Stress-State dependence of strain-hardening behavior in 2014 Al/15 vol Pct Al2O3 composite

The purpose of this investigation was to examine if the effective stress-strain function for discontinuously reinforced aluminum (DRA) matrix composites is independent of stress state, as they are for aluminum alloys. The rationale for such work is provided by the need to develop constitutive equations for applications in metal forming and forging problems. Experimental effectiveas curves at room temperature were determined for a particulate-reinforced composite, 2014 Al/15 vol pct A1₂O₃, and the matrix material, 2014 Al, under a variety of stress states. The tests consisted of uniaxial tension, equibiaxial tension (bulge test), and compression tests. To eliminate the effects of prior precipitation, all samples were given a solution-heat-treatment prior to tests. It was found that for the composite the effective yield stress in uniaxial tension was higher than that in equibiaxial tension but slightly lower than that in compression. However, the effective yield stresses for the matrix material in uniaxial tension and equibiaxial tension were nearly the same. The strain-hardening rate of the composite under equibiaxial tension was higher than that under either uniaxial tension or compression. It is suggested that nondeformable dead zones can develop around the particles during deformation whose shape changes with the applied stress state, and this is partly responsible for the observed differences in behavior. Formerly Graduate Student, the University of MichiganSenior Engineer     

Plasticity of continuous fiber-reinforced metals

Continuous parallel alumina fiber-reinforced metals produced by pressure infiltration are tested in tension/compression along the fiber axis with a goal of measuring the influence exerted by long fibers on the flow stress of their matrix. In this configuration, the equistrain rule of mixtures, modified to take into account stresses due to differential lateral contraction, can be used to back-calculate the matrix flow stress from that of the composite. This method provides the least physically ambiguous measurement of matrix flow stress in the composite; however, experimental uncertainty can be high. This uncertainty is evaluated in detail for the present experiments, in which matrix in situ stress-strain curves are measured for cast 3M NEXTEL 610 and DUPONT FIBER FP reinforced pure and alloyed aluminum- and copper-based matrices of varying propensity for recovery and cross-slip. Within experimental uncertainty, data show no enhanced matrix work-hardening rates such as those those that have been measured with tungsten fiber-reinforced copper. It is found that the fibers alter the matrix plastic flow behavior by increasing the flow-stress amplitude of the matrix, and by rendering initial yield in compression more progressive than in initial tension. Essentially, all observed features of matrix/fiber interaction can be rationalized as attributable to dislocation emission in the matrix caused by thermal mismatch strains within the material during composite cooldown from processing temperatures.     

Analysis of steady-state creep and creep fracture of directionally solidified eutectic γ/γ′-α alloy

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-l.6V-l.2Re (wt pct) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress isn = 7.5 ± 0.3, and the apparent activation energy for creep of the eutectic γ/γ′-α composite is determined to beQ = 517 ± 11 kJ mol₋₁. When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creepQ _c is calculated to be between 412 and 424 kJ mol₋₁ and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.     

Effect of Extrusion Temperature on the Plastic Deformation of an Mg-Y-Zn Alloy Containing LPSO Phase Using <Emphasis Type="Italic">In Situ</Emphasis> Neutron Diffraction

The evolution of the internal strains during in situ tension and compression tests has been measured in an MgY₂Zn₁ alloy containing long-period stacking ordered (LPSO) phase using neutron diffraction. The alloy was extruded at two different temperatures to study the influence of the microstructure and texture of the magnesium and the LPSO phases on the deformation mechanisms. The alloy extruded at 623 K (350 °C) exhibits a strong fiber texture with the basal plane parallel to the extrusion direction due to the presence of areas of coarse non-recrystallised grains. However, at 723 K (450 °C), the magnesium phase is fully recrystallised with grains randomly oriented. On the other hand, at the two extrusion temperatures, the LPSO phase orients their basal plane parallel to the extrusion direction. Yield stress is always slightly higher in compression than in tension. Independently on the stress sign and the extrusion temperature, the beginning of plasticity is controlled by the activation of the basal slip system in the dynamic recrystallized grains. Therefore, the elongated fiber-shaped LPSO phase which behaves as the reinforcement in a metal matrix composite is responsible for this tension–compression asymmetry.     

Thermally induced residual stresses in eutectic composites

The effect of thermally induced residual stresses on the yield behavior of a unidirectionally solidified eutectic, (Co, Cr)-(Cr, Co)₇C₃ is presented. At low temperatures, the yield stress is found to depend on the sense of the applied stress. The difference in yield stress between tension and compression is a function of temperature and disappears at a sufficiently high stress relaxation temperature. A straightforward analysis is presented that predicts the observed yielding behavior and a stress relaxation temperature that agrees well not only with the value obtained by observing the temperature dependence of the yield stress, but also with the value obtained from the thermal expansion behavior of the eutectic composite.     

