Temperature-induced Liquid-liquid Structural Transition Research Articles

Microstructure and properties of Sn-3.8Ag-0.7Cu-xCe lead-free solders with liquid-liquid structure transition and Ce addition**This work was supported by Guangdong Province Science and Technology Project (2016B090927008), the high-level leading talent introduction program of GDAS (project number 2016GDASRC-0106) and GuangZhou Science and Technology Project (201707010480).

Our previous work suggested that the Sn-3.8Ag-0.7Cu-xCe (x takes 0%, 0.2%, 0.5% and 1.0wt%) solder melt experienced a temperature-induced liquid-liquid structure transition (L-LST) which was reversible during two heating and cooling cycles far above the liquidus. In this work, the effects of liquid state manipulations and rare earth element Ce addition on solidified Sn-3.8Ag-0.7Cu-xCe structures and properties were investigated. The results revealed that the microstructure was remarkably refined, the spread ability and the shear strength were enhanced when the melt experienced L-LST and Ce addition. Meanwhile, 0.2 wt% Ce addition showed the most homogenous microstructure, leading to better properties. This work elicited that melt manipulations and Ce addition could be an effective way to optimize microstructure and properties of Sn-3.8Ag-0.7Cu-xCe soldered joints.

Composition dependent characteristic transition temperatures of Co-B melts

The temperature-dependent liquid structures of Co-B melts were studied by in-situ measuring the magnetization. A magnetization anomaly in term of the non-Curie-Weiss temperature dependence of magnetization was observed in the overheated state, demonstrating a temperature-induced liquid-liquid structure transition. This anomalous behavior was found to be a universal formula for the Co-B binary alloy system. The transition point (T0) and two paramagnetic Curie temperatures (θp(LI), θp(LII)) corresponding to two distinct kinds of liquids (i.e., high-magnetization liquid (HML) I and low-magnetization liquid (LML) II) were measured. With the increased concentration of Co, T0, θp(LI) and θp(LII) shift to higher temperatures and the Curie constants for the HML and LML decrease. Based on the location of T0, a guideline is drawn above the liquidus in the Co-B phase diagram, which could provide a significant guidance to the practical melt treatment.

Dependence of Solidification for Bi2Te3\u2212xSex Alloys on Their Liquid States

The resistivity versus temperature (ρ-T) behaviours of liquid n-type Bi2Te3−xSex (x = 0.3, 0.45 and 0.6) alloys are explored up to 1050 °C. A clear hump is observed on all ρ-T curves of the three studied Bi2Te3−xSex melts during the heating process, which suggests that a temperature-induced liquid-liquid structural transition takes place in the melts. Based on this information, the solidification behaviours and microstructures of the alloys with different liquid states are investigated. The samples that experienced liquid structural transition show that the nucleation and growth undercooling degrees are conspicuously enlarged and the solidification time is shortened. As a result, the solidified lamellae are refined and homogenized, the prevalence of low-angle grain boundaries between these lamellae is increased, and the Vicker Hardness is enhanced. Atom probe tomography analyses prove that there is no segregation or nanoprecipitation within the grains, but the Te-rich eutectic structure and the evolution of composition near the Te-matrix phase boundary are investigated in a sample that experienced liquid structural transition. Our work implies that the solidification behaviours of Bi2Te3−xSex alloys are strongly related to their parent liquid states, providing an alternative approach to tailor the thermoelectric and mechanical properties even when only a simple solidification process is performed.

Optimizing microstructures and mechanical properties of hypereutectic Al-18%Si alloy via manipulating its parent liquid state

Liquid state manipulations on solidified Al-18%Si structures and properties were investigated based on its liquid abnormal resistivity-temperature behaviors (LARTB), which suggested a temperature-induced liquid-liquid structural transition (TI-LLST). The primary Si crystals are dramatically refined if solidified from the melt after LARTB range, compared with those before and during it. And then, solidified structures are further ameliorated by suitably mixing the high-temperature melt with low-temperature melt in semisolid state, generating increased tensile strength and ductility synchronously. This work elicits that liquid manipulations for optimizing materials structures and properties can be more effective if information suggesting TI-LLST is detected beforehand.

