S.M.L. Sastry

Mechanical behavior of damage tolerant TiB whisker-reinforced in situ titanium matrix composites

The results of a systematic study of the effects of alloying and microstructure on the mechanical behavior of in situ titanium matrix composites are reported in this paper. In situ composites are produced by alloying with B which promotes the formation of TiB whiskers during rapid solidification processing. The composite powders are subsequently compacted and extruded to align the whiskers prior to systematic heat treatment in the β and/or α + β phase fields. The processing conditions for the development of in situ composites with attractive combinations of strength, ductility, damage tolerance and creep resistance are thus established. The improvements in the composite properties are rationalized using simple micromechanics principles. The paper highlights the potential for the microstructural design of composites using micromechanics and conventional physical metallurgy principles.

Plastic deformation of nanocrystalline Cu and Cu–0.2 wt.% B

Abstract Plastic deformation behavior of nanocrystalline copper with and without trace boron was studied. Fully dense defect-free compacts were produced and mechanical properties were measured. The Hall–Petch (H–P) relationship was followed down to the 25 nm, smallest grain size investigated. The H–P slopes were smaller in nanocrystalline copper than in conventional copper. The role of boron in altering grain growth, strain hardening, recovery, and recrystallization was investigated by comparison of results in nanocrystalline copper with and without trace boron.

Mechanical properties of nanocrystalline copper produced by solution-phase synthesis

Nanocrystalline copper powder was produced by a NaBH 4 reduction of CuCl in a simple solution phase room temperature reaction. Uniaxial hot pressing in a closed tungsten die was used to compact powder into dense specimens. Samples were analyzed by x-ray diffraction, precision densitometry, electron microscopy, energy dispersive x-ray analysis, and selected area diffraction. Mechanical properties of the consolidated samples were determined by microhardness measurements, three-point bending of rectangular specimens, and compression tests. Yield strength measured for nanocrystalline Cu in the present work was over two times that reported in literature for Cu with comparable grain size and over five times that of conventional Cu. Restricted grain growth observed in the hot-pressed samples and improved mechanical properties are attributed to the presence of boron. A unique method of obtaining homogeneous in situ nanosized reinforcements to strengthen the grain boundaries in nanocrystalline materials is identified.

Synthesis of titanium boride (TiB) 2 nanocrystallites by solution-phase processing

Nanocrystalline TiB 2 is prepared by reaction of NaBH 4 and TiCl 4 . The initial solution-phase reaction affords an amorphous precursor powder from which 5-100 nm TiB 2 crystallites are obtained upon annealing at 900-1100 °C. Crystallite sizes depend on the annealing temperature and other processing parameters. Crystallite morphology is size dependent; crystallites smaller than 12-15 nm are cuboidal, whereas crystallites larger than 12-15 nm are hexagonal platelets. The procedure affords gram quantities of the smallest available TiB 2 nanocrystallites.

Deformation behavior of a rapidly solidified fine grained Al-8.5%Fe-1.2%V-1.7%Si alloy

Abstract Deformation characteristics of a rapidly solidified dispersion strengthened Al-8.5% Fe-1.2% V-1.7% Si alloy processed by planar flow casting was studied by tension testing at 25–420°C, compression testing at 25°C and hardness tests. The as-processed alloy shows non-linear elastic behavior, yield drop, low uniform and total elongation, serrated yielding, stress relaxation, flow softening and anomalous strain rate dependence of ductility. The results indicate that greater dynamic recovery due to fine grains (

Gas-phase combustion synthesis of titanium boride (TiB 2 ) nanocrystallites

Two techniques are described for synthesizing nanometer-sized TiB{sub 2} particles by gas-phase combustion reactions of sodium vapor with TiCl{sub 4} and BCl{sub 3}: a low-pressure, low temperature burner and a high-temperature flow reactor. Both methods produce TiB{sub 2} particles that are less than 15 nm in diameter. The combustion by-product, NaCl, is efficiently removed from the TiB{sub 2} by water washing or vacuum sublimation. Material collected from the low-temperature burner and annealed at 1000{degree}C consists of loosely agglomerated particles 20 to 100 nm in size. Washed material from the high-temperature flow reactor consists of necked agglomerates of 3 to 15 nm particles. A thermodynamic analysis of the Ti/B/Cl/Na system indicates that near 100{percent} yields of TiB{sub 2} are possible with appropriate reactant concentrations, pressures, and temperatures. {copyright} {ital 1996 Materials Research Society.}

Consolidation of nanoparticles—development of a micromechanistic model

Abstract A micromechanism-based model is developed for nanoparticle densification during uniaxial and hydrostatic pressing. The model takes into account the effects on densification of agglomeration, bulk and surface impurities, fewer dislocations per particle and low stability of dislocations due to fine size, and other factors unique to nanoparticle systems. The model is applicable to a general nanoparticle system, and is capable of predicting density, densification rate(s), dominant densification mechanisms, and microstructural features, as a function of consolidation parameters. The present model is the first rigorous and comprehensive attempt to extend the Helle–Easterling–Ashby model to the nanocrystalline grain-size regime. Predictions of the model were compared with experimental data on nanocrystalline copper and nanocrystalline molybdenum disilicide and observations reported in literature. A good agreement of model predictions with experimental data for a wide range of material and processing variables was observed.

Consolidation of molybdenum disilicide based materials by hot isostatic pressing (HIP): comparison with models

Abstract Monolithic molybdenum disilicide (MoSi 2 ) powder and MoSi 2 powders blended with ductile and brittle reinforcements were consolidated by hot isostatic pressing (HIP). The extent of densification of the consolidated samples as a function of temperature, pressure and time was determined by precision density measurements. HIP diagrams were constructed based on theoretical models. Material properties, required as input for the HIP map software, were compiled or extrapolated from published literature, experimentally determined and in some cases were estimated from the rule of mixtures. The model predictions were compared with experimental data and dominant mechanisms of densification were identified.

Grain refinement of intermetallics by severe plastic deformation

Abstract Gamma titanium aluminides (γ-TiAl) of compositions Ti–(42, 48, and 52)at.% Al were deformed to strains >100% by equal channel angular extrusion (ECAE) at temperatures

Creep-fatigue life prediction of in situ composite solders

Eutectic tin-lead solder alloys subjected to cyclic loading at room temperature experience creep-fatigue interactions due to high homologous temperature. Intermetallic reinforcements of Ni3Sn4 and Cu6Sn5 are incorporated into eutectic tin-lead alloy by rapid solidification processes to formin situ composite solders. In this study, thein situ composite solders were subjected to combined creep and fatigue deformation at room temperature. Under cyclic deformation, the dominant damage mechanism ofin situ composite solders is proposed to be growth of cavities. A constrained cavity growth model is applied to predict creep-fatigue life by taking into account the tensile loading component as well as the compressive loading component when reversed processes can occur. An algorithm to calculate cavity growth in each fatigue cycle is used to predict the number of fatigue cycles to failure, based on a critical cavity size of failure. Calculated lives are compared to experimental data under several fatigue histories, which include fully reversed stress-controlled fatigue, zero-tension stress-controlled fatigue, stress-controlled fatigue with tension hold time, fully reversed strain-controlled fatigue, and zero-tension straincontrolled fatigue. The model predicts the creep-fatigue lives within a factor of 2 with the incorporation of an appropriate compressive healing factor in most cases. Discrepancy between calculated lives and experimental results is discussed.