Phase Transformations In Alloys Research Articles

Massive transformation in dual-laser powder bed fusion of Ti6Al4V alloys

In the present investigation, we fabricated additively manufactured Ti6Al4V parts using dual laser-powder bed fusion (DL-PBF) and identified the occurrence of a massive transformation. A detailed and systematic study of the massive transformation in the DL-PBF process by changing the energy and time offset of the lag laser beam and its effect on the microstructural and mechanical properties was undertaken. A mixture of martensite and massive phases was observed within the prior β grains in DL-PBF, and the dimensions of these massive grains increased with an increase in the lag laser beam energy, while acicular α′ martensite was present in the single Laser-PBF. The massive phase showed a featureless patch-shaped region, which consisted of thin laths, and they were associated with the prior β grain boundaries. The irregular patch-shaped regions showed inferior Vickers hardness when compared with acicular α′ martensite, and the ultimate tensile strength was lower in the DL-PBF than in the single Laser-PBF. The columnar prior-β grain was present in all the samples (both in the single- and dual-beam laser) and the dimensions of the grains varied with increasing the lag laser beam energy. These present results showed that the DL-PBF process significantly influenced the microstructural and mechanical properties of the Ti6Al4V materials, and provided a deep insight into massive phase transformation in AM titanium alloys and its effect on the mechanical properties.

Crystal-to-amorphous phase transformation in ternary shape memory alloys

This systematic study evaluates the role of important factors in crystal-to-amorphous phase transformation during severe plastic deformation in ternary shape memory alloys. Experiments coupled with thermodynamic calculations to obtain comprehensive interpretation on the amorphization in ternary shape memory alloys. Equiatomic binary NiTi together with ternary Ni–45Ti–5Hf and Ti–45Ni–5Cu (at%) alloys as alloy models were processed by high-pressure torsion (HPT) at room and cryogenic temperatures. The binary and Hf-added alloys revealed an amorphous structure with a few remaining nanocrystals, however, Cu-added ternary alloy showed resistance to the amorphization after HPT processing. The results revealed two key factors of preventing martensite-to-austenite transformation during deformation and the intrinsic potential of the material for amorphization drive crystal-to-amorphous phase transformation. Higher transformation temperatures, enthalpy and thermal hysteresis prevented martensite-to-austenite transformation, which therefore encouraged amorphization. In addition, a large negative heat of mixing, Gibbs energy for amorphization and a large atomic size mismatch are important factors in implementing crystal-to-amorphous phase transformation. This investigation is a step forward to a fundamental understanding of ternary or even compositionally complex shape memory alloys for microstructural engineering by providing a homogeneous ultrafine-grained structure due to amorphization and subsequent heat treatment to achieve superior shape memory effect and superelasticity.

Cu-doping induced tuning of magnetic properties and phase transformation in MnBi alloys

This study investigates the effect of Cu doping on the magnetic properties of MnBi alloy at sub-ambient temperatures. The Curie temperatures (Tc) were observed at 175.2 and 161.6 K for Mn52Bi45Cu3 and Mn51Bi45Cu4 alloys, respectively. The Mn55Bi45 sample demonstrates a notable saturation magnetization (Ms) of 72 emu/g, reaching 90% of the theoretical value, indicating the successful synthesis of high-purity ferromagnetic low-temperature phase MnBi. Compared to Mn55Bi45, all Cu-doped samples exhibit higher coercivity (Hc) within the temperature range of 10–200 K. In particular, the sample with 4 at. % Cu exhibits an Hc of 7.4 kOe at 10 K, which is 37 times higher than that of the MnBi sample (0.2 kOe), and it remains significantly higher than that of the 0–3 at. % Cu-doped samples, even at 200 K. Further details of this investigation are presented in this paper.