Influence of melt overheating treatment on solidification behavior of BiTe-based alloys at different cooling rates

Persistent efforts have been made to improve the performance of thermoelectric (TE) materials by different preparation methods. However, scarce attention has been paid to the effects of liquid state on solidified structures and properties of TE materials. In this work, the abnormal resistivity-temperature (ρ–T) behaviors of Bi2Te3 and Bi0.5Sb1.5Te3 melts during a definite temperature range in the first heating process were observed, which implied a temperature-induced liquid–liquid structural transition (TI-LLST) may exist in the liquid alloys. Based on this information, by melting specimens at different temperatures before and after TI-LLST and then solidifying at various cooling rates, it is found that the microstructures of solidified specimens are refined and the hardness elevate with cooling rate increasing for a specific composition. More significantly, the effects of crystal size decreasing and hardness increasing are further optimized by melt overheating treatment. In addition, this technique can also alter the crystallographic preferred orientation in some degree, which can also influence the transport of electrons and phonons. Since the TE and mechanical properties are highly susceptible to microstructures, the present work may provide an alternative approach to adjust the performance of TE materials and to improve the reliability and sustainability of TE devices in service.

Enhancing the thermoelectric performance of free solidified p-type Bi0.5Sb1.5Te3 alloy by manipulating its parent liquid state

In this work, an irreversible temperature induced liquid–liquid structural transition (TI-LLST) during the heating process was suggested by abnormal resistivity behaviors of Bi0.5Sb1.5Te3 melt. Based on this information, by manipulating the parent liquid state in free solidification, series of explorations were carried out on the solidified structures and TE properties of Bi0.5Sb1.5Te3 alloy. We found that, from the melt after TI-LLST, the solidified microstructures are evidently refined with more Te-rich eutectic strips precipitating on matrix boundaries. More significantly, hierarchical crystal defects are obviously increased, with tinier nanoprecipitates, more subgrains and boundaries, and much more densified lattice distortions. Consequently, the thermal conductivity, especially the lattice thermal conductivity, is decreased notably with the power factor increased moderately at high temperature. In addition, the ambipolar excitation temperature is shifted to higher value and the slope of thermal conductivity vs temperature is also declined. All these alterations contribute to a synergistic enhancement of the TE figure of merit (ZT) and the maximum figure shifts to higher temperature, with ZTmax = 0.78 at 442 K. The present work may provide a new hint for innovating preparation methods of bulk TE materials with simple free solidification.

Temperature-Induced Liquid-Liquid Transition in Metallic Melts: A Brief Review on the New Physical Phenomenon

Understanding the nature of liquid structures and properties remains an open problem for many fundamental and applied fields. It is well known that there is no other defined phase line above liquidus (TL) in phase diagrams of ordinary alloys. However, via resorts of internal friction, electric resistivity, thermal analysis, X-ray diffraction, solidification, etc., the results of our research on lots of single- and multiple-component melts show a novel physical image: temperature induced liquid-liquid structure transition (TI-LLST) can occur above TL. Moreover, the solidification behaviors and structures out of the melts that experienced TI-LLST are distinct from those out of the melts before TI-LLST. In this paper, some typical examples of TI-LLST and characteristic aspects of the TI-LLST are briefly reviewed, in which the main contents are limited in our own achievements, although other groups have also observed similar phenomena using different methods. In the sense of phenomenology, TI-LLST reported here is quite different from other recognized liquid transitions, i.e., there are only a few convincing cases of liquid P, Si, C, H2O, Al2O3-Y2O3, etc. in which the transition occurs, either induced by pressure or at a supercooled state and near liquidus.

Control of the phase composition and morphology of a Cu-Sb eutectic alloy via liquid-liquid structure transition

The current paper focuses on the solidification characteristics of a Cu-Sb eutectic alloy in its different liquid states. Liquid alloy resistivity-temperature patterns suggest an irreversible temperature-induced liquid-liquid structure transition (TI-LLST), and a reversible TI-LLST occurred during the heating-cooling runs. A set of solidification experiments was conducted based on the results. The irreversible TI-LLST caused an enhanced solidification undercooling, increased solidification rate, refined regular eutectic morphologies, and absence of a pre-eutectic Cu2Sb phase. The reversible TI-LLST resulted in different phase compositions and eutectic structures. The mechanisms behind these transitions are also briefly discussed.

Effect of Liquid Structural Transition on the Morphology of Solid/Liquid Interface during the Unsteady-State Unidirectional Solidification

The temperature-induced liquid-liquid structural transition has been observed and testified in different kinds of alloys. The effect of liquid-liquid transition on the morphology of solid/liquid interface was investigated by means of the unsteady-state unidirectional solidification. The results showed that the interface instability of Sn-1wt.%Pb was developed after the liquid structural change, which suggested that the solute distribution coefficient decreased due to the structural change of liquid Sn-1wt.%Pb and the solute on the frontier of solid/liquid interface enriched.

