Microstructure evolution in CoNiGa shape memory alloys

Magnetic shape memory CoNiGa alloys hold great promise as new smart materials due to the good ductility and a wide range of martensitic transformation (MT) temperatures as well as magnetic transition points. This paper reports the results of investigations on the equilibrium phase constitution and microstructure evolution in quenched or aged CoNiGa alloys using the optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The dendritic γ phase decreases as lowering of Ga content in studied two series of samples (Co₅₀Ni_50 − xGa_x, x = 0–50 and Co_100 − 2yNi_yGa_y, y = 15–35). Some γ′ precipitates with different morphologies were found in given alloys conducted with water quenching (WQ) at 800 °C or long-time ageing at 300 °C. After 800 °C quenching, the γ′ phase has a rod-like shape for the Co₅₀Ni₃₀Ga₂₀ alloy but shows a Widmanstätten morphology as Ga increases to 25 at%, and trends to be block structure in further high Ga content alloy. In the case of 300 °C aged alloys, the γ′ particles prefer to nucleate in interior of γ phase or at the interface of β–γ. We also presented an illustrative vertical section phase diagram keeping 50 at% Co, and isothermal section phase diagram at 1150 and 800 °C of the CoNiGa system. Based on the schematic ternary phase diagram, the composition scope which potentially holds over the magnetic pure martensite phase structure at room temperature (RT) was pointed out. It is believed that this optimized range alloys would play an important role in the functional materials design for application.     

Oxidation behavior of multiphase Mo5SiB2 （T2）-based alloys at high temperatures

Two MosSiB2 （T2）-based alloys with nominal compositions of Mo-12.5Si-25B and Mo-14Si-28B （molar fraction, %） were prepared in an arc-melting furnace, and their oxidation kinetics from 1 000 to 1 300 ℃ were studied. The microstructures of the alloys were characterized by X-ray diffractometry（XRD） and scanning electron microscopy（SEM） with energy dispersive spectroscopy （EDS）. The oxide scales of both alloys oxidized at 1 200 ℃ for l0 min, 2 h and 100 h were investigated by surface XRD and cross-sectional SEM-EDS. The results show that the matrix of both alloys consists of T2. The dispersions of Mo-12.5Si-25B alloy are Mo and Mo3Si, and the dispersions of Mo-14Si-28B alloy are MosSi3 （T1） and MoB. The cyclic oxidation kinetics data exhibit initial rapid mass loss followed by slow mass loss. The mass loss of Mo-12.5Si-25B alloy is much faster than that of Mo-14Si-28B alloy at 1 200 and 1 300 ~C. For l0 min exposure, both alloys form irregular and porous thin scale. For 2 h exposure, Mo-12.5Si-25B alloy forms irregular thin scale and the scale contains large cracks, and Mo-14Si-28B alloy forms sound and continuous scale. For 100 h exposure, Mo-12.5Si-25B and Mo-14Si-28B alloys form sound and continuous scale about 50-75 μm and 40-45 μm in thickness, respectively. The better oxidation resistance of Mo-14Si-28B alloy is due to a sound and continuous B-SiO2 layer formation in the early stage of oxidation.     

Effect of Y content on microstructure and mechanical properties of 2519 aluminum alloy

The effects of yttrium（Y） content on precipitation hardening, elevated temperature mechanical properties and morphologies of 2519 aluminum alloy were investigated by means of microhardness test, tensile test, optical microscopy（OM）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. The results show that the tensile strength increases from 485 MPa to 490 MPa by increasing Y content from 0 to 0.10%（mass fraction） at room temperature, and from 155 MPa to 205 MPa by increasing Y content from 0 to 0.20% at 300 ~C. The high strength of 2519 aluminum alloy is attributed to the high density of fine 0＇ precipitates and intermetallic compound AICuY with high thermal stability. Addition of Y above 0.20% in 2519 aluminum alloy may induce the decrease in the tensile strength both at room temperature （20 ℃） and 300℃.     

