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%.     

Influence of Sr addition on microstructure of the hypereutectic Zn–Al–Cu alloy

Journal of Thermal Analysis and Calorimetry - The purpose of the presented work is to answer the questions: how does the addition of strontium to the Zn–8Al–1Cu alloy crystallisation...     

Effect of grain refinements on the microstructure and thermal behaviour of Mg–Li–Al alloy

Journal of Thermal Analysis and Calorimetry - The light as-cast Mg–9Li–1.5Al alloys were manufactured and modified by 0.2 mass% Zr, commercial 0.2 mass% TiBor and 0.2 mass% AlSr master...     

DSC analysis of phase transformations during precipitation hardening in Al–Zn–Mg alloy (7020)

The hardening of the Al–Zn–Mg alloys during ageing process is based on very complex phase transformations. In order to contribute to the comprehension of these phenomena, we proceed to study the phase transformations of 7020 alloy using differential scanning calorimetry and X-ray diffraction analysis. The results confirm the formation of hardening phase GP zones, intermediate hardening metastable phase η′ and the equilibrium phase η. The calorimetric and X-ray diffraction results are in good agreement and confirm the successive precipitation/dissolution sequence. The dissolution of the precipitates is accompanied by the increase in the crystallographic lattice parameter due to the increase in solid solution concentration and by the softening of the material. On the contrary, the precipitation produces a lower concentration of the Zn/Mg solutes in the Al matrix, which generates a decrease in the lattice parameter value. These precipitates produce the hardening of the alloy. The sequence of phase formation and dissolution explains the evolution of the 7020 hardness as a function of the ageing temperature.

     

Electrochemical performance of Mg–9Al–1Zn alloy in aqueous medium

A systematic study on the corrosion and passivation behavior of AZ91D alloy in relation to the influence of concentration, temperature, pH, and immersion time was made in aqueous sulfate solution using electrochemical techniques including open-circuit potential, potentiodynamic polarization and impedance spectroscopy. It was found that the corrosion and pitting potentials (E _corr and E _pit) of the alloy drift to more active values with increasing either concentration (0.01–1.0 M) or temperature (278–338 K) of the test solution, suggesting that sulfate solution enhances the alloy dissolution, particularly at higher temperatures. On the other hand, values of the total film resistance (R _T) indicate that neutral solution (pH 7.0) supports the formation of a better protective layer on AZ91D surface than alkaline (pH 12.5) or acidic (pH 1.0) medium. The growth of a protective film on the alloy surface at short immersion times (up to ∼50 h) is evinced by a rapid positive evolution of E _corr and fast decrease in the corrosion rate (i _corr). However, for a long-term exposure (up to 500 h) E _corr drifts negatively and i _corr increases due to breakdown of the protective film, which causes a decrease in the alloy stability. Fitting the impedance data to equivalent circuit models suitable to each behavior assisted to explore the mechanism for the attack of the sample surface at various testing times. The results obtained from the three studied electrochemical techniques are in good agreement.     

Electrochemical and structural characterization of sputter-deposited Mg–Nb and Mg–Nb–Al–Zn alloy films

Nanocrystalline Mg–Nb and Mg–Nb–Al–Zn alloy films were deposited by dc magnetron sputtering on glass and quartz substrates in a wide range of niobium concentrations from 6 to 80 at.%. Structural, electrochemical and corrosion properties of the films were studied by X-ray diffraction, dc voltammetry, electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance. Development of body-centred cubic Nb structure in the Mg–Nb alloy matrix yielded the effects of lattice contraction, grain refining and electrochemical passivity. The measurements showed high corrosion resistance of the films in alkaline solutions when niobium content was one third or more. An increased corrosion resistance was achieved by introducing minor amounts of Al (ca. 2 at.%). In particular, such Al effect was pronounced at lower Nb concentrations (20 to 30 at.%). Semiconductor properties of spontaneously formed oxide on Mg–Nb alloy were studied by Mott–Schottky plots, which indicated highly doped n-type oxide structures on Mg–Nb surface. The paper fills some gap in understanding of niobium–magnesium systems, which show potential for applications in hydrogen storage, switchable mirrors and corrosion protection.     

Effect of morphology and precipitation of Si phase on thermal properties of Al–Si–Mg–Cu foundry alloy

Journal of Thermal Analysis and Calorimetry - In this study, CuAl13?xTax (% mass x?=?1; 1.5; 2; 2.5) shape-memory alloys were produced through arc-melting method. Phase...     

