Devitrification phase transformations in amorphous Al85Ni7Gd8 alloy

Non-isothermal devitrification phase transformations in amorphous Al₈₅Ni₇Gd₈ over the temperature range from 100 to 1300 °C were systematically investigated using differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) techniques. Continuous heating DSC scans revealed that the crystallization proceeds through multiple stages. The only crystalline phase formed in the first two stages is fcc-Al, appearing exclusively as dendritic single crystals. A metastable phase (τ_n) is formed in the 3rd stage, and another metastable phase (τ_u) is formed in the 4th stage, together with the equilibrium ternary compound τ₁. The equilibrium “binary” compound M₃Gd (M=Al, Ni) with 0.4 at.% Ni solubility is formed only in the 5th stage. Further heating initiates eutectic melting at 635 °C, followed by other melting events at higher temperatures, until fully liquid when T>919 °C. Isothermal annealing at 260 °C readily induces formation of another metastable phase (τ_m) and fcc-Al. Fcc-Al nanocrystal development and interpretation of isothermal DSC technique is discussed.     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

Multi-stage transformation in annealed Ni-rich Ti49Ni41Cu10 shape memory alloy

Multi-stage transformation (MST) in 500 °C annealed Ni-rich Ti₄₉Ni₄₁Cu₁₀ shape memory alloy (SMA) is investigated by differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as solution-treated alloy undergoes B2 ↔ B19 ↔ B19′ two-stage transformations. Ti(Ni,Cu)₂ precipitates are formed in 500 °C annealed specimens. Alloy annealed at 500 °C for 6–24 h exhibits MST. This MST is confirmed by DMA tests and is composed of B2₁ ↔ B19₁ ↔ B19′₁ and B2₂ ↔ B19₂ ↔ B19′₂ transformations corresponding to the regions near and far from Ti(Ni,Cu)₂ precipitates, respectively. Experimental results show that the more the annealing time, the more the B2₁ ↔ B19₁ ↔ B19′₁ transformations and finally only B2₁ ↔ B19₁ ↔ B19′₁ transformations retain with the transformation temperatures close to those of Ti₅₀Ni₄₀Cu₁₀ SMA.     

Effect of quenching on magnetostriction and microstructure of melt-spun Fe83Ga17 alloy

The effect of quenching on magnetostriction and microstructure of melt-spun Fe₈₃Ga₁₇ ribbons was investigated. The results show that magnetostriction of ribbons is greatly improved by heat treatment and the value of λ of ribbons reached nearly −2300 ppm after annealed at 700 °C for 3 h. The XRD analyses reveal that the microstructure of melt-spun Fe₈₃Ga₁₇ alloy ribbons was changed after heat treatment and the transition of A2 + DO₃ → A2 + DO₃ + DO₁₉ occurred at 700 °C for the ribbons. The magnetostriction of Fe₈₃Ga₁₇ ribbons is influenced by the emergence of DO₁₉ structure and the increase of ordered degree, and the variation of crystallinity of A2 phase is also related to the magnetostriction of Fe₈₃Ga₁₇ ribbons.     

Influence of annealing environment on microstructure and magnetic properties of amorphous Co75Fe5Zr10B10 ribbons

Rapidly solidified amorphous Co₇₅Fe₅Zr₁₀B₁₀ ribbon has been subjected to heat treatment at temperature of 300 °C for 10 min under three different atmospheric conditions, such as; air, vacuum and hydrogen-blowing atmospheres. Although no significant change in bulk structure of the ribbons was observed through X-ray diffraction analysis, substantial changes in surface microstructures are seen through SEM microstructural investigations. It is observed that the magnetic softness (coercivity, initial permeability, longitudinal permeability ratio [(Δμ/μ)%], sensitivity factor of (Δμ/μ)%, etc.) of the hydrogen-blowing atmospheric annealed ribbons is enhanced, compared to the as quenched, air and vacuum annealed samples. Moreover, a comparative study between these low temperature annealed samples has been reported.     

