Toop Model Research Articles

Design method of immiscible dissimilar welding (Mg/Fe) based on CALPHAD and thermodynamic modelling

Joining dissimilar metals is a major challenge in joining technology; the weldability of immiscible systems is especially challenging. In this study, a design methodology for dissimilar welding is suggested. The Miedema model and Toop model are developed to calculate the thermodynamics of quaternary alloy systems (Mg-Fe-Al-Cu). Finite element modelling (FEM) of temperature fields and the calculation of phase diagrams (CALPHAD) are combined to provide prerequisite information for modelling. As a test subject, laser welded lap configuration joints of AZ31B magnesium alloy and DP590 steel with a copper coating were put into the design scheme. The interfacial elemental diffusion and formation of intermetallics (IMCs) along the interface during the welding process are predicted. This simulation design scheme predicts the interfacial reaction kinetics and identifies whether the intermediate element works or not. The effects of the Cu coating thickness on the weld constitution, interfacial microstructures and mechanical properties were studied. Cu coating promotes the weld formation fostering the metallurgical reaction of the fusion zone (FZ) with the steel brazing interface. The mechanism of interfacial reactions during the welding-brazing process has been clarified. The Vickers hardness distribution across the interface shows that the Cu-IMCs are ductile.

Calorimetric Studies and Thermodynamic Modeling of Ag-Mg-Ti Liquid Alloys.

Due to the absence of thermodynamic data concerning the Ag-Mg-Ti system in the existing literature, this study aims to fill this gap by offering the outcomes of calorimetric investigations conducted on ternary liquid solutions of these alloys. The measurements were performed using the drop calorimetry method at temperatures of 1294 K and 1297 K for the liquid solutions with the following constant mole fraction ratio: xAg/xMg = 9/1, 7/3, 1/1, 3/7 [(Ag0.9Mg0.1)1-xTix, (Ag0.7Mg0.3)1-xTix, (Ag0.5Mg0.5)1-xTix, (Ag0.3Mg0.7)1-xTix)], and xAg/xTi = 19/1 [(Ag0.95Ti0.05)1-xMgx]. The results show that the mixing enthalpy change is characterized by negative deviations from the ideal solutions and the observed minimal value equals -13.444 kJ/mol for the Ag0.95Ti0.05 alloy and xMg = 0.4182. Next, based on the thermodynamic properties of binary systems described by the Redlich-Kister model and the determined experimental data from the calorimetric measurements, the ternary optimized parameters for the Ag-Mg-Ti liquid phase were calculated by the Muggianu model. Homemade software (TerGexHm 1.0) was used to optimize the calorimetric data using the least squares method. Next, the partial and molar thermodynamic functions were calculated and are presented in the tables and figures. Moreover, this work presents, for comparative purposes, the values of the enthalpy of mixing of liquid Ag-Mg-Ti alloys, which were calculated using Toop's model. It was found that the best agreement between the modeled and experimental data was observed for the cross-sections xAg/xTi = 19/1 [(Ag0.95Ti0.05)1-xMgx] and xAg/xMg = 9/1 [(Ag0.9Mg0.1)1-xTix]. The results of the experiments presented in this paper are the first step in the investigation and future evaluation of the thermodynamics of phases and the calculation of the phase diagram of the silver-magnesium-titanium system.

Metallurgical analysis of laser welded AZ31/5A06 lap joints with Ti–Ni interlayer

