Phase Transformations In Alloys Research Articles

Introducing dislocations locally in Al-supersaturated α2-Ti3Al single crystal via nanoscale wedge indentation

We investigated the nanoscale anisotropic deformation behavior of Al-supersaturated α2-Ti3Al single crystals, using wedge indentation with the loading axis parallel to the (0001) α2 basal plane. The deformability, plastic deformation mechanism, and introduced dislocation structures beneath the indentations were notably affected by the inclination angle between the indenter edge and basal plane. Notably, abnormal dislocation bands were introduced when the edge was parallel to the (0001) plane, which can potentially be utilized for manipulating the phase decomposition locally. The present study sheds light on the regulation of local plastic deformation and manipulation of nanoscale phase transformation in metals and alloys.

Temperature evolution associated with dynamic phase transformation in shape-memory TiNi alloys

ABSTRACTIn this paper, temperature evolution in shape-memory TiNi alloys was in situ measured in the process of phase transition under shock loading tests. It was shown that the temperature increased during loading, while unloading, it would keep the maximum value constant or decreased, depending on the strain rate. The corresponding dynamic response indicated that the hardening property is related to phase transition strain and the transformation path has remarkable strain-rate effect. The reason for this phenomenon is essentially due to the temperature dependence of the phase transformation stress. Moreover, the evolution of temperature was consistent with the change of stress, suggesting that this dynamic phase transition deformation is uniform. The theoretical calculation shows that the mechanism of temperature evolution results from the release of latent heat and the hysteresis dissipated energy. The results of this investigation provide insight into intriguing strain-rate-dependent phenomena intrinsic of TiNi alloys.

Energy dissipation of shock-generated stress waves through phase transformation and plastic deformation in NiTi alloys

The interaction between propagation of shock-induced stress waves and phase transformation in Nickel–Titanium (NiTi) shape memory alloys is studied experimentally and computationally. Molecular dynamics simulations at the atomistic scale are utilized to investigate how the local phase transformations dissipate the energy of shock waves as they travel in the material. It is shown that plastic deformations also happen at the vicinity of the regions at which the loading is applied. While plastic deformations also contribute in the energy dissipation, our results show that the energy dissipation is prominently induced by the reversible phase transformation. These results affirm that shape memory alloys are superior candidates for dissipating the energy of shock loads, with minimal permanent deflections remaining on the structure after the load is applied. Experiments are also performed in which precise and controlled shock loads (generated by the pressure of the liquid jet produced by the collapsing of a spark-generated bubble) are applied to two bars made of NiTi and Aluminum. Our results show that both the measured strain and lateral displacements are drastically damped in NiTi bars, which confirms the dissipation of stress shock ways due to phase transformation in these alloys.

Effect of quenching rate and its coupling model on precipitation and strength of Al–Cu–Cd alloys

The mechanical properties of Al–Cu–Cd alloys primarily depend on the heat treatment conditions, including quenching and aging treatment. The quenching factor analysis method has been widely used to describe the effect of continuous quenching on the properties of alloys, and the precipitate strengthening model focuses on the aging treatment process by calculating the evolution of physical parameters of precipitates affecting the properties of alloys. In this paper, a coupling model is proposed to predict the evolution of yield strength of Al–Cu–Cd alloys at different quenching rates and different aging times. To couple the two models, the equivalent isothermal quenching temperature is proposed, and the effects of Cd in the solid phase transformation of aluminum alloys is considered. The yield strength of Al–Cu–Cd alloys under different aging times and different quenching rates is determined experimentally. The predictions of the coupling model show good agreement with the experimental measurements.

Influence of precipitation on phase transformation and mechanical properties of Ni-rich NiTiNb alloys

Ni-rich NixTiyNbz (x/y = 1.0645/x-y ≈ 3, z = 0, 2, 4, 6, 9 at.%) alloys were designed and prepared in this work. Based on these alloys, the precipitation evolution of Ni-rich NiTiNb alloys and its influence on phase transformation and mechanical properties were investigated. Ni4Ti3 precipitates are observed in Nb0 and Nb2 alloys after aging and furnace cooling. Ni4Ti3 precipitation sharply increases Ms in Nb0 alloy, however, causes no obvious change of Ms in Nb2 alloy. With increasing Nb addition, large numbers of β-Nb precipitates densely distribute among eutectic regions of Nb6 and Nb9 alloys. β-Nb precipitates and eutectic β-Nb particles decrease Ms. The critical yield stresses of all Ni-rich NiTiNb alloys are determined by Ms, and present same minus linear relationship with Ms. Precipitation and element variation change Ms and then influence mechanical properties. Meanwhile, this result is demonstrated by thermodynamic analysis.

