation

HfNbTaTiZr refractory high entropy alloy has emerged as a prospective structural material suitable for high-temperature applications owing to its remarkable combination of high strength, good tensile ductility and excellent high-temperature properties. However, the insufficient understanding of the mechanical responses of this refractory high entropy alloy has hindered the improvement of performance in alloy design and potential for engineering applications. Therefore, the orientation-dependent tensile behaviors of HfNbTaTiZr refractory high entropy alloy are investigated from nanoscale using molecular dynamics simulations. The simulation results show that the mechanical responses under uniaxial tension are strongly correlated to the crystallographic orientation with respect to the loading direction, and the highest Young's modulus and yield strength are achieved along [111] direction. In addition, the deformation twinning and phase transformations are charactered. The tensile behavior oriented along [001] direction is dominated by the BCC-FCC phase transformation, while that oriented along [110] or [111] direction by the BCC-HCP phase transformation. The results reveal the critical role of tensile direction in generating specific crystalline microstructures under high-strain-rate loading conditions, which can enlighten new design strategy in engineering refractory high entropy alloys with specific orientations for extreme environments.

Structure and magnetism of AlCoCrCuFeNi high-entropy alloy

We report on the investigation of magnetic and structural properties of AlCoCrCuFeNi, which is known to crystallize in a dual phase solid solution: the face-centred cubic (FCC) or the body-centred cubic (BCC). The results of neutron (NPD) and synchrotron powder diffraction (SXRD) allow to partially resolve magnetic information coming from BCC and FCC phases, which is impossible in the bulk magnetic measurements. Electron diffraction (PED) revealed that AlCoCrCuFeNi forms dendritic microstructure with the Cu-rich FCC phase and the Ni-rich BCC phase. Lattice parameters obtained from PED method are in good agreement with parameters obtained after refinement on the basis of powder X-ray diffraction measurements. The local crystal and electronic structure around Co was studied using Co K X-ray Absorption Spectroscopy (XAS). The magnetic measurements show that AlCoCrCuFeNi reveal a ferromagnetic transition at about 330 K and displays magnetic hysteresis loop at the room temperature. Results from NPD suggest that the magnetic moment is mostly located in the BCC subsystem. The alloy shows soft magnetic properties. Saturated magnetizations (Ms), remanence ratio (Mr/Ms) and coercivity (Hc) of the cast are estimated to be 45.10 emu/g, 5.1 % and 56 Oe at 300 K, respectively. Finally, the BCC-FCC phase transformation up to 673 K (400 °C) was investigated using temperature dependent NPD, where a possible second BCC phase was identified.

FCC-BCC phase transformation induced simultaneous enhancement of tensile strength and ductility at high strain rate in high-entropy alloy

FCC-BCC phase transformation-induced plasticity (TRIP) has been extensively studied in high-entropy alloys (HEAs) to customize their mechanical properties through compression/tension loading or thermal fabrication processes. In this study, we employed a combination of molecular dynamics (MD) and Monte-Carlo (MC) simulations to investigate the effects of TRIP on the uniaxial strain tensile deformation of Co25Ni25Fe25Al7.5Cu17.5 HEA. Our results demonstrate that a complete FCC-BCC phase transformation occurs, in accordance with the N-W relationship, resulting in a simultaneous enhancement of strength and ductility. This is attributed to the HEA's significantly low stacking fault energy and pronounced lattice distortion (LD). However, short-range order (SRO) acts as an obstacle on atomic sliding, which further reduces the degree of phase transformation, leading to an increase in Young's modulus but a decrease in ductility. Furthermore, an increase in strain rate can promote the occurrence of the phase transformation to a certain extent but also leads to an increase in the degree of disorder defects. We also found that the HEA maintains excellent thermal stability up to 900 K, but the amount of phase transformation decreases with increasing initial temperature. Our systematic study of FCC-BCC transformation, considering the effects of SRO, LD, strain rate, and temperature, provides insights into tailoring the mechanical properties of HEAs for practical design purposes.