The presence of brittle Mg–Al intermetallic compounds (IMCs) in Mg/Al welded joints significantly degrades their mechanical properties. Understanding of the joint interfacial metallurgical reactions is essential for improving the performance of Mg/Al joints. Various single element interlayers, such as Ti, Ni, Zn, Fe, Ce and Cu, have been introduced to improve the joint performance, and the Miedema model has been applied to analyze the priority of phase generations in binary systems. However, the metallurgical reactions in the Mg/Al interface with bi-alloy interlayers cannot be fully explained using only a binary system. In this study, laser welding of AZ31-5A06 using a Ti–Ni interlayer was comprehensively investigated Then, the aggregation tendency of the elements in the Mg–Ni–Ti–Al system was explained using the Toop model and the ternary phase diagram was used to analyze the solidification process in the interlayer. With the increasing of laser power, the interlayer became from intact to incomplete, and finally completely destroyed. As a result, the Mg–Al IMCs with a thickness of 4.5 μm (1617 W) increased to 17–21 μm (1815 W) and 130 μm (2013 W) respectively. The distribution of Mg–Al IMCs also changed from being distributed between Mg-interlayer (1617 W) to being discontinuously distributed within the joint (1815 W) and continuously distributed within the joint (2013 W). When the interlayer remained intact at 1617 W, the maximum shear strength of the joint was 145.76 MPa, and the fracture exhibited a mixed fracture pattern consisting of brittle and ductile fracture. At 1617 W, the distribution of elements at the joint interface revealed that the interlayer was composed of Al, Ni, and Ti without Mg, which successfully blocked the diffusion between Mg and Al. The Gibbs free energy calculated from the Toop model indicated that the minimum Gibbs free energy of an Al–Ni–Ti system was −59.3 kJ/mol, which is lower than that of a Mg–Ni–Ti system (−48.1 kJ/mol); therefore, the Al–Ni–Ti system would be preferentially generated in the joint. The composition of the interlayer was mainly concentrated in the α-Ti and β-Ti phase region of the liquid-phase projection phase diagram and in the TiAl+τ3-Al3NiTi2 and Ti3Al+τ3-Al3NiTi2 phase regions of the isothermal cross-section phase diagram. Consequently, the interlayer consists of multiple phases such as Ti3Al, TiAl, and Al3NiTi2. The Ti3Al and TiAl with superior strength and deformability in the interlayer inhibited the generation of Mg–Al IMC, and the partially melted Ni layer facilitated the connection between Mg/Ni/Ti, thus improving the strength of Mg–Al dissimilar alloy joints.

Calorimetric study of the enthalpy of mixing in the liquid systems Li-Zn and Li-Sn-Zn

Partial and integral molar enthalpies of mixing of binary liquid Li-Zn alloys and ternary liquid Li-Sn-Zn alloys were determined by drop calorimetry at 823 K. The binary system was investigated up to xZn = 0.85 and the integral mixing enthalpy was described with a Redlich-Kister-polynomial. Li-Zn shows an exothermic behavior and a minimum molar mixing enthalpy of about ΔHmix = -11.7 kJ⋅mol−1 at xZn = 0.6. The mixing enthalpy along nine ternary sections of Li-Sn-Zn (A: xLi/xSn ≈ 9:11, B: xLi/xSn ≈ 1:4, C: xSn/xZn ≈ 9:1, D: xSn/xZn ≈ 7:3, E: xSn/xZn ≈ 1:1, F: xLi/xZn ≈ 1:3, G: xLi/xZn ≈ 1:1, H: xLi/xZn ≈ 3:2, I: xLi/xZn ≈ 7:3) was investigated. Solid pieces of Li, Sn, and Zn respectively were dropped to the liquid binary starting alloys in the calorimeter at 823 K. The integral values at the crossing points of the various sections show excellent agreement. The course of the partial and integral enthalpy values along the sections A, D, E, F, G, H, and I indicate multiple-phase regions. The occurrence of such partially liquid and de-mixed liquid regions is discussed and compared with a calculated version of the isothermal section at 823 K which exhibits a large ternary liquid miscibility gap. The experimental ternary data allocated to the fully liquid monophasic regions was numerically fitted based on a Redlich-Kister-Muggianu polynomial for substitutional solutions including ternary interactions. In addition, a comparison with enthalpy values from the extrapolation methods based on binary data according to the Muggianu and Toop model, respectively, is shown. Both models fail to accurately reproduce the experimental values. Iso-enthalpy plots of calculated integral mixing enthalpy for stable and metastable fully homogeneous liquid alloys for the entire ternary system are shown.

Effect of boron on surface tension of liquid Fe-3.0%Si alloys

Based on Butler and Toop models, the surface tensions of liquid Fe-3.0%Si (wt.%, same as below) alloys with different Boron (B) additions were calculated. Meanwhile, the surface tensions of Fe-3.0%Si and Fe-3.0%Si-0.5%B alloys at 1550–1650 °C were also measured with the sessile drop method. The calculated results indicated that the surface tension of liquid Fe-3.0%Si alloys decreased with the increase of B content from 0.5% to 2%, which is in good agreement with the measured results. Si and B with lower surface tension tend to aggregate on the surface of the liquid alloy, while Fe with higher surface tension tends to aggregate inside the liquid alloy. With temperature increased from 1550 to 1650 °C, the surface concentration of each element is approaching the ideal concentration. Finally, a linear fit is used to correct β values. When β values are between 0.762–0.867, the calculated results of Fe-3.0%Si and Fe-3.0%Si-0.5%B alloys are close to the experimental results.