Structure and property of Ge/Si nanomultilayers prepared by magnetron sputtering

Ge/Si nanomultilayers were prepared using magnetron sputtering deposition and adjusting the growth conditions, such as the substrate temperature, sputtering pressure, sputtering power and annealing temperature. The surface topography and microstructure of the nanomultilayers were characterized by X-ray diffraction, Raman spectrometry and AFM. The favorable pressure of working gas was about 0.6 Pa in our experimental conditions. The surface of the as-deposited film is compact and smooth when the sputtering power is 2 W/cm². The as-deposited film is amorphous at room temperature, however, the film is crystalline at the deposition temperature of 300 °C. When the annealing temperature is 500 °C, the Ge/Si nanomultilayers transform into GeSi alloy because the thermal annealing activates Ge/Si atomic interdiffusion. At the annealing temperature of 700 °C, the interdiffusion increases and the amount of Ge in the germanosilicide phase had been decreased compared to that of the sample annealed at 600 °C. In addition, Ge may have segregated from the germanosilicide and lead to the formation of Ge nanocrystals. For the sample annealed beyond 800 °C, the strong agglomeration and the formation of Ge nanocrystals are present.     

Growth and microstructural stability of epitaxial Al films on (0001) α-Al2O3 substrates

Al films were grown epitaxially on single-crystal α-Al₂O₃ substrates by magnetron sputtering and molecular beam epitaxy, respectively. The microstructure and thermal stability of these films were analysed in detail using X-ray diffraction methods and electron microscopy techniques. The films consist of two twin-related growth variants, related by a 180° rotation around the <111> film normal resulting in a {111} Al || (0001) α-Al₂O₃, and ± < > Al || < > α-Al₂O₃ orientation relationship. The Al variants are separated by Σ3 { } Al twin boundaries possessing a rigid body translation of the {111} Al planes across the boundary plane in order to reduce their energy. Motion of the twin boundaries was observed by annealing plan-view samples in situ in a transmission electron microscope. The twin boundaries advance in jerky motion at velocities of several μm/s at temperatures of ˜400 °C, resulting in grain coarsening. In all cases, heat treatments resulted in increased area fraction of one twin variant, which finally will result in single-crystal films upon further annealing.     

On the four-phase reactions in the Ti–Ni–Al system

Two four-phase reactions of transition type in the Ti–Ni–Al system were studied on several alloys, which were annealed at carefully set temperatures and quenched. The phase constitution was established by XRD and EPMA analyses. Due to sluggish reaction kinetics, the transition temperatures were defined by annealing and quenching techniques as no DTA signals could be received. For the reaction NiAl + TiNiAl  TiNiAl₂ + TiNi₂Al, the transition temperature was found to be 925 °C ± 15 °C and for the reaction TiNiAl + Ti₃NiAl₈  TiAl₂ + TiNiAl₂, the transition temperature was found to be 990 °C ± 15 °C. Furthermore we confirmed the three-phase field TiNi₂Al + Ti₃Al + Laves phase (TiNiAl), as reported at 900 °C by Huneau et al. in 1999.     

Martensitic transformation of Ni64Al34Re2 shape memory alloy

As-rolled and annealed Ni₆₄Al₃₄Re₂ shape memory alloy (SMA) exhibits B2 → L1₀ (3R) martensitic transformation with M_s temperature up to about 210 °C. Experimental results indicate that the annealing temperature is the major factor that affects the M_s temperature. It is found that adding 2 at.% Re to replace Al in Ni₆₄Al₃₆ binary SMA can significantly refine the alloy's grain size and enhance the softening behavior during transformation. Meanwhile, Re has the same trend as Ni to affect the M_s temperature, but it has a less effect than Ni. The lattice constants and microstructures of NiAl-B2 phase, NiAl-L1₀ (3R) martensite and Ni₃Al-L1₂ phase are almost similar to those of Ni–Al binary SMAs.     