Insights into the structure sustainable evolution and crystallization kinetics of amorphous Mg60Ni30La10 alloy

Nanocrystalline materials have wide potential applications due to the special properties, which can be easily prepared by nanocrystallization of the amorphous alloys. In this work, phase transformations of the melt-spun amorphous Mg60Ni30La10 alloy were characterized by in-situ XRD and TEM. The results showed that during crystallization Mg2Ni first formed from the amorphous matrix followed by LaMg3, LaMg2Ni and LaMgNi4. The phase transformation was completed rapidly, with nucleation progressing more prominently than grain growth. Nucleation and grain growth during crystallization were discussed according to Avrami index n(α) based on Kissinger equation. In addition, the classic Miedema theory was introduced to estimate the crystallization temperature and the nucleation frequency. Nucleation behaviors are closely related to the phase structures formed during the amorphous crystallization from the view of thermodynamics. This work provided a preliminary basis for controlling the evolution of amorphous/nanocrystalline structure of alloys.

Effects of laser welding on the martensitic transformation of a shape-memory Ti-Ni alloy

The objective of this study is to investigate the martensitic transformation behavior of a Ti-Ni alloy subjected to a laser welding process. Materials characterization techniques were used to analyze the influences of this welding procedure on the phase transformation behavior. This study can provide valuable insights for the improvement of Ti-Ni alloy welding processes used for technological applications, such as in the aerospace industry. In this study, samples were prepared under three conditions: thermal treatment at 400°C, thermal treatment at 500°C, and as-received. The characterization techniques used were: Optical Microscopy (OM), Differential Scanning Calorimetry (DSC), Electron Dispersion Spectroscopy (EDS), X-Ray Diffraction (XRD), and Vickers Microhardness (HV). The results allow for analysis of thermoelastic martensitic transformation and variations in the compositions of the alloy elements. The research, in general, aims to repurpose welding waste in Ti-Ni alloys for the production of mini sensors. The enthalpy was higher for the alloy treated at 500°C and welded. The research findings led to the observation of martensitic phase transformation in the laser-welded Ti-Ni alloy, which suggests that it can be used as a sensor/actuator.

Crystalline-amorphous-crystalline two-step phase transformation and the resulting supra-nano structure in a metastable iron-based alloy

In this work, we report a crystalline-amorphous-crystalline (CAC) two-step solid-state phase transformation (SSPT) in a 15Cr-2Ni iron-based alloy that results in nanostructures. Microstructurally, the CAC process leads to a localized supra-nano structure having almost equal content of an amorphous matrix and nanocrystalline inclusions less than 6 nm. Dynamically, it is manifested as a glass-transition-like variation of Young's modulus with a sharp surge in internal friction. Through in situ heating and Neutron Scattering experiments, a reduction in the intensities of the austenite diffraction peaks is observed, which is not associated with any intensity increase in other peaks, suggesting the collapse of austenite lattice during heating. The CAC two-step SSPT occurs at temperatures far below the melting point, and the introduction of the dislocations is the indispensable condition giving rise to such a transition. Based on experimental results, we suggest the physical picture of CAC two-step SSPT and a phenomenological model to describe it. Through the analysis, we argue that the CAC two-step SSPT provides a new route to fabricate nanostructured alloys.

The effect of Ge substitution on phase transformation and magnetic properties of MnBi alloys

The Mn55Bi45-xGex intermetallic compound with x ≤ 4 was prepared and compared by annealed ribbons and powder metallurgy. Substituting Ge in the MnBi alloy significantly alters the phase transformation temperatures, enhances the coercivity (Hc), and improves the squareness. As the Ge concentration increased from 0 at.% to 4 at.%, both the crystalline size of low-temperature phase MnBi in annealed ribbons and ball-milled powders demonstrated a gradual decrease, while lattice strain exhibited an upward trend. The Hc of Mn55Bi41Ge4 powder reaches 14.1 kOe, which is 33 % higher than that of pure MnBi powder, making MnBi-Ge an ideal candidate magnet for the next generation of rare-earth-free magnets.