Predicting the Concentration Dependence of the Surface Tension of Ternary Systems

A comparative analysis of semi-empirical methods based on the Kohler, Bonnier, and Toop models for predicting the surface tension (ST) of ternary systems has been performed. Surface tension–concentration dependence calculations have been carried out in the indium–tin–lead and indium–tin–gallium systems. The ternary indium–tin–lead system is characterized by the fact that the excess surface tension isotherms of all the side binary melts are symmetrically shaped with respect to the equimolar composition, and such systems are called symmetric. In the ternary indium–tin–gallium system, the excessive surface tension isotherms of side binary indium–gallium and tin–gallium systems have pronounced asymmetry with respectto the equimolar composition, and such systems are called asymmetric. It has been shown that a method based on the Kohler model rather precisely describes the concentration dependence of the surface tension ofsymmetric ternary systems. However, this method does not predict the concentration dependence of the surface tension of asymmetric systems. It has been revealed that the Bonnier and Toop models can predict thesurface tension–concentration dependence for asymmetric ternary systems with strong asymmetry in the excess surface tension isotherms of two side binary systems independently of the degree of complexity in these isotherms within overall error of experiment.

Thermodynamic and kinetic analysis of ternary intermetallics formation in Al–Cu–La alloy

The thermodynamic and kinetic analysis of rare earth (RE) intermetallics formation in Al–Cu–La alloy was carried out in this study. The formation enthalpy, excess entropy, excess Gibbs free-energy and activity of each component of Al–Cu–La ternary alloy were calculated combining Miedema model with Toop model. The calculated results show that the formation enthalpy, excess entropy and excess Gibbs free-energy of Al–Cu–La ternary alloy are all negative in the whole alloy composition range. The activities of Al, Cu and La are all tiny in the central zone of composition triangle of Al–Cu–La ternary alloy, indicating that Al, Cu and La components are easy to form ternary intermetallics. Moreover, the parabolic growth kinetics equation of RE intermetallics was derived integrating the Fick's second law with the mass balance law. According to the parabolic growth rate, the thickness of the RE intermetallics increases in proportion to the square root of the diffusion time.

Calculation of Thermodynamic Properties of Mg-Zn-Al Alloy

The thermodynamic properties of Mg, Zn, Al binary alloys and ternary alloys were calculated by Miedema model and Toop model, and magnesium was selected as the asymmetric component. The results show that the mixing enthalpy, excess entropy and excess Gibbs free energy of Al-Zn binary alloy are all greater than zero. The thermodynamic values of Mg-Al and Mg-Zn binary alloys are relatively similar, and both are less than zero. Within the range of composition variation, the excess Gibbs free energy of Mg-Zn-Al ternary alloy is basically negative. In the Mg-poor and Mg-rich regions, the thermodynamic value of the alloy changes greatly. Excess Gibbs free energy and mixing enthalpy of magnesium-zinc-aluminum alloy can be reduced by adding zinc and aluminum.

Thermodynamic, surface and viscous properties of ternary Al–Fe–Mn alloy using theoretical modellings

ABSTRACT Thermo-physical properties of ternary Al-Fe-Mn alloys were studied using various theoretical models. The excess free energy of mixing of binary sub-systems of the ternary alloy was computed using an exponentially optimised interaction parameter in the framework of the R-K polynomial. The excess free energy of mixing of the ternary alloy was computed using different theoretical models. The surface properties of the binary and ternary alloys were calculated using Butler equation. The values of surface tensions so obtained were then used to calculate the excess surface tensions of the binary sub-systems. The optimised parameters for the excess surface tension of the binary sub-systems were then used to calculate the surface tension of the ternary alloy in the framework of different modelling equations. The viscosities of the binary sub-systems were computed using the Kaptay, Kozlov-Romanov-Petrov and Moelwyn-Hughes models and the viscosity of ternary alloys was obtained using the Chou, Toop, Kohler and Kaptay models.