Phase constitution near the interface zone of diffusion bonding for Fe3Al/Q235 dissimilar materials

The characteristics of phase constitution near the interface of Fe₃Al/Q235 diffusion bonding are researched by means of scanning electron microscope, X-ray diffraction and transmission electron microscope, etc. The test results indicated that an obvious diffusion transition zone forms near the Fe₃Al/Q235 interface as a result of vacuum diffusion bonding (heating temperature 1050–1080 °C, holding time 60 min and pressure 9.8 MPa). The maximum value of the Al content in the transition zone was 16.6 wt.% (about 28.5 at.%). The micro-hardness in the diffusion transition zone was HM 200–400. The transition zone consists of Fe₃Al and α-Fe(Al) solid solution. There was no brittle phase of high hardness near the diffusion interface. This is favorable to the enhancing of the toughness of Fe₃Al/Q235 diffusion joint.     

Improving leakage currents of Pt/Ba0.8Sr0.2TiO3/Pt capacitors using multilayered structures

Pt/Ba_0.8Sr_0.2TiO₃ (BST)/Pt capacitors fabricated by the sol–gel process generally show abnormally high leakage currents. In this paper, we report the reduction of this leakage current in multilayered sol–gel Pt/BST/Pt thin film capacitors. The multilayered structure also provided the flexibility of adjusting the dielectric constant of the film. The thin films were fabricated by a step-by-step annealing scheme at 750 °C except that the top and bottom layers were annealed at less than 750 °C. The observed results are explained by an amorphous/polycrystalline structure, which was confirmed by scanning electron microscopy and X-ray diffraction analysis.     

Preparation of TiAl3-Al composite coating by cold spraying

TiAl₃-Al coating was deposited on orthorhombic Ti₂AlNb alloy substrate by cold spraying with the mixture of pure Al and Ti as the feedstock powder at a fixed molar ratio of 3:1 when the spraying distance, gas temperature and gas pressure for the process were 10 mm, 250 °C and 1.8 MPa, respectively. The as-sprayed coating was then subjected to heat treatment at 630 °C in argon atmosphere for 5 h at a heating rate of 3 °C/min and an argon gas flow rate of 40 mL/min. The obtained TiAl₃-Al composite coating is about 212 μm with a density of 3.16 g/cm³ and a porosity of 14.69% in general. The microhardness and bonding strength for the composite coating are HV525 and 27.12 MPa.     

An investigation of thermal aging effects on the mechanical properties of a Ni3Al-based alloy by nanoindentation

Ni₃Al-based intermetallic alloys are composed of a γ network and γ′ domains. In general, through precipitation, the γ′ phase is used as a hardening source for this alloy. This study examined how γ′ acts as a hardening material in a cast Ni₃Al-based intermetallic alloy. Specimens cut from a centrifugally cast tube were aged in Ar at elevated temperatures for up to 1600 h. Hardness tests were then performed in air at room temperature. Vickers microhardness and nanoindentation tests were carried out on specimens aged at 900 °C and 1100 °C. The microstructures were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microhardness of bulk Ni₃Al decreased dramatically with increasing thermal aging time at 900 °C, but the nanohardness measured by the nanoindenter did not significantly decrease. The nanohardness data suggested that the hardening effects were caused by the precipitation of the γ′ phase on the γ and γ′ cells.     

High-temperature phase transition and magnetic property of LaFe11.6Si1.4 compound

The LaFe_11.6Si_1.4 compounds are prepared by arc-melting and then annealed at different high temperatures from 1323 K(5 h) to 1623 K(2 h). The powder X-ray diffraction and metallographic microscopy show that 1423 K and 1523 K are two curial temperatures, at which large amount of 1:13 phase begins to form and the most amount of 1:13 phase is obtained, respectively. With annealing temperature increasing to 1573 K and 1623 K, a new phase of La₅Si₃ is detected in LaFe_11.6Si_1.4 compound. According to the DSC curve of as-cast LaFe_11.6Si_1.4 compound and the X-ray patterns of annealed LaFe_11.6Si_1.4 compounds, the high-temperature phase transition process is analyzed. For studying the influence of different high-temperature and short-time annealing on the Curie temperature, thermal and magnetic hysteresis, magnetocaloric effect of LaFe_11.6Si_1.4 compound annealed at different temperatures are also investigated.     

