Effect of cooling rate and Al content on solidification characteristics of AZ magnesium alloys using cooling curve thermal analysis

The solidification behavior of AZ Magnesium alloys in various cooling conditions was investigated using a computer-aided cooling curve thermal analysis method. In each case, the cooling curve and its first and second derivative curves have been plotted using accurate thermal analysis equipment and solidification characteristics were recognized from these curves. The cooling rates used in the present study range from 0.22 to 8.13 °C s^?1. The results of thermal analysis show that the solidification parameters of AZ alloys such as nucleation temperature (T _N,α), nucleation undercooling (?T _N,α), recalescence undercooling (?T _R,α), range of solidification temperature (?T _S) and total solidification time (t _f) are influenced by variation of cooling rate. Also, the effect of Al content on these parameters was studied. Microstructural evaluation was carried out to determine the correlation between the cooling rate and secondary dendrite arm spacing.     

Thermal analysis study on the grain refinement of Al–15Zn–2.5Mg–2.5Cu alloy

Computer-aided cooling curve analysis is a reliable method to characterize the solidification behavior of an alloy. In this study, the effect of Al–5Ti–1B grain refiner on the solidification path, microstructure and macrostructure of a new Al–Zn–Mg–Cu super high-strength aluminum alloy containing high amounts of zinc was investigated using thermal analysis technique. The grain size measurement showed that Al–5Ti–1B reduces the grain size from 1402 to 405 μm. Solidification parameters in the liquidus region were in a good accordance with microstructural results. The addition of 1 mass% of Al–5Ti–1B grain refiner decreased ΔT _N from 9.1 to 7.7 °C. It also diminished recalescence undercooling from 1.42 to 0.32 °C. The grain refinement also altered dendritic structure of the alloy from a coarse, elongated and non-uniform to a rosette and more uniform shape. Moreover, the grain refiner resulted in a more uniform distribution of eutectic structure between dendrite arms. Furthermore, the grain refinement enhanced fraction of solid at dendrite coherency point from 21 % for unrefined alloy to 25 % for the alloy containing 1 mass% Al–5Ti–1B. In the same trend, the addition of 1 mass% Al–5Ti–1B reduced the amounts of porosity from 2.3 to 1.8 %.     

Computer-aided cooling curve thermal analysis of near eutectic Al–Si–Cu–Fe alloy

The effects of bismuth (Bi), antimony (Sb) and strontium (Sr) additions on the characteristic parameters of the evolution of aluminium dendrites in a near eutectic Al–11.3Si–2Cu–0.4Fe alloy during solidification at different cooling rates (0.6–2 °C) were investigated by computer-aided cooling curve thermal analysis (CA-CCTA). Nucleation temperature ( $ T_{\text{N}}^{{\alpha {\text{ - Al}}}} $ ) is defined with a new approach based on second derivative cooling curve. The results showed that $ T_{\text{N}}^{{\alpha {\text{ - Al}}}} $ increased with increasing cooling rate but both the growth temperature ( $ T_{\text{G}}^{{\alpha {\text{ - Al}}}} $ ) and the coherency temperature (T _DCP) decreased. Increase in the temperature difference for dendrite coherency ( $ T_{\text{N}}^{{\alpha {\text{ - Al}}}} - T_{\text{DCP}} $ ) with increasing cooling rate indicate a wider range of temperature before the dendrite can impinge on each other and higher fraction solid ( $ f_{\text{S}}^{\text{DCP}} $ ). Additions of Bi, Sb and Sr to the base alloy produced only a minor effect on $ T_{\text{N}}^{{\alpha {\text{ - Al}}}} $ . Additions of Bi and Sb resulted in an increase in fraction solid and an increase of 30 % in the value of $ T_{\text{N}}^{{\alpha {\text{ - Al}}}} \, - \,T_{\text{G}}^{{\alpha {\text{ - Al}}}} $ to almost 13 °C.     