Crystallographic analysis of Ga+ irradiation-induced phase transformation in austenitic stainless steels

Focused ion beam processing with Ga+ ion sources is widely applied for nanoscale fabrication of materials and preparation of specimens for advanced microstructural observation by methods such as transmission electron microscopy and 3D atom probes. It is important to suppress specimen damage and microstructural changes associated with Ga+ irradiation, but limited information is available on the mechanism of microstructural changes. In this study, we explored the mechanism of Ga+ irradiation-induced phase transformation of an austenitic stainless steel. The irradiation-induced fcc-bcc phase transformation was characterized by microstructural observations of surfaces and cross sections of specimens and analyses of crystallographic orientation relationships. The results of microstructural analysis showed that the formation of the irradiation-induced bcc phase on the specimen surface increased with increasing irradiation dose. At the same irradiation dose, the bcc transformation initiated sooner, and the total amount of transformation was higher, for irradiation of the (111)fcc planes than the (001)fcc planes. Microstructural analysis of cross-sections showed that irradiation (8.0 × 1016 ions/cm2) of the (001)fcc and (111)fcc planes caused formation of a transformation phase to depths of 100 nm and 60 nm, respectively; the bcc transformation extended to deeper regions in the (001)fcc planes than the (111)fcc planes. In addition, these transformation areas extended over a wider area than the areas of high Ga concentration. The crystallographic orientation relationship before and after the transformation was not a specific orientation relationship, such as Kurdjumov–Sachs or Nishiyama-Wasserman, and its distribution differed depending on the direction of Ga+ irradiation. The mechanism of fcc-bcc phase transformation of SUS304 steel by Ga+ ion irradiation was explained by chemical and stress effects due to the injection of Ga+ ions. Furthermore, the transformation behavior, which depended on the irradiation direction, was discussed in terms of the (111)fcc orientation angle relative to the irradiation direction.

Deformation Behavior of Two-Phase Gradient Nanograined Fe95Ni5 Alloys under Different Types of Loading

In this paper, we used molecular dynamics simulations to study the atomic mechanisms of phase transformations, plasticity features, and mechanical properties of two-phase Fe95Ni5 (at. %) samples with a gradient nanograined structure under uniaxial deformation and shear. The simulated samples with a uniform distribution of Ni atoms are composed of fcc grains from 10 to 30 nm in size, which in turn contain bcc interlayers in the form of lamellae of various distribution and size. It was shown that uniaxial loading or shear causes the bcc-fcc phase transformation in the lamellae. In the vast majority of cases, phase transformations are initiated at the junction of lamellae and grain boundaries. Deformation-induced phase transformations in lamellae occur at the front of bands propagating from grain boundaries. Grains larger than ~15 nm can have several bands or regions with differently orientated fcc lattices, whose meeting results in grain fragmentation. It was found that the atomic volume increases abruptly during the bcc-fcc structural phase transformation. The Kurdyumov–Sachs orientation relation is valid between the initial bcc and formed fcc structures. It was shown that the volume fraction and spatial distribution of the bcc phase significantly affect the yield stress of the sample. The yield stress can be increased by forming the bcc phase only in large-grained layers. This behavior is associated with the fragmentation of large grains, and consequently with grain refinement, which, in accordance with the Hall–Petch relation, improves the strength of the material.

The BCC-FCC Phase Transformation Pathways and Crystal Orientation Relationships in Dual Phase Materials From Al-(Co)-Cr-Fe-Ni Alloys