Thermodynamic, surface and transport properties of ternary Al–Sn–Zn liquid alloy and its sub binaries

ABSTRACT The excess free energies of sub-binary Al–Sn, Sn–Zn and Zn–Al alloys and ternary Al–Sn–Zn alloy were computed using the optimised exponential parameters of R–K polynomial at 973, 1073, 1173 and 1273 K. The computed values of free energy of mixing, enthalpy of mixing and activity of the ternary alloy were compared with the available experimental data. The surface concentrations of the monomers of the ternary Al–Sn–Zn alloy were computed using Butler model. The surface tension of the ternary liquid alloy was calculated in the framework of Chou, Kohler, Toop and Bulter models using optimised parameters for the excess surface tension. The viscosity of the sub-binary systems have been computed using Kaptay equation with the help of calculated results of enthalpy of mixing, and the viscosity of the ternary alloy were computed in the framework of Chou, Kohler and Toop model using optimised parameter for the excess viscosity.

Thermodynamic properties of Ga-In-Li alloys - a potential material for liquid metal batteries

With the use of the drop calorimetry method, measurements of the mixing enthalpy change were carried out for the liquid Ga-In-Li liquid solutions. The experiments were conducted for five different cross sections and at temperatures higher than 1000 K. The alloys investigated were as follows: (Ga0.75Li0.25)1-xInx at 1005 K, (In0.75Li0.25)1-xGax at 1005 K, (Ga0.50Li0.50)1-xInx at 1005 K, (Ga0.25Li0.75)1-xInx at 1026 K and (In0.30Li0.70)1-xGax at 1031 K. The mixing enthalpy change, measured for all the cross sections of the Ga-In-Li system, is characterized by a negative deviation from the ideal solutions. Based on the optimized thermodynamic parameters of binary liquid solutions in the form of the Redlich-Kister equation and the experimental data determined in these studies, the ternary interaction parameters of the Toop model were elaborated. An agreement between the calculated and experimental data can be established as well, taking into account the degree of measurement difficulty.

Optimization of thermodynamic and surface properties of ternary Ti–Al–Si alloy and its sub-binary alloys in molten state

Thermodynamic and surface properties of the ternary system were studied at different higher temperatures (1973 K, 2073 K, 2173 K and 2273 K). The thermodynamic properties were studied using the Chou, Kohler and Toop models. Meanwhile, the surface properties were investigated using the same models plus the Butler model. The thermodynamic and surface properties of the ternary alloys computed from different models were compared with one another. The surface concentration of the components of the ternary Ti–Al–Si alloy was computed using Butler's equation.

Prediction of thermodynamic and surface properties of ternary Ti–Si–Fe liquid alloy

Thermodynamic and surface properties of the ternary Ti–Si–Fe liquid alloy have been assessed at temperatures 1873 K, 1973 K, 2073 K and 2173 K. For this purpose, the optimized linear temperature dependent coefficients of Redlich–Kister (R–K) polynomials for excess free energy of mixing for sub-binary systems Ti–Si, Fe–Si and Ti–Fe have been computed considering available experimental and literature data. The thermodynamic properties of ternary liquid alloy, such as activity have been studied using general solution model (GSM) and excess free energy of mixing has been studied using GSM, Kohler and Toop models. In surface properties, surface concentration has been computed using Butler’s equation and surface tension of the liquid alloy has been studied using all the four above mentioned models feeding the database of liquid Fe–Si, Si–Ti and Ti–Fe sub-binary systems. The values of surface tension computed using GSM and Toop model are in well agreement with each other at all concentrations. Moreover, the computed values of excess free energy of mixing and surface tension decrease at elevated temperatures from all corners and at all considered cross-sections indicating the decrease in ordering tendency of the alloy.

Thermodynamic and surface properties of liquid Ti–Al–Fe alloy at different temperatures

ABSTRACT Thermodynamic and surface properties of Ti-Al-Fe liquid ternary alloy have been assessed at temperatures 1873 K, 1973 K, 2073 K and 2173 K. The thermodynamic properties, such as activity have been studied using general solution model (GSM) and excess free energy of mixing has been studied using GSM, Kohler and Toop models. In surface properties, surface concentration has been computed using Butler’s equation and surface tension of the liquid alloy has been studied using all the four above mentioned models feeding the database of liquid Ti-Al, Al-Fe and Ti-Fe sub-binary systems. The values of surface tension computed using GSM and Toop model are in well agreement with each other at all concentrations. Moreover, the computed values of excess free energy of mixing and surface tension decrease at elevated temperatures from all corners and at all considered cross-sections indicating the decrease in mixing tendency of the alloy.

