Characteristic Transformation Temperatures Research Articles

Microstructure and texture control of Ni-Mn-Ga magnetic shape memory alloys manufactured by laser powder bed fusion

The Laser Powder Bed Fusion (LPBF) process was used to manufacture Ni-Mn-Ga polycrystalline samples. The initial powders with a fine particle size of about 20 µm were firstly prepared by a ball milling from melt-spun ribbons. The microstructure and texture evolution of Ni-Mn-Ga alloy manufactured by LPBF were investigated employing SEM, TEM, and synchrotron radiation diffraction. Using two different scanning strategies, optimization of laser parameters and post-processing heat treatment, a homogeneous microstructure with a strong crystallographic texture was obtained. The localized heating/cooling conditions allow obtaining a layered structure with preferred <100> fiber orientations along the growth direction. The crystal structure and crystallographic texture drastically change when the laser oscillation mode is chosen. The strong crystalline anisotropy and layered-type microstructure have a significant impact on twining flow behavior giving rise to an anisotropic mechanical response. The variant reorientation during mechanical training is realized by the so-called orthogonal shearing. It is also shown that the resulting crystal structure and characteristic transformation temperatures are strongly dependent on chemical composition related to Mn losses and internal stresses along with chemical order creating metastable phases due to the extremely high cooling rate upon the 3D printing. Therefore, all the above parameters are thoroughly monitored during the three-stage fabrication process.

Influence of applied stress on shape memory characteristics of Ni50Ti45Cu5 (at.%) alloy subjected to thermomechanical cycling

Shape memory alloys have made rapid progress in many domains, primarily biomedical (endovascular stents, orthodontic archwires), and engineering (smart actuators, robotics, hydraulic couplings). The selection of a shape memory alloy for the indented application is based on its characteristic phase transformation temperatures. These characteristic temperatures are influenced by myriad parameters, such as composition, microstructure of the alloy, defect density, etc. When an shape memory alloy under an external load is subjected to cyclic operations to perform useful work, for example, actuators, these characteristic temperatures are modified. This study, therefore, aims to understand the influence of external loading on the shape memory characteristics of a Ni50Ti45Cu5 (at.%) alloy. A wire of 1.43 mm diameter and length of 100 mm was subjected to heating and cooling between its phase transformation temperatures in a cyclic manner under constant stress (of up to 60 MPa). The maximum recovery strain, actuation/retraction rate, and the stress influence coefficient were determined and compared with those of the other Ni-Ti and Cu-based shape memory alloys. The results show that raising the load level causes an increase in the transition temperatures, especially the Ms (martensite start temperature) rather than the other phase transformation temperatures (martensite finish (Mf), austenite start (As), austenite finish (Af)). It also significantly affects the recovery strain and the rate of retraction during forward transformation and the symmetry of operation.

Surface Effect on Phase Transformation of Single Crystal NiTi Shape Memory Alloys Studied by Molecular Dynamics Simulation

Surface effect on phase transformation of single crystal (SC) NiTi shape memory alloys (SMAs) with various thicknesses and various Ni contents is studied by molecular dynamics simulation. For the SMAs with various thicknesses considering surface effect, the residual strain, the average atomic potential energy, and the four characteristic phase transformation temperatures all increase, and fewer twinned martensite boundaries are left after cooling compared to the sample without considering surface effect. Moreover, with increasing thickness of the sample, the surface effect is gradually weakened, and there is no obvious regularity in phase transformation characteristics. For the SMAs with various Ni contents considering surface effect, segmental martensite phase transformation, phase transformation fluctuation, and wider phase transformation temperature ranges are observed, whereas these phenomena do not occur in the sample without considering surface effect. With the increasing of Ni contents, for both cases of considering surface effect or not, the critical stress of phase transformation, average atomic potential energy, the modulus of detwinned martensite increase, but the phase transformation temperature and modulus of twinned martensite decrease.

Martensitic Transformation Temperatures and Hall Effect in Ni47-xMn41+xIn12 (x = 0, 1, 2) Alloys.

At present, the question of the relationship between the characteristic martensitic transformation temperatures (MTT) and the electronic parameters of a system has not been fully studied. In the present work, an attempt to establish a similar relationship using the example of the concentration of charge carriers, n, was made. The field dependences of Hall resistivity ρH and magnetization M of the magnetocaloric Ni47-xMn41+xIn12 (x = 0, 1, 2) alloys were measured at T = 4.2 K and in magnetic fields of up to 80 kOe. The MTT were obtained from the temperature dependences of electrical resistivity and magnetization. It was observed that the MTT correlate strongly with both the valence electron concentration e/a and the electronic transport characteristics, which are the coefficient of the normal (NHE) R0 and anomalous (AHE) RS Hall effect and the concentration of charge carriers n.