The alloy system Al-(Co)-Cr-Fe-Ni contains compositional ranges where a solid state BCC-FCC phase transformation leads to dual-phase materials composed of face-centered cubic (FCC) and body-centered cubic (BCC) phases with nearly equal volume fraction. The microstructure arising from this transformation at slow cooling rates is the classical Widmanstatten structure, with FCC-laths and colonies growing from grain boundaries into the parent BCC-B2 grain. Very distinct microstructures are obtained, when Widmanstatten growth is kinetically suppressed e.g. during continuous cooling with high cooling rates. These novel microstructures are associated with the spinodal decomposition of the parent BCC-B2 such that FCC growth occurs during the spinodal decomposition or upon annealing from a metastable, fully spinodal state. We review the microstructures at case as function of the imposed cooling regimes for the Co-free medium entropy alloy AlCrFe2Ni2. One of them, termed ultrafine vermicular microstructure, involves a characteristic and novel crystal orientation relationship (OR) between FCC and BCC. We identify the common planes and directions of this OR using electron backscatter diffraction maps to be {111}^FCC∥{121 }^BCC and 〈1 01〉^FCC∥〈1 01 〉^BCC, respectively. Embedded is a second OR with {13 1 }^FCC∥{1 03}^BCC and 〈101〉^FCC∥〈010〉^BCC. We further show that the vermicular FCC phase contains a high amount of lattice strain and sub-grain boundaries with disorientation angles in the range from 2 to 12°, as a result of the solid-state phase transformation.

Discontinuous plastic flow coupled with strain induced fcc–bcc phase transformation at extremely low temperatures

A popular class of materials massively used at cryogenic temperatures comprises the stainless steels of different grades, such as 304, 304L, 316, 316Ti, 316L, 316LN etc. Such materials are metastable at extremely low temperatures, and usually undergo plastic strain induced phase transformation. In addition, these materials applied in the proximity of absolute zero exhibit the so-called discontinuous (intermittent, serrated) plastic flow (DPF). It consists in frequent, abrupt drops of stress against strain, characterized by increasing amplitude of the stress oscillations. Strong coupling between both phenomena: DPF and phase transformation is observed. Recent experiments performed by means of stainless steel samples tested in liquid helium (4.2 K) clearly indicate strong strain localization during DPF, in the form of shear bands propagating along the sample. However, as soon as the phase transformation process takes place, the motion of shear bands is hindered by formation of secondary phase. A physically based constitutive model developed in the present paper reflects coupling between the discontinuous plastic flow and the plastic strain induced phase transformation in the temperature range 0–T1. The model involves nonlinear mixed hardening, that occurs during the 2nd stage of each serration (stress–strain oscillation). The hardening is based on two mechanisms: interaction of dislocations with the inclusions of secondary phase, evolution of tangent stiffness operator due to changing proportions between the primary and the secondary phases. Nonlinear hardening strongly increases the stress level during each serration, which affects production of the internal lattice barriers, and the amount of the accumulated plastic strain. This, in turn, affects intensity of the phase transformation (full coupling). The constitutive model and its numerical version allow to reproduce the observed serrations, which is crucial for its application in the design of components operating at extremely low temperatures.

Radiation-induced bcc-fcc phase transformation in a Fe[sbnd]3%Ni alloy

The issue of neutron irradiation embrittlement of Reactor Pressure Vessel steels must be considered for Nuclear Power Plant life extension. This phenomenon partly arises from the existing interactions between dislocations and nanometric clusters composed of Cu, P, Si, Mn and Ni. The latter alloying element, playing a key role in the evolution of solute enriched clusters under irradiation, is the focus of this publication. To assess the effect of Ni on microstructure evolution under irradiation, particle accelerator based experiments were conducted. An under-saturated Fe3at.%Ni alloy was irradiated with self-ions, at 673 K, up to ∼1.2 dpa. Then, the microstructural damage was characterized, at the atomic scale, by conventional Transmission Electron Microscopy, Scanning Transmission Electron Microscopy coupled to Energy Dispersive X-ray Spectroscopy and Electron Energy Loss Spectroscopy, while chemical features were investigated by Atom Probe Tomography. Informations obtained by combining these coupled techniques provide evidence for the formation of a FCC phase, containing 25 at.%Ni, which can be either the disordered γ phase or the ordered L12 type Fe3Ni phase. The metastable or stable state of this FCC phase is discussed in the light of what is known from the literature. It is the first time that this BCC-FCC phase transformation is observed in an under-saturated α-FeNi alloy and this likely occurred via a Radiation Induced Precipitation (RIP) mechanism. Ni atom segregation is observed on cavities, dislocation lines and dislocation loops. The latter constitute nuclei for precipitates, leading to the formation of an additional segregation site for Ni: the precipitate FCC - matrix BCC nearly coherent interface. Similar mechanisms are argued to be operating also in high Ni RPV steels under neutron irradiation.

