Effect of high-temperature quenching on the magnetostructural transformations and the long-range atomic order of Ni–Mn–Sn and Ni–Mn–Sb metamagnetic shape memory alloys

The influence of high-temperature thermal treatments on the martensitic transformation and the magnetic properties of Ni–Mn–Sn and Ni–Mn–Sb metamagnetic shape memory alloys have been investigated by calorimetric and magnetic measurements. Contrary to Ni–Mn–Ga and Ni–Mn–In systems, the martensitic transformation and Curie temperatures of Ni–Mn–Sn and Ni–Mn–Sb alloys are found to be unaffected by the increasing quenching temperature. Neutron diffraction measurements confirm the null effect of quenching on the next-nearest-neighbors atomic order due to the negligible L2₁ atomic disorder achieved with high-temperature annealings. The analysis of long-range order also suggests that no L2₁–B2 ordering transition takes place in the studied alloys, thus indicating an unusually high stability of the L2₁ structure. The obtained results show that the magnetostructural properties of Ni–Mn–Sn and Ni–Mn–Sb alloys cannot be properly tuned by means of standard thermal treatments.     

Processes of atomic ordering in microcrystalline ternary alloys based on Ni3Mn

Stable and metastable phases are formed in the production of microcrystalline alloys By quenching from the melt, The atomic ordering of these phases has not been completely studied until recently. In ternary systems with two ordered phases having different Kurnakov temperatures TK (the temperature of the order-to-disorder transition), for example, an intetmetallic compound and an ordinary superstructure, preferential formation of a superstructure with a highT _K is expected and the alloying atoms occupy vacancies in the ordered lattice of the intermetallic compound. The present work concerns neutron diffraction analysis of atomic ordering in the ternary systems Ni₃Mn-Ni₃Al, Ni₃Mn-Ni₃Si, and Ni₃Mn-Ni₃Ti.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 2–4, August. 1995.     

Simulation study on non-linear effects of initial melt temperatures on microstructures during solidification process of liquid Mg7Zn3 alloy

The non-linear effects of different initial melt temperatures on the microstructure evolution during the solidification process of liquid Mg₇Zn₃ alloys were investigated by molecular dynamics simulation. The microstructure transformation mechanisms were analyzed by several methods. The system was found to be solidified into amorphous structures from different initial melt temperatures at the same cooling rate of 10 × 10¹² K/s, and the 1551 bond-type and the icosahedron basic cluster (12 0 12 0) played a key role in the microstructure transition. Different initial melt temperatures had significant effects on the final microstructures. These effects only can be clearly observed below the glass transition temperature T_g; and these effects are non-linearly related to the initial melt temperatures, and fluctuated in a certain range. However, the changes of the average atomic energy of the systems are still linearly related with the initial melt temperatures, namely, the higher the initial melt temperature is, the more stable the amorphous structure is and the stronger the glass forming ability will be.     

Structure and properties of cast and splat-quenched high-entropy Al–Cu–Fe–Ni–Si alloys

The effect of the composition and cooling rate of the melt on the microhardness, phase composition, and fine-structure parameters of as-cast and splat-quenched (SQ) high-entropy (HE) Al–Cu–Fe–Ni–Si alloys was studied. The quenching was performed by conventional splat-cooling technique. The cooling rate was estimated to be ~10⁶ K/s. Components of the studied HE alloys were selected taking into account both criteria for designing and estimating their phase composition, which are available in the literature and based on the calculations of the entropy and enthalpy of mixing, and the difference between atomic radii of components as well. According to X-ray diffraction data, the majority of studied Al–Cu–Fe–Ni–Si compositions are two-phase HE alloys, the structure of which consists of disordered solid solutions with bcc and fcc structures. At the same time, the Al_0.5CuFeNi alloy is single-phase in terms of X-ray diffraction and has an fcc structure. The studied alloys in the as-cast state have a dendritic structure, whereas, after splat quenching, the uniform small-grained structure is formed. It was found that, as the volume fraction of bcc solid solution in the studied HE alloys increases, the microhardness increases; the as-cast HE Al–Cu–Fe–Ni–Si alloys are characterized by higher microhardness compared to that of splat-quenched alloys. This is likely due to the more equilibrium multiphase state of as-cast alloys.     

Identifying the role of nanoscale heterogeneities in pitting behaviour of Al-based metallic glass

To clarify the correlation of nanoscale heterogeneity with corrosion in Al-based metallic glasses, three model alloys with a single nanoscale α-Al, Al₃Ni or Al₁₁Ce₃ phase embedded in amorphous Al-Ni-Ce alloy matrix were obtained directly by melt quenching. The results indicated that the high pitting corrosion resistance of AM alloys was not deteriorated by nanocrystalline α-Al precipitation; whereas the pitting potential was slightly decreased and considerably reduced relative to their amorphous state due to the precipitation of nanocrystalline Al₃Ni or Al₁₁Ce₃ respectively. Such a pitting sensitivity of different types of heterogeneities attributes to the nano-scale pit initiation events.     