Lattice dynamics of Mg-Sc lightweight shape memory alloys

Lattice vibrations play a significant role in the martensitic phase transformation of MgSc alloys, thus in the shape memory effects as well. In this article, the phonon spectra and thermodynamic properties of MgSc alloys with different structures were systemically studied by first-principles methods. The phonon spectra of Mg-18.75 at%Sc and Mg-20.83 at%Sc in austenite phase are fundamentally different, and the experimental martensitic transition behavior of MgSc alloys with different compositions can be well explained by the lattice dynamics. The occurred Fermi surface nesting at specific compositions leads to the appearance of phonon imaginary frequency, thus resulting in the lattice instability. The entropy change and latent heat obtained from harmonic phonon calculations suggests that the MgSc alloys are promising for the application in low-temperature elastocaloric cooling, and that the entropy changes are comparable to the well known cooling materials such as BaTiO3 and NiTi.

Repetitive Nanosecond Laser-Induced Oxidation and Phase Transformation in NiTi Alloy

NiTi shape memory alloys, known as Nitinol, are highly valuable in medical fields for their unique attributes, including superelasticity, wear resistance, and biocompatibility. Laser treatment provides precise control over surface characteristics, enhancing biocompatibility. This study focuses on the effects of laser irradiation on NiTi alloy surfaces, particularly considering the number of laser scans and their impact on surface features. Even at low laser power, multiple high-frequency scans significantly alter surface roughness and induce phase transformation. After 16 repeated laser irradiations, amorphous Ti oxide transforms into crystalline anatase. Remarkably, anatase can further transform into rutile due to the influence of Ni nearby and TiO, due to insufficient oxygen content. The most notable outcome is the formation of a thick Ti oxide layer, causing unbound Ni to emerge on the surface, resulting in a Ni oxide layer. These findings highlight the importance of precisely adjusting laser parameters to achieve tailored surface properties for medical applications, addressing challenges and enhancing biocompatibility.Graphical abstract

Influence of high temperature ternary and quaternary additions on the phase transformation and actuation fatigue characteristics of NiTi shape memory alloys

Influence of high temperature ternary and quaternary additions on the phase transformation and actuation fatigue characteristics of NiTi shape memory alloys

Microstructure evolution and phase transformation mechanism of Ti–46Al–8Nb–2.5V alloy based on phase transformation geometric models

The microstructure of conventionally solidified and rapidly solidified Ti–46Al–8Nb-2.5V alloys was analyzed with phase transformation geometric models, revealing the phase transformation mechanisms. The β→α→γ phase transformation processes follow Burgers and Blackburn orientation relationships (ORs), respectively, from which the calculated transformation between the β/β0 and γ phases follow K–S OR. Whlie, the direct β/β0→γ phase transformation can be directly observed in the microstructure, which follows a variety of ORs, mainly following the K–S OR but also following the N–W or Bain ORs, most of which deviate from the strict ORs. In cases where N–W and Bain ORs are followed, there is a large dislocation spacing between the habit planes of the β/β0 and γ phases, resulting in low static energy at the interface and narrow elongated morphology of precipitated phases. Conversely, when following K–S OR for β/β0 → γ phase transformation, interfacial migration energy is relatively lower and grain precipitated phases is larger. The rapidly solidified microstructure of Ti–46Al–8Nb-2.5V alloy consists of the massive α2 phase, massive γ phase, and basket-weave β0 phase closely resembling those found in β-type titanium alloys. After annealing at service temperature, the rapidly solidified alloy forms a microstructure highly similar to that of conventionally solidified TiAl alloy. Various metastable phases, such as the Ti2Al, L12 and Ti1.4Al phases, precipitate in the blocky γ phase, which provides a theoretical basis for studying the phase transformations of TiAl alloys in service.

In situ measurement of austenite grain growth and recrystallization using laser ultrasonics

The development of next generation process models and advanced high-strength steel products for thin slab casting and direct rolling requires quantification of microstructure evolution during thermomechanical processing. Laser ultrasonics is a non-contact in-situ method to record grain growth, recrystallization and phase transformations in metals and alloys. Here, we will present an improved experimental design that facilitates a continuous microstructure measurement through the various stages of simulated hot rolling from reheating to runout table cooling using a Gleeble thermomechanical simulator equipped with a laser ultrasonics for metallurgy (LUMet) system. Austenite grain growth and static recrystallization after hot deformation are quantified based on attenuation of the ultrasound waves whereas austenite decomposition can be recorded with the changes in ultrasound velocity during the phase transformation. Further, the LUMet results for a microalloyed low carbon steel are validated with conventional techniques including optical and electron microscopy as well as double-hit tests. These experimental studies demonstrate the capabilities of laser ultrasonics in the identification of both normal and abnormal grain growth, non-recrystallization temperature, recrystallization, and austenite decomposition kinetics in a single test for a given processing path, as well as its potential for accelerated optimization of process control under industrial rolling conditions.

