Icosahedral-phase formation and stability in Ti Zr Co alloys

We present the first report of icosahedral phase i-phase formation in rapidly quenched alloys of Ti Zr Co . Electron diffraction patterns of i TiZrCo 53 27 20 contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti 3d transition metal Si O and Ti Zr Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti Zr Ni i-phases, showing none of these features. The quasilattice constant for a i TiZrCo, 0 51 nm, is close to that of i TiZrNi . The i-phase in rapidly q quenched Ti Zr Co alloys is deeply metastable and transforms exothermically o to the hexagonal Laves phase about 500 C. The Laves phase transforms to the o T bcc solid solution phase b-Ti and the Ti Ni-type fcc structure for 630 C. A 2 reversible transformation between the b-Ti and the hexagonal solid solution phase a-Ti is observed on temperature cycling; the Ti Ni-type fcc phase is 2 stable over the entire temperature range.     

Friction and wear behaviour of Inconel 718 alloy fabricated by selective laser melting after heat treatments

ABSTRACT

The effect of heat treatments (solution and double aging) on friction and wear behaviour of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) were studied. After heat treatment of solution and double aging, the friction and wear of the alloy have been improved. The worn surfaces of heat-treated IN718 alloy became smoother, and micro-ploughing became shallower. After solid solution hearting, a portion of the Laves phase ((Ni, Fe, Co)₂(Nb, Ti, Mo)) dissolves into the matrix, after which the γ″ phase (Ni₃Nb) andthe equilibrium phase of the δ phase (Ni₃Nb) precipitates during double ageing. After solution and double aging, nano-scale γ′ (Ni₃(Al, Ti)) and γ″ phases distribute homogeneously in the matrix, which improves the material hardness and wear resistance of the SLMed IN718 alloy.     

Thermal stability of nanocrystalline fcc and hcp Ni(Si) synthesized by mechanical alloying of Ni90Si10

The thermal stability of nanocrystalline fcc and hcp Ni(Si), obtained by mechanical alloying of Ni₉₀Si₁₀, has been studied. The allotropic transformation from fcc to hcp Ni(Si) is accompanied by a volume expansion of 8.6% and is observed when fcc Ni(Si) reaches a critical crystallite size of 10nm. The hcp phase transforms to stable fcc Ni(Si) at 573K. It has been identified that the lattice distortion in nanometre-sized crystallites from the equilibrium configuration and the decrease in the interfacial energy with grain refinement act as self obstacles in controlling the grain growth of nanocrystalline materials.     

Synthesis of amorphous and quasicrystal phases by mechanical alloying of Ti 45 Zr 38 Ni 17 powder mixtures,and their hydrogenation

Mechanical alloying of Ti 45 Zr 38 Ni 17 powder mixture forms an amorphous phase, but subsequent annealing causes the formation of an icosahedral ( i ) phase. The maximum hydrogen concentration that can be loaded at 573K at a hydrogen pressure of 3.8MPa is the same (\[H]/\[M] 1.5) for the amorphous and i -phase powders. With hydrogenation, the i -phase is almost stable, forming no hydrides, whereas the amorphous phase transforms to a fcc hydride. The activation energy for hydrogen desorption for the i -phase is about 127kJmol -1, which is lower than that for the amorphous phase, suggesting that the i -phase powder may have better properties for hydrogen-storage applications.     

Structural and thermal investigations of a high-strength Cu-Zr-Ti-Co bulk metallic glass

Cu 55 Zr 30 Ti 10 Co 5 bulk metallic glass exhibits a high compressive fracture strength of 2.31 GPa and Young's modulus of 130 GPa, values that are higher than those of other Cu- and Zr-based metallic glasses. The addition of Co to the ternary Cu-Zr-Ti alloy stabilizes the supercooled liquid. On heating, the Cu 55 Zr 30 Ti 10 Co 5 metallic glass devitrified and formed an intermediate intermetallic compound prior to reaching equilibrium by diffusion-controlled growth at constant nucleation rate. In the fully annealed state the structure consists of the equilibrium Cu 10 Zr 7 phase with the slightly reduced lattice parameters a = 0.933 nm, b = 0.928 nm and c = 1.254 nm and a small fraction of an unidentified phase.     