Effect of cooling rate on the microstructure and solidification characteristics of Al2024 alloy using computer-aided thermal analysis technique

The aim of this work was to investigate the effect of different cooling rates on the microstructure and solidification parameters of 2024 aluminum alloy. Solidification characteristics are recognized from the cooling curve and its first and second derivative curves which have been plotted using thermal analysis technique. In this study, a mold having high cooling rate was designed and used to simulate the direct-chill casting process. The results of thermal analysis show that the characteristic parameters of Al2024 alloy are influenced by cooling rate. The cooling rates used in the present study range from 0.4 to 17.5 °C s^?1. Increasing the cooling rate affects the undercooling parameters both in liquidus and eutectic solidification regions. Investigations showed that solidification parameters such as nucleation temperature, recalescence undercooling temperature, and range of solidification temperature are influenced by variation of cooling rates. Microstructural evaluation was carried out to present the correlation between the cooling rate and dendrite arm spacing.     

Effects of Zr addition on solidification characteristics of Al–Zn–Mg–Cu alloy using thermal analysis

The effect of Zr as a grain refiner on the solidification behavior, micro- and macrostructure of a new Al–Zn–Mg–Cu aluminum super-high strength alloy containing high Zn content was studied. The addition of 2 mass% Zr reduced the grain size from 1500 to 190 μm. Moreover, the dendritic structure of the alloy altered from a coarse, elongated and non-uniform morphology to a rosette-like shape and more uniform one. The parameters of liquidus region of cooling curve obtained from thermal analysis were in a good correlation with grain size results. The maximum of first derivative in the liquidus region was introduced beside recalescence undercooling which could predict the grain refinement level even after disappearing of recalescence in the cooling curve. Furthermore, the addition of 1 mass% Zr enhanced fraction of solid in dendrite coherency point from 21 to 31% and lessened the amounts of porosity from 2.3 to 1.4%.     

Thermal characterization of the interaction of poly(3-hydroxybutyrate) with maleic anhydride

In this research, the effects of Al–5Ti–1B grain refiner and Al–10Sr modifier were studied on solidification characteristics and microstructural features of 319 aluminum alloy. Important solidification events such as recalescence and nucleation undercooling temperature and aluminum–silicon eutectic depression temperature have been evaluated using cooling curve and its first derivative curve obtained from thermal analysis of a sample. The aim of this article is to show the ability of the thermal analysis technique to predict some key parameters controlling solidification and casting process. It has been found that the thermal analysis is the identified method for a rapid on-line monitoring of metallurgical characteristics of aluminum alloy melts without conventional metallographic examination.     

Al-Ni-Y三元共晶合金高压凝固组织

利用光学显微镜,扫描电镜,X射线衍射仪对不同压力(2,4,6 GPa)下凝固的Al-1Ni-3Y(%,原子分数)共晶合金组织形貌和相组成进行研究,并且与常压条件下合金凝固组织进行对比。实验结果表明,与常压凝固相比,在超高压力条件下凝固的合金的显微组织形貌发生了很大的改变,但是合金的相组成没有发生变化。常压凝固的合金具有共晶团组织,而高压下凝固的合金具有枝晶状亚共晶组织。高压凝固合金显微组织中出现了大量的初生α-Al相,这表明高压条件下凝固的合金共晶点发生偏移,使得常压共晶成分的合金在高压下变成亚共晶成分合金。同时,共晶组织的形貌发生了很大变化,常压下凝固的合金的组织为Al3Y相与α-Al相形成的菊花状的二元共晶和Al23Ni6Y4相与α-Al相形成的层片状的二元共晶,而高压下这两种共晶组织逐渐转化为离异共晶组织。随着压力的增大,共晶第二相的体积分数减小,α-Al的晶格常数增加。     

Thermodynamic investigations of 1-ethyl-3-methylimidazolium tetrafluoroborate and cycloalkanone mixtures

The densities, ρ, speeds of sound, u, and heat capacities, (C _P)_mix, for binary 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + cyclopentanone or cyclohexanone (2) mixtures within temperature range (293.15–308.15 K) and excess molar enthalpies, H ^E, at 298.15 K have been measured over the entire composition range. The excess molar volumes, V ^E, excess isentropic compressibilities, \( \kappa_{\text{S}}^{\text{E}}, \) and excess heat capacities, \( C_{\text{P}}^{\text{E}}, \) have been computed from the experimental results. The V ^E, \( \kappa_{\text{S}}^{\text{E}} \) , H ^E, and \( C_{\text{P}}^{\text{E}} \) values have been calculated and compared with calculated values from Graph theory. It has been observed that V ^E, \( \kappa_{\text{S}}^{\text{E}} \) , H ^E, and \( C_{\text{P}}^{\text{E}} \) values were predicted by Graph theory compare well with their experimental values. The V ^E, \( \kappa_{\text{S}}^{\text{E}}, \) and H ^E thermodynamic properties have also been analyzed in terms of Prigogine–Flory–Patterson theory.     