Electroplated Fe-Co-Ni films prepared in ammonium-chloride-based plating baths

We electroplated Fe-Co-Ni films in ammonium-chloride-based plating baths, and investigated the effect of the Co content on the magnetic properties and the structural ones of the as-plated films. The coercivity increased abruptly when the Co content become more than 60 at.%. As the rough surfaces were observed in the high Co content region, we considered that degradation of the surface is a factor of the abrupt increase in the coercivity. From the XRD analysis, we found that another factor of the abrupt increase is fcc-bcc phase transformation, and concluded that we need to keep the fcc structure to obtain Fe-Co-Ni films with low coercivity.

Coupled constitutive model of damage affected two-phase continuum

A broad class of metastable materials, including selected alloys (e.g. stainless steels) used for applications in radiation environment (particle accelerators), is characterized by simultaneous occurrence of the plastic strain driven phase transformation and evolution of nano/micro damage. Plastic flow in such materials is usually accompanied by dynamic evolution of microstructure, resulting from the strain induced fcc-bcc phase transformation. Two-phase continuum is composed of austenitic matrix (fcc) and martensitic inclusions (bcc), represented by type Eshelby ellipsoidal entities embedded in ductile matrix. The matrix remains entirely plastic, and contains micro-cracks and micro-voids representing ductile damage. On the other hand, the inclusions are characterized by much higher yield stress and their behavior is generally brittle. Thus, brittle damage develops in the inclusions. The origin of damage is mechanical (manufacturing and load induced defects), and related to the source of radiation (primary or secondary particles flux). Among the lattice defects induced by radiation, the clusters of nano/micro voids are accounted for. The constitutive model takes into account the evolution of mechanically and radiation induced nano/micro damage in the presence of microstructure evolution, reflected by the plastic strain driven dynamic change of proportions between the matrix and the inclusions. The model is multiscale since the processes that occur at different scales (micro, meso, macro) are addressed, and coupled, since both phenomena: phase transformation and damage are described by coupled equations. Application to irradiated corrugated shells, components of thermo-mechanical compensation systems, are presented.

Orientation Relationships in Al0.7CoCrFeNi High-Entropy Alloy.

A detailed microstructural evaluation was executed on the crystallographic texture as well as the mechanisms for nucleation, phase transformation, and grain growth in a Al0.7CoCrFeNi high-entropy alloy. The microstructure and crystallographic orientations were characterized by electron backscatter diffraction, and the chemical composition variations by energy-dispersive X-ray spectroscopy. The cast Al0.7CoCrFeNi alloy started in the BCC phase and partially transformed into the FCC phase. It was found that the Pitsch orientation relationship (OR) dominates the nucleation mechanism of the FCC phase; however, deviations with respect to the Pitsch OR are observed and are attributed to the differently sized atoms forming an ordered B2 phase in the alloy causing lattice distortions. The dual phase BCC-FCC microstructure contains FCC Widmanstätten plates oriented parallel to the {110}BCC planes of the parent grain. It was found that the crystal orientation distribution after the BCC-FCC phase transformation is confined and is explained as a product of the governing mechanisms.

Advanced High Strength Steels: Improved Properties by Design of Textures and Microstructures

Texture and microstructure formation during annealing of a 75% cold rolled AHS steel, reheated at a rate of 10°C/s to ∼3000°C/s were studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. It was found that at ultra-high heating rates, the bcc-fcc phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase, which is the product of intercritical austenite transformation in as-quenched samples, i.e., after the double α-γ-α’ transformation, is similar to the initial cold rolling texture, which is an indication for the texture memory effect. After ultra-fast reheating with average reheating rates of 1000°C/s and higher, a significant grain refinement was observed with an average calculated ferrite grain diameter of ∼2 μm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the heating rate. The same effect is observed in 75% cold rolled quenching and partitioning steels for heating rates of 400°C/s, 800°C/s and 1000°C/s. In both cases the combination of fine grains, and weak RD/ND-fiber textures produce an increase of strength and ductility compared to material annealed using a conventional heating rate of ∼10°C/s.