Molecular Dynamics Simulation of NiTi Shape Memory Alloys Produced by Laser Powder Bed Fusion: Laser Parameters on Phase Transformation Behavior.

In this study, the deposition, powder spreading, and laser fusion processes during the laser powder bed fusion (L-PBF) process were studied using molecular dynamics (MD) simulation. The effect of Ni content on the characteristic phase transformation temperatures was also investigated. Shape memory effect and superelasticity of NiTi alloys with Ni content ranged from 48.0% to 51.0% were analyzed. By employing MEAM potentials, the effects of the laser power, spot diameter, and scanning speed on the molten pool size and element evaporation were studied. Simulation results showed that a larger spot diameter renders a higher Ni content in the molten pool, also a larger molten pool. A faster scanning speed leads to a higher Ni content in the molten pool, and a smaller molten pool. The element is difficult to evaporate using small laser power and a large spot diameter. The element in the molten pool expresses a great evaporation effect when the Es is larger than 0.4 eV/ų. According to Ni content within the molten pool during laser fusion, characteristic phase transition temperatures in single crystalline NiTi alloys with variant Ni content were investigated by employing a 2NN-MEAM potential. Characteristic phase transition temperature changes as the Ni content increases from 48.0% to 51.0%. Austenite boundaries and Ni content in the boundary were found to be the keys for controlling the characteristic phase transformation temperature.

Thermo-magnetic loading effects on high-frequency dynamic behaviour of magnetic shape memory alloys

This paper theoretically studies the thermo-magnetic loading effects on the long-term (>100 s, reaching the steady state) high-frequency (>100 Hz) dynamic behaviour of magnetic shape memory alloys actuated by cyclic magnetic fields. The material's dynamic behaviour at different levels of ambient heat-transfer, ambient temperature, applied magnetic field frequency and amplitude are simulated by a dynamic model incorporating both magnetic-field-induced martensite reorientation and temperature-driven martensitic phase transformation. Analytical expressions of the material's long-term steady-state behaviour (i.e., stable strain amplitude and temperature) as a function of ambient thermal conditions and magnetic loading conditions are further derived. It is found from model simulations and analytical calculations that weak ambient heat-transfer, high ambient temperature, and high magnetic field amplitude (to trigger martensite reorientation) lead to large net heat generation from dissipative martensite reorientation and thus increase material's stable temperature, which results in reduced twinning stress and increased stable strain amplitude. It is also found that the material's stable temperature can reach the characteristic phase transformation temperature triggering the martensite-to-austenite transformation. In this case, weaker ambient heat transfer, higher ambient temperature and higher magnetic field frequency result in larger heat generation rate accelerating the martensite-to-austenite transformation. Therefore, less martensite remains in the material and the material's stable strain amplitude becomes smaller.

TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation

ABSTRACT The martensitic phase transformation in Ti40.4Ni48Hf11.6 shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO2/Si composites. The shape memory properties of magnetron sputtered Ti40.4Ni48Hf11.6 films annealed at 635°C – 5 min are influenced by film thickness and the underlying substrate. Decreasing TiNiHf film thickness from 21 μm to 110 nm results in the reduction of all characteristic transformation temperatures until a critical thickness is reached. Particularly, Ti40.4Ni48Hf11.6 thin films as low as 220 nm show transformations above room temperature when deposited on SiO2 buffer layer, which is of great interest in nano-actuation. In comparison, 220 nm films on Si substrates are austenitic at room temperature, and thus not suitable for actuation. Thermal fatigue tests on TiNiHf/SiO2/Si bimorphs demonstrate better functional fatigue characteristics than freestanding films, with an average reduction of 15°C after 125 cycles, with temperature stabilization subsequently. Experimental bi-directional actuation results are promising in the development of bistable actuators within a PMMA/TiNiHf/Si trimorph composite, whereby the additional PMMA layer undergoes a glass transition at 105°C. With the aid of constitutive modeling, a route is elaborated on how bistable actuation can be achieved at micro- to nanoscales by showing favorable thickness combinations of PMMA/TiNiHf/Si composite.