Interdiffusion of polymers with glassy bulk

Mechanical properties developing at the contact zone of two samples of linear amorphous polymers with vitrified bulk have been employed to analyze the process of segmental diffusion of reptating chains across polymer?Cpolymer interfaces. The diffusion coefficients (D) of this process have been estimated for polymers with different chain architecture and investigated with respect to healing temperature (T). It has been shown that logD????1/T at $ T < T_{\text{g}}^{\text{bulk}} $ ( $ T_{\text{g}}^{\text{bulk}} $ is the glass transition temperature of the polymer bulk), which implies that the process of segmental diffusion under healing conditions investigated is a thermally activated Arrhenius-like process describing as D?=?D ₀ exp[?E _a/(RT)], where D ₀ and E _a are the activation energy and the pre-exponential factor, respectively. It has been found that logD ₀????E _a, which points to the validity of the so-called kinetic compensation effect for the interdiffusion process at the contact zones of polymers with glassy bulk when an increase in E _a is ??compensated?? by the corresponding increase in D ₀.     

Magnesium–lithium alloys with TiB and Sr additions

The ultralight hypoeutectic α-phase Mg–4.5Li–1.5Al alloy and hypereutectic β-phase Mg–12Li–1.5Al alloy in as-cast state were fabricated and subjected to modification by 0.2 mass% TiB and 0.2 mass% Sr grain modifiers. The crystallisation sequence of Mg–Li–Al alloys has been investigated in detail by using thermal-derivative analysis and microstructural observations. The presented work includes the effects of grain refiners on grain size and microstructure and thermal events registered during crystallisation of ultralight Mg–Li–Al alloys by recording and analysis of the temperature vs time, i.e. as T_N, T_α, T_β, T_η(LiAl) and T_SOL. Microstructure and phase observation has been done by light microscope, X-ray diffraction and energy dispersive X-ray spectroscopy. The changes of characteristic temperature points for phase transformation are studied in detail. Due to the addition of 0.2 mass% TiB and 0.2 mass% Sr, the grain structure of the alloy was refined, and mechanical properties were improved. When a TiB and Sr added simultaneously, the average grain size of the analysed alloys strongly decreases. When the TiB or Sr content was severally added, a low effect of improvements of mechanical properties was observed. With the TiB and Sr content, the liquidus and solidus decrease gradually.

     

Raman Spectroscopic Study of Aluminum Silicate Complextion in Acidic Solutions from 25 to 150°C

Raman spectroscopic measurements were performed on aqueous acid to neutral silica-bearing solutions (0.005 ≤ m _Si ≤ 0.02, 0 ≤ pH ≤ 8) and Al–silica solutions at temperature from 20 to 150°C. At 20°C, the spectrum of silica-bearing solutions exhibits only the bands of water and a completely polarized band at 785 cm^?1. This band is attributed to the ν₁ band of the tetrahedral Si(OH)₄ molecule. In ${\text{Si(OH)}}_{\text{4}} {\kern 1pt} {\kern 1pt} - {\kern 1pt} {\text{AlCl}}_3 {\kern 1pt} - {\kern 1pt} {\text{HCl}}$ solutions, the intensity of this band decreases with increasing Al concentration, temperature, and pH. This decrease can be explained by the formation of an inner sphere complex between Al³⁺ and Si(OH)₄ according to the reaction: ${\text{Al}}^{{\text{3 + }}} {\text{ + H}}_{\text{4}} {\text{SiO}}_{\text{4}}^{\text{0}} ({\text{aq}}){\text{ }} \Leftrightarrow {\text{ AlH}}_{\text{3}} {\text{SiO}}_{\text{4}}^{{\text{2 + }}} {\text{ + H}}^{\text{ + }} $ The fraction of complexed silica deduced from raman spectroscopic measurements is in good agreement with that calculated for the similar solution compositions and temperatures using the complexation constant generated by Pokrovski et al. ⁽²³⁾ from potentiometric measurements. At ambient temperature, the formation of aluminum silicate complex is weak and does not account for more than ca. 5 % of the total Al in most natural waters. As temperature increases, this complex becomes more significant and can dominate Al speciation in acid (pH ≤ 2) hydrothermal solutions.     