On the Choice of the Geometrical Extrapolation Models for the Mg-Al-Sr System Based on Experimental Investigation

Abstract In the current work, solidification curves were deduced experimentally from Differential Scanning Calorimetric (DSC) using heat transfer model, then compare our experimental work with different extrapolation solidification curves models predicted based on thermodynamic properties of binary sub-systems. The motivation of this work is to better predict ternary thermodynamic properties for binary phase diagram and the error of predicting arises from misjudging the asymmetric component in the asymmetric geometric model, It was found that Kohler gives a better agreement than Muggianu (symmetric model). Tested alloys show that, choosing strontium (Sr) element as the asymmetric component in Mg-Al-Sr system gives the best prediction for solidification curves. In addition, the solidification curves calculated using Toop model for the ternary system (Al or Mg is asymmetric) shows the poorest fit solidification curves construed from DSC data.

Thermodynamic Analysis of Al‐RE Phase Formation in AZ91‐RE (Ce, Y, Gd) Magnesium Alloy

The Al‐RE intermetallic compounds formed by adding rare earth elements into AZ magnesium alloy have important effects on the microstructure and properties of alloy. Based on classical thermodynamic theory and Miedema model and Toop model, the heat of formation of compounds in Mg‐Al‐RE (Ce, Y, Gd) ternary alloy and Gibbs free energy of Al‐RE (Al4Ce, Al2Y, and Al2Gd) compounds are calculated theoretically, the order of formation of compounds in alloys and the formation process of Al‐RE compounds are analyzed. The results show that, Al‐RE (Al4Ce, Al2Y, and Al2Gd) compounds are preferentially formed in Mg‐Al‐RE (Ce, Y, Gd) alloys, Al‐RE compounds are mainly formed before the crystallization of α‐Mg solid solution; meanwhile, with the crystallization of α‐Mg solid solution, Al‐RE compounds can still be formed sustainably. The order of Al‐RE compounds formation ability is Al4Ce, Al2Gd, and Al2Y, but the order of formation ability is Al2Gd, Al2Y, and Al4Ce, when adding the same mass fraction of rare earth (Ce, Y, Gd). In as‐cast structure of AZ91‐0.9 wt%; RE (Ce, Y, Gd) experimental alloys, Al‐RE compounds mainly distributes at grain boundaries of alloys at two different scales, and the amount of β‐Mg17Al12 phase in alloys decreases relative to that of alloys without RE.

An investigation of influencing of Sb and Bi contents on surface tensions associated with Pb-free Sn-Zn-Sb-Bi quaternary and sub-quaternary solder alloys

ABSTRACTThe surface tensions of ternary and quaternary systems of Sn-based Pb-free solder alloys have been calculated using geometric models, such as Muggianu, Kohler, Chou’s general solution model (GSM), Toop, Guggenheim, ideal Butler and Butler models. It is observed from the calculation carried out in the present work that Sb and Bi contents decrease the surface tension of the solder alloys Sn-Zn-Sb-Bi. It is inferred from the statistical analysis that the best agreement between the experimental results and the corresponding calculated values of the surface tensions is generally observed in GSM and Muggianu models among the geometric models. Whereas Muggianu model is the most appropriate. Relatively good agreements have been observed between models considered in this study and experimental data.

Butt laser welding-brazing of AZ31Mg alloy to Cu coated Ti-6Al-4V with AZ92 Mg based filler

Butt laser welding-brazing of Mg to Ti with Mg based filler was performed with the assistance of Cu coating. The thickness of Cu coating was varied in the study, to investigate its influence on microstructure and mechanical properties of the joint. Two main regions were distinguished along the Mg/Cu coated Ti interface. At the upper interface, the bonding mechanism evolved from mechanical bonding into a “Ti3Al and AlCu2Ti” metallurgical bonding, and into a “Ti3Al + Ti2Cu + AlCu2Ti” metallurgical bonding as Cu coating thickness varied from 0 μm to 19.7 μm to 24.9 μm. At the lower interface, mechanical bonding changed into the IMC layer consisted of Ti3Al, Ti2Cu and AlCu2Ti when Cu coating thickness reached 28.2 μm. The chemical potential calculated by the developed Toop model suggested that Cu played a role in promoting the mutual diffusion between Al and Ti. Ti tended to react with Al first compared with Cu suggesting Ti-Al compounds were produced easily at Mg/Ti interface. In the case of Cu coating thickness of 19.7 μm, the joint load reached a peak value of 3457 N, as high as 85.35% of that of Mg base metal. The fracture mode changed from interfacial failure to fusion zone failure when the coating thickness reached 19.7 μm.