Thermal parameters determination of Co–Al–W as-cast alloy homogenization by DTA analysis

The γ–γ′ Co-based superalloys are newly developed class of refractory alloys which may replace commercial Ni-based superalloys owing to their favorable properties at high temperature. In case of new Co-based superalloys, the heat treatment aims to obtain microstructure composed of appropriate volume fraction of small cuboidal γ′-Co₃(Al,W) precipitates within the γ-Co matrix. However, due to a high tendency to interdendritic segregations of alloying elements, the alloys based on Co–Al–W system should be normally homogenized before further steps of heat treatment (solutionizing and aging). In this study, thermal analysis was applied for determination of temperature range for primary heat treatment of the Co–9Al–9W (at.%). The differential thermal analysis (DTA) measurements were carried out on the thermal analyzer NETZSCH STA 449 F3 Jupiter. On the base of obtained results, respectively, solvus of γ′ phase and solidus temperatures were determined, as well as the thermal range of Co₃W (DO₁₉) phase precipitation. As a consequence, the heat treatment without homogenizing (only solution and aging) was proposed as a most suitable way to obtain beneficial microstructure.     

Thermodynamic study of the eutectic Mg49–Zn51 alloy used for thermal energy storage

The eutectic Mg₄₉–Zn₅₁ (mass%) alloy has been identified as a suitable material for latent heat thermal energy storage. Within this scope, the exhibited solid–solid and solid–liquid phase transitions have been carefully characterized. A detailed thermodynamic study focused on the specific heat of the investigated alloy is also provided. The C _p behaviour, very important in the thermal energy storage frame, is theoretically modelled and experimentally validated by quasi-isothermal modulated differential scanning calorimetry measurements. Different intermetallic phases of the Mg–Zn binary system have also been successfully described within this approach in the complete temperature range.     

Crystallization behavior of Mg–Cu–Y amorphous alloy

Amorphous Mg₆₁Cu₂₄Y₁₅ ribbons were manufactured by melt-spinning at wheel speeds in the range 5?C20?ms^?1. The crystallization behavior of amorphous ribbons was investigated by a combination of differential scanning calorimetry (DSC) and X-ray diffractometry. DSC measurements showed that the amorphous ribbons exhibit distinct glass transition temperature and wide supercooled liquid region before crystallization. During continuous heating three exothermic peaks and two endothermic peaks were observed. The characteristic thermodynamic parameters such as T _g, T _x , ??T _x , and T _rg are around 432?C439, 478?C485, 46?C54?K, and 0.55?C0.56, respectively. Isothermal annealing DSC traces for this amorphous alloy, the first crystallization peak showed a clear incubation period and Avrami exponent was found to be 2.30?C2.74, which indicate that the transformation reaction involved nucleation and three-dimensional diffusion controlled growth. Mechanical properties of the as-quenched and subsequently annealed ribbons were examined by Vickers microhardness (HV) measurements. Results showed that microhardness of the as-quenched ribbons were about 309?HV. However, the results also showed that microhardness of the rapidly solidified ribbons increases with the increasing temperature.     

DSC study of the kinetic parameters of the metastable phases formation during non-isothermal annealing of an Al–Si–Mg alloy

Kinetics of β″ and β′ precipitations in an AlSiMg have been studied under non-isothermal conditions using differential scanning calorimetry (DSC) technique. The variation of the activation energy as a function of transformed fraction is determined using two isoconversional methods of Kissinger–Akahira–Sunose (KAS) and Friedman. The results obtained using the two methods show a change in the activation energy for both metastable phases precipitations as a function of transformed fraction. The results obtained from KAS method as compared with those obtained from Friedman method, show some major disagreements between the two methods. The growth exponent, determined by Ozawa method, decreases as a function of temperature for both phases.     

DSC analysis of Cu–Zn–Sn shape memory alloy fabricated via electrodeposition route

Cu–Zn–Sn shape memory alloy strips with composition range of 13.70–46.30 mass% Sn were fabricated by electrodepositing Sn on a shim brass surface and then subsequently annealed at a constant temperature of 400 °C for 120 min under flowing nitrogen. Subjecting the Sn-plated strips to differential scanning calorimetry (DSC) analysis revealed that the austenitic start (A _s) temperature was essentially constant at 225 °C while the martensite start (M _s) temperature was consistently within the 221.5–222 °C interval. Austenite to martensite phase transformation showed two distinct peaks on the DSC thermogram which can be attributed to the non-homogeneity of the bulk Cu–Zn–Sn ternary alloy. The latent heats of cooling and heating were found to increase with the mass% Sn plated on the shim brass. Effect of annealing temperature was also investigated wherein strips with an essentially constant composition of 26 mass% Sn were annealed at a temperature range of 350–420 °C for 120 min under flowing nitrogen. Varying the annealing temperature has no significant effect on the transformation temperatures of the ternary alloy.     

