Incomplete Phase Transformation Research Articles

In-Situ Study of Temperature- and Magnetic-Field-Induced Incomplete Martensitic Transformation in Fe-Mn-Ga

Significant interest in the stoichiometric and off-stoichiometric Fe2MnGa alloys is based on their complex phase transition behavior and potential application. In this study, temperature- and magnetic-field-induced phase transformations in the Fe41.5Mn28Ga30.5 magnetic shape memory alloy were investigated by in situ synchrotron high-energy X-ray diffraction and in situ neutron diffraction techniques. It was found that incomplete phase transformation and phase coexistence behavior are always observed while applying and removing fields in Fe41.5Mn28Ga30.5. Typically, even at 4 K and under 0 T, or increasing the magnetic field to 11 T at 250 K, it can be directly detected that the martensite and austenite are in competition, making the phase transition incomplete. TEM observations at 300 K and 150 K indicate that the anti-phase boundaries and B2 precipitates may lead to field-induced incomplete phase transformation behavior collectively. The present study may enrich the understanding of field-induced martensitic transformation in the Fe-Mn-Ga magnetic shape memory alloys.

Comprehensive shape memory alloys constitutive models for engineering application

The current shape memory alloys (SMA) constitutive models have the problems of complicated calculation, inaccurate results, measuring difficult parameters, and inconvenient application, which brings about challenges for designing, analyzing, and simulating SMA’s macroscopic behavior in engineering. This paper proposed practical SMA constitutive models for accurate engineering calculation, which apply to both the single loading-unloading process and the multiple loading-unloading cycles. The more accurate SMA theoretical tensile curve and temperature-stress phase transformation diagram is introduced to derive the constitutive model. The more accurate calculation method of elastic modulus, phase transformation modulus, and critical phase transformation stress is proposed in this paper. The proposed constitutive model considers the incomplete phase transformation phenomenon and the variation of critical phase transformation stress under multiple loading and unloading conditions. It can accurately evaluate the critical phase transformation stress and stress-strain relationships at various temperatures. Tensile tests under various working conditions were conducted. The results demonstrated that the proposed constitutive model could well simulate the SMA’s macroscopic mechanical behavior at various temperatures, loads, and multiple loading-unloading cycles with accuracy, practicality, and efficiency.

Formation mechanism of partial stacking faults by incomplete mixed-mode phase transformation: A case study of Fe-Ga alloys

• The faulted {111} distance of partial stacking faults is shorter than that of the typical SFs in deformed FCC materials. • The partial stacking faults have lower ordering degree than the well-stacked product phase. • The formation of partial stacking faults depends on both atomic ordering and lattice shear strain of the parent phase. Partial stacking faults (PSFs) formed by incomplete mixed-mode phase transformation have been found to exhibit unfixed slip distance of closely-packed planes unlike those of the deformation-induced stacking faults (SFs) with fixed distance. Though engineering PSFs can yield appealing properties, such as the enhanced damping capacity, understanding of the interaction between lattice distortion and atomic diffusion and their influences on forming PSFs is still far from being clear. Herein we performed a case study on aged Fe-Ga alloy that undergoes a mixed-mode phase transformation from body-centered cubic (BCC) to ordered face-centered cubic (FCC). The TEM investigations showed that the faulted {111}-FCC distance of the PSFs is shorter than a /6<112> of the typical {111}-<112> SFs in deformed FCC materials and the PSFs have disordered Fe and Ga arrangements. Further studies revealed that such PSFs will not be completely dissociated at FCC twin boundaries (TBs) even after long term isothermal aging. Consequently, the formation of PSFs can be ascribed to the transformation-dependent atomic ordering and lattice shear strain of the parent BCC lattice, where the diffusion-controlled glides of the PSFs-associated dislocations will accelerate atomic diffusions due to the dislocation-pipe effect along <112>-FCC direction, but may hinder the atomic diffusions across the {111}-FCC TBs due to the retarding effect. This study may add important insight into the defects process during mixed-mode phase transformation and broaden the knowledge of the interaction between concurrently-happened lattice distortion and atomic diffusion.