The influence of silica nanoparticles on thermal degradation and mechanical properties of nanocomposites based on aliphatic polyurethanes

Nanocomposites based on aliphatic polyurethanes have recently attracted a lot of attention regarding economical and ecological aspects, due to their improved thermal and mechanical properties. The aim of this paper was to investigate the influence of silica nanoparticles, differing in size and specific surface, on thermal stability and degradation, lifetime and mechanical characteristics of the obtained nanocomposites. Two series of nanocomposites based on aliphatic polyurethanes were obtained by using a single-step procedure and by addition of silica nanoparticles of types A380 or N999 at different loadings (0,0 0.15, 0.5, 1,0 and 3.5 wt.%). It was found that the increase in heating rate caused shifting of the onset temperature to higher values (from 283 to 312 ?C). According to the shape of DTG curves, it was observed that the degradation mechanism of prepared nanocomposites consists of two overlapping processes, related to the scission of hard and soft building blocks. Based on DTG results, the addition of larger N999 silica nanoparticles induced lower thermal degradation, shifting the maximum rate temperatures of the first and second degradation stages to lower values, and caused the change in the degradation mechanism. Addition of smaller silica nanoparticles (A380) did not significantly affect the mechanism of the degradation reaction, indicating homogeneity of the obtained nanocomposites. The presence of A380 nanoparticles improved thermal stability of nanocomposites, by increasing the onset temperature from 286 ?C for the pristine elastomer to 303 ?C for the sample containing 3.5 wt.% of silica. Existence of interactions of A380 silica nanoparticles with hard and soft phases was observed, based on the increase in the maximum rates of the first and second degradation steps. The activation energy of thermal degradation of polyurethanes modified with A380 silica nanoparticles was obtained by using the Flyn-Wall and Toop models. Dependence of the activation energy and the lifetime of nanocomposites based on aliphatic polyurethanes on the silica content were estimated. The highest Ea values (determined for 1 and 5 % weight loss) were found for nanocomposites containing 0.5 and 0.15 wt. % of A380 silica nanoparticles (121 and 161.2 kJ/mol). A negative effect of the silica addition on mechanical properties of nanocomposites was observed. The polyurethanes containing smaller SiO2 particles (A380) had a higher tensile strength, elongation at break and hardness as compared to the elastomers filled with larger silica nanoparticles (N999).

Effect of Nb, Y and Zr on thermal stability of nanocrystalline Al-4.5 wt.% Cu alloy prepared by mechanical alloying

In the present study, effect of Nb, Y and Zr (having low equilibrium solid solubility in Al) in the formation of nanocrystalline Al-4.5 wt.% Cu-1at.% X (X = Nb, Y & Zr) solid solutions by mechanical alloying and their thermal stability have been investigated. The mechanically alloyed samples were annealed in batches for 1 h at different temperatures ranging from 150 to 550 °C. The thermal stability has been studied through X-ray diffraction (XRD) phase analysis and variation of microhardness as a function of temperature. Transmission electron microscopy (TEM) and SAED analysis showed that 1 at.% Y and 1 at.% Nb could be dissolved into Al-4.5% Cu solid solutions after mechanical alloying up to 8 h of milling. The spontaneity of formation of Al-4.5%CuY, Al-4.5%CuNb and Al-4.5%CuZr solid solutions has been explained from the change of Gibbs free energy as per Miedema's and Toop's models. It demonstrated that the summative energy due to the reduction in crystallite size and accumulation of dislocation density exceeds the theoretical energy barrier required for the formation of the Al-4.5%CuY, Al-4.5%CuNb disordered solid solutions. The grain sizes were found to retain <100 nm even after annealing at 550 °C. This is attributed to the stabilization of metastable grains by segregation of large size solute atoms (Nb, Y and Zr) along grain boundaries and/or Zener pinning by intermetallic precipitates like Al3Nb in Al CuNb, Al3Y and Al3Y5 in AlCuY and Cu10Zr7, Al3Zr and Cu5Zr in AlCuZr alloys.