γ-TiAl合金定向凝固过程中与氧化钇陶瓷材料的界面反应

研究液态金属冷却定向凝固条件下,Ti-47%Al合金与Y2O3/Al2O3双层结构陶瓷管内层Y2O3的相互作用。分析熔体温度和相互作用时间对界面反应、合金组织和成分的影响。结果表明：虽然Y2O3层受到一定程度的侵蚀和溶解,但Y2O3层有效地阻隔化学稳定性较差的Al2O3外层与TiAl合金熔体接触,避免Al2O3与TiAl之间的化学反应。合金熔体受到一定程度的污染且含有少量夹杂物。在一定保温时间内,合金中的杂质含量（Y和O）和夹杂物（Y2O3）体积分数均随着过热温度的提高而增加,但随着保温时间的进一步延长,杂质含量和夹杂物体积分数趋于恒定值,不再显著变化。     

Effects of rapid quenching on microstructures and electrochemical properties of La0.7Mg0.3Ni2.55Co0.45Bx（x=O-0.2） hydrogen storage alloy

In order to improve the electrochemical performances of La-Mg-Ni based electrode alloys with PuNi3-type structure, a trace of boron was added in La0.7Mg0.3Ni2.55Co0.45 alloy. The La0.7Mg0.3Ni2.55Co0.45Bx（a=0, 0.05, 0.1, 0.15 and 0.2） alloys were prepared by casting and rapid quenching. The electrochemical performances and microstructures of the as-cast and quenched alloys were investigated. The effects of rapid quenching on the microstructures and electrochemical performances of the above alloys were investigated. The results show that the as-cast and quenched alloys are composed of （La, Mg）Ni3 phase, LaNi5 phase and LaNi2 phase. A trace of the Ni2B phase exists in the as-cast alloys containing boron, and the Ni2B phase in the B-contained alloys nearly disappears after rapid quenching. Rapid quenching increases the amount of the LaNi2 phase in the B-free alloy, but it decreases the amount of the LaNi2 phase in the boron-containing alloys. The effects of rapid quenching on the capacities of the boron-containing and boron-free alloys are different. The capacity of the B-free alloy monotonously decreases with increasing quenching rate, whereas the capacities of the B-contained alloys have a maximum value with the change of the quenching rate. The rapid quenching can improve the stability of La-Mg-Ni based electrode alloy but lowers the discharge plateau voltage and decreases the plateau length. The effect of rapid quenching on the activation capabilities of the alloys was complicated.     

Gaseous and electrochemical hydrogen storage kinetics of nanocrystalline Mg2Ni-type alloy prepared by rapid quenching

The nanocrystalline Mg₂Ni-type Mg₂Ni_1−xCu_x (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by direct melt quenching technique. The structures of the as-cast and quenched alloys were investigated by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the alloys was tested by using constant current to charge and discharge the electrode. The results indicate that the substitution of Cu notably rendered the grain refinement of the as-cast alloys without altering the major phase Mg₂Ni. All the as-quenched alloys exhibit a nanocrystalline structure without the presence of any amorphous phase. It is found that the substitution of Cu for Ni and rapid quenching significantly ameliorated the gaseous and electrochemical hydrogen storage kinetics of the nanocrystalline Mg₂Ni_1−xCu_x (x = 0-0.4) alloys. Furthermore, both the rapid quenching treatment and the Cu substitution results in a notable increase in the hydrogen diffusion coefficient (D) as well as the limiting current density (I_L) but an obvious decline in the electrochemical impedance.     

The microstructure and properties of rapidly quenched Ti-rich Ti-Ni binary alloys with shape memory effects

The structure, phase composition, and martensitic transformations in binary titanium-rich Ti-Ni alloys with shape memory effects, produced by ultrarapid quenching using melt jet spinning, have been studied using electron microscopy, X-ray diffraction, and measurements of some physicomechanical properties in a wide temperature range. The alloys with a Ti content that exceeded the stoichiometric composition by 5% and more can be produced in an amorphous state. The alloys with a smaller deviation from the stoichiometry, as well as the Ti₅₀Ni₅₀ alloy, are crystallized in a submicrocrystalline state and undergo a B2 → B19’ martensitic transformation at temperatures above room temperature. They have high strength and plastic properties and demonstrate narrow-hysteresis shape-memory effects.     