Features of the HDDR process in R–Fe–B ferromagnetic alloys (R is a mixture of Nd, Pr, Ce, La, Dy and others)

Features of the conventional hydrogenation, disproportionation, desorption, recombination (HDDR) and solid-HDDR processes in some R–Fe–B (R is a mixture of Nd, Pr, Ce, La, Dy) ferromagnetic alloys were studied in the temperature range 20–990 °C and pressure range from 1×10⁻³ Pa to 0.1 MPa. This was carried out by means of differential thermal analysis (DTA), X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) methods. The hydride of the initial phase is formed by heating to 115 °C. The disproportionation of the alloys occurs in the temperature range from 320 to 800 °C. Φ-phase constitutes the base of the initial alloys. Among the disproportionation products, R-hydride, -Fe and two borides (Fe₂B and R_1.1Fe₄B₄) were revealed. The initial phase in all the alloys is recovered after heating in vacuum to a temperature of 990 °C. Full hydrogen desorption occurs in two temperature ranges with the peaks at 200–320 and 630–715 °C.     

Effects of Yb addition on microstructures and mechanical properties of 2519A aluminum alloy plate

The effects of Yb content on the microstructures and mechanical properties of 2519A aluminum alloy plate were investigated by means of tensile test,optical microscopy,transmission electron microscopy,scanning electron microscopy and X-ray diffractometer.The results show that addition of 0.17% (mass fraction) Yb increases the density of θ' particles of the 2519A alloy plate and reduces the coarsening speed rate of θ' phase at 300 ℃.Therefore,tensile strength is enhanced from 483.2 MPa to 501.0 MPa at room temperature and is improved from 139.5 MPa to 169.4 MPa at 300 ℃.The results also show that with the addition of 0.30% (mass fraction) Yb,the mechanical properties increase at 300 ℃ and decrease at room temperature.With Yb additions,the Al7.4Cu9.6Yb2 phase is found whilst the segregated phases of as-cast alloys along grain boundaries become discontinuous,thin and spheroidized.     

Fatigue fracture of high-strength Al-Zn-Mg-Cu alloy

X-ray diffractometry(XRD), optical microscopy(OM), scanning electron microscopy(SEM) were used to study the fatigue fracture of the T7451 Al-Zn-Mg-Cu alloy (470 °C, 60 min+115 °C, 8 h+165 °C, 16 h). The study reveals mainly the microscopic structure of the alloy in the process of crack formation and crack growth. The fatigue fracture is characterized by three zones: fatigue crack source zone, fatigue crack propagation zone and fatigue fracture zone. The fatigue damage preferably incubates at the fractured inclusion particles at or near (about 25 μm) the specimen free surfaces, and these brittle Fe-rich intermetallic inclusion particles are (7–10) μm×(11–14) μm in size. Some features such as “feather-like”, “river and range” and boundary extrusions can be observed in the fatigue propagation zone, and in the fatigue fracture zone the surface is rough and uneven.     

Effect of Rare Earth Sc Addition on Microstructure and Mechanical Properties of an Al-Cu-Mg-Ag Alloy

Al-5.3Cu-0.8Mg-0.6Ag alloys containing 0, 0.1, 0.3 and 0.5 (mass. %) Sc were prepared by ingot metallurgy and thermomechanical treatment. The effect of Sc addition on the precipitation and microstructure of the alloys has been investigated using mechanical testing, optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been shown that trace Sc element refines the grains of the casting alloys and the average grain size decreases from 85 μm to 30 μm. Increasi...     

Le système binaire Pd---Si

The Pd---Si phase diagram has been revised in details (178 alloys) on carefully annealed specimens by microprobe analysis, X-rays diffraction and DTA. The Pd₂Si intermetallic phase (m.p. 1404 °C) is confirmed to be stoichiometric with the C22-structure, but within the ranges of composition 33.3–33.9 and 34.5 at.% Si it diversifies in three very similar crystalline structures formed peritectically at 1053–1083 and 1072 ± 2 °C respectively. PdSi only exists in a very short range of temperature (888–908 °C melting congruently), whereas Pd₅Si (m.p. 856 °C) disappears below 811 °C, likewise producing four very narrow and similar phases at 16 at.% Si (below 811 °C), 17.7 (below 795 °C), 18.2 at.% Si (between 819 and 764 °C) and 21.0 at.% Si (between 792 and 753 °C), all these phases being formed by peritectoïd transformations. The only Pd₃Si-phase (congruent m.p. 1074 °C) remains stable at low temperature.

Résumé

L'étude très détaillée du diagramme de phase Pd---Si (178 alliages) révèle que la phase intermétallique Pd₂Si (Fus. 1404 °C) est stoechiométrique avec une structure C22. Cependant, elle se diversifie en trois structures cristallines ordonnées étroites, qui se forment péritectiquement à 1053–1083 et 1072 ± 2 °C, dans un domaine restreint de composition 33,3–33,9 et 34,5 at.% Si respectivement. Le composé PdSi (Fus. congruente 908 °C) existe dans un très faible domaine de température (888–908 °C). Pd₅Si (Fus. 856 °C) disparaît au-dessous de 811 °C en donnant naissance à quatre phases très proches et très similaires à 16 at.% Si (au-dessous de 811 °C), 17,7 (au-dessous de 795 °C), 18,2 at.% Si (Pd₉Si₂) (entre 819 et 764 °C) et 21,0 at.% Si (entre 792 et 753 °C), toutes ces phases sont issues de transformations péritectoïdes. Seule la phase Pd₃Si se maintient sans autre modification à basse température (Fus. congruente 1074 °C).     

Processing of Fe3Al and FeAl alloys by reaction synthesis

The effects of powder particle size, alloy composition, and reaction atmosphere on reaction synthesis of binary Fe---Al alloys were studied. Reactions were observed in an open (air) furnace, under static vacuum (in an evacuated quartz tube) and in a dynamic vacuum furnace. Reactions occurring in the open furnace and in the evacuated quartz tube were recorded using high-speed video equipment. High-speed videotapes of reaction synthesis of compacts formed from 45 μm Fe and 10 μm Al particles reacted in air and under static vacuum revealed that an unusual ‘two-stage’ reaction exists in this system under these conditions. Compacts formed from 9 μm Fe and 3 μm Al powder particles do not exhibit a two-stage reaction under any of the conditions examined in this work. The first stage of the two-stage reaction lasts several seconds and starts at round 650 °C. The second stage begins at about 900 °C, reaching temperatures between 1250 and 1350 °C. The progress of the reaction to the second stage is sensitive to the alloy composition and reaction atmosphere. The reaction behavior is explained in terms of thermodynamics and heat transfer, which control the delicate balance between heat accumulation and heat loss during reaction synthesis.     

Influence of Sc on high temperature strengthening behavior of Ti-6A1-4V alloy

Vacuum arc melting technique was used to prepare Ti-6A14V alloy containing Sc （0.3% and 0.5%, mass fraction）. The ingots were melted twice by vacuum self-consumable electrode arc furnace. Forging of ingots was started in β-phase region and finished in high （a＋β-phase region. Annealing after forging was performed in low （a＋β）-phase region for 30 min. Isothermal high temperature compression tests were conducted using thermal simulation machine under Ar atmosphere at 850℃ and 1 000 ℃, and the strain rate were 0.001, 0.01, 0.1 and 1.0 s^-1. Optical microscope（OM）, scanning electron microscopy（SEM）, energy dispersive spectrum（EDS） and transmission electron microscope（TEM） were used to study the microstructure evolution during high temperature deformation. The results show that, the peak stress value of alloys increases with increasing Sc content after deformation at 850℃, however, there is no obvious strengthening of Sc when the alloys are deformed at 1 000 ~C. Sc exists as Sc203 forms by internal oxidation during forging procedure, only minor Sc solutes in matrix. At 850 ~C, the interaction between dislocation and participated particles and twinning mechanism controls the deformation procedure accompanied recrystallization. At 1 000 ℃, the deformation of alloys containing Sc is mainly controlled by twinning, while the deformation of alloy without Sc is not only controlled by twinning, but also the interaction between dislocation and precipitated particles inside the twinning lamellar.     

Surface alloying of an Mg alloy subjected to surface mechanical attrition treatment

A 100  200 μm thick Al-enriched surface alloyed layer was formed on an AZ91D Mg alloy subjected to surface mechanical attrition treatment and diffusion coating at temperature as low as 400 °C. Transmission electron microscopy observations indicated the formation of a large volume fraction of pearlite-like lamellar microstructure within the surface alloyed layer, which was identified to be Mg₁₇Al₁₂ precipitates (γ phase) in Mg solid solution matrix. The Al-enriched alloyed layer enhanced the wear resistance of the alloy in comparison with the un-treated AZ91D Mg alloy substrate under the same dry sliding wear condition. Examination of the worn surface indicated that the enhanced wear resistance of the alloyed layer was mainly attributed to the strengthening effect of γ phase.     