Enhancing microstructure and mechanical properties of nickel aluminium bronze alloy through tin addition

This article describes the changes in the microstructure, cooling curve characteristics and mechanical properties of cast Nickel Aluminium Bronze alloy (NAB) alloy that were produced by the addition of various amounts of Tin (Sn). The solidification parameters were recorded using a computer-aided cooling curve analysis setup, and optical and scanning electron microscopes were utilised to study the evolution of the microstructure. The chemical composition of different phases generated in the NAB alloy with and without Tin was investigated using an X-ray diffraction technique. With the addition of tin, the alloy's microstructure changed from columnar to equiaxed grain structures, and the ideal microstructure was produced at a Tin concentration of roughly 1.0 weight percent. The formation of the high temperature α and the grain boundary Sn rich phases across the alloy microstructure as a result of further addition has a considerable impact on the alloy's increased hardness (upto 69%) and tensile strength (upto 28.4%) compared to untreated NAB alloy. Influence of Sn on microstructure transformation is confirmed by the decline in alloy nucleation temperatures, the reduction in undercooling intensity, and the decrease in cooling rate during solidification. The addition of Tin to the NAB alloy caused morphological changes in the kappa (K) phases, which are also reported in the this article. In addition to this, the research makes an attempt to describe the mechanism underlying the formation of equiaxed grains and phase transformations in Sn-treated NAB alloys.

Modeling the material behavior of heterogeneous materials considering a two‐scale FE–FFT‐based simulation framework

AbstractIn various engineering applications, components subjected to high mechanical or multi‐physical loads such as thermal, thermo‐mechanical, or electro‐thermal loads are predominantly made of metals or composites. These materials are characterized by polycrystalline or multi‐phase microstructures which determine the overall material response. However, since the microscopic material behavior is directly related to the distribution, size, and morphology of the individual grains or phase constituents, precise predictions of the macroscopic material response require simulations of the microstructural behavior. Hence, this work considers a two‐scale finite element (FE) and fast Fourier transform (FFT)‐based simulation framework, which is an efficient alternative to the classical method for the simulation of periodic unit cells. Assuming scale separation, the macroscopic and microscopic boundary value problems are first solved individually and subsequently linked by performing a scale transition in terms of averaging the macroscopic quantities over the corresponding local fields. While the macroscopic solution is computed by means of the finite element method, the microscopic boundary value problem, which is embedded as a periodic unit cell in each macroscopic integration point, is solved using an FFT‐based simulation method. In general, these microscopic simulations allow to model grain‐scale phenomena such as martensitic phase transformation or plastic deformation caused by dislocation glide which can be observed in polycrystalline materials. In this work, we present a two‐scale FE–FFT‐based model to capture the martensitic phase transformations in shape memory alloys. In order to demonstrate the applicability of the model, a numerical example is given.

The Influence of Ultrasonic Activation on Microstructure, Phase Transformation and Mechanical Properties of Porous Ni-Ti Shape Memory Alloys via Self-Propagating High-Temperature Synthesis.

Porous Ni-Ti shape memory alloys (SMAs) have been widely studied in biomedical and engineering applications. Porous Ni-Ti SMAs were obtained via self-propagating high-temperature synthesis (SHS), and their microstructure, phase transformation, and mechanical properties were investigated. This article presents the results of a study of changes in the microstructure, phase transformation, and mechanical properties of porous Ni-Ti SMAs when Ni and Ti metal powders were preliminarily subjected to ultrasonic activation at various periods. It was determined that the porosity of the obtained alloy samples was 62-68 vol%. The microstructure was composed of the main matrix Ni-Ti phase and the accompanying Ti and Ti2-Ni phases. The results show that the hardness 34.1-86.8 HB and elastic modulus 4.2-10.8 GPa increased with an increase in the ultrasonic activation time of the samples. The phase transformation temperature of the Ni-Ti shape memory alloy remained almost unchanged under the influence of ultrasonic treatment.