Effect of the deviation of the chemical composition from the stoichiometric composition on the structural and phase transformations and properties of rapidly quenched Ti50 + xNi25 ? xCu25 alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase composition and structure of an almost stoichiometric alloy Ti₅₀Ni₂₅Cu₂₅. The alloys of the quasi-binary section TiNi-TiCu to be studied, which exhibit in the initial ascast state thermoelastic martensitic transformations B2 ↔ B19 and related shape-memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti_{50 + x} Ni_{25 − x} Cu₂₅ alloys was varied with respect to titanium and nickel within x ≤ ±1% (from Ti₄₉Ni₂₆Cu₂₅ to Ti₅₁Ni₂₄Cu₂₅). It has been shown that the rapid quenching from the melt at a cooling rate of 106 K/s provides amorphization for all the alloys under consideration. Heating to 723 K or higher temperatures leads to the devitrification of the amorphous alloys with the formation of a polycrystalline structure of the B2 austenite. The mechanical properties of the alloys have been measured in the initial amorphous state and after subsequent heat treatment. It has been established that, depending on the degree of deviation of the alloy from the stoichiometric composition, which leads to solid solution decomposition in the process of nanocrystallization upon heat treatment, there occur regular changes in the mechanical properties and shape-memory effects of the alloys. The characteristic temperatures of the onset and finish of the process of crystallization from the amorphous and amorphous-crystalline states and the critical temperatures of the onset and finish of the forward and reverse thermoelastic martensitic transitions have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition of the alloy has been constructed.     

Spinodal decomposition of Ni–Nb–Y metallic glasses

Phase-separated Ni–Nb–Y metallic glasses were prepared by rapid quenching from the melt. The early stages of decomposition were characterized in Ni–Nb–Y alloys with Ni contents of more than 60 at.%. Strongly correlated chemical fluctuations with a nanometer length scale were found to exist in the as-quenched state. The observed fluctuation lengths range from 5 to 12 nm, depending on the actual composition of the glass. The “frozen-in” early stages of decomposition occur in the deeply undercooled melt due to the reduction in the critical temperature of liquid–liquid phase separation with Ni content. Annealing of the phase-separated Ni₇₀Nb₁₅Y₁₅ glass below the crystallization temperature leads to an increase in the amplitude of the fluctuations. However, the wavelength was unchanged, which provides evidence for the spinodal character of the decomposition.     

In situ evaluation of the transformation behaviour of NiTi-based high temperature shape memory alloys

Fine grained polycrystalline NiTi shape memory alloys containing 15 at.% Hf and Zr and zero or 3 at.% Cu fabricated by ingot metallurgy were investigated using in situ synchrotron X-ray diffraction in order to examine the viability of producing stable and affordable high temperature shape memory alloys. The alloys produced had a high thermal hysteresis, in excess of 70 °C but A_f temperatures of over 250 °C were obtained for Ni₅₀Ti₃₅Hf₁₅. 3 at.% Cu additions did not significantly reduce the per-cycle degradation of transformation temperatures but did reduce the transformation temperatures. The evolution of the lattice parameters during the first five thermal cycles was observed. Negative thermal expansion was found in the b_B19′ cell direction in all the alloys examined and significant deviations in the lattice parameters in the region of transformation were found. A per-cycle evolution in the end-point B19′ lattice parameters was observed, but no such evolution was found for the B2 phase, which is rationalised by appealing to the increase in population of interface dislocations.     

The local environmental effects on the magnetism of short-range clustered CuNi alloys

The locally self-consistent multiple-scattering method has been applied to spin-polarized LDA calculations for ferromagnetic Cu_0.2Ni_0.8alloys. The sample used to simulate the alloys involved up to 256 atoms per cell and was constructed to be short-range clustering with the experimentally measured short-range order parameters. The magnetic cross section for neutron scattering was also calculated and compared with the neutron scattering data. The magnetic moments of Ni atoms were found to be sensitive to local atomic configurations.     

Formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al Composites by Directed Melt Oxidation of Al-Si-Zn Alloy

Observations are presented on the initiation and growth of Al₂O₃/Al composites by the directed melt oxidation of Al-Si alloys containing metallic Zn or using external dopant ZnO. Thermal gravimetric analysis, optical microscopy, and x-ray diffraction analysis were employed to characterize the progress of oxidation and the nature of oxidation products. Both Zn and ZnO dopants were able to initiate the directed melt oxidation of Al-Si alloys without any Mg being present. Al₂O₃/Al composites were produced when the alloying Zn concentration exceeding 3 wt.%. The incubation period of the oxidation process for Al-Si-Zn alloys was shortened markedly and the amount of composite products increased with the increasing of Zn content in the alloy. In addition, doping with ZnO powder resulted in dense composite formation. A macroscopically planar surface and a fine microstructure promote oxidation growth in Al₂O₃/Al composites. Doping with ZnO powder offers a significant advantage over using metallic Zn for the directed melt oxidation of Al-Si alloy.     