The FCC to BCC phase transformation kinetics in an Al0.5CoCrFeNi high entropy alloy

In this paper, thermal expansion method is used to characterize the phase transformation process of Al0.5CoCrFeNi high entropy alloy (HEA), and the FCC-BCC phase transformation kinetics, microstructure and mechanical properties are investigated. Results show that during continuous heating process, three stages of phase transformation process for Al0.5CoCrFeNi HEA are characterized by thermal expansion curves, and the third stage is corresponding to FCC-BCC phase transformation. The FCC-BCC phase transformation kinetics is analyzed using the thermal expansion experiments carried out at different heating rates. The FCC-BCC phase transformed fraction, f, as a function of temperature is determined from thermal expansion curve. The activation energies, Q, determined by KAS model show an increasing trend which is from 144.4 kJ/mol at initial stage (f = 0.2) to 209.2 kJ/mol when f = 0.8, indicating the barrier for transformation is increasing. The Avrami exponent, n, determined by modified JMA model, varies with transformed fraction, indicating the FCC-BCC phase transition firstly is interface controlled with decreasing nucleation rate (f < 0.05), and changed to diffusion controlled with an increasing nucleation rate (f < 0.15), then diffusion controlled with decreasing nucleation rate. The microstructure and mechanical properties of the corresponding FCC-BCC phase transformation process are also studied.

Strain localization during discontinuous plastic flow at extremely low temperatures

• The paper deals with an extended, physically based, multiaxial constitutive model of discontinuous plastic flow, including new feature of strain localization at near 0 K temperatures in polycrystalline materials. • Another new feature defines the drop of stress being entirely determined by dislocations density before and after the serration. • Regular way of nucleation and free unconstrained glide of slip bands is observed within the sample, in the absence of phase transformation. • Contrary to former views, strong coupling of discontinuous plastic flow with fcc-bcc phase transformation in metastable materials is observed. The phenomenon of strain localization in the course of discontinuous plastic flow (DPF) at extremely low temperatures is investigated. DPF is observed mainly in fcc metals and alloys strained in cryogenic conditions, practically down to absolute zero. These materials undergo at low temperatures a process similar to dynamic strain ageing, manifested by the so called serrated yielding (DPF). DPF is attributed to the mechanism of local catastrophic failure of lattice barriers (including Lomer–Cottrell locks), under the stress fields related to the accumulating edge dislocations. Failure of LC locks leads to massive motion of released dislocations, accompanied by step-wise increase of the strain rate (macroscopic slip) and drastic drop of stress. Recent experiments indicate strong strain localization in the form of shear bands propagating along the sample. The plastic power dissipated in the shear band is partially converted to heat, which results in a local drastic increase of temperature promoted by the so-called thermodynamic instability (nearly adiabatic process). The Dirac-like temperature function is measured by two thermometers located in the gage length of the sample. Spatio-temporal correlation indicates smooth shear band propagation, as long as the process of phase transformation remains on hold. A physically based multiaxial constitutive model presented in the paper describes both DPF and strain localization, accompanied by temperature distribution represented by Green-like solution of heat diffusion equation. The model accounts for the thermodynamic background, including phonon mechanism of heat transport, accompanied by specific heat vanishing with the temperature approaching absolute zero. Experimental identification of parameters of the constitutive model is carried out. A projection of the model to the range where the phase transformation takes place is discussed.

Dynamical simulations of an electronically induced solid-solid phase transformation in tungsten

The rearrangement of a material's electron density during laser irradiation leads to modified nonthermal forces on the atoms that may lead to coherent atomic motions and structural phase transformation on very short time scales. We present ab initio molecular dynamics simulations of a martensitic solid-solid phase transformation in tungsten under conditions of strong electronic excitation. The transformation is ultrafast, taking just over a picosecond, and follows the tetragonal Bain path. To examine whether a solid-solid bcc-fcc phase transformation could occur during laser irradiation, we use two-temperature molecular dynamics (2T-MD) simulations with a specially developed potential dependent on the electronic temperature. Our simulations show that the occurrence of the solid-solid phase transformation is in competition with ultrafast nonthermally assisted melting with the strength of the electron-phonon coupling determining the lifetime of the new solid phase. In tungsten the melting transition is predicted to occur too rapidly for the fcc phase to be detectable during laser irradiation.

