Solid state SiC/Ni alloy reaction

The solid state reaction between silicon carbide and a model superalloy consisting of 70 at. pct Ni, 20 at. pct Cr, and 10 at. pct Al was studied between 700 °C and 1150 °C for times ranging from “0” hours to 330 hours. Reaction couples consisting of SiC/Ni, SiC/Cr, and SiC/NiCr were also studied. The reactions were carried out in air with the materials, in the shape of discs, maintained in contact under a pressure of 7 MPa. A reaction was detected with SiC and the model alloy at all temperatures studied, and the reaction was diffusion controlled with an activation energy of 184 kJ/mole. In the ceramic the reaction was dominated by the diffusion of Ni into the ceramic forming a banded structure consisting of alternating layers of δ-Ni₂Si and a two phase mixture of graphite and δ. On the metal side, the reaction was very dependent on the presence of alloying elements, with pure Ni reacting to the greatest extent, followed by the binary NiCr alloy, and finally by NiCrAl. The growth and presence of the phases detected in these reactions is consistent with phase equilibria concepts.     

Multiple matrix cracking in a fiber-reinforced titanium matrix composite under high-cycle fatigue

An experimental study of multiple matrix cracking in a fiber-reinforced titanium alloy has been conducted. The focus has been on the effects of stress amplitude on the saturation crack density and the effects of crack density on hysteresis behavior. Comparisons have been made with predictions based on unit cell models, assuming the sliding resistance of the interface to be characterized by a constant interfacial shear stress. In addition, independent measurements of the sliding stress have been made using fiber pushout tests on both pristine and fatigued specimens. D.P. WALLS, Graduate Student, formerly with the Materials Department, University of California, Santa Barbara     

A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced NiAl composite

A composite of NiAl reinforced with continuous zirconia-toughened alumina (PRD-166) fibers was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was investigated by optical and transmission electron microscopy and energy-dispersive spectroscopy (EDS). Exposure of the fiber to the molten metal caused ZrO₂ particles in the fiber to move to the surface, thus permitting dissolution of ZrO₂ into the molten metal. The dissolved Zr reacted with A1₂O₃ of the fiber and formed ZrO₂ particles in some regions at the fiber/matrix interface. Vacuum annealing did not result in any noticeable change in the microstructure. Air annealing led to the precipitation of ZrO₂ within the matrix near the fiber/matrix interface. A thin layer of A1₂O₃ was observed to envelop the ZrO₂ particles and cover the fiber. During air annealing, Al oxidized preferentially, thereby continually reducing the Al content of the β-NiAl. This caused a progressive change in the microstructure of the matrix from β-NiAl to premartensitic microstructure, to martensitic structure, followed by nucleation and growth of Ni₃Al, to the development of a two-phase microstructure consisting of Ni₃Al cuboids dispersed in a disordered α-Ni(Al) and, subsequently, the formation of single-phase α-Ni(Al). The orientation relationship between Ni₃Al and NiAl was . Internal oxidation of α-Ni(Al) led to precipitation of A1₂O₃ particles which subsequently reacted with Ni, in the presence of O, to form NiO · A1₂O₃ spinel. The Ni was oxidized to formβ-NiO. Titanium-containing, platelike precipitates with a {111} habit plane were occasionally observed in NiO. Some larger NiTiO₃ particles were also formed within NiO. Diffusion of O through the interphase and grain boundaries of the fiber is believed to be responsible for the rapid oxidation of the composite.     

Time-dependent deformation in an enhanced SiC/SiC composite

Time-dependent deformation in an enhanced SiC/SiC composite has been studied under constant load at high temperatures of 1200 °C, 1300 °C, and 1400 °C. Creep damage evolution was evaluated by a Young’s-modulus change of partial unloading and microscopic observation. The addition of the glassy phase in the matrix is very effective for protecting the composite from oxidation. The transient creep is dominant in creep life at all the temperatures. An empirical equation is proposed to describe creep behavior of the composite. It is found that creep activation energy increases with creep time at stresses lower than matrix cracking stress, but the activation energy remains constant at stresses higher than the matrix cracking stress. The creep strain rate of the composite is considered to be controlled by creep of fibers based on examining the time, strain, stress, and temperature dependencies of creep strain rates.     

Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite

The oxidation behavior of Ti₅Si₃ has been studied in air in the temperature range of 1200 °C to 1400 °C. The oxidation kinetics is slower than that predicted by the parabolic-rate law equation at 1200 °C, but is sharply enhanced beyond a temperature of 1300 °C. The oxidation kinetics of a Ti₅Si₃-8 wt pct Al alloy and a Ti₅Si₃-20 vol pct TiC composite at 1200 °C has also been investigated and compared to that of Ti₅Si₃. Alloying with Al does not alter the oxidation resistance much, but the presence of TiC reinforcements enhances the rate of oxidation significantly. The oxidation products have been identified and the mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Microstructure study on the effect of titanium in the aging treatment of an Al-Zn-Mg alloy

A study has been made of the effect of Ti additions on the microstructure in two stage and single stage aging of an Al-5 Zn-2 Mg alloy. The influence of aging treatment was examined for alloys with and without Ti addition. The alloys were solution heat treated at 510°C, air cooled then aged at various temperatures below the GP zone solvus for short times before aging above the GP zone solvus at 200°C. The Ti addition alloy yields a small precipitate free zone (PFZ ∼ 0.3 μ) and a higher density of precipitate near the edge of the PFZ than in the midgrain region. The ternary alloy without Ti addition produces a wide range of PFZ sizes (0.4 to 4 μ) depending upon the aging temperature below the GP zone solvus. Significant changes in the morphology of the matrix precipitate, the amount of grain boundary precipitate, and the width of the PFZ, were observed in single stage aging at 200°C as a result of the Ti addition. These results could be explained in terms of Ti interacting with vacancies or solute atoms, causing the changes in the vacancy and/or solute concentration profiles. A model based upon solute and vacancy concentrations coupled with a time at temperature effect has been developed to allow the interpretation of the observed two-stage aging results in an Al-Zn-Mg alloy. Formerly Graduate Research Assistant, Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York.     

A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced Fe3Al composite

A zirconia-toughened alumina fiber-reinforced Fe₃Al-based intermetallic composite was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was studied by optical, scanning, and transmission electron microscopy. Fiber/molten metal interaction during pressure casting resulted in the rejection of ZrO₂ from the fiber into the molten metal. The fiber/matrix interface in the cast composite was in some areas covered with thin ZrC and Fe₂AlZr layers. Vacuum annealing resulted in the dissolution of Fe₂AlZr and precipitation of ZrC and (Ti, Nb)C particles within the matrix. The density of carbides was very low. Air annealing led to the oxidation of ZrC to ZrO₂, Fe₂AlZr to a mixture of A1₂O₃ + ZrO₂, and preferential growth of α-Al₂O₃ over the ZrO₂. Depletion of Al from the matrix as a result of oxidation gave way to the precipitation of (a) coarse (Fe, Al)₂(Nb, Al) particles and (b) fine cuboidal-shaped particles within the matrix during slow cooling from the oxidizing temperature. Oxidation of the matrix ended with the conversion of Fe(Al, Cr) into (Fe, Al, Cr)₂O₃. The Fe₂O₃ was observed to wet the grain boundaries of the A1₂O₃ fiber, which resulted in the disintegration of the fiber. Zr-containing plate-like precipitates with a {10-14} habit plane were occasionally observed in Fe₂O₃. Diffusion of oxygen through the fiber and/or the fiber/matrix interface is believed to be responsible for the rapid oxidation of the composite.     

Observation of fatigue damage process in SiC fiber-reinforced Ti-15-3 composite at high temperature

Unnotched SiC (SCS-6) fiber-reinforced Ti-15-3 alloy composite is subjected to a tension-tension fatigue test in a vacuum of 2×10⁻³ Pa at 293 and 823 K with a frequency of 2 Hz and R=0.1. Direct observation of the damage evolution process during the test is carried out by scanning electron microscopy (SEM). Test temperature dependent and independent fatigue damage behaviors are observed. The early stage fiber fractures observed at the polished surface are not influenced by the test temperature; however, matrix crack initiation and propagation behaviors differ greatly with temperature. The evolution of interface wear damage also differs with temperature, becoming more severe at 823 K, and the interface wear damage zone increases with the increase of the number of fatigue cycles. The macroscopic fatigue damage appears as a modulus reduction associated with interface sliding, matrix crack propagation, and plastic deformation of the matrix. The deformation zone of the composite tested at 823 K spreads more than that at 293 K. The fatigue life of the composite tested at 823 K is longer than that at 293 K. This behavior is related to the difference in spread of the damage zone in the matrix.     

A study of grain boundary precipitate-free zone formation in an Al−Zn−Mg alloy

The effect of quench rate and boundary type on the width of precipitate-free zones in an Al-6.86 wt pct Zn-2.35 wt pct Mg alloy that was aged at two different temperatures was studied by transmission electron microscopy. The width of the precipitate-free zones increased with decreasing quench rate, but by a considerably smaller extent than that predicted from calculations using previous models based upon quenched vacancy profiles. The simple tilt-type boundaries were associated, in general, with narrower precipitate-free zones than those occurring at complex-type boundaries. The width of precipitate-free zones was also observed to decrease with increasing precipitate coverage in the boundary. A mechanism of precipitate-free zone formation based on the modification of the as-quenched vacancy profile by the vacancies generated due to grain boundary precipitation was proposed to explain the observed results. C. R. SHASTRY, formerly Graduate Student, Materials Division, Rensselaer Polytechnic Institute, Troy. N. Y.     

A structural study of oxidation in an Al2O3 fiber reinforced titanium aluminide composite

A composite of titanium aluminide, reinforced with ZrO₂ toughened Al₂O₃ fibers, has been produced by pressure casting. The stability of the microstructure during vacuum and air annealing was investigated by scanning and transmission electron microscopy (TEM). Processing resulted in partial dissolution of ZrO₂ of the fiber into the molten metal which solidified into a two phase lamellar α₂ + γ matrix. Vacuum annealing caused dissolution of Al₂O₃ fiber into the matrix, transformation of the matrix into γ and precipitation of ZrO₂ and Al₂Zr within γ. During air annealing diffusion of O through the fiber resulted in the formation of Al₂O₃ coated ZrO₂ nodules extending from the fiber surface into the matrix in some regions and in the other regions the growth of Al₂O₃ into the γ of the lamellar matrix. An A15 phase with the metal composition, Ti₃Al₂, was found between the external oxide scale and the metal matrix.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

Interactions between SiC fibers and a titanium alloy during infrared liquid infiltration

The rapid interaction between SiC fibers (SCS-0) and a liquid titanium alloy has been investigated using an infrared processing technique. Experimental results revealed that disso-lution of the fibers in the alloy occurred within seconds without forming a continuous layer of reaction products at the interface. Thermodynamic analysis indicated that SiC is unstable in the presence of liquid titanium and dissociates into Si and C in the metal. With increasing carbon concentration, TiC will form when the carbon solubility limit in Ti is exceeded. The Ti-rich corner of the Ti-Si-C phase diagram with supercooled liquid Ti at 1300 °C was used to illustrate the tendency of TiC formation in this system. The fiber-matrix interface comprised two distinct morphologies: uniform dissolution fronts and scalloped dissolution fronts. The uniformly dis-solved domains are believed to be caused by an isothermal dissolution mechanism controlled by a zeroth-order chemical reaction, whereas the scalloped interfaces are believed to be caused by an accelerated dissolution mechanism resulting from localized heating. A model employing heat balance and reaction kinetics indicates that the conditions for accelerated dissolution are satisfied by the observed dissolution rates in the scalloped domains.