Solidification microstructures and carbides morphology in rapidly solidified Fe−Al−Cr−C alloys

The combined effects of alloying additions and heat treatment on the evolution and development of the microstructures of Fe₃Al-based Fe−Al−Cr−C alloys produced by melt suction processing have been examined. Particular emphasis has been placed on the distribution and morphology of carbides in rapidly solidified Fe₃Al-based intermetallics. The results suggest that the formation of intrinsically hard and brittle Fe₃AlC and Cr₇C₃ type carbides depends on the alloying content. These carbides tend to form preferentially along the grain boundaries where more continuous and coarse networks are observed, especially for alloys having higher Cr and C content. These networks are fragmented as a result of heat treatment at 1200 °C and subsequent air cooling and quenching.     

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and selected-area electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti₅0Ni₂₅Cu₂₅ alloy. Based on the method of rapid quenching of the melt (free-jet melt spinning), alloys of the quasi-binary TiNi-TiCu section have been prepared, which in the initial as-cast state exhibited the thermoelastic martensitic transformations B ₂ ? B ₁₉ and related shape-memory effects. The chemical composition of the Ti_{50 + x} Ni₂₅Cu_{25 ? x} alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti₄₉Ni₂₅Cu₂₆ to Ti₅₁Ni₂₅Cu₂₄). It has been established that quenching at a cooling rate equal to 10⁶ K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shape-memory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their characteristic temperatures have been determined, and a diagram of the dependence of the characteristic temperatures on the chemical composition of the alloys has been constructed.     

Point defects in binary Laves phase alloys

Point defects in the binary C15 NbCr₂ and NbCo₂, and C14 NbFe₂ systems on both sides of stoichiometry were studied by bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000°C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with model predictions. Thermal vacancies exhibiting a maximum at the stoichiometric composition were observed in NbCr₂ Laves phase alloys after quenching from 1400°C. However, there are essentially no thermal vacancies in NbFe₂ alloys after quenching from 1300°C. Anti-site hardening was found on both sides of stoichiometry for all the three Laves phase systems studied. Neither the anti-site defects nor the thermal vacancies affect the fracture toughness of the Laves phases significantly.     

Effect of rapid quenching on the microstructure and electrochemical characteristics of La_（0.6）Ce_（0.4）Ni_（3.6）Co_（0.65）Mn_（0.4）Al_（0.2）Ti_（0.05_（FeB）_（0.1） hydrogen storage alloy

In order to improve the cycling stability of AB₅ type alloy electrodes, rapid quenching technology and new alloy composition design were employed. A hydrogen storage alloy with nominal composition La_0.6Ce_0.4Ni_3.6Co_0.65Mn_0.4Al_0.2Ti_0.05(FeB)_0.1 was prepared by vacuum magnetic levitation melting under high purity argon atmosphere, followed by rapid quenching at different cooling rates. XRD results show that all alloys exhibit the single-phase CaCu₅-type structure. Electrochemical tests indicate that rapid quenching can slightly improve the cycling life of the alloy. Nevertheless, the high-rate dischargeability of the quenched alloys is lower than that of the as-cast alloy.     

Investigation of microcrystalline NiAl-based alloys with high-temperature thermoelastic martensitic transformation: II. Construction of isothermal diagrams of decomposition of a supersaturated β solid solution of Ni65Al35 and Ni56Al34Co10 alloys

Diagrams of the onset of decomposition of two functional microcrystalline alloys (Ni₆₅Al₃₅ and Ni₅₆Al₃₄Co₁₀) with a thermoelastic reversible martensitic transformation prepared by ultrarapid quenching from the melt have been constructed based on the results of isothermal measurements of electrical resistance during various annealings. A multi-stage nature of the diffusive decomposition of a β solid solution supersaturated with Ni has been revealed, and the temperature range of its maximum thermal stability has been found. The retardation effect of cobalt on the decomposition of high-nickel martensitic Ni-Al alloys has been determined.     

Influence of rapid quenching on hydrogen storage characteristics of nanocrystalline Mg2Ni-type alloys

Nanocrystalline Mg2Ni-type alloys with nominal compositions of Mg20Ni10–xCux(x=0,1,2,3,4,mass fraction,%) were synthesized by rapid quenching technique.The microstructures of the as-cast and quenched alloys were characterized by XRD,SEM and HRTEM.The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system.The hydriding and dehydriding kinetics of the alloys were measured using an automatically controlled Sieverts apparatus.The results show that all the as-quenched alloys hold the typical nanocrystalline structure and the rapid quenching does not change the major phase Mg2Ni.The rapid quenching significantly improves the electrochemical hydrogen storage capacity of the alloys,whereas it slightly impairs the cycling stability of the alloys.Additionally,the hydrogen absorption and desorption capacities of the alloys significantly increase with rising quenching rate.