Kinetics of biaxial dome formation by transformation superplasticity of titanium alloys and composites

By thermally cycling through their transformation temperature range, coarse-grained polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism is presently investigated under biaxial stress conditions during thermal cycling of unalloyed titanium, Ti-6Al-4V, and their composites (Ti/10 vol. pct TiC _p , Ti-6Al-4V/10 vol. pct TiC _p , and Ti-6Al-4V/5 vol. pct TiB _w ). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify the operation of this deformation mechanism in all experimental materials and compare the biaxial results directly to new uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with experimentally measured profiles.     

Yield criteria and strain-rate behavior of Zr57.4Cu16.4Ni8.2Ta8Al10 metallic-glass-matrix composites

We have examined the yielding and fracture behavior of Zr_57.4Cu_16.4Ni_8.2Ta₈Al₁₀ metallic-glass-matrix composites with a small volume fraction (∼4 pct) of ductile crystalline particles under quasi-static uniaxial tension and compression and dynamic uniaxial compression. The yield stress of the composite is the same for quasi-static tension and compression, consistent with a von Mises yield criterion. The measured average angle between the shear bands and the loading axis in quasi-static compression is 47±2 deg, significantly larger than the value of ∼42 deg typically reported for single-phase metallic glasses. Finite element modeling (FEM) shows that the measured value is consistent with both the von Mises criterion (48±4 deg) and the Mohr-Coulomb criterion (46±5 deg). The fracture surface angles, however, are 41±1 deg (compression) and 54±2 deg (tension), in good agreement with observations of single-phase metallic glasses. At low strain rates (<10⁻¹ s⁻¹), the yield stress is independent of strain rate, while at higher strain rates (>10⁰ s⁻¹), the failure stress decreases with increasing strain rate, which again is similar to the behavior of single-phase glasses. These results indicate that while the presence of the particles has a significant effect on the yield behavior of the composites, the fracture behavior is largely governed by the properties and behavior of the amorphous matrix.     

Study of the interface and its effect on mechanical properties of continuous graphite fiber-reinforced 201 aluminum

The formation of fiber-matrix interfacial reaction zone and its impact on mechanical properties of Gr/201 Al composite (41 vol pct fiber) was evaluated in the as-received condition and after heat treatment in vacuum at 450°C, 500°C, and 545°C temperatures for one day, and at 545°C for one week. After heat treatment the microstructures of matrix and interface were studied by transmission electron microscopy. This study revealed the presence of interfacial constituents Al₄C₃, Al₄O₄C, and TiB₂. The mean fiber-matrix reaction zone thickness showed an increase with increasing heat treatment temperature and time. The effects of heat treatment on interfacial shear strength, monotonic and cyclic tension/compression properties were evaluated. The results show that the interfacial shear strength not only depends on chemical reaction but also depends on the thickness of the reaction zone. An increase in reaction zone size reduces mechanical bonding considerably (thermal induced stresses). The growth of reaction zone was very detrimental to monotonic and cyclic tension/tension fatigue behavior. The mechanism of failure in tension/tension fatigue was the initiation of cracks at the interface and their subsequent propagation in the matrix. It was concluded that the reaction zone was the controlling factor in tension/tension fatigue. In contrast, the results showed that compressional fatigue was matrix dependent and was little sensitive to the size of the fiber/matrix interfacial reaction zone.     

Copper-zirconium tungstate composites exhibiting low and negative thermal expansion influenced by reinforcement phase transformations

A fully-dense Cu-75 vol pct ZrW₂O₈ metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW₂O₈ particles. A small amount of the high-pressure γ-ZrW₂O₈ phase was created during the cooldown and depressurization following densification; near complete transformation to γ-ZrW₂O₈ was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25 °C and 325 °C was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal cycles. The measured thermal expansion coefficients within specific temperature ranges varied from −6 · 10⁻⁶ K⁻¹ to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/contraction behavior could be justified by considering the evolution of phase transformations taking place in the ZrW₂O₈ phase, which were observed by in-situ synchrotron X-ray diffraction measurements.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