Helium retention and thermal desorption from defects in Fe9Cr binary alloys

In order to study the fundamental processes of helium retention and thermal desorption from the structural material of future fusion reactors, thermal desorption measurements were performed to investigate helium trapping from defects in binary Fe9Cr model alloys irradiated by 3 keV and 0.2 keV He ions. Interstitial type dislocation loops, vacancies and vacancy clusters were produced by irradiation with 3 keV helium ions, which acted as the sink trapped the helium atoms. Helium thermal desorption peaks from dislocations, helium-vacancies were obtained by thermal desorption spectroscopy at ∼540 °C, in the range from 205 °C to 478 °C, respectively. Simple first order dissociation kinetics are used to estimate the activation energies associated with the desorption groups. A sharp desorption peak was observed at ∼865 °C due to the BCC-FCC phase transformation for specimens under all examined implantation conditions.

Ion-Irradiation-Assisted Phase Selection in Single CrystallineFe7Pd3Ferromagnetic Shape Memory Alloy Thin Films: From fcc to bcc along the Nishiyama-Wassermann Path

When processing Fe-Pd ferromagnetic shape memory thin films, selection of the desired phases and their transformation temperatures constitutes one of the largest challenges from an application point of view. In the present contribution we demonstrate that irradiation with 1.8 MeV Kr(+) ions is the method of choice to achieve this goal: Single crystalline Fe(7)Pd(3) thin films that are grown with molecular beam epitaxy on MgO (001) substrates and subsequently irradiated with ions reveal a phase transformation along the whole phase transformation path ranging from fcc austenite to bcc martensite. While for 10(14) ions/cm(2) a fcc-fct phase transformation is observed, increasing the fluence to 5 × 10(14) ions/cm(2) and 5 × 10(15) ions/cm(2) leads to a phase transformation to the bcc phase. Pole figure measurements reveal an orientation relationship for the fcc-bcc phase transformation according to Nishiyama and Wassermann.

Metastable bcc-Ni and bcc-NiFe Single-Crystal Films Prepared on GaAs Single-Crystal Substrates With Different Orientations

Ni and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> (at. %) epitaxial thin films are prepared on GaAs single-crystal substrates of (100), (111), and (110) orientations by ultra-high vacuum RF magnetron sputtering. The film growth behavior and the crystallographic properties are studied by reflection high energy electron diffraction and X-ray diffraction. Metastable bcc-Ni and metastable bcc-NiFe single-crystals of (100), (111), and (110) orientations respectively nucleate on GaAs substrates of (100), (111), and (110) orientations, where the bcc structure is stabilized through hetero-epitaxial growth. With increasing the film thickness, the bcc structure in Ni and NiFe films starts to transform into more stable fee structure. The bcc-fcc phase transformation orientation relationships observed for Ni and NiFe films on GaAs(100) and (111) substrates are fcc{100}{011} ||bcc{100}〈 001〉, whereas those for Ni and NiFe films on GaAs(110) substrates are fcc(111)[112̅] ||bcc(110)[11̅0] and fcc(111)[1̅1̅2] ||bcc(110)[11̅0]. The resulting films thus consist of mixtures of bcc and fee crystals. Metastable bcc structure is more stable in Ni film than in NiFe film on all the GaAs substrates. The lattice constants of bcc-Ni and bcc-NiFe crystals are determined to be α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-Ni</sub> = 0.292 nm and α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bcc-NiFe</sub> = 0.291 nm, respectively. The in-plane magnetization properties of Ni and NiFe films grown on GaAs(110) substrates are reflecting the magnetocrystalline anisotropies of bcc-Ni and bcc-NiFe crystals, respectively.