On the formation of topological entanglements during the contact of glassy polymers

Fracture energy (G) of the symmetric amorphous polystyrene (PS)–PS interfaces that were partially healed at temperatures (T) below the glass transition temperature of the bulk ( $ T_{\text{g}}^{\text{bulk}} $ ) has been measured at ambient temperature and compared with those reported in the literature (G ₀) for the symmetric PS–PS interfaces that were fully healed at T?>? $ T_{\text{g}}^{\text{bulk}} $ . It has been shown that G developed at T?<? $ T_{\text{g}}^{\text{bulk}} $ corresponds to G ₀ for the polymers having the molecular weight larger than the entanglement molecular weight. This behaviour indicates that topological entanglements can be formed across the contact zone of the polymers with glassy bulk via the interdiffusion of the chain segments located in the viscoelastic contact layer.     

Thermo Physical and Spectroscopic Properties of Binary Liquid Systems: Ethanoic Acid/Propanoic Acid/Butanoic Acid with 2-Methylaniline

Densities (ρ), speeds of sound (u), and viscosities (η) are reported for binary mixtures of 2-methylaniline with carboxylic acids (ethanoic acid, propanoic acid and butanoic acid) over the entire composition range of mole fraction at T?=?(303.15–318.15) K and at atmospheric pressure (0.1 MPa). The excess properties such as excess molar volume (V _m ^E ), excess isentropic compressibility (κ _S ^E ) and excess Gibbs energy of activation of viscous flow (G^*E) are calculated from the experimental densities, speeds of sound and viscosities. Excess properties are correlated using the Redlich–Kister polynomial equation. The partial molar volumes, \( \bar{V}_{\text{m,1}} \) and \( \bar{V}_{\text{m,2}} \), partial molar isentropic compressibilities, \( \bar{K}_{\text{s,m,1}} \) and \( \bar{K}_{\text{s,m,2}} \), excess partial molar volumes, \( \bar{V}_{\text{m,1}}^{\text{E}} \) and \( \bar{V}_{\text{m,2}}^{\text{E}} \), and excess partial molar isentropic compressibilities, \( \bar{K}_{\text{s,m,1}}^{\text{E}} \) and \( \bar{K}_{\text{s,m,2}}^{\text{E}} \), over whole composition range, partial molar volumes, \( \bar{V}_{\text{m,1}}^{ \circ } \) and \( \bar{V}_{\text{m,2}}^{ \circ } \), partial molar isentropic compressibilities, \( \bar{K}_{\text{s,m,1}}^{ \circ } \) and \( \bar{K}_{\text{s,m,2}}^{ \circ } \), excess partial molar volumes, \( \bar{V}_{\text{m,1}}^{{ \circ {\text{E}}}} \) and \( \bar{V}_{{{\text{m}},2}}^{{ \circ {\text{E}}}} \), and excess partial molar isentropic compressibilities, \( \bar{K}_{\text{s,m,1}}^{{ \circ {\text{E}}}} \) and \( \bar{K}_{\text{s,m,2}}^{{ \circ {\text{E}}}} \), of the components at infinite dilution have also been calculated from the analytically obtained Redlich–Kister polynomials. The excess molar volume V^E results are analyzed using the Prigogine–Flory–Patterson theory. Analysis of each of the three contributions viz. interactional V^E(int.), free volume V^E(fv.) and characteristic pressure p* to V^E showed that the interactional contributions are positive for all systems while the free volume and characteristic pressure p* contributions are negative for all the binary mixtures. The results are analyzed in terms of attractive forces between 2-methylaniline and carboxylic acids molecules. Good agreement is obtained between excess quantities and spectroscopic data.     

Viscosity of ternary mixtures. II. Water-tert-butyl alcohol-alkali halides

The viscosities of binary alkali halide-water systems and of ternary alkali halide-tert-butyl alcohol-water systems have been measured at 25°C in the water-rich region. The relative viscosities of the ternary solution are expressed by an extended form of the Jones-Dole equation $$\begin{gathered} \eta /\eta _0 = 1 + {\text{A}}_E {\text{m}}_E^{1/2} + {\text{B}}_E {\text{m}}_E + {\text{B}}_N {\text{m}}_N + {\text{D}}_{EE} m_E^2 \hfill \\ + {\text{D}}_{NN} {\text{m}}_N^2 + {\text{D}}_{EN} {\text{m}}_E {\text{m}}_N + ... \hfill \\ \end{gathered} $$ wherem _E andm _N are the molalities of the electrolyte E and nonelectrolyte N expressed in mole-kg^?1 of water. The parameterA _E accounts for the long-range ionic forces, andB _E andB _N are the Jones-DoleB coefficients of E and N. It is shown, in particular, that theD _EN term is additive for different ionic pairs and that it can be correlated to the entropic coefficient of pair interaction. TheD _EN coefficients thus seem to reflect some pair interaction contribution to the excess viscosity of ternary mixtures.     

Analysis of (NH4)6Mo7O24·4H2O thermal decomposition in argon

Nanometric carbides of transition metals and silicon are obtained by using precursors. Control of the course of these processes require data concerning transformations of single precursor, transformations of precursor in the presence of reducing agent and synthesis of the carbide. In this work, the way of investigating such processes is described on the example of thermal decomposition of (NH₄)₆Mo₇O₂₄·4H₂O (precursor) in argon. The measurements were carried out by TG–DSC method. The solid products were identified by XRD method, and the gaseous products were determined by mass spectrometry method. There was demonstrated that the investigated process proceeded in five stages. Kinetic models (forms of f(α) and g(α) function) most consistent with experimental data and coefficients of Arrhenius equation A and E were determined for the stages. The Kissinger method and the Coats–Redfern equation were applied. In case of the Coats–Redfern equation, the calculations were performed by analogue method. In this way good consistency between the calculated and determined conversion degrees α(T) at practically constant values of A and E were obtained for distinguished stages and different sample heating rates.     

A feature on ensuring safety of superfine explosives

To probe the dependence of particle size on the safety of nitroamine explosives, coarse RDX and HMX were comminuted to nanometer particles by an improved superfine mill. Their thermolysis characteristics were studied by thermal analysis and described via calculating some thermodynamic and kinetic parameters such as the activation free-energy (ΔG ^≠), activation enthalpy (ΔH ^≠), activation entropy (ΔS ^≠), apparent activation energy (E), critical temperature of thermal explosion (T _b), and critical heating rate of thermal explosion ( $ ({\text{d}}T/{\text{d}}t)_{{T_{\text{b}} }} $ ). After comminuted, the values of T _b and $ ({\text{d}}T/{\text{d}}t)_{{T_{\text{b}} }} $ were increased. However, the values of ΔH ^≠, ΔS ^≠, ΔG ^≠, and E for nanoexplosives were close to those of microexplosives, which mean decreasing particle size into nanometer did not distinctly influence the thermolysis characteristic of nitroamines. The safety of the nanoexplosives was practically assessed by testing their impact, friction, and shock sensitivities. Results indicated that nano nitroamines presented obviously higher safety than the micro-counterparts. Especially in Small Scale Gap Test, the shock sensitivity of nano-RDX and nano-HMX decreased by about 45 and 56% compared with that of micro-RDX and micro-HMX, respectively.     

Solid–liquid equilibria, thermochemical and microstructural studies of binary organic monotectic and eutectic alloy

The phase diagram of 1,4-dibromobenzene (DBB) with pyrogallol (PG) shows the formation of a monotectic and a eutectic alloys at 0.12 and 0.99?mol fractions of DBB, respectively. The phase equilibrium shows the large miscibility gap region with the upper consolute temperature 159.0?°C at 0.55?mol fraction of DBB. Growth kinetics of pure compounds and their monotectic and eutectic at different undercooling (??T) obey Hillig?CTurnbull??s equation: v?=?u (??T) ⁿ . Thermodynamic parameters such as enthalpy of mixing, entropy of fusion, interfacial energy, roughness parameters and excess thermodynamic functions were computed based on enthalpy of fusion values obtained from DSC studies. The Cahn wetting condition is applicable for monotectic alloy. The optical microphotographs of binary alloys show lamellar and dendritic features.     

