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.     

A new high-pressure phase transition in a zirconium-niobium alloy

It is reported for the first time that a g M y transformation can be induced in a g -stabilized zirconium alloy subjected to shock pressure. The y phase formed in the g matrix has been found to have a plate shape akin to martensitic plates. The lattice correspondence between the g and y structures has been found to be the same as that produced by thermal treatment. The formation of the plate-shaped y phase is explained in terms of a mechanism involving shear on <112> planes of the bcc lattice and the mechanical instability of the g phase.     

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.     

Formation of a 'splitting pattern' associated with L1 2 precipitates in Ir-Nb alloys

A cuboidal L1 2 phase forms in the fcc matrix of an Ir-Nb alloy. Under some conditions, a rod- or plate-like fcc structure can be observed in this phase. This is similar to the 'splitting pattern' observed in some Ni-based alloys. To understand the formation mechanism of this structure, samples were heated under different conditions. Microstructure evolution during ageing was observed by transmission electron microscopy. We observed a splitting pattern only when the L1 2 precipitates were already formed in the as-cast condition after additional heat treatment at the ageing temperature. In this case, the composition of the L1 2 phase was not equilibrated. We conclude that the splitting pattern appears owing to a kinetic phenomenon when the L1 2 phase changes to the equilibrium composition.     

Structure of a cubic phase related to Cd?Mg?rare-earth quasicrystals

The structure of a Cd₆₈Mg₁₂Dy₂₀ crystalline phase denoted as the φ-phase, which has a composition close to that of the Cd₆₆Mg₂₁Dy₁₃ icosahedral quasicrystalline phase, has been investigated by electron diffraction and scanning transmission electron microscopy (STEM). The φ phase has a fcc lattice with a = 21.6Å. High-angle annular dark-field STEM with Z contrast confirms that the phase has the Cd₄₅Sm₁₁-type structure. The atomic cluster in the structure is shown to be characterized by a Friauf polyhedron with tetrahedral symmetry.     

New 1/1 cubic approximant phases in the Al-Cu-Ru-Si system studied by electron diffraction and high-resolution electron microscopy

Two types of new Al-Cu-Ru-Si 1/1 cubic approximant phases, both of which have cell parameters a=12.68A have been found in an as-cast Al_58.5Cu₁₈Ru_13.5Si₁₀ alloy. The two phases have sc and bcc structures respectively and are finely mixed in transmission electron microscopy observations. It is proposed that the sc structure is caused by ordering of atoms in the bcc structure.     

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

Cu-based icosahedral quasicrystal formed in Cu-Ga-Mg-Sc alloys

The first-reported Cu-based icosahedral quasicrystal has been found as an almost single phase in Cu 48 Ga 34 Mg 3 Sc 15 alloy annealed at 1043K for 61h. On the basis of our experiments, this icosahedral quasicrystal is expected to be one of the equilibrium phases in this alloy system. Powder X-ray diffraction and electron diffraction experiments revealed that the quasicrystal exhibits a high degree of structural perfection and has a primitive type quasilattice with a six-dimensional lattice parameter a 6D =0.6938nm. The existence of a Cu 3.7 Ga 2.3 Sc-type structure, which is a bcc structure with diffraction symmetry m3, suggests that the Cu-Ga-Mg-Sc quasicrystal is to be classified into a new structural type to which Cd-based icosahedral quasicrystals and the Zn-Mg-Sc quasicrystal belong. This new type has a characteristic local atomic configuration different from both Mackay-type and Bergman-type quasicrystals.     

Influence of test temperature on the size effect in molybdenum small-scale compression pillars

Previous research has shown that body-centred cubic (bcc) metals exhibit a smaller size dependence of strength than what is commonly observed in face-centred cubic (fcc) metals. This work investigates compression testing of focused ion beam-manufactured molybdenum pillars ranging in size from 300?nm to 5?μm, both above and below its critical temperature at 300 and 500?K. At 500?K the size effect is found to be consistent with what is observed in fcc metals, owing to the increased mobility of screw dislocations.     

Kinetics of diffusion via divacancies in a concentrated random fcc alloy

The diffusion kinetics in a concentrated fcc alloy are described by means of divacancies. The model chosen is the random alloy model with bound divacancies. We show that for the fcc structure the Manning formalism developed originally for monovacancies can be used intact to describe diffusion via divacancies. Monte Carlo simulation results for both tracer and collective correlation factors are in good agreement with the results of the formalism for a wide range of the component atom exchange frequencies with the divacancy except for the slower-moving component.     

Unexpected nanoscale phase separation in sputterdeposited Ni-25 at.%Al thin films

Thin films of nominal composition Ni-25at.%Al, sputter deposited from a target of the intermetallic compound Ni 3 Al on unheated substrates, exhibit an unexpected phase separation, in contrast with other intermetallic-forming systems such as Ti-Al which are deposited as compositionally homogeneous amorphous films under similar conditions. Precipitates of a novel tetragonal phase, a few nanometres in size, were formed in the matrix consisting of a fcc Ni-rich Ni(Al) phase and a hcp Ni-rich Ni(Al) phase. Ni-Al films of the same composition deposited on heated substrates exhibited the formation of a single, chemically long-range-ordered Ni 3 Al phase with the L1 2 structure, the thermodynamically stable phase for this composition.     