High-yield synthesis of CsPbBr3 nanoparticles: diphenylphosphine as a reducing agent and its effect in Pb-seeding nucleation and growth

Based on the reported nucleation mechanisms for CsPbX3 and II–VI/IV–VI quantum dots, CsPbBr3 nanoparticles with a higher reaction-yield (up to 393% mass-increment) were synthetized by the hot-injection method. The introduction of diphenylphosphine (DPP) as a reducing agent improved nanoparticle nucleation and growth, giving out evidence for Pb-seeding in CsPbBr3 nanoparticles formation. Additionally, a clear influence of the DPP in a CsPbBr3-Cs4PbBr6 incomplete phase transformation was observed, marked by the appearance of several PbBr2 nanoparticles. This indicated the need for an improved ratio between the stabilizing agents and the precursors, due to the increased number of nucleation sites produced by DPP. The resulting CsPbBr3 nanoparticles showed high quality, as they displayed 70%–90% photoluminescence quantum yield; narrow size distribution with an average nanoparticle size of ∼10 nm; and the characteristic cubic morphology reported in previous works. This increment in CsPbBr3 nanoparticles’ reaction yield will contribute to making them a more attractive option for different optoelectronic applications.

Evolution of hysteresis characteristic of shape memory alloys at incomplete phase transformation cyclic loading

The phase transformation ratchetting of shape memory alloys (SMAs) at incomplete phase transformation cyclic loading is experimentally and theoretically investigated. To this end, two different kinds of incomplete phase transformation cyclic loading tests on NiTi wires are performed, i.e. incomplete transformation cyclic loads are respectively applied at the stages of forward martensitic transformation and reverse martensitic transformation. When the cyclic load of incomplete transformation is applied in the forward martensitic transformation stage, a novel phenomenon is discovered: although there is no greater stress to drive the anstenite turn to martensite, the SMAs can still gradually undergo martensitic transformation and accumulation until martensite reaches saturation. The hysteretic behavior finally reaches a shakedown state where the strain–stress curve no longer changes with the number of cycles. When the cyclic load of incomplete transformation is applied in the reverse martensitic transformation stage, a similar phenomenon is obtained. According to the analysis of the temperature evolution during the deformation process of the SMAs, combined with the relationship between the phase transformation yield stress and the temperature of SMAs, the experimental results are reasonably explained. This research is of great significance for a more comprehensive grasp of the mechanical behavior of SMAs.

Effect of the Initial Columnar‐Grained Inhomogeneity of Electrical Steels on the Transformation Temperature

The effects of various initial structures and heating and cooling rates on the columnar‐grained transformation temperature are studied by a thermal dilatometer and electron backscatter diffraction. The results show that deformation can reduce the thermal hysteresis more effectively than decrease the heating rate, and the phase transformation hysteresis of the deformed sample is the smallest during heating, followed by the columnar‐grained sample with a slow heating rate, whereas the columnar‐grained sample with a fast heating rate has the largest phase transformation hysteresis. Moreover, the dilatational amount changes greatly with cooling rate. The coarse columnar grains can severely restrain dilatation. The smaller dilatational amount is related to the incomplete phase transformation of the columnar grains and also related to the suppression of the dilatational amount of the small grains by the surrounding columnar grains. In contrast, the largest dilatational amount is caused by the relatively sufficient phase transformation. In addition, the changes of the transformation temperature and the dilatational amount are mainly induced by the grain size effect and the inhomogeneity of the structures, but {100} texture can affect the uniformity of grain size.