Phase transformation and mechanical properties of HfPd ultra-high temperature shape memory alloys

The effects of chemical composition on the structural, functional and mechanical properties of hafnium palladium (HfPd) alloy system were investigated. Five selected compositions of Hf50-xPd50+x (x = − 0.5, 0, 0.5, 2.5 and 5) alloys were studied to reveal their potentials as high-temperature shape memory alloys to be employed in the applications to operate above 400 °C. All the alloys showed reversible phase transformations at temperatures above 400 °C. The large hysteresis of more than 120 °C was observed for HfPd alloys which is higher than most of the other conventional high-temperature SMAs. Thermal hysteresis increases with Pd content, as it was 124 °C for Hf51.5Pd49.5 alloy and 158 °C for the Hf45Pd55 alloy. The alloys do not show superelasticity and have very low recoverable strain. It was also shown that hardness increased (to above 400 HV) with increasing Pd content. The alloys fail when they are compressed up to 1500 MPa.

A cellular automaton model integrated with CALPHAD-based thermodynamic calculations for ferrite-austenite phase transformations in multicomponent alloys

A cellular automaton (CA) model, which is directly coupled with thermodynamic calculations based on the CALPHAD method, is developed for the simulation of ferrite (α)-austenite (γ) phase transformations involving the partitioning and long-range diffusion of both interstitial and substitutional elements in multicomponent alloys. A data management scheme is proposed, which renders the efficient coupling between the kinetic simulation using the CA approach and thermodynamic calculations. The present CA model is applied to simulate the cyclic phase transformations between the α- and γ-phases for a quaternary Fe-0.02C-0.2Mn-0.1Si (wt%) alloy in both one- and two-dimensions. The simulated curves of α-volume fraction varying with temperature clearly illustrate the different α-γ transformation stages. The transformation kinetics and concentration profiles of Mn and Si obtained by the 1-D CA simulation agree well with DICTRA predictions. The 2-D simulation of a single circular grain is found to have shorter stagnant stages and faster transformation kinetics than that of the 1-D planar case. This can be attributed to the different growth geometries of 1-D planar and 2-D circular, and the effect of substitutional element Mn on the migrating interface. The non-uniform distribution of C and residual spikes of Mn and Si can be clearly delineated in the 2-D multi-grain simulation. It is shown that the data management scheme proposed in the present work significantly reduces the simulation time for the required thermodynamic data by two orders of magnitude approximately, which makes it feasible to perform valid simulations of microstructural evolution and solute distributions during α-γ phase transformations in steels with more than three components.

Phase transformation and dynamic recrystallization behaviors in a Ti55511 titanium alloy during hot compression

Hot compression experiments of a Ti55511 titanium alloy are conducted to study the phase transformation characteristics and dynamic recrystallization (DRX) behavior. It is found that the fraction of α phase decreases with increasing the strain rate or deformation amount. Meanwhile, the original lamellar α phases easily transform into the spheroidal and bulk α phases at larger deformation amounts or higher strain rates. Most of α phases disappear and the final microstructures are mainly β phases when the temperature is over 800 °C. The DRX degree rises with raising the deformation amount or decreasing the strain rate. However, the DRX behavior becomes weaken with raising the deformation temperature. α phases are evenly distributed around β phases during hot compressive deformation, which not only restrains the growth of β phases but also promotes the DRX process of β phases. In addition, the geometric DRX takes place within α phases, while the continuous DRX occurs within β phases during the hot compression.