Microstructural characteristics of Nd2Fe14B/α-Fe nanocomposite ribbons bearing Nb element

The effects of Nb on the microstructure and magnetic properties of (Nd_0.9Dy_0.1)_9.5Fe_79-xCo₅Nb_xB_6.5 (x = 0, 1) nanocomposite magnets were investigated. A fine and uniform microstructure was achieved for the ribbons annealed at 710°C for 4 min, enhancing the interaction coupling between grains and improving the magnetic properties. The results of three-dimensional atom probe (3DAP) indicated that Fe-Nb-B intergranular phase existed at the grain boundaries, suppressing the grain growth during the crystallization process. The coercivity was improved from 224 to 643 kA/m for the modification of the microstructure.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

Oxidation mechanism and resistance of ZrB2–SiC composites

The oxidation mechanism of ZrB₂–SiC composites was investigated based on a combination of theory and experiments. The oxidation reactions, microstructure evolution, scale stability and temperature limit were examined in our research and a good correspondence was obtained between theoretical predictions and experimental results. Microstructure evolution and stability are significantly dependent on both temperature and composition. SiO₂ is thermochemically stable below 1800 °C and will lose its protective properties at temperatures above 2300 °C. The temperature limit for ZrB₂–SiC composites is strongly dependent on the vapor pressure of the gaseous products and volume content of ZrB₂.     

Atom probe tomography study of the decomposition of a bulk metallic glass

The microstructural evolution of a Pd₄₀Ni₄₀P₂₀ bulk metallic glass that was isothermally annealed at 260 °C for 14 h, and then aged at 340 °C for times up to 1280 min has been studied. Differential scanning calorimetry (DSC) curves of the aged samples show an endothermic peak at approximately 370 °C in addition to the ubiquitous glass transition. The endothermic peak appears after 20 min aging and disappears after 320 min aging. The corresponding X-ray diffraction (XRD) data show no Bragg peaks that could indicate the formation of a crystalline phase. Near-atomic-resolution atom probe tomography (APT) was used to study changes in the atomic spatial distributions as a function of aging time. The chemical environment around each of the atomic species, and the tendencies for solute clustering and chemical short range ordering, were determined from statistical analysis of the APT data. Clustering and possible phase separation are identified by APT after only 20 min aging at 340 °C, which correlates with the appearance of the peak in the DSC signal. Crystallization is apparent in the APT and XRD data after aging for 320 min. The study suggests that the amorphous Pd₄₀Ni₄₀P₂₀ annealed at a temperature 40 °C above T_g phase separates into two or more amorphous phases. The endothermic peak in the DSC trace is produced by the dissolution of the phase separation.     

Microstructural evolution of spinodally formed Fe35Ni15Mn25Al25

The microstructural evolution of a b.c.c.-based, spinodally formed alloy Fe₃₅Ni₁₅Mn₂₅Al₂₅ has been studied as a function of annealing time at 550 °C using atom probe tomography and transmission electron microscopy, including energy-filtered imaging. The sizes, crystal structures, orientation relationships and compositions of the phases present were determined as a function of annealing time. The hardness showed complicated behavior as a function of annealing time, consisting of initial hardening, followed by softening and finally, by a rapid hardening behavior. The hardness is controlled both by the coarsening of the spinodally formed phases, and the precipitation and growth of β-Mn structured particles.     

Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Cu₆Sn₅ exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu₆Sn₅ at temperatures lower than 186 °C to hexagonal η-Cu₆Sn₅. We recently discovered that the hexagonal structure of Cu₆Sn₅ in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS. This report further confirm the phase stabilising effect of Ni by analysing samples of Cu₆Sn₅ extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry.