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Results are presented of compressive deformation tests performed on particulate-reinforced CoAl-TiB2 composites in the temperature range 1100-1300 K. Hot-pressed and postdeformation microstructures were characterized by TEM and by optical microscopy. It was found that the addition of TiB2 particles improves the deformation resistance of the matrix, due to dislocation-particle interactions.
The kinetics of the olivine-spinel transformation in Ni2SiO4 were investigated in an in situ X-ray diffraction experiments in which synchrotron radiation was used as an X-ray source. The starting material was Ni2SO4 olivine which was hot-pressed in situ at 980 C and 2.5 GPa; during the transformation, X-ray diffraction patterns were collected at intervals of 30 or 120 sec. The kinetic data were analyzed using Cahn's (1956) model. The activation energy for growth at 3.6-3.7 GPa was estimated as 438 + or - 199 kJ/mol. It is shown that, in order to make significant extrapolations of the kinetic data to a geological scale, the dependence of the rates of both nucleation and growth on temperature and pressure must be evaluated separately.
The reaction of Cu, a potential interfacial compliant layer for the Ti3Al plus Nb/SiC composite, with SiC, SCS-6 fiber, and the Ti3Al plus Nb matrix was examined at two temperatures: 1223 and 1273 K. Reaction of Cu with SiC resulted in the formation of a CuSi solution and free carbon, the reaction product being molten at 1273 K. Hot pressing the SCS-6 fiber in a Cu matrix at 1273 K resulted in cracking and delamination of the outer carbon-rich coating, thus allowing the Cu to penetrate to the SiC-carbon coating interface and react with SiC. In contrast, no such damage to the outer coating was observed at 1223 K. There was excessive reaction between Cu and the Ti3Al plus Nb matrix, the reaction product being molten both at 1223 and 1273 K. An interlayer of Nb between Cu and Ti3Al plus Nb matrix prevented the reaction between the two.
Zone-leveled and hot-pressed n- and p-type Si80Ge20 alloys were irradiated with neutrons to a fluence of 4 x 1018 n/sq cm and to a fluence of 5.4 x 1019 n/sq cm at a temperature of approximately 200-300 C. The effect of neutron irradiation on the thermoelectric properties of these alloys was evaluated. The carrier concentration and mobility (and hence the resistivity) were measured at room temperature while the thermal diffusivity was measured at 177-192 C both before and after the irradiation and after each subsequent 2-h heat treatment at 350 C, 600, and 1000 C. The irradiation increased the resistivity significantly, but the thermal conductivity decreased only by about 10-15 percent. This tends to indicate that the radiation produced only small defects (single pairs and small vacancy chains). The samples all returned to almost exactly their preirradiation state after the 1000 C anneal. This indicates that SiGe alloys can be operated in this neutron fluence at high temperatures without a degradation of thermoelectric properties.
Efforts to improve the thermoelectric efficiency of silicon-germanium alloys are discussed. These efforts are essentially focused on substantially enhancing carrier concentration levels and thus increasing the power factor values of n-type materials. With the help of thermodynamic phase diagram calculations, a combination of various crystal growth and dopant saturation techniques is used for this purpose. Bulk polycrystal growth of double-doped materials is achieved by zone leveling while a new method (the traveling heater method) for high-quality single-crystal growth of SiGe is being developed. Thin films of homogeneous single-crystalline multidoped Si80Ge20 have also been successfully grown from low-temperature melts to investigate combined dopant solid solubility variations. Very high carrier concentration levels were obtained by (P+As)-combined diffusion in zone-leveled and hot-pressed materials. The good overall understanding of doping mechanisms which has been achieved should enable optimization of the thermoelectric properties over this extended carrier concentration range. Reductions in thermal conductivity are being sought by introduction of very small phonon scattering centers (inert material) in the initial powders-precursors of Si-Ge alloys. This approach should benefit both n- and p-type materials.
The 1000-K and 1200-K time-dependent deformation of 100-line-oriented and non-100-line-oriented single crystals of Ni-40Al (made by a modified Bridgman technique) was examined over a large range of strain rates (from 0.1 to 10 to the -7th per sec). The results were compared with those for polycrystalline Ni-40Al made by hot pressing XD synthesized powder. The results from measurements of slow-plastic-strain-rate properties of the two materials show that single crystals offer no strength advantage over polycrystalline material. Both forms were found to deform via a dislocation climb mechanism.