The effects of cobalt elements addition on Ti2Ni phases, thermodynamics parameters, crystal structure and transformation temperature of NiTi shape memory alloys

By adding cobalt into NiTi-based shape memory alloy, the change of phase transformation, microstructure, and magnetic property of alloys has been investigated in austenite and martensite phases. NiTi and NiTiCo alloys with various rates of cobalt element, 2-6at.%, were prepared by arc-melting furnace, which had a high-vacuum level to avoid contamination, and then samples homogenized at 1323 K for 24h. Subsequently, at room temperature each crystal structure, the microstructural phases, change in chemical composition ratio, and magnetization of the alloys, were investigated by using XRD, optical microscope, SEM-EDX, and PPMS equipment, respectively. The results of these measurements showed that, at room temperature, NiTiCo alloys with cobalt compositions of 0, 2, and 3 (at.%) exhibit the austenite phase, while the rest of the alloys show the martensite phase. Also, the enthalpy change for the first three alloys increased with increasing cobalt addition, however its value decreased suddenly for NiTiCo alloys with 4, 5, and 6 (at.%) cobalt doping.

How can we make use of synchrotron radiation to study phase transformations in magnesium alloys containing long-period stacking ordered (LPSO) structures?

How can we make use of synchrotron radiation to study phase transformations in magnesium alloys containing long-period stacking ordered (LPSO) structures?

Structural and Phase Transformation in a Cold-Deformed Titanium Alloy During Aging

The change of structure, phase composition and properties during aging of VT22(Ti-5Al-5Mo-5V-1Fe-1Cr) titanium alloy rods, pre-quenched from

Structure and magnetic properties of (Sm0.9Zr0.1)Fe11Ti alloys with ThMn12-type structure

Abstract Rapidly increasing demand for high-energy permanent magnets and volatility of the rear-earth market encourage a search of hard magnetic materials that can compete with those based on the Nd2Fe14B phase. Sm-Fe compounds with the ThMn12-type crystal structure are considered as promising candidates for it. However, their synthesis and achievement of theoretically predicted magnetic properties are still challenging tasks that have not been solved yet. This paper addresses this problem. Its aim was to synthesize and to study magnetic properties of the (Sm0.9Zr0.1)Fe11Ti compound with ThMn12-type structure. Initially amorphous alloy was obtained by melt spinning and its subsequent heat treatments were performed. Structural and phase transformations of the (Sm0.9Zr0.1)Fe11Ti alloy with annealing temperatures were studied along with magnetic properties. For the optimally annealed alloy features of magnetization reversal were discussed and temperature dependences of coercivity and maximum energy product were obtained.

Phase and Structural Transformations of High-Carbon Alloy of the Fe–V–C System During Chemical-Thermal Processing and Deformation

Phase and Structural Transformations of High-Carbon Alloy of the Fe–V–C System During Chemical-Thermal Processing and Deformation

Effect of dual aging treatments on phase transformation and microstructure in a pre-deformed Ti2AlNb-based alloy containing O + β/B2 structures

The effect of dual aging treatment on phase morphology, composition, and microstructure evolution of a pre-deformed Ti2AlNb-based alloy were investigated in this study. The 1040 °C-pre-deformed alloys were dual aged at different temperature in O + β/B2 region followed by 750 °C-aged treatment. The experimental results show that coarse O lath and lamellar O + β/B2 were formed when the aging temperature is lower than 950 °C while globular O appeared when aging temperature is higher than 950 °C. The size and content of the O phase increase and decrease with the aging temperature, respectively. Widmanstätten O + β/B2 laths were obtained after secondary aging treatment at 750 °C when the first aging temperature is higher than 910 °C. Complete lamellar O + β/B2 precipitation and coarse O lath, in particular, were obtained in the 910 °C and 750 °C dual aged alloy with no Widmanstätten O + β/B2 laths appeared. The phase content is closely related to the aging temperature. When the 970 °C-aged alloy put into the secondary aging at 750 °C, coarse O laths which were fragmented or partially spheroidized and globular O during 970 °C-aged process were dissolved and the Widmanstätten O began to precipitate inside.

Energy-based analysis of temperature oscillation at the shakedown state in shape memory alloys

This paper documents an energy-based analysis of temperature oscillation during cyclic stress-induced phase transformation in shape memory alloys (SMAs). The conversion of total hysteresis work into dissipated heat and stored energy in SMAs is discussed, and the temperature oscillation at the shakedown state in cyclically loaded SMAs is modeled. To validate the theoretical model, both strain and stress-controlled tensile tests on pseudoelastic NiTi wires are performed under cyclic loading and an infrared camera is used to record the surface temperature of the wire. The results show that depending of the loading type, strain or stress-controlled loading, the amplitude of temperature oscillation during cyclic stress-induced phase transformation evolves differently with the frequency: The amplitude of the temperature oscillation under strain-controlled loading gradually reaches a saturated value independent of the frequency; however, for stress-controlled loading, the saturated temperature amplitude is frequency-dependent since it reduces significantly if the frequency is increased.