In-situ strain measurement of shape memory alloy fiber during the austenitic and martensitic phase transformation

Fibers made from shape memory alloys perform strain recovery during phase transformation of crystal structure. Controlled parameters such as heating time, holding temperature, and cooling time dominate the micro deformation of shape memory alloy fiber (SMAF). Mechanical contraction occurs at high temperature (austenitic phase), and strain-relief happens at low temperature (martensitic phase). But the rapid change of temperature causes unfinished or incomplete phase transformation, which induces residual strain within the SMAF. The accumulated defects shorten the specimen’s life during cyclic loading and lower the limit of fatigue failure. This study uses digital image correlation to provide the in-situ response temperature-strain relations during the phase transformation and quantizes the residual strain within the SMAF. Experiments accurately control the holding temperature of SMAF in three conditions, which are (a) under, (b) around, and (c) beyond the phase transformation temperature of the specimen, and systematically analyzes the self-accommodation and strain-recovery of SMAF in the three experiments. The in-situ measurement of microscale deformation can help the end-user optimize the control parameters and increase the life and performance of SMAF.

A one-dimensional constitutive model for NiTi shape memory alloys considering inelastic strains caused by the R-phase transformation

In some experiments, an intermediate phase, i.e., the rhombohedral phase (R-phase), can be observed in the phase transformation process between austenite and martensite for NiTi shape memory alloys. To predict phase transformations among these three phases, a one-dimensional constitutive model for shape memory alloys considering the inelastic strains caused by the R-phase transformation is proposed. Firstly, a temperature-stress phase diagram reflecting martensitic transformation as well as R-phase transformation is provided. On the basis of this phase diagram, the novel model for shape memory alloys is developed in the framework of thermo-mechanics taking into account various inelastic deformation mechanisms, involving the independent phase transformation (e.g., the phase transformation between pure austenite and pure R-phase or the one between pure R-phase and pure martensite) and the mixed phase transformation (e.g., the phase transformation between mixed phases of ‘austenite + R-phase’ and pure martensite). Through numerical examples, it is demonstrated that the proposed model is capable to predict the deformation properties of shape memory alloys associated with the R-phase transformation, including shape memory effect and pseudo-elasticity, as well as internal sub-loops due to the incomplete phase transformation. Furthermore, the numerical results are compared with experimental results and show good agreement with the experimental ones, which confirms the feasibility of the proposed model.

Thermomechanical modeling of nonlinear internal hysteresis due to incomplete phase transformation in pseudoelastic shape memory alloys

This paper presents a thermomechanical model for pseudoelastic shape memory alloys (SMAs) accounting for internal hysteresis effect due to incomplete phase transformation. The model is developed within the finite-strain framework, wherein the deformation gradient is multiplicatively decomposed into thermal dilation, rigid body rotation, elastic and transformation parts. Helmholtz free energy density comprises three components: the reversible thermodynamic process , the irreversible thermodynamic process and the physical constraints of both. In order to capture the multiple internal hysteresis loops in SMA, two internal variables representing the transition points of the forward and reverse phase transformation, $$\phi _s^f$$ and $$\phi _s^r$$ , are introduced to describe the incomplete phase transformation process. Evolution equations of the internal variables are derived and linked to the phase transformation. Numerical implementation of the model features an Euler discretization and a cutting-plane algorithm. After validation of the model against the experimental data, numerical examples are presented, involving a SMA-based vibration system and a crack SMA specimen subjected to partial loading–unloading case. Simulation results well demonstrate the internal hysteresis and free vibration behavior of SMA.

Effects of powder dispersion on reactive flash sintering of 8 mol% yttria-stabilized zirconia and MgAl2O4 composites

A three-phase system consisting of alumina (Al2O3), magnesia (MgO) and 8 mol% yttria‐stabilized zirconia (8YSZ) was reactive flash sintered to produce MgAl2O4-8YSZ composites. We show that a critical volume fraction of the ionic conductor 8YSZ is required to promote the flash phenomenon, indicating a typical percolation behavior. The easy path for conduction could be reached with a smaller critical volume fraction when using agglomerated powders for producing the green bodies. Furthermore, microstructure and X-ray diffraction analyses demonstrate that both sintering and phase transformation largely depend on the mixing level of the precursors. Although the presence of agglomerates resulted in a decrease of the flash onset temperature, poor sintering and incomplete phase transformation were verified. This behavior is attributed to inhomogeneous heat distribution within those samples during the flash state, which could be explained by the electrical current percolation.

Utilizing Incomplete Phase Transformation to Characterize Elastocaloric Effect of Shape Memory Alloys

This study reports a correction to assist the elastocaloric effect characterization of shape memory alloys and obtain the latent heat of stress‐induced martensitic transformation. The incomplete phase transformation analysis (IPTA) correction is developed based on the assumptions of linear transformation plateau of stress–strain and identical heat capacities for austenite and martensite phases. Taking Ni50.8Ti49.2 alloy as a demonstration, an integrated test rig is built to validate IPTA correction and study the heat leak effect. Using water as heat transfer fluid and heat leak compensation it is possible to further improve the accuracy of imposing IPTA correction. The predictions of latent heat from direct and indirect approaches with IPTA correction are validated from the experimental data. The required minimum martensitic phase fraction is only 10–20% when applying the IPTA correction, compared with 84–93% in traditional approaches. The IPTA correction extends the precondition of complete transformation to incomplete transformation in elastocaloric characterization.

Rapid self-sustaining consolidation of titanium silicide (Ti5Si3) via transient liquid phase reaction induced by an electric discharge

The fabrication of Ti5Si3 in the form of a solid product directly from an elemental 37.5 at.% Si and 62.5 at.% Ti powder mixture was carried out by two different powder metallurgy routes. The first was by uniaxial pressing of the reactant powder mixture with sequent vacuum-sintering, and the second was by electric discharge sintering (EDS) of reactant powder mixture. The pressing process combined with vacuum-sintering produced a porous compact with multi phases of titanium silicide such as Ti5Si3, Ti5Si4, TiSi2, and TiSi, including elemental Ti, which indicated an incomplete phase transformation into Ti5Si3. On the other hand, the EDS induced the phase transformation mostly into Ti5Si3 with a small amount of Ti5Si4 in <180 μsec, which had a sequent consolidation into a solid compact with an average crystallite size of 30.4 nm and a lattice parameter of a = 7.42 Å and c = 4.91 Å. The significantly higher hardness value of the EDS compacts can be the result of the high density and the fine microstructure stemming from the homogeneous dissolution of the elements and the constrained grain growth. The formation of Ti5Si3 solid compact from the stoichiometric Ti and Si powder mixture by EDS can be dominated by the solid to liquid phase transformation mechanism.

Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure, Mechanical Properties, and Bio-corrosion Behavior of Mg–5Zn–1.5Y Cast Alloy

The influence of long-term solution treatment for various intervals on the microstructure, mechanical properties, and corrosion resistance of the as-cast Mg–5Zn–1.5Y alloy was investigated. Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations. Tensile and hardness tests, as well as polarization and immersion tests, were performed to evaluate the mechanical properties and corrosion behavior of the as-cast and heat-treated alloy, respectively. Results show that solution treatment transforms I-phase (Mg3Zn6Y) into W-phase (Mg3Zn3Y2), as well as dissolves it into the α-Mg matrix to some extent; therefore, the amount of W-phase increases. Moreover, prolonged solution treatment decreases the volume fraction of the phases. In the first stage of solution treatment for 14 h, the tensile properties significantly increase due to the incomplete phase transformation. Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy, it improves the corrosion resistance due to the transformation of I-phase into W-phase. In fact, it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the α-Mg matrix, resulting in the reduction in galvanic microcells between the matrix and compounds. It is found that the optimum time for long-term solution treatment is 14 h, which improves both corrosion behavior and mechanical properties.

Evaluation of strength property variations across 9Cr-1Mo steel weld joints using automated ball indentation (ABI) technique

The variations of strength properties across 9Cr-1Mo steel weld joints fabricated by different arc welding processes such as shielded metal arc welding (SMAW), tungsten inert gas (TIG) and activated tungsten inert gas (A-TIG) have been evaluated employing automatic ball indentation (ABI) technique. ABI tests were conducted at 298K across various zones of the weld joints comprising of base metal, weld metal, heat affected zone (HAZ) and intercritical HAZ (ICHAZ) regions. The flow curves obtained from ABI tests were correlated with corresponding conventional tensile test results. In general, the tensile strength decreased systematically across the weld joint from weld metal to base metal. Inter critical HAZ exhibited the least strength implying that it is the weakest zone. The incomplete phase transformation in the ICHAZ during weld thermal cycle caused the softening. The A-TIG weld metal exhibited higher UTS and strain hardening values due to higher carbon in the martensite. The strain hardening exponent exhibited only slight variation across the various regions of the weld joints. A-TIG weld joint exhibited higher weld metal and HAZ strength, marginally higher UTS to YS ratio in the weld metal and HAZ compared to that of the other two processes. Hence, among the three welding processes chosen, A-TIG welding process is found to be superior in producing a 9Cr-1Mo steel weld joint with better strength properties.

Spark Plasma Sintering of α/β Si3N4 Ceramics with MgO-Al2O3 and MgO-Y2O3 as Sintering Additives

In the present work, the α/β Si3N4 ceramics were fabricated by spark plasma sintering (SPS) at 1400-1500 °C for 6 min with 3wt.%MgO + 5wt.%Al2O3 and 3wt.%MgO + 5wt.%Y2O3 as sintering additives. The results showed that the phase composition, microstructure and mechanical properties of α/β Si3N4 ceramics were highly dependent on the type of sintering additive. The incomplete phase transformation from α to β occurred in the presence of an oxynitride (Mg-Al(Y)-Si-O-N) liquid phase. Compared with MgO-Al2O3, MgO-Y2O3 can significantly improve the β conversion rate of as-sintered α/β Si3N4 ceramics. And the as-sintered ceramics using MgO + Al2O3 as sintering additives had higher mechanical properties.

Laser-induced local phase transformation of CIGSe for monolithic serial interconnection: Analysis of the material properties

The change of electrical conductivity in chalcopyrite (i.e., Cu(Inx, Ga1−x)Se2 or CIGSe) solar cells induced by nanosecond laser pulses is investigated as a function of the elemental composition and its spatial distribution. The underlying laser induced phase transformation process, which results in a decomposition of the CIGSe semiconductor and a modification of its elemental composition, is utilized to form the monolithic series interconnection between front and back contact in CIGSe based thin film solar cells. The results show a dependence of the composition of the CIGSe layer and the resulting series resistance on the applied laser fluence. Lower series resistance is primarily related to an enhanced fraction of copper, gallium and zinc in the laser transformed zone resulting from selective vaporization of absorber elements. For intermediate laser fluences (~0.36J/cm2) a patterning process is established that allows reliable and high-quality series interconnection. Both, lower and higher laser fluences result in high series resistances due to incomplete phase transformation or damages of the back contact, respectively.

Magnetic and magnetocaloric properties of Er-Co-Al thin-film alloys

Er-Co-Al thin films with the compositions of ErCo1.52Al0.36, ErCo1.69Al0.76, and ErCo1.87Al0.16 were fabricated by magnetron-sputtering, and their magnetic and magnetocaloric properties were characterized in the asdeposited state and post-annealed state at 1073 K. The X-ray diffraction results indicate that the microstructure in the as-deposited state consists of an amorphous phase, which then transforms into a cubic Laves phase with C15 symmetry on annealing. The phase transformation, however, is incomplete, evidenced by the existence of Co precipitates and some oxides of Er and Al in the annealed samples. A compensation-like behavior is observed in the as-deposited samples, probably due to the sperimagnetic structure, but it disappears after annealing. The magnetic entropy change is largest for the annealed ErCo1.87Al0.16 sample, in which the composition is closest to the pseudo-binary Er(Co,Al)2 compound. The observed value, however, is rather small (3.2 J/kg K in a field sweep of 0–50 kOe), due the incomplete phase transformation from the amorphous to the cubic Laves phase during annealing.