Thermodynamic analysis of Fe contamination in Cu-Mo alloys processed by mechanical alloying

ABSTRACT

This work aims to verify if Fe atoms coming from balls and jars enter into solid solution of Cu-Mo powders during mechanical alloying. The powders were milled and its structure and composition were characterised by scanning and transmission electron microscopy, and X-ray diffraction. Gibbs free of mixing values for Cu-Mo-Fe system were calculated using the Miedema’s model and showed that Cu-rich corner exhibit smallest energy values (<10?kJ?mol^?1). The observed coherency relation Cu(111) || Mo(101) || Fe(101) confirmed formation of solid solution. Fe atoms have entered in solid solution with Cu and Mo and do not remain as particles of second phase.     

On the orientation relationship between a2 precipitates and the B2 phase in a Ti-47at.%Al-2at.%W-0.5at.%Si alloy

An k -phase Mn-Al-C single crystal was grown and annealed to obtain an intermediate stage of the k M transformation. The crystal was analysed by transmission electron microscopy, and direct evidence for the operation of the shear mode was obtained. The transformation occurs in two steps from the hcp structure ( k ) over the orthorhombic structure ( k ' phase) to the tetragonal phase. The study reveals the morphologies of k ' and , the necessity for reordering during the second transformation step ( k ' M ) which is not a purely martensitic transformation, and the selection of variants which is controlled by stresses through the selection of one k ' variant.     

Investigation of {111} stacking faults and nanotwins in epitaxial BaTiO3 thin films by high-resolution transmission electron microscopy

{111} stacking faults and nanotwins in epitaxial BaTiO₃ thin films on MgO substrates have been investigated by high-resolution transmission electron microscopy. In many cases, the stacking faults and nanotwins were found to be accompanied by partial dislocations. These partial dislocations can be classified as two different types, analogous to the situation in the fcc structure. One is of the Shockley type with the Burgers vector (a/3)<112>. The other is of the Frank type with the Burgers vector (a/3)<111>. The movements of both types of partial can lead to the {111} stacking faults and the {111} twins observed in these films.     

Observation of planar stacking faults in a Ti-rich two-phase Ti-Al alloy after deformation at elevated temperatures

It is a common observation that in two-phase Ti-Al-based binary alloys, deformation of the gamma phase occurs by 1/2<110]-type ordinary dis-location activity and twinning associated with 1/6<112] type partials. In the present study the microstructure of a new Ti-Al-based alloy (Ti-47at.% Al-2at.%Mn-2at.%Nb+ 0.8 vol.% TiB₂) with a duplex microstructure consisting of primary equiaxed gamma grains and lamellar alpha₂+ gamma colonies was studied by transmission electron microscopy (TEM) after deformation at elevated temperatures. Planar stacking faults were found in the gamma laths. A detailed contrast analysis by TEM shows that these planar stacking faults lying on {111} planes are bound by all the fcc variants of the Shockley partial dislocations of type 1/6<121>, in contrast with the observations in stoichiometric binary TiAl alloys, where only 1/6<112]-type Shockley partials are found to be associated with the true twins. It is proposed that the addition of ternary and quaternary elements such as Mn and Nb promotes the other variants of the fcc-like dissociations (not common in L1₀structure) in the present alloy.     

Structure of a phase induced with Xe-ion irradiation-implantation in γ-TiAl

A phase transformation in γ-TiAl intermetallic alloy was found to be induced with 50keV Xe-ion irradiation-implantation at doses larger than 2.2 x 10¹⁸ ions m^-2 at room temperature. The structure and the chemical composition of the induced phase were investigated with high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The zones of the induced phase have sizes up to about several tens of nanometres. The phase has a hexagonal structure with a = 0.286 nm and c = 0.462nm. The crystallographic orientation relationship between the phase (P) and the gamma-TiAl matrix is (001)_P//(111)_γ and \[100]_P//[011]_γ. The \[Al]/[Ti] atomic composition ratio in the phase is analysed to be 56/44, slightly different from that of the matrix, 51/49. These results suggest that the induced phase is an Al solid solution of α-Ti alloy phase, which has different structural parameters and chemical composition from those of the reported phase. It is suggested that the size of the ions is important in the phase transformation.     

Composition and magnetic character of nanometre-size Cu precipitates in reactor pressure vessel steels: Implications for nuclear power plant lifetime extension

The sensitivity of positrons to nanometre-size Cu precipitates in Fe alloys has enabled us to apply a novel spin-polarized element-specific method to probe the composition and magnetic character of the precipitates responsible for reactor pressure vessel (RPV) steel embrittlement. The results clearly show that the precipitates are non-magnetic and place an upper limit of about 10at% on their Fe content. The practical implication of this result is that the Cu precipitate contribution to RPV steel embrittlement saturates is not expected to contribute further during lifetime extension. Our study demonstrates that polarized positrons can be used as a powerful probe of the magnetic character of nanoscale materials, even those embedded in a strongly magnetic host.     

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.