Effect of liquid–liquid structure transition on solidification and wettability of Sn–0.7Cu solder

The temperature dependence of electrical resistivity ( ρ– T) of a Sn–0.7Cu (wt.%) melt was measured and an abnormal change was found on the ρ– T curve within the range of 825–1066 °C. The result suggests that the melt has experienced a temperature-induced liquid–liquid structural transition (TI-LLST), and the transition is reversible after the first cycle heating. Based on the result of TI-LLST, solidification experiments and spreadability tests were carried out on the Sn–0.7Cu alloy to investigate the effect of liquid structure transition on solidification microstructure and wettability. The results show that the microstructure was refined and the wettability was improved when the samples solidified from the melt experienced TI-LLST.

Influence of temperature-induced liquid–liquid structure transition on directional solidification microstructure of Sn–Bi40wt-% alloy

Prior investigation suggested that a temperature-induced liquid–liquid structure transition (TI-LLST) could occur in Sn–Bi40wt-% alloy around 879°C. Based on the results of TI-LLST, the Sn–Bi40wt-% alloys are melted and held at the temperature above and below TI-LLST, respectively, and then solidified from the same temperature. The aim of this work was to explore the influence of TI-LLST on directional solidification microstructure by using the Bridgman method. The results show that dendritic arm spacing decreases and the microstructure is refined markedly for the samples solidified from the melt which experienced the TI-LLST.

On the correlation between solidified microstructures and liquid structural states in Bi-40 wt%Te alloy

The compound-forming alloy Bi-40 wt%Te was selected to explore effects of the temperature-induced liquid–liquid structure transition (TI-LLST) on solidification. The resistivity–temperature pattern of the molten alloy suggested that an irreversible TI-LLST occurred from 694.6 to 723.2 °C as temperature elevated. When solidified from the melt undergoing the TI-LLST, the solidification undercooling of the primary phase Bi 2Te 3 was enlarged and its growth manner was obviously changed, resulting in the spirally curling refined branches, contrary to the orientated thick rod-like crystals solidified from the melt failed to undergo the TI-LLST. Moreover, the volume fraction of the peritectic phase increased from 30.6% to 56.4%, which implied that the phase competition behavior during the peritectic reaction was also changed.

Influence of a Liquid Structural Change on the Solidification of the Alloy CuSn30

The effect of a liquid structural change on the solidification of the alloy CuSn30 is discussed. The temperature dependence of the electrical resistivity (ρ-T) of a CuSn30 (wt.%) melt was measured, and an abnormal change was found on the ρ-T curve within a range of 855°C to 1040°C, which suggests that the melt may have undergone a temperature-induced liquid-liquid structural transition (TI-LLST). The existence of the TI-LLST was verified by means of differential scanning calorimetry. Furthermore, the melt that experienced the TI-LLST needed higher undercooling to nucleate, and the eventual grain size was evidently refined. Analysis of the atomic bonding mechanism of the CuSn30 melt at temperatures above melting point suggest that the structural transition can be attributed to the destruction of the inherited chemical short-range orders or clusters, and that this destruction alone results in the homogeneity of the melt. In turn, the homogeneity of the melt inhibits the nucleation and growth of the primary phase, promotes a peritectic reaction, and eventually refines the solidification microstructures.

Kinetics of Liquid Structure Transition of Sn—(40wt%)Bi Melt

The kinetics of temperature-induced liquid–liquid structure transition (TI-LLST) process in Sn–(40wt%)Bi melt is investigated in isothermal and continuous heating experiments with electrical resistivity method. The time evolution pattern of the electrical resistivity suggested the transition mechanism of TI-LLST for Sn–(40wt%)Bi melt in accordance with the autocatalytic reaction model, which is an indication of nucleation-growth type. With the calculated reaction rate constant KT and apparent activation energy ΔE, we deduce the reason for the characteristics of TI-LLST. The present result may be beneficial for further understanding of the nature of TI-LLST.

Temperature-induced liquid–liquid transition process in eutectic Pb–Sn melt explored fromkinetic viewpoint

Up to now, there have been few studies of the temperature-induced liquid–liquidstructure transition (LLST) kinetics for atomic systems. In this paper, throughisothermal and heating experiments, we have investigated the kinetics of theLLST process in Pb–Sn61.9 wt% melt by measuring the electrical resistivity andheat flux. The time evolution of the heat flux and electrical resistivity can bedescribed by the Johnson–Mehl–Avrami model with an Avrami exponent of 3.87,which is an indication of nucleation-growth-type ordering of the nonconservedorder parameter. What is more, we have calculated the reaction rate constantKT and apparentactivation energy ΔE, and deduced the reason for the characteristics of temperature-induced LLST. The resultof this research could be beneficial for a further understanding of the essence of LLST.