Evolution of microstructure and crystallographic texture of Ni-Mn-Ga melt-spun ribbons exhibiting 1.15% magnetic field-induced strain

The microstructure and texture evolution of 10M Ni-Mn-Ga melt-spun ribbons were thoroughly evaluated by high-energy synchrotron radiation and electron backscatter diffraction. The as-spun ribbons were subjected to annealing treatment in order to tailor microstructure, atomic order degree, and crystallographic texture. The optimum annealing treatment at 1173 K for 72 h produced a homogenous <100> fiber texture and induced grain growth to the size that spans the entire ribbon thickness. This in turn reduced microstructural constraints for twin variant reorientation in the direction perpendicular to the ribbon surface. On the other hand, a homogenous radial microstructure ensured in-plane stress/strain compatibility giving rise to strain accommodation during variant reorientation. Particular attention was also given to the evaluation of atomic order, which to the largest extent controls the characteristic transformation temperatures. It also lowered the twinning stress to a level sufficiently low for martensitic variant reorientation under magnetic field. As a result, 1.15% magnetic field-induced strain without the aid of mechanical training in the self-accommodated state was achieved.

Thermostructural shape memory effect observations of ductile Cu-Al-Mn smart alloy

The Cu-Al-Mn shape memory alloy (SMA) with a new different composition was fabricated by arc melting method. The characteristic shape memory effect (SME) property of Cu-Al based SMA was revealed by performing thermostructural measurements. The differential scanning calorimetry (DSC) tests were taken to observe the reversible martensitic phase transformation peaks of the alloy as evidence of SME property of the alloy. To determine the basic thermodynamical parameters of the alloy, these endothermic and exothermic transformation peaks were analyzed by the tangent differentiation method that was performed automatically by the DSC analyzing program over a manually selected part on the DSC curve and by these analyses the characteristic martensitic transformation temperatures (working temperatures) that found below 100°C and the enthalpy change values of the alloy were directly obtained. The other kinetic transformation parameters of the alloy - the entropy change, hysteresis, and equilibrium temperature - were also determined. The common high-temperature behavior of the Cu-Al based Heusler alloys was detected by differential thermal analysis (DTA) measurement. The XRD and metallography tests that were conducted at room temperature showed the presence of M18R and the dominant 2H martensite structures that formed in the alloy and this dual martensitic structure was also prescribed by determining the theoretical e/a ratio of the alloy. Furthermore, the microhardness tests on the alloy demonstrated the high ductility feature of the alloy. All results demonstrated that the CuAlMn alloy exhibiting a shape memory effect property can be useful in smart alloy applications.

Martensitic Transformation, Thermal Analysis and Magnetocaloric Properties of Ni-Mn-Sn-Pd Alloys

Martensitic transition and magnetic response of Ni50−x Pdx,y Mn36 Sn14−y (x = 0, 1, 2 and y = 0, 1) Heusler alloys were analysed. The crystalline structure of each composition was solved by X-ray diffraction pattern fitting. For x = 1 and 2, the L21 austenite structure is formed and, for y = 1, the crystallographic phase is a modulated martensitic structure. From differential scanning calorimetry scans, we determine characteristic transformation temperatures and the entropy/enthalpy changes. The temperatures of the structural transformation increase with the addition of Pd to replace Ni or Sn, whereas the austenitic Curie temperature remains almost unvarying. In addition, the magneto-structural transition, investigated by magnetic measurements, is adjusted by suitable Pd doping in the alloys. The peak value of the magnetic entropy changes reached 4.5 J/(kg K) for Ni50Mn36Sn13Pd1 (external field: 50 kOe).

Comparison of high strength steels with different aluminium and manganese contents using dilatometry

In this paper, the properties of three low carbon steels containing different percentage of manganese (1.5 or 3 wt%) and aluminium (1.5 or 2 wt %) were tested using dilatometric analysis and metallog-raphy. The steels had chemical compositions typical for TRIP (Transformation induced plasticity) steels and the two steels with an increased manganese content already belonged to the third genera-tion of AHS (advanced high strength) steels. The dilatometric measurement was employed not only to determine the characteristic transformation temperatures but also to simulate the heat treatment procedure commonly used for TRIP steels. Resulting microstructures obtained through this simula-tion were evaluated and compared. The two main alloying elements were proved to have a significant effect on the transition temperatures and thus on the processability and overall resulting properties of the material.

Modifying Thermal and Structural Characteristics of CuAlFeMn Shape Memory Alloy and a Hypothetical Analysis to Optimize Surface-Diffusion Annealing Temperature

In this experimental research study, quaternary Cu-based shape memory alloy with a chemical composition of 73.23Cu-21.93Al-3.36Fe1.48Mn (at.%) and a valence electron concentration (e/a) value of 1.487 was produced by arc melting method. After production, the identical samples of alloy were all subjected to thermal aging at 150 °C for 1 h and then cooled in different coolant materials. A varied number of thermal and structural tests and analyses were carried out successfully to study the alterations in the alloy samples and to see the effects of the aging and quenching/cooling media on alloy’s characteristic martensitic transformation temperatures, eutectoid points, some other kinetic parameters, and structural features. Among these tests, the DSC analyses showed the variations between the thermodynamical parameters such as the martensitic transformation temperatures of the alloy samples that occurred due to the effects of both aging and various quenching environments. Differential thermal analysis (DTA) measurements showed that some eutectoid point shifts (~ 20-25 °C) occurred on the DTA thermograms of the alloy samples which happened mainly due to the thermal aging. A detailed hypothetical analysis and discussion related to the eutectoid temperature made upon DTA results, since the eutectoid temperature has a key role in some thin-film SMA applications for selecting optimum annealing temperature to make good diffusion of the metal layer on the semiconductor surface without causing a deterioration occurs in SMA properties. The results of structural XRD tests showed that the main martensite peak, the intensities of all peaks, and crystallite size values were changed by the effect of aging. In addition, the intensities of all peaks and crystallite size values were also changed by thermal aging and also by quenching the alloy samples in different environments. All obtained results have reflected that modifying the characteristic parameters of this CuAlFeMn SMA by such ways used here may be helpful in various macroscale SMA-based applications and also in designing miscellaneous micro/nanoscale electromechanical systems (MEMS and NEMS) where different modified SMA properties have been demanding.

Preparation of Cu–Al–Ni shape memory alloy strips by spray deposition-hot rolling route

The present work describes the preparation of near-full density Cu–Al–Ni shape memory alloy (SMA) strips via a novel processing route consisting of ‘spray deposition’ of atomised liquid Cu–Al–Ni alloy with a jet of argon gas followed by hot-rolling densification of the deposited preform. The subsequent homogenisation of the hot rolled Cu–Al–Ni strips resulted in complete martensitic structure in the finished strip, consisting of self-accommodated plates of β1′ and γ1′ martensites. The characteristic transformation temperatures and shape memory effects of Cu–Al–Ni strips were studied. It has been demonstrated that the Cu–Al–Ni SMA strips, prepared in the present work, resulted in relatively finer grain size with better combination of strength and ductility compared to other techniques based on conventional casting method.

Evolution of crystallographic orientation, precipitation, phase transformation and mechanical properties realized by enhancing deposition current for dual-wire arc additive manufactured Ni-rich NiTi alloy

Ni-rich NiTi alloys were deposited using the in-situ alloying wire arc additive manufacturing (WAAM) method, with varying deposition currents from 80 A to 120 A. The effects of deposition current on the crystal orientation, precipitation, phase transformation and mechanical properties of the WAAM-deposited NiTi alloys were investigated. The results show that increasing the deposition current during the WAAM process would result in noticeable coarsening of B2 grain and an increased volume fraction of high angle grain boundaries (HAGBs). Also, the texture intensity gradually decreased with increasing deposition current. The fabricated components are dominated by the B2 phase with quantities of Ni4Ti3 precipitates in all samples. When increasing the deposition current during the WAAM process, the size of Ni4Ti3 precipitates generally increased and gradually decomposed into a stable Ni3Ti phase which could be detected in the sample produced at 120 A. Furthermore, all of the characteristic phase transformation temperatures increased with the deposition current while the ultimate tensile strength dropped from 927.9 MPa to 613.8 MPa and elongation reduced from 8.7 % to 5.6 %. The cyclic loading-unloading tests revealed that similar trends for the evolution of irreversible strain (εir), recoverable strain (εre), recovery ratio, and elastic energy storage efficiency (η) during cycling were obtained in all samples processed with different deposition currents. The highest εre of 3.2 % and the highest recovery ratio of 53.9 % were obtained in the sample processed with 80 A at an applied stress of 700 MPa for ten cycles. The change of mechanical properties with varying deposition current is due to a combination of factors including precipitation hardening effect, grain refinement effect, and crystal orientation. These results can be useful for optimizing WAAM process parameters to fabricate NiTi components with acceptable structural properties.

