Compositing routes are reviewed for the fabricaiton of metal and metalceramic matrices combined with ceramic reinforcements and/or ductilizing phases. The important role of micromechanics in elucidating microstructural design principles to guide the processing “pathways” is emphasized. Specific processing techniques are described including incorporation of particles and fibers into melts, melt infiltration into preforms, powder metallurgy and melt oxidation.

Advances in kinetic energy storage devices have opened up a new approach to powder processing of High Temperature Composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar generator. The high-energy high-rate “lMJ in 1s” pulse permits rapid heating of a conducting powder in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible using standard powder processing approaches. This paper will describe the consolidation results of two high temperature composite materials, (W-Ni-Fe)/B4C and (Ti3 Al+Nb)/SiC. The focus of this study was the identification of the reaction products formed at the matrix/reinforcement interface as a function of input energy and applied stress. Input energies beyond a threshold value for each system were required to produce detectable reaction products. In the (W-Ni-Fe)/B4C system, the reaction products formed at 4000 kJ/kg input energy under 420 MPa applied stress were a series of complex carbides and borides including W2C, FeWB, Fe3C, Fe6W6C and Ni4B3. The intermetallic Fe7W6 was also observed. In the (Ti3Al+Nb)/SiC system, the reaction products observed at 3400 kJ/kg and 210 MPa were TiC and TiSi2.

The advantages of reinforcing metals with ceramic particles to produce metal matrix composites are well known. The behavior of discontinuously reinforced intermetallic compounds, however, has not been extensively studied. Martin Marietta Laboratories has produced a new generation of discontinuously reinforced titanium aluminide composites using a proprietary casting process known as XD™ technology. These new materials possess enhanced properties at room and elevated temperatures and may be cast, extruded, or forged. The effects of matrix composition, reinforcing phase, and thermal mechanical processing on properties have been studied using optical and various electron microscopy and mechanical and physical property measurement techniques to characterize the alloys. To date, most work has been done on a two-phased lamellar Ti-45 a/o Al alloy reinforced with TiB2 ceramic having an equiaxed morphology. Data on temperature dependence of the dynamic Young's modulus, coefficient of thermal expansion, deformation and fracture behavior, and microstructure are presented.

The fiber-matrix interface is believed to play a central role in the load-supporting and crack-growth resisting capabilities of fiber-reinforced high temperature composites. In metal matrix composites, this interface can be very complex: new phases may be present due to reactions between the matrix and reinforcement, compositional gradients due to interdiffusion occur, and the outer fiber surface is usually coated to avoid handling damage, improve fiber wetability, and/or control matrix reactivity. Attempts at measurements of fiber and interface strength from studies of actual composite samples are thwarted in part by the difficulty of determining the local distribution of stress and strain in these highly inhomogeneous materials. Furthermore, the inability to determine the point, during loading, where failure occurs in the interfacial zone also complicates the problem in these opaque materials. It is not surprising, therefore, that the fundamental relationships between the structure and properties of interfaces, on the one hand, and the load-supporting/crack-resisting properties of bulk metal matrix composities, on the other, have not been determined to date.

The effect of temperature on the mechanical properties and microstructures has been evaluated for heavily cold drawn Cu-20% Nb and Cu-20% Ta composites. The strengths of the composites decrease with increasing temperature, with the decrease becoming most pronounced at temperatures above about 300°C and at larger draw ratios. Cu-20% Ta composites are stronger than Cu-20% Nb composites throughout the temperature range studied (22–600°C) with the improvement increasing with increasing temperature. Resistivity measurements and substructure analyses showed that at temperatures where softening accelerated, resistivity decreased indicating a substructural change which was observed to be coarsening of the Nb and Ta filaments in the composites.

Composites containing alumina fibers in Ni3Al+B or an Ni3Al, Cr+B alloy have been prepared by several powder metallurgical processes. Tensile tests reveal some ductility and a mixed fracture mode in both monolithic and composite alloys. Previous reports of inadequate bonding of fibers in Ni3 Al+B have been confirmed.

Many intermetallic compounds possess properties which make them excellent candidates for high temperature use in advanced gas turbine and aerospace applications. One method proposed for increasing damage tolerance in these brittle materials is to artificially composite them with high temperature fibers as utilized in both ceramic and glass composites. Fabrication of these composites is a formidable problem. One method of fabricating these structures, termed here Transient Liquid Phase Consolidation, TLPC, is demonstrated for a number of intermetallic/reinforcing fiber combinations. Thermal stability of the fibers in the intermetallic matrices was observed with FP Alumina being the most stable. Ambient temperature tensile property evaluations were made on monolithic, chopped and aligned FP fiber reinforced Al3 Ta with the aligned structure having the highest ultimate strength and the chopped fiber composite the greatest pseudo plastic response.