On Gold(I) Complexes and Anodic Gold Dissolution in Sulfite–Thiourea Solutions

Processes involving gold(I) complexes were studied in sulfite–thiourea (TU) solutions. It is shown that at pH >5 the complex [\( {\text{AuTU}}_{2}^{ + } \)] undergoes irreversible decomposition followed by deprotonation and formation of a solid phase. From the data of pH in mixed solutions, the equilibrium constants were evaluated: \( {\text{Au}}({\text{SO}}_{3} )_{2}^{3 - } + i{\text{TU}} \rightleftharpoons {\text{Au}}({\text{SO}}_{3} )_{2 - i} {\text{TU}}_{i}^{2i - 3} + i{\text{SO}}_{3}^{2 - } \), log₁₀ β ₁ = ?1.2, log₁₀ β ₂ = ?3.6. Some aspects of the anodic dissolution of gold in mixed sulfite–thiourea solutions are considered. With the help of the carbonate buffer system the change of the anodic current density j _a was studied at high pH; j _a (pH) has a maximum at pH 11.6–11.9 for E _a = 0.3–0.6 V (vs. NHE). At pH > 12.0, the j _a values decrease sharply. Possible mechanisms of anodic gold dissolution, as well as the role of sulfite, are discussed.     

Thermodynamic characterization of chromium tellurate

The standard Gibbs energy of formation of chromium tellurate, Cr₂TeO₆ was determined from the vapour pressure measurement of TeO₂(g) over the phase mixture Cr₂TeO₆(s) + Cr₂O₃(s) in the temperature range 1,183–1,293 K. A thermogravimetry (TG)-based transpiration technique was used for the vapour pressure measurement. This technique was validated by measuring the vapour pressure of CdCl₂(g) over CdCl₂(s). The temperature dependence of the vapour pressure of CdCl₂(g) could be represented as logp (Pa) (±0.02) = 12.06 ? 8616.3/T (K) (734 ? 823 K). A ‘third-law’ analysis of the vapour pressure data yielded a mean value of 185.1 ± 0.4 kJ mol^?1 for the enthalpy of sublimation of CdCl₂(s). The temperature dependence of vapour pressure of TeO₂(g) generated by the incongruent vapourisation reaction, $ {\text{Cr}}_{ 2} {\text{TeO}}_{ 6} (\rm s) \to {\text{Cr}}_{ 2} {\text{O}}_{ 3} (\rm s) + {\text{TeO}}_{ 2} (\rm g) + 1/2\,{\text{O}}_{ 2} (\rm g) $ could be represented as logp (Pa) (±0.04) = 18.57 – 21,199/T (K) (1,183 – 1,293 K). The temperature dependence of the Gibbs energy of formation of Cr₂TeO₆ could be expressed as $ \{ \Updelta G_{\text{f}}^{ \circ } ({\text{Cr}}_{ 2} {\text{TeO}}_{ 6} ,{\text{ s}}){\text{ (kJ}}\,{\text{mol}}^{ - 1} )\pm 4. 0 {\text{\} = }} - 1 6 2 5. 6 { \,+\, 0} . 5 3 3 6\,T({\text{K}}) \, (1{,}183 - 1{,}293\,{\text{K}}). $ A drop calorimeter was used for measuring the enthalpy increments of Cr₂TeO₆ in the temperature range 373–973 K. Thermodynamic functions viz., heat capacity, entropy and Gibbs energy functions of Cr₂TeO₆ were derived from the experimentally measured enthalpy increment values. $ \Updelta H_{{{\text{f}},298\,{\text{K}}}}^{ \circ } ({\text{Cr}}_{ 2} {\text{TeO}}_{ 6} ) $ was found to be ?1636.9 ± 0.8 kJ mol^?1.