Influence of surface modification on electrochemical performance of high voltage spinel ordered-LiNi0.5Mn1.5O4 exposed to 5.3 V for 100 h before and after surface modification with ALD method

To quantify the effect of high voltage on the electrochemical properties of high-potential spinel ordered-LiNi0.5Mn1.5O4, it was intentionally exposed to 5.3V for 100h that can ensure the decomposition of electrolyte. After this treatment, the bulk structure did not change, but electrochemical properties of the sample were severely degraded; polarization became large and capacity loss was substantial. Polarization was caused by formation of a thick insulating passivation layer on the surface of the sample that was measured by impedance spectroscopy. The capacity loss can be partially caused by incomplete phase transformation during discharging as a result of loss of electrical contact due to the presence of the thick passivation layer on the surface of particles. This indicates that the phase transformation depends on the applied current. The other cause for the capacity loss can be from the inactiveness of transition metals in the surface that was measured by XPS. Thick passivation layer on the surface can have inactive transition metals leading to permanent capacity fading. Hence, to control the electrode stability in high voltage spinel LiNi0.5Mn1.5O4, a bare LNMO sample coated with Al2O3 by Atomic Layer Deposition (ALD) were prepared, then exposed to 5.3V for 100h. After this surface treatment, the Al2O3-coated sample showed much better electrochemical performance than the bare sample. During the exposure, the bare sample underwent intensive surface reactions with very large generated current density and large charge-transfer resistance. In contrast, the coated sample experienced much weaker surface reactions with low charge-transfer resistance even though the applied potential, 5.3V was much higher than the stable upper voltage limit (∼4.5V) of conventional electrolyte. The coating effectively protects the surface of the material from surface reactions such as oxidation of the electrolyte; therefore Al2O3-coated LNMO shows reasonable electrochemical properties after exposing at 5.3V for 100h. This finding demonstrates that detrimental effects of the exposure at high potential on the electrochemical properties strongly depends on surface characteristics. This understanding can be used to stabilize high-voltage positive electrode materials.

Formation of Packet (Lath) Martensite in Iron-Nickel Alloys

The kinetics of isothermal formation of packet martensite in Fe – Ni alloys is considered. Atheory describing the formation of a packet of martensite crystals is presented as an autocatalytic process initiated by formation of the first laths of a future packet and of blocks in it. The theory predicts progressive change of the “instantaneous” exponent of power in the Avrami equation from 2 or 3 to 1, which has been observed indeed for alloy Fe – 14.95% Ni. The kinetics of incomplete phase transformations is described within the concept that the reaction is stopped due to decrease in the thermodynamic stimulus of the transformation caused by accumulation of defects in the initial phase. An equation describing the C-curve of the transformation is obtained.

Interpreting the effect of de-solvation on structure of molecular crystals.

The structure effects of varying hydration/solvation levels in molecular crystals can sometimes be difficult to characterize with analytical techniques based solely on Bragg diffraction or other more common methods such as thermal analysis or spectroscopy. It can be demonstrated that supplementary scattering features such as diffuse and satellite intensities can provide insight into these processes even just from a qualitative standpoint. Understanding the nature of these processes is of great importance to many industrial applications (e.g. pharmaceuticals, bulk characterization). One previous caveat is that interpretations for these features are often restricted to experts, yet our work demonstrates this is not the case. For our examples, real-space models can be used to calculate both diffuse diffraction and inter-modulation satellite features. In one case study what we observe when the water leaves the crystal is an incomplete phase transformation and each crystallite in the bulk can be explained as being made up of disordered layers of the different structure phases with an intermediate phase layer resulting in satellite reflections. Another example demonstrates how in a crystal which has one solvent replaced for another, temperature sensitive phase transitional phenomena are induced. Models based on disorder diffraction features not only provide further insight into understanding of the de-solvation process but also could account for observed changes in PXRD from bulk. This can be related to similar material behaviors observed in diffraction patterns from more complex host-guest systems.