Magnetic properties, martensitic and magnetostructural transformations of ferromagnetic Ni–Mn–Sn–Cu shape memory alloys

The influence of Cu doping on structural and magnetic properties of Ni50−xMn36Sn14−yCux,y (x = 0, 1, 2 and y = 1 at.%) ribbons produced by melt spinning has been investigated. The crystalline structures of the alloys were determined by X-ray diffraction (XRD) patterns analysis XRD measurements. Cu addition in specific sites shifts structural transition temperatures. The L21 austenite is found for x = 0, 1 and 2 and modulated martensitic structure for y = 1. Characteristic transformation temperatures were obtained from differential scanning calorimetry scans. It is found that the addition of Cu for Ni stabilizes the austenite phase (increasing martensitic start temperature from 194 to 228 K), whereas replacing small amounts of Cu for Sn stabilizes the modulated martensite phase (increasing martensitic transformation temperature from 194 to 325 K). The transformation temperatures generally increases as the Cu content increases. Therefore, the magnetostructural transition, analysed by vibrating sample magnetometry, is tuned by appropriate Cu doping in the alloys. Likewise, both martensitic and austenitic states exhibit ferromagnetic behaviour.

An exploratory research of calorimetric and structural shape memory effect characteristics of Cu–Al–Sn alloy

The ternary CuAlSn alloy was fabricated as cast ingot by arc melting method and the traditional homogenization and fast cooling (quenched in iced-brine water) SMA processes were carried out on small-cut samples of the ingot alloy. Then, the differential thermal and structural EDX, XRD measurements and analyses were made for this monotype alloy samples. The DSC thermograms graphed at different heating/cooling rates revealed the deep endo/exo peaks indicating martensitic phase transitions occurred in the alloy. The alloy's behavior in high temperature region was elicited by DTA results. The thermodynamical parameters such as characteristic phase transformation temperatures, hysteresis, enthalpy and entropy changes, equilibrium temperature were determined either by taking directly from DSC data or by calculations made on them. XRD measurement made in room conditions showed the diffraction patterns belong to the existing martensite phase in the alloy, plus the crystallite size of the alloy was computed by using XRD data.

Heat treatment and quenching media effects on the thermodynamical, thermoelastical and structural characteristics of a new Cu-based quaternary shape memory alloy

In this study, Cu-based shape memory alloy (a smart material) with a new quaternary Cu-22.78Al-2.59Fe-2.44Mn (at%) composition was produced by arc melting and the samples cut from this cast-ingot alloy were homogenized at 900 °C for 1 h and quenched in traditional iced-brine water. Then except keeping one as the reference the others were all heat-treated (aged) at 200 °C for 1 h and then quenched/cooled in several different media. To find out the effects of the aging and varied quenching/cooling media types on the characteristic martensitic transformation temperatures, thermodynamical parameters and structural properties of this CuAlFeMn shape memory alloy (SMA) the thermal and X-ray measurements were carried out. The differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements were made at different heating/cooling rates for all of the different types of the alloy samples. Thermal results showed that the characteristic forward martensite to austenite and backward phase transitions occurred at varied temperatures in between 44 and 176 °C and the different samples had different characteristic transformation temperatures and hysteresis due to the different quenching media effects. All of the samples exhibited high temperature shape memory alloy (HTSMA) behavior since all of their austenite start temperatures came up over 100 °C. The highest average austenite phase finish temperature belonged to the sample quenched in boiling water and the lowest belonged to the sample quenched in liquid nitrogen. The least amount of transformation entalphy and entropy change values were found for the aged sample quenched in liquid nitrogen which is the fastest coolant medium among others and caused the neatest martensite and interface formations and this was also understood from this sample having the lowest Gibbs free energy difference (ΔG). The X-ray analyses of the CuAlFeMn SMA samples showed the formations of β1′(M18R) and γ1′(2H) in the alloy and a main peak plane change occurred by aging. Besides this, there has been an adverse magnitude ranking correspondency found between average crystallite size and entropy change values of the aged samples. The analyses showed that the characteristic thermodynamical, thermoelastical and structural parameters of the CuAlFeMn high temperature shape memory alloy are strongly affected from the conditions of composition, aging temperature and quenching/coolant medium type used and the different combinations of those conditions can lead to their own distinctive genuine consequences.