Microstructure of the surface zone in a heat-treated cermet material

The microstructure of a Ti(C, N)-(Ti, W)(C, N)-(Ti, Ta)(C, N)-WC-Mo₂C-Co cermet with a heat-treatment induced surface gradient has been studied. The post-sintering heat-treatment was performed at 1200 °C in a nitrogen atmosphere and the resulting microstructure was compared to that of the as-sintered material. The microstructures were characterised with optical microscopy, X-ray diffraction, electron microprobe analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, electron energy-loss spectroscopy and energy-filtered transmission electron microscopy. It was found that a 50-μm deep nitrogen-rich surface zone was introduced by the heat treatment. η-phase was observed in the surface zone. The nitrogen gradient caused diffusion of titanium towards the surface, thereby forcing the cobalt-rich binder to be transported inwards. The composition of the binder phase and outer rim of the carbonitride grains showed only minor changes with depth. The variations in binder composition may be a direct consequence of the heat treatment, but are also likely to be influenced by the formation of η-phase. A nitrogen-rich phase was present in the surface zone of the heat-treated material both as thin layers (≈ 70 nm) surrounding the carbonitride grains and as small parts of some carbonitride grains. In addition, the Ti(C, N) cores in the surface region were found to have different N/(C + N) ratios.     

Solid-phase equilibria in the Ti-rich part of the Ti---Al system

The solid-state-phase equilibria in the Ti-rich portion of the Ti---Al system (0–50%Al) have been investigated at temperatures between 800°C and 1415°C by electron probe microanalysis (EPMA) and by transmission electron microscopy (TEM) on diffusion couples as well as on annealed alloys. The TEM observations on the diffusion couples show that the phase field ₂, (ordered hexagonal DO₁₉) extends up to the β (bcc A2) phase field, leading to two peritectoid transformations, β + → ₂ at T = 1210 ± 10°C and β + ₂→ at T = 1160 ± 10°C (: hexagonal A3). The diffusion-couple experiments yield the tie lines of the observed phase equilibria. The present results confirm the existence of a eutectoid transition → ₂ + λ (λ: ordered fcc Ll₀) at about T = 1120 ± 10°C. An updated phase diagram based on these data is presented.     

Optimisation of precipitation for controlling recrystallisation of wrought Fe3Al based alloys

A Fe–26Al–5Cr (at.%) single-phase (:A2/B2/D0₃) alloy and two-phase (+TiC) alloys with different amounts of TiC particles have been hot rolled at 800 °C and the kinetics of static recrystallisation have been studied. In the alloys with a high amount of TiC, needle-like TiC of more than 1 μm in length formed during cooling after homogenisation in the single-phase region and coarsened during hot rolling. The large particles cause particle stimulated nucleation (PSN) and hence accelerate recrystallisation. In order to accomplish both strengthening by precipitates and inhibition of recrystallisation that deteriorates room-temperature ductility, a thermo-mechanical treatment consisting of hot deformation with a low amount of precipitates and a subsequent heat treatment for further precipitation is proposed. This process is difficult to carry out in the (Fe–26Al–5Cr)–TiC system due to the high precipitation temperature of TiC. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC.     

Ni-Cr-Ti protective coating

Ni-Cr-Ti protective films were prepared by a plasma beam sputter deposition process at 250 °C. Ni-Cr-Ti thin films (50 nm) have been used to study the microstructure by transmission electron microscopy and the grain growth during heating in the heating stage of an electron microscope and after baking in an oxygen flow in an oven. Energy-dispersive spectroscopy analysis in the transmission electron microscope was used to study compositional variations in the films. Ni-Cr-Ti films 300 nm thick deposited on polished hot-worked tool steel (AISI H11) substrates have been studied by Auger electron spectroscopy and depth profiling before and after heat treatment in oxygen in an oven at temperatures from 600 to 800 °C. Basic microstructural, morphological, compositional and mechanical properties have been studied on Ni-Cr-Ti protective coatings 3 μm thick. Oxidation and electrochemical properties have also been studied.     