Mechanical-milling-induced amorphization of Se: A crystallite destabilization model

Complete solid-state amorphization has been realized in elemental Se by means of mechanical milling of crystalline Se powder. Quantitative X-ray diffraction and thermal analyses were employed to characterize the amorphization process and indicated that the amorphization onset corresponds to a critical crystallite size and a drop in microstrain. During the major amorphization process, the remaining crystallite size remains unchanged with a constant lattice expansion. A new kinetics model of crystallite destabilization is proposed for the solid-state amorphization which satisfactorily explains the experimental observations.     

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni₅₀Al₅₀ amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10^?2 m/s. The heat of the intermixing with the formation of Ni₅₀Al₅₀ amorphous alloy can be estimated at approximately ?0.34 eV/at. Next, the crystallization of the Ni₅₀Al₅₀ amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately ?0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.     

Titanium segregation mechanism in deformed vanadium-titanium alloys

Deformed V-Ti alloys with 0.3-5at.% Ti have been characterized by transmission electron microscopy and X-ray diffraction. A mechanism for Ti segregation during annealing, related to the formation of deformation twins, has been identified. Energy-dispersive X-ray spectroscopy indicates strong local changes in Ti content along the twins. Samples post-deformation annealed at 1473K show Ti segregation and the formation of Ti-rich precipitates which have been identified using electron microdiffraction and X-ray spectroscopy. X-ray diffraction measurements reveal changes in the lattice parameter of the alloys induced by the Ti segregation and the precipitation processes.     

Synthesis of a single phase of high-entropy Laves intermetallics in the Ti–Zr–V–Cr–Ni equiatomic alloy

The high-entropy Ti–Zr–V–Cr–Ni (20 at% each) alloy consisting of all five hydride-forming elements was successfully synthesised by the conventional melting and casting as well as by the melt-spinning technique. The as-cast alloy consists entirely of the micron size hexagonal Laves Phase of C14 type; whereas, the melt-spun ribbon exhibits the evolution of nanocrystalline Laves phase. There was no evidence of any amorphous or any other metastable phases in the present processing condition. This is the first report of synthesising a single phase of high-entropy complex intermetallic compound in the equiatomic quinary alloy system. The detailed characterisation by X-ray diffraction, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the existence of a single-phase multi-component hexagonal C14-type Laves phase in all the as-cast, melt-spun and annealed alloys. The lattice parameter a = 5.08 Å and c = 8.41 Å was determined from the annealed material (annealing at 1173 K). The thermodynamic calculations following the Miedema’s approach support the stability of the high-entropy multi-component Laves phase compared to that of the solid solution or glassy phases. The high hardness value (8.92 GPa at 25 g load) has been observed in nanocrystalline high-entropy alloy ribbon without any cracking. It implies that high-yield strength (~3.00 GPa) and the reasonable fracture toughness can be achieved in this high-entropy material.     

Adoption of near-coincident-site lattice orientations by contacting monolayer rafts of metallic nanoparticles with different superlattice periodicities

A hexagonal raft of monodisperse alkane-thiol-stabilized Au nanoparticles has been self-assembled from solution on to an amorphous C substrate and then subsequently a second layer of monodisperse but differently sized gold nanoparticles deposited on top of the first. Detailed analysis of electron micrographs obtained from various regions of this bilayer revealed the presence of several distinct epitaxial interface structures. A simple near-coincident-site lattice model is used to rationalize the existence of the observed characteristic nanoparticle interface structures.     

Rapid dendritic growth within an undercooled Ni–Cu–Fe–Sn–Ge quinary alloy

A liquid quinary alloy with composition Ni–5%Cu–5%Fe–5%Sn–5%Ge has been prepared from a containerless state by undercooling. Dendritic growth of α-Ni phase took place with a velocity of 28 m s^?1 at the maximum degree of undercooling, which was as high as 405 K (0.24T _L). All of the four solute elements Cu, Fe, Sn and Ge exhibited a significant solute trapping effect during the rapid dendrite growth. Segregation-less solidification is consequently realized when the degree of undercooling is sufficiently large. The lattice constant of α-Ni solid solution phase is found to increase with the amount of multicomponent solute trapping.     

site determination of dilute Cu in beta-phase NiAl by 63Cu nuclear magnetic resonance and X-ray emission channelling patterns

A sharply bimodal lattice site distribution of dilute Cu in both Ni-deficient and Al-deficient beta-phase Ni-Al alloys has been observed in the 63Cu nuclear magnetic resonance (NMR) spectra. These NMR results are correlated with results derived from incoherent channelling patterns (ICPs) formed by variations in characteristic X-ray emission as a function of incident fast electron orientation. Statistical analysis of ICP data, generated near a 210 zone axis, indicates that Cu occupies substitutional sites; Cu is exclusively on the Al sublattice sites in the Al-deficient alloy, whilst the partition ratio of Ni : Al sublattice sites for Cu in the Ni-deficient alloy is about 80 : 20. Comparison of ICP contrast from Cu X-ray emission with ICPs from the host lattice enables the two peaks in the 63Cu NMR spectrum to be individually identified as originating from Cu on Ni sublattice sites, and from Cu on Al sublattice sites. The respective NMR line intensities from the Ni-deficient alloy yields a Ni : Al sublattice site occupancy ratio which is in good agreement with the partitioning ratios derived from channelling patterns     

A novel lattice correspondence of B19 → B19′ transformation in Ti–Ni–Cu shape memory alloy thin film

In this study, we found a novel lattice correspondence of the B19–B19′ transformation in a Ti–Ni–Cu thin film: (1?1?1)_B19′//(0?0?1)_B19, [0, 1, 1]_B19′//[1?0?0]_B19. Near the coarse precipitate and the grain boundaries, the B19′ martensite forms with the novel lattice correspondence to product the (1?1?1) type I twinning instead of the usual (0?0?1) compound twinning. Crystallographic analyses show that the novel lattice correspondence results from the local stress concentration.     

Aluminium precipitates in the primary silicon of as-sprayformed hypereutectic aluminium-silicon alloys

Precipitates have been observed in the primary silicon of as-spray-formed hypereutectic aluminium-silicon alloys with and without various alloy additions. The precipitates have been investigated using transmission electron microscopy and high-resolution transmission electron microscopy. They are aluminium based and generally adopt a spheroidal morphology, with diameters in the range of approximately 5-10nm, unless located on a stacking fault or grain boundary. Spray forming is a rapid solidification process and precipitation occurs because of the low solid solubility of aluminium in silicon (maximum of 0.016 +/- 0.003at.% at 1190 C). The morphology of the precipitates suggests that they form from aluminium or aluminium-rich liquid. The orientation relationships of several precipitates in one of the primary silicon grains have been determined.     

Amorphization of cubic zirconia by caesium-ion implantation

Stabilized cubic zirconia is a promising candidate material for use as an inert fuel matrix for 'burning' excess Pu in light-water nuclear reactors. Zirconia is also considered to be an excellent nuclear waste form for direct geological disposal. Both applications are based on zirconia's high solubility for actinides, high chemical durability and high stability under irradiation. We report the first evidence of solid-state amorphization of yttria-stabilized cubic zirconia (YSZ) by 400keV Cs-ion implantation to 1 x 10²¹ ions m^-2 at room temperature. Amorphization of YSZ is caused by the large size incompatibility and low mobility of Cs ions in the YSZ structure, reflecting a relatively low solubility of Cs in YSZ. Nevertheless, the Cs concentration at which amorphization of YSZ occurred (about 8at.%) is well above the value that will typically be reached in an inert fuel matrix.     