The crack healing behavior of a commercial, MgO-containing, hot pressed Si3N4 was studied as a function of temperature in oxidizing and inert annealing environments. Crack healing occurred at a temperature 800 C or higher due to oxidation regardless of crack size, which ranged from 100 microns (indentation crack) to 1.7 mm (SEPB precrack). The resulting strength and apparent fracture toughness increased at crack healing temperature by 100 percent and 300 percent, respectively. The oxide layer present in the crack plane was found to be highly fatigue resistant, indicating that the oxide is not solely silicate glass, but a mixture of glass, enstatite, and/or cristobalite that was insensitive to fatigue in a room temperature water environment.
XD synthesis, powder blending, and hot pressing techniques have been utilized to produce NiAl composites containing 4, 7.5, 15, and 25 vol pct alumina whiskers and hybrid composite materials with 15 vol pct Al2O3 + 10 or 20 vol pct, nominally 1 micron TiB2 particles. The resistance to slow plastic flow was determined at 1200 and 1300 K via compression testing in air under constant velocity conditions. The stress-strain behavior of the intermetallic composites depended on the fraction of second phases where the 4 and 7.5 percent Al2O3 materials flowed at a nominally constant stress after about 2 percent deformation, while all the other composites exhibited diffuse yielding followed by strain softening. The flow stress-strain rate properties increased with volume fraction of Al2O3 whiskers except for the 4 and 7.5 percent materials, which had similar strengths. The hybrid composite NiAl + 15Al2O3 + 10TiB2 was substantially stronger than the materials simply containing alumina. Deformation in these composites can be described by the Kelly and Street model of creep in perfectly bonded, rigid, discontinuous fiber materials.
Experimental and theoretical work has been conducted at the Jet Propulsion Laboratory to improve the thermoelectric properties of n-type SiGe materials. Particular emphasis has been placed upon the understanding of the differences in dopant solid solubilities when multidoping with both Ga and P instead of P alone, as in standard SiGe alloys. A set of various experimental techniques for obtaining heavily doped hot-pressed samples coupled with thermodynamic theoretical considerations was used to relate microstructure and composition to electrical and thermal transport properties. Application of these results to high-temperature heat treatments of several SiGe/GaP samples achieved substantial improvements of the thermoelectric figure of merit Z, with average values close to 1 x 10-3K-1 over the 600-1000 C temperature range. A set of systematic anneals on heavily doped SiGe/GaP materials is reported in an attempt to consistently reproduce the optimized thermoelectric properties.
Spark erosion was used to produce ultra-fine particles of SiGe thermoelectric material and boron nitride, an inert phonon-scattering material. A homogeneous powder was made by mixing the two powders. The mixture was hot pressed to produce a thermoelectric material with uniformity dispersed, ultra-fine, inert, phonon-scattering centers. It is shown that, in samples with inert boron nitride or silicon nitride, thermal conductivity of a SiGe alloy can be reduced by about 25 percent while maintaining the electrical properties of the samples. Annealing of all the samples at 1525 K caused grain growth to over a micron, eliminating the detrimental effect attributable to small grains. Only in the sample with boron nitride the thermal conductivity did remain well below that for standard p-type SiGe (about 25 percent), while the electrical resistivity and Seebeck coefficient were very close to the values for standard p-type 80/20 SiGe.
The tensile creep and creep-recovery behavior of a hot-pressed unidirectional SiC-fiber/Si3N4-matrix composite was investigated at 1200 C in air, in order to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The data accumulated indicate that the fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa was periodic fiber fracture and that the creep life and the failure mode at 250 MPa were strongly influenced by the rate at which the initial creep stress was applied. Cyclic loading significantly lowered the duration of primary creep and the overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.
Chemical stability of TiB2 reinforcement in NiAl (45 at percent Al) and Ni3Al (24 at percent Al) matrices has been theoretically and experimentally investigated. Calculations were made using thermodynamic properties of the systems to predict behavior at temperatures between 1173 and 1573 K. Experimental investigation of hot-press consolidated TiB2 particulate/prealloyed matrix powder blends were conducted using energy dispersive X-ray analysis, XRD, AES, and TEM. The theoretical and experimental analyses suggest that TiB2 is chemically stable in both matrices up to 1573 K; however, TiB2 was found to be less active in NiAl than in Ni3Al due to lower nickel activity in NiAl.