Investigation of varied quenching media effects on the thermodynamical and structural features of a thermally aged CuAlFeMn HTSMA

In this research work, a Cu-based HTSMA (high temperature shape memory alloy) with a new quaternary Cu-21.52Al-3.49Fe-1.95Mn (at%) composition was produced by arc melting and the samples cut from this cast-ingot alloy were aged all at 250 °C thermally for 1 h and then quenched or cooled in several different coolant media. In order to determine the effects of the quenching/cooling media on the characteristic martensitic transformation temperatures, thermodynamic parameters and structural features of the HTSMA alloy the thermal analyses were executed by differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements at different heating/cooling rates for every sample quenched/cooled in different medium. The characteristic direct and reverse martensitic phase transitions occured at high temperatures (above 100 °C) were appeared on the all heating/cooling DSC curves. The DSC analyses also showed that the highest transformation temperatures were determined for the sample quenched in boiling water and the lowest ones for the sample that quenched in iced-brine water. Among the coolant quenching media, the liquid nitrogen and boiling water with having the fastest coolant effects on the aged hotter alloy were found that they led alloy to have the least entalphy and entropy change amounts during martensitic phase transformations of this aged alloy. Also these fastest coolant media caused the formations of finest and neatest martensite structures in the alloy with the lowest degree of atomic orders. The X-ray analyses were made at room conditions to determine the structural features, the diffraction planes and their corresponding phases of the aged CuAlFeMn alloy samples. For all of the samples, the prevailing martensite planes were observed as the peaks resulted from β1′(18R) planes and the largest crystallite size was determined for the alloy sample quenched in liquid nitrogen which implied that the sample became to have the finest single monolith crystallite with the lowest degree of atomic order, this came out just like the indications and results of DSC analyses. Furthermore, it can be said that using liquid nitrogen or boiling water as a coolant quenching medium after heat treatments on HTSMAs can give better results than using traditional iced-brine water quenching medium. From all results, it is found that the type of quenching medium causes significant distinct effects on the heat treated CuAlFeMn HTSMA, so the different requirements demanded in SMA related applications can be afforded by choosing the appropriate aging and quenching method.

Molecular dynamics simulation on the shape memory effect and superelasticity in NiTi shape memory alloy

Molecular dynamics (MD) simulations Ni-Ti based on a second nearest neighbor modified embedded-atom method potential were performed to study atomic-scale mechanical behavior and microstructural evolution of Ni-Ti alloy at different temperatures. By using four calculated characteristic transformation temperatures, the optimum simulated temperature ranges were determined. At temperatures lower than Ms, a thermally induced self-accommodating martensitic phase consisting of three variants formed. Each of the two variants was twin structures. In the subsequent loading process, the most favorable variant grew with the movement of interfaces among the variants, which produced remnants of the residual strain after unloading. After heating up to over Af, the residual strain disappeared and the shape recovered to the original. The shape memory effect under austenite state was also performed, wherein the sample exhibited a typical stress–strain temperature curve and remained in a martensitic state after unloading. After heating at a high temperature, the martensite transformed back into its parent phase accompanied with the recovery of residual strain. The material exhibited superelastic behavior above Af, and the critical transformation stress increased with increased temperature. Some parts of B2 structures remained at the end of the plateau stage and continued transforming into martensites during the hardening stage for both T > Af and Ms <T <Af. The critical stresses for transformation satisfied the Clausius–Clapeyron relationship at T > Ms.

A new correlation between the characteristics temperature and glass-forming ability for bulk metallic glasses

A new criterion or parameter χ given by $$\left( {\frac{{T_{\text{x}} - T_{\text{g}} }}{{T_{\text{l}} - T_{\text{x}} }}} \right) \times \left( {\frac{{T_{\text{x}} }}{{T_{\text{l}} - T_{\text{x}} }}} \right)^{a}$$ (wherein Tg is the glass transition temperature, Tx the onset crystallization temperature, Tl the liquidus temperature or the offset temperature of melting and a a fitting exponent) has been suggested to predict the glass-forming ability (GFA) of bulk metallic glasses (BMGs). Regression analysis between the critical diameter Dmax and this criterion yielded a value of a = 1.47. There exists a linear relationship between χ and Dmax with a coefficient of determination R2 = 0.43, and this correlation can be expressed by the approximation formula: $$D_{\hbox{max} } = \left( {0.33 \pm 0.30} \right) + \left( {15.0 \pm 0.70} \right)\chi$$ . The comparisons of the χ parameter with the currently available parameters based on characteristic transformation temperatures of BMGs clearly confirmed that the new criterion exhibits the best ability for appraising the GFA of the alloys. Finally, evaluated glass-forming tendencies of four previously reported Zr-based BMGs using this χ parameter were compared with their corresponding experimental results.