A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

Grain refinement of a Fe3Al-based alloy using κ-Fe3AlC precipitate particles stimulating nucleation of recrystallization

Effects of particle distribution level on recrystallization were investigated in a Fe₃Al-based alloy containing coarse κ-Fe₃AlC precipitate particles. Volume fraction of 10–12% of rod-like κ particles with different size and interparticle spacing was introduced within the Fe₃Al matrix by changing the cooling rate from 1200 °C, which is above the precipitation temperature of the κ phase. These samples were warm rolled at 700 °C to a total reduction of 75%. Annealing of the warm rolled samples produced complete recrystallized structures. The average recrystallized grain size against interparticle spacing showed a valley-shaped curve with a minimum size of 20 μm. Orientation analyses of the warm rolled samples with high resolution EBSD method revealed that the valley shape of the curve may be explained by the particle stimulated nucleation density of recrystallization around κ particles, dependent on the particle distribution.     

Phase transformation and morphology of the intermetallic compounds formed at the Sn–9Zn–3.5Ag/Cu interface in aging

The morphology and phase transformation of the intermetallic compounds (IMCs) formed at the Sn–9Zn–3.5Ag/Cu interface in a solid-state reaction have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The monoclinic η′-Cu₆Sn₅ transforms to the hexagonal η-Cu₆Sn₅ and the orthorhombic Cu₅Zn₈ transforms to the body-centered cubic (bcc) γ-Cu₅Zn₈ as aged at 180 °C. The scallop-shaped Cu₆Sn₅ layer is retained after aging at 180 °C for 1000 h. In the solid-state reaction, Ag is repelled from η′-Cu₆Sn₅ and reacts with Sn to form Ag₃Sn, and the Cu₅Zn₈ layer decomposes. Kirkendall voids are not observed at the Sn–9Zn–3.5Ag/Cu interface even after aging at 180 °C for 1000 h.     

Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: Part II-coarsening of Al3(Sc1−xZrx) precipitates

The coarsening behavior of four Al(Sc,Zr) alloys containing small volume fractions (<0.01) of Al₃(Sc_1−xZr_x) (L1₂) precipitates was investigated employing conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The activation energies for diffusion-limited coarsening were obtained employing the Umantsev–Olson–Kuehmann–Voorhees (UOKV) model for multi-component alloys. The addition of Zr is shown to retard significantly the coarsening rate and stabilize precipitate morphologies. HREM of Al(Sc,Zr) alloys aged at 300 °C reveals Al₃(Sc_1−xZr_x) precipitates with sharp facets parallel to {1 0 0} and {1 1 0} planes. Coarsening of Al-0.07 Sc-0.019 Zr at.%, Al-0.06 Sc-0.005 Zr at.% and Al-0.09 Sc-0.047 Zr at.% alloys is shown to be controlled by volume diffusion of Zr atoms, while coarsening of Al-0.14 Sc-0.012 Zr at.% is controlled by volume diffusion of Sc atoms.     

Microstructural evolution and mechanical properties of a Cr---Cr2Hf alloy

The cast, homogenized and forged microstructures of a two-phase Cr---Cr₂Hf alloy (Cr-6.5 at.% Hf) were characterized via optical, scanning and transmission electron microscopy. The as-cast microstructure was substantially broken down by the hot working operation and subsequent heat treatment led to further refinement in the second-phase morphology. In the forged condition, the eutectic Cr₂Hf had the C14 structure; in addition, precipitation on a fine scale was observed in the Cr phase due to solid state decomposition, and these fine precipitates were tentatively identified as C15 Cr₂Hf. Extended heat treatments at high temperatures (1273–1373 K) led to the metastable C14 eutectic phase transforming to the C36 phase. Compression specimens from the forging, tested in the temperature range 293–1473 K, exhibited a yield strength-temperature profile characteristic of BCC alloys. Four-point bend tests were conducted as a function of temperature and strain rate to obtain an estimate of the ductile-to-brittle transition temperature. Notched bend specimens tested as a function of temperature provided a toughness of 7 MPa√m at 293 K that increased almost linearly with temperature to 15 MPa√m at 873 K. The resulting fracture surfaces were examined in the scanning electron microscope. The measured properties were correlated with the observed microstructures; microstructural changes that accompany the thermal-mechanical processing of such an alloy were understood.