We present a first model that enables prediction of the density (and its time evolution) of a monotape lay-up subjected to a hot isostatic or vacuum hot pressing consolidation cycle. Our approach is to break down the complicated (and probabilistic) consolidation problem into simple, analyzable parts and to combine them in a way that correctly represents the statistical aspects of the problem, the change in the problem's interior geometry, and the evolving contributions of the different deformation mechanisms. The model gives two types of output. One is in the form of maps showing the relative density dependence upon pressure, temperature, and time for step function temperature and pressure cycles. They are useful for quickly determining the best place to begin developing an optimized process. The second gives the evolution of density over time for any (arbitrary) applied temperature and pressure cycle. This has promise for refining process cycles and possibly for process control. Examples of the models application are given for Ti3Al + Nb, gamma TiAl, Ti6Al4V, and pure aluminum.
The rapid solidification of aluminum alloys as powder and the subsequent fabrication processes can be used to develop and tailor alloys to satisfy specific aerospace design requirements, including high strength and toughness. Laboratory procedures to produce aluminum powder-metallurgy (PM) materials are efficient but require evidence that the laboratory methods used can produce a product with superior properties. This paper describes laboratory equipment and procedures which can be used to produce tough aluminum PM sheet. The processing of a 2124 + 0.9 percent Zr aluminum alloy powder is used as an example. The fully hardened sheet product is evaluated in terms of properties and microstructure. The key features of the vacuum hot press pressing operation used to consolidate the powder are described. The 2124 + 0.9 percent Zr - T8 temper aluminum sheet produced was both strong (460-490 MPa yield strength) and tough (Kahn Tear unit-propagation- energy values over three times those typical for ingot metallurgy 2024-T81). Both the longitudinal and longitudinal-transverse directions of the sheet were tested. The microstructure was well refined with subgrains of one or two micrometers. Fine dispersoids of Al3Zr in the precipitate free regions adjacent to boundaries are believed to contribute to the improved toughness.
An investigation to determine the heat-shock resistant properties of two molybdenum disilicide turbine blades under centrifugal loads imposed by turbine rotation is presented. Molybdenum disilicide turbine blades fabricated by hot-pressing techniques withstood heat-shock conditions under blade centrifugal stresses up to 5350 pounds per square inch. Additional development is required before the heat-shock resistant properties of molybdenum disilicide are satisfactory for turbine-blade application.
Semicrystalline polyimides with controlled molecular weights synthesized in process that yields polyimides in powder form. Powders with desirable melt-flow properties formed in reaction vessels, without grinding. Commercially attractive for fabrication of adhesive bonds, compression molding of shaped parts, and deposition onto reinforcing fibers for subsequent hot pressing into polyimide-matrix/fiber composites.
Diffusion is one of the most fundamental physical phenomena. It relates to the manufacturing of almost all materials. Therefore, the precise knowledge of diffusivities of relevant materials is necessary for the optimization of these processes. On the other hand, such knowledge is indispensable to understanding the diffusion mechanism and structure of liquid metal in the field of physical metallurgy. Many diffusion experiments on solid metal were undertaken and a lot of data were accumulated. But on liquid metals there are few measurements because of experimental difficulties, such as thermal convection, induced by very slight temperature gradients in the specimen. Under microgravity conditions, the movement of liquid metal due to density differences and thermal convection must be suppressed markedly. Therefore, the transportation process will be controlled only by the diffusion of constituent atoms. These conditions offer the optimum environments for the diffusion experiment. The experiments are briefly described. Diffusion couples are prepared by attaching end to end two rods of different pure metals using hot press equipment. These couples are inserted into a graphite crucible and then doubly enclosed by two silica ampoules, respectively. The specimen is inserted into a tantalum cartridge, which is welded in a vacuum using an electron-beam apparatus. These cartridges are charged in the continuous heating furnace in space and held at a given temperature for a predetermined time and then solidified. The concentration distribution of elements in these specimens will be measured along the rod axis by means of an electron probe microanalyzer. The counterdiffusivity of the liquid metal binary system is determined on the basis of the compensated concentration distribution profile for thermal contraction.
The weldability of W-Cu composite sheet was investigated using simulated and welded joints. The welded joints were produced in a vacuum hot press. Tensile test results showed that simulated joints can provide strength and failure mode data which can be used in joint design for actual weldments. Although all of the welded joints had flaws, a number of these joints were as strong as the W-Cu composite base material.