Structural and Phase Transformations in Alloys under the Severe Plastic Deformation

Structural and Phase Transformations in Alloys under the Severe Plastic Deformation

Paths of phase transformations in titanium alloys based on four-dimensional structural description

Paths of phase transformations in titanium alloys based on four-dimensional structural description

Role of Zr and thermomechanical treatment on phase transformation and functional properties of NiTi-based shape memory alloys

A systematic investigation was conducted to evaluate the role of Zr addition on the martensitic phase transformation behavior and properties of NiTi-based shape memory alloys. Equiatomic binary NiTi alloy and ternary Ni50Ti45Zr5 and Ni50Ti40Zr10 alloys (at.%) were processed by a simple thermomechanical treatment including cold rolling followed by post-deformation annealing at 450 °C for 30 and 60 min. The microstructural investigation and thermal analyses reveal that martensitic phase transformation is suppressed by adding Zr. Solid solution strengthening due to Zr addition, strain hardening due to cold rolling and formation of (Ti,Zr)2Ni precipitates in a dual-phase microstructure during annealing leads to a significant increase in hardness in the thermomechanically treated alloys (e.g., up to 550 Hv for Ni50Ti45Zr5 alloy). The stress-strain curve of martensitic Ni50Ti50 alloy reveals the essential characteristics of shape memory effect, however, Ni50Ti45Zr5 alloy shows typical features of pseudoelasticity with significantly high stress plateau (∼600 MPa) and remarkable recovered strain of 3.5% during unloading.

Effect of phase transformation of CoCrFeNiAl high-entropy alloy on mechanical properties of WC-CoCrFeNiAl composites

WC-CoCrFeNiAl composites were fabricated via SPS using commercial CoCrFeNiAl and WC powders, with the optimal addition content of CoCrFeNiAl determined. Furthermore, the influence of phase transformation in CoCrFeNiAl high-entropy alloy on the mechanical properties of WC-CoCrFeNiAl composites was investigated. The results indicate that the hysteresis diffusion effect of CoCrFeNiAl HEA can significantly impede the growth of WC grains. Moreover, during the sintering process, a BCC-to-FCC phase transformation occurs in CoCrFeNiAl HEA. The phase transition of HEA can be regulated by adjusting the sintering temperature, resulting in a decrease in hardness and an increase in fracture toughness of WC-CoCrFeNiAl composites as HEAs undergo phase transformation. The Vickers hardness and fracture toughness values of WC-10CoCrFeNiAl composites sintered at 1250 °C are comparable to those of WC-10Co hard alloy, with respective values of 17.64 GPa and 12.3 MPa m1/2.

Composition- and temperature-dependence of β to ω phase transformation in Ti-Nb alloys

ω phases have shown great effects on the superelasticity and modulus of metastable β-Ti alloys. In this study, the microstructure evolution during cooling and aging for β → ω phase transformation is investigated by integrating a thermodynamic database with phase field simulations. Our CALPHAD calculations based on an available thermodynamic database give the Gibbs energies of metastable β (Nb-lean β1 + Nb-rich β2 produced via spinodal decomposition) and ω phases in Ti-Nb. Informed by the results, our phase field simulations show that the formation mechanisms of ω exhibit dependence on the composition and temperature. The ω can form in Ti-26 at.% Nb without the assistance of spinodal decomposition. Further analysis shows that the precursory spinodal decomposition in the β phase occurs in Ti-50 at.% Nb, and could induce geometrically confined ω. The novel transformation pathway could create unique morphology of ω. This study could elucidate new insights into the ω phase transformation in Ti-Nb alloys and metastable β-Ti alloys having spinodal decomposition.