M.J. Kaufman

Site-specific growth of Zno nanorods using catalysis-driven molecular-beam epitaxy

We report on catalyst-driven molecular beam epitaxy of ZnO nanorods. The process is site specific, as single crystal ZnO nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300–500 °C. The nanorods are uniform cylinders, exhibiting diameters of 15–40 nm and lengths in excess of 1 μm. With this approach, nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics.

Phase equilibria and transformations in intermediate titaniumaluminum alloys

The phase equilibria and solid state phase transformations observed in TiAl alloys between 40 and 50 at.% Al are reported. It is shown that the three-phase invariant between the high temperature α phase (disordered h.c.p.) and the lower temperature ordered α2-Ti3Al (D019) and γ-TiAl (L10) phases is eutectoid in nature (α → α2 + γ) but that it is difficult to impossible to produce this reaction experimentally due to differences in the transformation kinetics of the two product phases. The decomposition of the high temperature α phase is shown to be a strong function of composition and cooling rate and the results are used to construct schematic CCT diagrams. It is also shown that γ precipitation can be suppressed in 40, 42 and 45Al alloys by rapid quenching from the α field but not in the 48 and 50Al alloys; in the latter, the γ tends to be either lamellar or massive in nature. Finally, the nature of the discontinuous coarsening of the lamellar (α2 + γ or α + γ) structures to produce serrated, interlocking grain boundaries and the lack of heterogeneous nucleation of γ at grain boundaries is described and related to the thermal histories, driving forces, and crystallography of the system. The various results are then used to shed light on the behavior observed in previous studies on alloys in this system.

Microstructures and phase relationships in the Ti3Al + Nb system

Alloys based on the {alpha}/sub 2/-Ti/sub 3/Al compound (hexagonal) DO/sub 19/ are currently experiencing limited use as advanced aerospace materials. To date, the alloys with the most desirable properties contain additions of {beta} stabilizers, such as Nb, Mo and V, which promote the formation of a two-phase mixture of {alpha}/sub 2/+{beta} or {alpha}/sub 2/+B2 (where B2 refers to the ordered CsCl structure). Unfortunately, the phase relationships in these systems have not been established in sufficient detail to allow their more widespread application. Recently, there has been a series of investigations aimed at alleviating this deficiency in the ternary Ti-Al-Nb system. These studies have clearly indicated the existence of the ordered B2 phase, which, in the higher Nb alloys, can be retained at room temperature by rapid cooling from the liquid or solid state. The authors describe (TiNb)/sub 3/Al alloys (from 0 to 30 at. pct. Nb) were studies after conventional and nonequilibrium (i.e., rapid solidification) processing with an emphasis on providing further insight into the transformation sequences and phase equilibria in these alloys.

Icosahedral Al-Mn and related phases: resemblance in structure

Etudes preliminaires des relations cristallographiques entre les phases icosaedrique, T, T″ et Al 4 Mn par microscopie electronique en transmisssion a haute resolution

An analytical electron microscopy study of the recently reported “Ti2Al phase” in γ-TiAl alloys☆

Abstract A variety of experimental techniques has been utilized to establish that the phase reported previously to be a new stable Ti2Al phase (1–3) is instead Ti2AlN. The space group of Ti2AlN has been determined to be P63/mmc with lattice parameters, a = 0.304 nm and c = 1.369 nm, in agreement with the results of Jeitschko, et al. (13). Also, since this phase is observed only in alloys which do not contain α2-Ti3Al, it was suggested that the solubility of N in α2 must be rather large, while that in the γ(TiAl) phase is low. Finally, in light of the improved creep properties of γ alloys containing Ti2AlN precipitates (3), it is suggested that an increase in the volume fraction of this phase, by increasing the N and/or C contents, in these alloys might be used to enhance the mechanical properties of this normally brittle compound.

Intrinisic second-phase particles in powder-processed MoSi2

Abstract MoSi 2 has been produced by hot pressing of commercial and in-house powders and also by arc melting of pure molybdenum and silicon. It is shown that the “glassy” phase observed in powder-processed MoSi 2 consists of amorphous SiO 2 and that formation of this phase is intrinsic to powder processing. In addition, the Mo 5 Si 3 phase, which is often present in MoSi 2 , is found to be a carbon-stabilized form of approximate composition CMo 5 Si 3 .

