Philip Nash

Factors affecting particle-coarsening kinetics and size distribution

The Lifshitz-Slyozov-Wagner (LSW) theory was developed to model kinetics of precipitate growth from supersaturated solid solutions. The theory corresponds to a zero volume fraction approximation but has been modified for finite volume fractions in order to correspond to real situations. The LSW theory has been applied to study coarsening of grains in liquid-phase sintering and to the coarsening of pores in solid-state sintering systems. There are some additional factors not considered in the LSW theory which can influence the coarsening kinetics depending on the system. It is important, therefore, to incorporate these factors into a coarsening model for better analysis of experimental data. The experimental evidence for the effects of these additional factors is reviewed together with the theoretical modifications made to the basic LSW theory in order to incorporate these factors.

Solid-state syntheses and properties of Zn4Sb3 thermoelectric materials

Abstract Various solid-state synthesis techniques have been used in order to produce high-efficiency e-Zn 4 Sb 3 bulk specimens. The techniques used were: sintering of cold compacts followed by hot pressing; direct synthesis by hot pressing; and sinter-forging. The phase transformations in this alloy system during solid-state syntheses were systematically investigated using DSC, XRD and SEM. Single-phase Zn 4 Sb 3 powders and crack-free bulk specimens having high density were successfully synthesized, but excess Zn was found to be necessary to compensate for Zn loss during processing. The thermoelectric properties at room temperature were evaluated for the hot consolidated compacts and compared with the published results of other studies. The thermoelectric properties of single-phase Zn 4 Sb 3 materials showed reasonable values. Solid-state synthesis utilizing the hot consolidation process offers potential processing routes to produce bulk Zn 4 Sb 3 .

Formation of metastable L12 phases in Al3Zr and Al-12.5%X-25%Zr (X ≡ Li, Cr, Fe, Ni, Cu)

The intermetallic Al3Zr crystallizes in the tetragonal DO23 structure but can be produced as a metastable cubic L12 structure by rapid solidification. We have studied the effects of mechanical alloying, a low-temperature isothermal processing method, and ternary additions of lithium, chromium, iron, nickel and copper on the phase stability of the L12 phase. Mechanically alloyed Al3Zr is single-phase L12 which is stable up to 550 °C. The additions of lithium or chromium increase the stability of the L12 phase to 750 and 740 °C, respectively. The addition of iron or nickel leads to the formation of amorphous, rather than L12, phases. Mechanically alloyed Al-12.5%Cu-25%Zr is single-phase L12 and this phase is stable up to at least 1300 °C.

Fabrication of in-situ grown graphene reinforced Cu matrix composites.

Graphene/Cu composites were fabricated through a graphene in-situ grown approach, which involved ball-milling of Cu powders with PMMA as solid carbon source, in-situ growth of graphene on flaky Cu powders and vacuum hot-press sintering. SEM and TEM characterization results indicated that graphene in-situ grown on Cu powders guaranteed a homogeneous dispersion and a good combination between graphene and Cu matrix, as well as the intact structure of graphene, which was beneficial to its strengthening effect. The yield strength of 244 MPa and tensile strength of 274 MPa were achieved in the composite with 0.95 wt.% graphene, which were separately 177% and 27.4% enhancement over pure Cu. Strengthening effect of in-situ grown graphene in the matrix was contributed to load transfer and dislocation strengthening.

Mechanical alloying and thermoelectric properties of Zn4Sb3

Thermoelectric Zn4Sb3 bulk specimens were produced by mechanical alloying of elemental powders and consolidated by hot pressing. Single phase Zn4Sb3 was not obtained using a nominal stoichiometric composition, but near single phase Zn4Sb3 with remnant elemental Zn having a relatively high density was produced using a nominally 11.7 at.% Zn rich powders. Phase transformations during mechanical alloying were systematically investigated using XRD and SEM. Thermoelectric and transport properties were evaluated for the hot pressed specimens and compared with results of analogous studies.

Thermodynamic Calculation of Phase Equilibria in the Ti–Co and Ni–Sn Systems

A thermodynamic model for the titanium-cobalt system has been developed utilizing measured enthalpies of mixing of the liquid and evaluated phase-diagram data. The free energies of the liquid, bcc, fcc, and hcp solid solutions, and TiCo, Ti2Co, TiCo2, and TiCo3 compounds were calculated for a temperature of 400 K. The model and measured heats of crystallization have been used to predict the free energy of the metastable amorphous phase at 400 K, needed for comparison with experimental results on the mechanical alloying of Ti and Co. The predicted glass-forming range for alloys prepared by mechanical alloying is from 10 to 81.5 at. % Co. We adopted a similar approach for modeling the Ni–Sn system to calculate the free energies of Ni3Sn, and Ni3Sn2, and the liquid (amorphous) and fcc solid solutions in the nickel-rich region at 240 K. In this system the inclusion of the magnetic contribution to the free energy of the Ni-rich fcc solid solution is important in interpreting the results of mechanical alloying. We propose a simple transformation of the free-energy curves, which assists in the graphical identification of the glass-forming ranges.

Deformation mechanisms and ductility of mechanically alloyed NiAl

Abstract An NiAl-based alloy has been produced by mechanical alloying and hot extrusion, resulting in material which is fully dense, with a homogeneous distribution of oxide particles and with a fine grain size of less than 1 μm. Mechanical properties of the mechanically alloyed (MA) NiAl were studied by compression testing from room temperature to 1300 K. At room temperature, the alloy exhibited high yield strength (1380 MPa) and considerable compressive ductility (greater than 11.5%). Transmission electron microscopy of the compressed specimens was carried out. In order to determine the Burgers vectors of slip dislocations a rigorous procedure was followed. The 〈100〉 slip was found to be predominant but strong evidence of 〈110〉 slip was also gathered. The occurrence of the slip vectors satisfies the general requirement for plasticity and contributes to the notable compressive ductility. Cast and hot extruded NiAl has been also investigated for comparison with the MA material. At room temperature, it exhibited a poor ductility (2.3%), low yield strength (400 MPa) and only 〈100〉 slip dislocations were observed. The 〈100〉 slip provides three independent slip systems, an insufficient number for general plasticity. The different behavior of cast and MA NiAl is believed to be a result of distinct textures, 〈111〉 and 〈110〉 respectively, exhibited by these differently processed materials.

The role of microstructure on strength and ductility of hot-extruded mechanically alloyed NiAl

Mechanical alloying followed by hot extrusion has been used to produce very fine-grained NiAl-based alloys containing oxide dispersoids. The dispersoids affect the progress of recrystallization during hot extrusion and contribute to the preservation of the 〈110〉 deformation fiber texture. The 〈110〉 texture enables the activation of 〈110〉 〈100〉 and 110 〈110〉 slip systems. The occurrence of 〈100〉 and 〈110〉 slip dislocations satisfies the von Mises criterion for general plasticity and is postulated to contribute to notable room-temperature compressive ductility of the mechanically alloyed (MA) materials. Another factor likely affecting the compressive ductility is the predominant occurrence of low-angle grain boundaries. The attractive dislocation — dispersoid interactions lead to a ductility trough observed at 800 K in the MA materials. The MA NiAl materials are strong at both ambient and elevated temperatures due to fine grain and the presence of dispersoids and interstitial atoms.

Mechanical properties of NiAl–AlN–Al2O3 composites

Abstract The mechanical properties of NiAl-based composites containing a dispersion of AlN particles and Al2O3 fibers were studied. NiAl matrix powder containing a dispersion of about 5 vol.% AlN was synthesized by mechanical alloying (MA) of elemental nickel and aluminum under a nitrogen gas atmosphere. Nextel 610 short fibers were used as the second reinforcement in the composite. Composites containing 5, 15 and 30 vol.% of the Al2O3 fibers in addition to the AlN dispersion particles were fabricated by hot pressing a dry blend of the MA NiAl powder and the fibers. The as-fabricated microstructures revealed that the matrix is fully dense and bonded well with the randomly distributed Al2O3 fibers. Neither chemical reaction nor cracks were observed at the fiber/matrix interface. Compressive behavior of the composites and a monolithic counterpart was studied at 1300 K and strain rates between 8.5×10−4 and 2.8×10−6 s−1. The 0.2% yield stress of the composites increases with fiber volume fraction at all strain rates. The strain rate–flow stress behavior at 1300 K indicates that the strength of NiAl–5%AlN–30%Al2O3 approaches that of NASAIR 100, a first generation Ni-base single crystal superalloy.

High-temperature deformation and defect chemistry of (La1−xSrx)1−yMnO3+δ

The creep behavior of (La{sub 1{minus}x}Sr{sub x}){sub t{minus}y}MnO{sub 3+{delta}} has been studied as a function of oxygen partial pressure (P{sub O{sub 2}}) and Sr concentration. Polycrystalline samples (x = 0.1, 0.2, 0.3) were deformed at 1,523 K in constant-crosshead-speed compression tests in various atmospheres (10{sup {minus}2} {le} P{sub O{sub 2}} {le} 10{sup 5} Pa). The material deformed by grain-boundary sliding accommodated by lattice diffusion with some possible cavitation and/or interface reaction control. The defect-chemistry model which is standard in the literature could not explain the dependence of the stress on P{sub O{sub 2}} and the Sr concentration. A modified defect-chemistry model shows that cation vacancies controlled the creep rate at P{sub O{sub 2}} {le} 10{sup 5} Pa for x = 0.3 and at low P{sub O{sub 2}} for x = 0.1 and 0.2, and that oxygen vacancies were rate-controlling at high P{sub O{sub 2}} for x = 0.1 and 0.2.