Electron-microscope study of a non-stoichiometric phase in InSb heated in a vacuum

Abstract

When thin-foil specimens of InSb are heated in the vacuum of an electron microscope, a depletion of Sb takes place because of the preferential evaporation of Sb. This results in the formation of a novel non-stoichiometric (In-rich) solid phase having a cubic structure with a lattice parameter of 5·9 Å, which is about 10% smaller than that of InSb. It has been shown that the twin-orientation relationship exists between the matrix and the non-stoichiometric phase.     

Formation of decagonal and approximant phases in the Al-Ni-Ru system

Al-Ni-Ru alloys with a wide composition range have been synthesized and examined by X-ray and electron diffraction experiments. In this system, two types of decagonal (d) phases are formed, namely basic-type and superlattice-type d phases. Annealing experiments have shown that the basic-type d phase is stable at high temperatures and that its single-phase region is located in a small composition range around Al 75 Ni 15 Ru 10 . On the other hand, the superlattice-type d phase is metastable and has been found to form only in the samples before annealing. Besides the d phases, a high-order crystal approximant phase with lattice parameters a = 99.5 Å and b = 84.6 Å has been found. The structural relation between the approximant phase and the d phase is discussed in terms of a phason tensor.     

Abnormally high residual dislocation density in pure aluminum after Al/Ti/Al laminate annealing for seven days

We report an abnormally high residual dislocation density in aluminium in an Al/Ti/Al laminate annealed at 873 K for seven days. The residual dislocation density reaches 7.5 × 10¹⁴ m^?2, higher than that in aluminum after severe plastic deformation processes such as accumulative roll bonding and high-pressure torsion. It is proposed that the high residual dislocation density may result from obstruction of the movement of TiAl₃ nanoparticles by the grain boundary and Ti atoms conglomerating at vacancies distributed in the aluminium matrix at a high temperature for a sufficient time to allow a relatively stable crystal.     

Spectroscopic studies of a thermochromic centre in synthetic diamonds grown by the temperature-gradient method

Abstract

Synthetic diamonds grown by the temperature-gradient method at temperatures just above the melting point of the solvent-catalyst often have an unattractive dull-green colour. This green colour, which is prominent at room-temperature, disappears if the diamond is cooled to below 120 K and is restored on warming the crystal back to room temperature. We show that the behaviour of the absorption band responsible for the colour is consistent with the transition occurring from the upper level of a centre with a split ground state, for which transitions from the lower level are forbidden. Temperature-dependence measurements indicate that the energy splitting in the ground state is about 50 meV. The broad featureless absorption band, with a maximum near 1·9eV, is assumed to be vibronic in origin. The absence of structure in the band indicates that the electronic transition is strongly coupled to the lattice and, as expected, the zero-phonon line, estimated to be at around 1eV, is too weak to detect. The same absorption band is also shown to be present in some individual crystals of synthetic diamond abrasive grit.     

Formation of omega and Ti2Ni phases in a residual Ti–Ni–Ti multilayer between stainless steel and zirconia after bonding at 1173 K

Stainless steel (316L) and 8 mol pct Y₂O₃-stabilized zirconia (8Y-ZrO₂) were bonded using a Ti–Ni–Ti multilayer at 1173 K (900 °C) for 1 h. Cross-sectional transmission electron microscopy specimens were prepared by an innovative focused ion beam plus lift-out technique. In addition to acicular α-Ti, the dissolution of Fe, Cr, and Ni diffusing outwards from 316L into β-Ti led to the precipitation of the omega (ω) phase with different variants in the residual Ti foil between 316L and Ni. The ω-phase was not found in the residual Ti foil between Ni and 8Y-ZrO₂, while Ti₂Ni precipitates were precipitated in some α-Ti grains owing to the exclusion of Ni from the β-Ti.