Nonequilibrium Diffusional Phase Transformations in Alloys Induced by Migration of Grain Boundaries and Dislocations

The main scenarios of nonequilibrium diffusional transformations induced by moving defects (dislocations, grain boundaries) in alloys under severe plastic deformation are considered. It has been shown that the phase state locally changes in the area of a defect where thermodynamic properties of alloy are locally changed, and the attained state is frozen after the displacement of a defect due to the difference between the rates of bulk diffusion and diffusion on a defect. For this reason, an alloy shifts from the state of its thermodynamic equilibrium under treatment, thus different nonequilibrium states, such as the disordering of alloy, the dissolution of equilibrium phase precipitates, the appearance of nonequilibrium phases, and the formation of regular structures, are possible depending on the type of the system. These effects may take place if the treatment of an alloy is performed at moderate temperatures, when diffusion is frozen in the bulk and rather active on defects. The phenomena of phase and structural instability developing under severe plastic deformation at moderate temperatures are considered within the framework of the proposed model.

Features of the Formation of Structure and Phases in Single-Crystal High-Temperature Alloys and their Effect on Mechanical Properties

Features of the structure and phase transformations in single-crystal nickel alloys 1, 2 after long high-temperature exposures were considered in this paper. The results and studies of the residual mechanical properties of alloys after exposure are given. Nanophase hardening of single-crystal heat resistant nickel alloys was also considered. The method of heat treatment of single-crystal high-temperature nickel alloys was developed based on the studies. It provides an increase in the strength properties of alloys by 20...30%, due to the formation of bulk nanophases (γn+ γ'n). The data obtained on structural and phase changes in high-temperature nickel alloys make it possible to evaluate the stability of the structural state of alloys.

Analysis of Phase Transformation in Nickel-Base Alloy after Ageing

Analysis of Phase Transformation in Nickel-Base Alloy after Ageing

Systematic Phase-Field Study on Microstructure Formation During Brazing of Mar-M247 with a Si-Based AMS4782 Filler

Although advanced solidification processes like high temperature brazing have gained more and more industrial importance, they still are often not well understood, and defining optimal process conditions for practical use in many cases can be achieved only by expensive experimental trial and error methods. On the other hand, the phase-field method has become more and more powerful for the simulation of solidification and phase transformations in technical alloys. This paper presents phase-field simulations for high temperature brazing of the Ni-base superalloy Mar-M247 using a Si-based AMS4782 filler. The applied phase-field model can be online-coupled to Calphad databases which deliver the thermodynamic and mobility data for this complex multicomponent multiphase alloy. Unknown or uncertain physical parameters like diffusion coefficients have been calibrated by comparing to optical micrographs and EDX element mappings obtained from laboratory braze experiments for the same material combination. By variation of the process conditions the effects of the brazing temperature, the brazing time, and the thickness of the braze gap on precipitation of brittle phases and formation of stray grains as well as changes in the microstructure of the base material itself have been systematically investigated. Results are presented and discussed.

Onset of Θ' to Θ Phase Transformation in Al-Cu Alloys

The θʹ to θ phase transformation that occurs during overaging in Al-Cu alloys is detrimental for their mechanical properties. Experiments suggest that the onset of this transformation is dependent on the temperature and duration of thermal exposure, but the microstructural conditions which trigger this transformation are still unclear. In this study, the conditions associated with the onset of θʹ to θ phase transformation are predicted using thermodynamic analysis. We compare the predicted microstructural conditions with both new and previously reported microstructural observations, and also test the predictions using phase field modeling. This multi-pronged approach suggests that the tendency of a θʹ particle to transform into the θ phase depends primarily on temperature, particle size, and particle aspect ratio. We define a critical aspect ratio below which θʹ to θ phase transformation becomes thermodynamically favored and show how this aspect ratio increases with both temperature and particle volume. With this approach, the intimate link between phase stability, morphological stability, and coarsening resistance for θʹ precipitates in Al-Cu alloys is illustrated.