The massive transformation in TiAl alloys: Mechanistic observations

Abstract The massive α → γm transformation, as observed using analytical transmission electron microscopy, in Ti49Al, Ti48Al2Nb2Mn, Ti55Al25Ta and Ti50Al20Ta alloys is described. Conventional solution heating and quenching experiments have been combined with the more rapid quenching possible using electron beam melting in order to provide further insight into the early stages of the transformation in these alloys. It is shown that the γ develops first at grain boundaries as lamellae in one of the grains and that these lamellae intersect and spread into the adjacent grain in a massive manner. Consequently, there is no orientation relationship between the massive gamma (γm) and the grain being consumed whereas there is the expected relation between the γm and the first grain which is inherited from the lamellae. It is further shown that the γm grows as an f.c.c. phase after initially growing with the L10 structure. Furthermore, it is shown that the massive f.c.c. phase then orders to the L10 structure producing APDB-like defects which are actually thin 90° domains separating adjacent domains that have the same orientation yet are out of phase. The advancing γm interface tends to facet parallel either to one of its four {111} planes or to the basal plane in the grain being consumed by impinging on existing γ lamellae. Thin microtwins and α2 platelets then form in the γm presumably due, respectively, to transformation stresses and supersaturation of the γm with titanium for alloys containing ∼48% Al; indeed, there is a local depletion in aluminium across the α2 platelets as determined using fine probe microanalysis.

Microstructural modification of MoSi2 through aluminum additions

The objective of this work was to demonstrate the possibility of influencing one or more aspects of the microstructure of powder processed MoSi[sub 2] via in-situ displacement reactions. Since silica is frequently present in MoSi[sub 2] produced by powder processing followed by consolidation and is generally considered as an undesirable phase, an in-situ process that could eliminate silica was selected. There are three main routes to eliminate or control silica in powder processed MoSi[sub 2] namely (i) silica reduction via a suitable deoxidant, (ii) powder processing and consolidation in extremely clean conditions, and (iii) chemically etching the powder surface prior to consolidation under clean conditions. Although there are a number of elements that can reduce silica, aluminum was selected in this work since it has some solubility in MoSi[sub 2] and, when dissolved in larger amounts, it promotes the formation of the hexagonal (C40), alumino-silicide Mo(SiAl)[sub 2] phase.

The effects of chromium on NiAl intermetallic alloys: Part I. microstructures and mechanical properties

Abstract The effects of Cr on the microstructure and mechanical properties of NiAl have been characterized. It is shown that Cr is an efficient solid solution strengthener in NiAl, effectively tripling the 0·2% yield strength at the 1 at. % level; this is also its maximum solubility. The presence of Cr also increases the BDTT of NiAl by approximately 150K and reduces the already limited ductility to essentially zero. Larger alloying additions produce fine α-Cr precipitation. However, the associated hardening contribution due to precipitation is relatively minor compared with solid solution effects. Independently of Cr content, stoichiometric deviation was observed to produce significant increases in strength and BDTT. It is shown that hardening due to both intrinsic and extrinsic substitutional defects is ultimately a function of the solubility, site preference, and alloying strategy (e.g., a particular Ni/Al ratio, or Cr substituted solely for either Ni or Al) of the Cr addition.

Optical properties of Zn1−xMgxO nanorods using catalysis-driven molecular beam epitaxy

Abstract We report on the optical properties of (Zn,Mg)O nanorods grown by catalyst-driven molecular beam epitaxy. The process is site-specific, as single crystal (Zn,Mg)O nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO 2 -terminated Si substrate surface. Growth occurs within a flux of Zn, Mg, and O 2 /O 3 mixture at substrate temperatures of 400–500 °C. With the addition of Mg, the nanorod morphology becomes more uniform relative to the pure ZnO nanomaterials synthesized under similar conditions. The (Zn,Mg)O nanorods are cylindrical, exhibiting diameters of 15–40 nm and lengths in excess of 1 μm. The (Zn,Mg)O nanorods exhibit a strong photoluminescence response, showing a slight shift to shorter wavelengths due to Mg incorporation.