The deformation mechanisms that can occur in advanced composites at high temperature are reviewed and the implications of different deformation modes in the constituent phases for the creep behaviour of the composite are considered. A generalised description of creep of composites in terms of internal state variables that have clear physical significance is presented.

As part of an investigation into the micro-mechanisms of crack-tip shielding associated with the growth of fatigue cracks in metal-matrix composites, simple models are developed for the role of crack bridging in high-strength aluminum alloys reinforced with SiC particulate (AI/SiCp). Based on experimental observations of crack growth, crack-tip shielding and crack-path morphology in these alloys, the bridges are found to be associated with uncracked ligaments in the wake of the crack tip, and are modelled in terms of approaches based on a critical crack-opening displacement or critical tensile strain in the ligament.

To enhance the high temperature strength of aluminides, NiAl–iB2 composites with particulate contents up to 30 vol. pct. were made by XDTM synthesis and hot pressed to full density. Microstructures of these composites were characterized by optical, scanning and transmission electron microscopy (TEM). The average size of the TiB2 particles was about 1 μm, and the average grain size of the NiAl matrix was on the order of 10 μm. Elevated temperature compression testing was conducted on these composites in air at 1200 and 1300 K with strain rates varying from −10−4 to −10−7 s−1. Flow strengths were found to increase with increasing TiB2 content; for example, the 20 vol. pct. TiB2 composite was three times stronger than unreinforced NiAl. Post test TEM analysis showed that the primary feature of the dislocation substructure of deformed NiAl was well defined subgrain boundaries, whereas the structure of the higher volume fraction composites consisted of a very high density of tangled dislocations, loops and subgrain boundaries connecting particles. These observations suggest that TiB2 particles can stabilize a completely different dislocation structure than that normally found in NiAl.

This work is aimed at developing understanding of ductile phase toughening in powder-processed intermetallic γ-TiAl alloys. The nominally ductile phases studied include Nb and Ti6Al4V, in the form of pancake shaped particles. Toughening is primarily due to the formation of crack face bridging zones. Toughness increases in the composites with a Ti6Al4V phase are limited due to particle embrittlement during processing. In this case, toughening is associated with the formation of short, low ductility/high strength bridge zones. Larger toughness increases and significant resistance curves are observed in composites containing Nb particles. In this case, long, high ductility/low strength bridge zones are observed. The behavior of the Nb composites is consistent with current understanding of constrained deformation of embedded particles and bridge zone toughening models. Additional toughening was observed in the Nb composites due to crack deflection and blunting in orientations where the crack intersects the broad pancake face.

In the Nb-Si system, the terminal Nb5 Si3 phase and the Nb Si phase are virtually immiscible up to approximately 2033K. This system offers the potential of producing composites consisting of a ductile refractory metal phase and a strong intermetallic phase. In-situ composites containing different volume fractions of the ductile Nb phase were produced via vacuum arc-casting. Microhardness testing as well as smooth bend bar testing was conducted at temperatures ranging from 298K to 1673K in an attempt to determine microstructural effects on the yield strength and smooth bar fracture strength. Notched bend specimens were similarly tested to determine the effects of the ductile phase (i.e. Nb) on enhancing the notched bend toughness. It is shown that the Nb phase often behaves in a ductile manner during testing, thereby toughening the in-situ composite. The mechanism of toughening appears to be due to crack bridging.

Internal friction behavior in cast 8-ply [0°1 P55Gr/Mg-0.6%Zr alloy and P55Gr/Mg-1%Mn composites as a function of vibratory strain amplitude was measured at 80 kHz using a Marxtype piezoelectric composite oscillator. Both the matrix and composite exhibited strain amplitude independent internal friction below ε ≈ 10−6, while significant strain amplitude dependence was noted at higher strain levels. A maxima in damping was observed for most of the specimens tested. Heat treatment to enlarge grain size was found to increase both the strain amplitude independent and dependent internal friction of the composite. Strain amplitude dependence of the internal friction, including the existence of the maxima, was explained by the Granato-Lucke (G-L) dislocation internal friction model. Dislocation densities obtained from various TEM images from the fiber-matrix interface were compared to values predicted by G-L theory.

The elevated temperature modulus and strength of aluminum, titanium, and hybrid aluminum/titanium metal matrix composites were investigated. Aluminum (6061-F) and titanium (Ti-6AI-4V) metal matrix composites reinforced with AVCO silicon carbide or boron fibers were vacuum hot pressed and their tensile properties evaluated to temperatures in excess of 300°C. Microstructure, fracture modes and mechanical properties were characterized to assess the effect of fibers and matrix on composite strength and modulus as a function of temperature. Finally, a comparison of specific strength and modulus is provided as a function of temperature. In general, the metal matrix composites exhibited low density (<2.8 g/cm3), high modulus (200 GPa), and strengths equivalent to 1250 MPa at 250–300°C. The effect of fiber orientation on axial stiffness was investigated using boron fiber reinforced aluminum (6061-F).

For the modeling of ply deformation behavior the orthotropic, thermal viscoplasticity theory based on overstress is used. It can represent creep, relaxation and rate sensitivity as well as monotonic and cyclic loadings. The theory is “unified” since creep and plasticity are not separately modeled. No yield surfaces and loading/unloading conditions are employed. The laminate theory for in-plane loading maintains the geometric assumptions of classical laminate theory. The elasticity law, however, is replaced by the thermal, orthotropic viscoplasticity law. Numerical experiments illustrate the predictions of the theory for an angle-ply and a cross-ply laminate subjected to a temperature increase, temperature hold and subsequent return to the original temperature. The ply and laminate stresses are calculated as a function of time for unconstrained and constrained conditions using postulated properties close to a real metal matrix composite. Redistribution of ply stresses and relaxation are found. In some cases, nearly permanent residual ply stresses are present after completion of the temperature cycle.

The tensile behavior of IN90211 was characterized at strain rates between 0.0001/sec and 340/sec at temperatures between 425 and 475 °C. At strain rates below 0.1/sec, the strain rate sensitivity m is about 0.027, with corresponding low elongation (<100%). At strain rates above 0.1/sec, the strain rate sensitivity increases to 0.26. A maximum elongation of 500% was obtained at 475 °C at a strain rate of 2.5/sec. Grain boundary sliding and rotation was observed on the highly elongated specimens and fracture surfaces exhibited intergranular fracture. Experimental data in the high strain rate regime (superplastic) revealed the existence of a temperature dependent threshold stress that seemed unrelated to the low stress deformation regime. This result is consistent with stress relaxation experiments. These threshold stresses are generally lower than those typically observed in other oxide dispersion strengthened (ODS) alloys. This observation is not expected from conventional superplastic creep theory.

During the past 25 years materials scientists have been able to make a major change in the way materials are considered for application. In the past designers have worked with data representing the properties of homogeneous, isotropic materials and designed their components to fit written accepted ranges of “design allowables”. More recently, however, the concept of composite materials has permitted almost limitless tailoring of composites to create entirely new designs never previously possible. By choice of types of material constituents, their relative percentages and their orientation the designer can now work closely with the materials scientist to optimize system performance. This philosophy has firmly taken hold in the family of fiber reinforced polymer matrix composites and more recently has made metal matrix composites an industrial reality.

Ceramic matrix composites have been produced by utilizing polymer pyrolysis as the processing technique. The precursor, polyvinylsilane, is a viscous, thermosetting polymer which yields a predominantly SiC ceramic material when pyrolyzed. This organometallic polymer in combination with SiC fibers and SiC whiskers was used to fabricate ceramic matrix composites. One of the major problems with a brittle/brittle composite system in which strong coupling exist between the fibers and matrix is the characteristic catastrophic failure that occurs once the strain to failure of one of the constituents is exceeded. This brittle behavior can be altered by the application of a suitable barrier layer between the fiber and matrix. Due to the success of a barrier layer between fiber and matrix in producing higher performance composites, multiple barrier layers were used to further improve the performance of the ceramic composite.

The fracture behavior of a 3-D braided Nicalon fiber-reinforced SiC matrix composite processed by chemical vapor infiltration (CVI) has been investigated. The fracture toughness and thermal shock resistance under various thermomechanical loadings have been characterized. The results obtained indicate that a tough and durable structural ceramic composite can be achieved through the combination of 3-D fiber architecture and the low temperature CVI processing.

A SiC whisker reinforced HIPped RBSN material fabricated with a Y2O3 sintering aid was characterized using TEM. The matrix is > 90% β-Si3 N4 with a Y-Si-O-N glassy phase at the Si3 N4 grain boundaries and about the SiC whiskers. The SiC whisiers are heavily faulted and have a well defined core. Si3 N4 precipitates are observed in the core region after composite fabrication. A preliminary mechanism for the growth of the SiC whisker, based on the VLS mechanism is proposed.