Cooling curve thermal analysis of Al–Mg2Si–Cu–xSr composite

Journal of Thermal Analysis and Calorimetry - In situ composites are today being considered for industrial use, owing to the fewer production steps involved, lower production cost, and better...     

Effect of cooling rate on the microstructure and solidification parameters of Mg–3Al–3Nd alloy

Journal of Thermal Analysis and Calorimetry - The effect of cooling rate on the microstructure and solidification parameters of Mg–3Al–3Nd alloy was investigated by thermal analysis....     

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.     

Study of the Cu–Li–Mg–H system by thermal analysis

Finite fossil-fuel supplies, nuclear waste and global warming linked to CO₂ emissions have made the development of alternative/‘green’ methods of energy production, conversion and storage popular topics in today’s energy-conscious society. These crucial environmental issues, together with the rapid advance and eagerness from the electric automotive industry have combined to make the development of radically improved energy storage systems a worldwide imperative. CuMg₂ has an orthorhombic crystal structure and does not form a hydride: it reacts reversibly with hydrogen to produce Cu₂Mg and MgH₂. However, CuLi _x Mg_2−x (x = 0.08) has a hexagonal crystal structure, just like NiMg₂, a compound known for its hydrogen storage properties. NiMg₂ absorbs up to 3.6 wt% of H. Our studies showed that not only CuLi _x Mg_2−x absorbs a considerable amount of hydrogen, but also starts releasing it at a temperature in the range of 40–130 °C. In order to determine the properties of the hydrogenated CuLi _x Mg_2−x , absorption–desorption, Differential scanning calorimeter and thermo-gravimetric experiments were performed. Neutron spectra were collected to elucidate the behavior of hydrogen in the Li-doped CuMg₂ intermetallic. Using DFT calculations we were able to determine the best value for x in CuLi _x Mg_2−x and compare different possible structures for the CuLi _x Mg_2−x hydride.     

The study on microstructure and electrochemical properties of Al–Mg–Sn–Ga–Pb alloy anode material for Al/AgO battery

The microstructure of Al–Mg–Sn–Ga–Pb quinary aluminum alloy anode material and the influences of its electrochemical properties and self-corrosion rate in 4 mol/l NaOH +10 g/l Na₂SnO₃ medium were studied. The microstructure and morphology were characterized by metallographic microscope, transmission electron microscope, and scanning electron microscopy. The electrochemical properties were tested by electrochemical workstation, and the self-corrosion rate of Al alloy anode was studied by methods of recovery H₂ gas by discharge water. The results show that homogenization has not much impact on the electrochemical properties and the corrosion rate of the cast aluminum alloy anode material; besides, return annealing treatment of the cold-rolled Al–Mg–Sn–Ga–Pb quinary aluminum alloy anode material can reduce the rate of self-corrosion and make Al anodic potential shift negative steadily and improve the properties of the material.

     

Effects of the preparation method and calcination temperature on Cu–Cr–Mg–Al catalysts for the dehydrogenation of cyclohexanol

Novel dioxomolybdenum(VI) complexes bearing bis(ferrocenylcarbaldimine) ligands were prepared in good yield and characterized by spectroscopy and elemental analysis. The complexes were found to be excellent catalysts for the homogeneous epoxidation of cyclohexene and styrene using tert-butyl hydroperoxide (TBHP) as oxidant. The complexes can be recovered and reused.     

Thermal behavior of Mg–Al–CO3 layered double hydroxide characterized by emanation thermal analysis

Emanation thermal analysis (ETA) was used to characterize microstructure changes during heating of Mg–Al–CO₃ layered double hydroxide (LDH) in the temperature range of 293–1473 K. It was confirmed by ETA that the formation of an intermediate phase with grafted CO₃^2– anions in the hydroxide layers took place in the temperature range of 508–523 K and the formation of Mg–Al mixed oxide (MO) occurred in the range 623–773 K. The small peak of the emanation rate at 603 K indicated the degradation of the layered structure and the broad peak in the range of 1073–1273 K characterized the onset of the separation of the decomposition products of MO into MgO and Mg₂Al₄O₇. The ETA results revealed that dehydration of the product with grafted CO₃^2– anions occurred at lower temperatures than that of the initial Mg–Al–CO₃ LDH.