Mathematical modeling of the extrusion of 6061/Al2O3/20p composite

An integrated approach, involving laboratory experiments, extrusion plant trials, and finite element modeling (FEM) has been adopted for the study of the extrusion of the metal matrix composite (MMC) 6061/Al₂O₃/20p. Gleeble compression tests were performed to develop the constitutive equation of the MMC under industrial extrusion process conditions. Extrusion plant trials were conducted to measure load and temperature and to obtain samples for microstructural analysis. Metal flow, with respect to particle behavior in the deformation zone, was examined microscopically. An FEM based on the commercial code DEFORM was adopted for the simulation of the extrusion of the MMC; the constitutive equation developed was incorporated into the model. Using an updated Lagrangian formulation, both the transient and steady-state regions of extrusion were modeled. Load and temperature predictions resulting from this model agree well with the measured values in the upsetting stage and in the steady-state region. Temperature predictions agree to within less than 3 pct of the measured values. The FEM predictions of temperature, stress, strain, and strain-rate distribution were correlated with the particle behavior and low-speed cracking during extrusion: large shear deformation promotes particle fracture in the deformation zone, and tensile stress generation in the die land zone of the billet leads to low-speed cracking of the MMC during extrusion. The latter occurs at low temperature in the front end of the billet at the beginning of the extrusion process due to heat loss to the cold die.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Interfacial sliding near a free surface in a fibrous or layered composite during thermal cycling

This paper presents a simple shear lag model of interfacial sliding at a free surface in a layered or continuous fiber composite. The interface is characterized by a critical interfacial shear stress, τ₀, which might represent the critical stress for frictional sliding at a weakly bonded interface, or the shear flow stress of a thin, ductile interface layer at a well bonded interface. We calculate the history during heating and cooling of the relative normal displacement of the reinforcing inclusions and the matrix on a free surface cut normal to the inclusions. The calculated history is shown to depend on both the absolute value and the temperature dependence of τ₀, as well as on the magnitudes of the bulk residual stresses. Analytical results are obtained for the first few heating and cooling cycles and the equilibrium hysteresis loop under thermal cycling of uniform amplitude. The variety of possible displacement histories suggests that they are a rich source of information about τ₀ and the residual stresses. A discussion of feasible experiments and some results for continuous fiber titanium and titanium aluminide composites are presented in a companion paper.     

Work of fracture in aluminum metal-matrix composites

The effect of isothermal exposure and thermal cycling on the toughness of B/Al (1100), B/Al (6061), and A1₂O₃/A1 composites has been investigated. In B/Al (1100), isothermal exposure at 773 K for 45 × 10⁴ s (125 hours) reduced toughness, measured by the work of fracture, from 76 kJm^-2 to 10 kJm^-2, and a similar reduction occurred after equivalent thermal cycling. The corresponding reduction in toughness after isothermal exposure in B/Al (6061) was from 44.5 kJm^-2 to 8 kJm^-2; however, the effect of thermal cycling was less detrimental. In the FP-A1₂O³/A1 composite, the work of fracture was insensitive to both forms of thermal treatment. Changes in the toughness of the B/Al composites have been correlated with and analyzed in terms of modifications to matrix, fiber, and interface properties, in particular, matrix softening, interface reaction products, and fiber notch sensitivity. The latter currently on The latter currently on     

Copper-zirconium tungstate composites exhibiting low and negative thermal expansion influenced by reinforcement phase transformations

A fully-dense Cu-75 vol pct ZrW₂O₈ metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW₂O₈ particles. A small amount of the high-pressure γ-ZrW₂O₈ phase was created during the cooldown and depressurization following densification; near complete transformation to γ-ZrW₂O₈ was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25°C and 325°C was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal cycles. The measured thermal expansion coefficients within specific temperature ranges varied from −6·10⁻⁶ K⁻¹ to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/contraction behavior could be justified by considering the evolution of phase transformations taking place in the ZrW₂O₈ phase, which were observed by in-situ synchrotron X-ray diffraction measurements.     

Thermal fatigue of a SiC/Ti-15Mo-2.7Nb-3Al-0.2Si composite

The influence of thermal cycling and isothermal exposures in air on the residual ambient temperature strength of SCS-6/Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent) metal-matrix composites comprised of [0]₄ and [0/90]_s laminates has been determined. A maximum temperature of 815 °C was used in thermal cycling and isothermal exposure. Temperature range, cycle count, maximum/minimum temperature, environment, and hold time at temperature were systematically varied. Postexposure ambient-temperature tension testing, scanning electron and optical microscopy, and fractography were performed on selected specimens to determine the degree of damage. A reduced residual strength was noted in thermal fatigue with increasing cycle count, maximum temperature, and hold time for all specimens tested in air. Isothermal exposures at 815 °C also substantially reduced residual ambient-temperature strength. Considerably less reduction in strength occurred in inert environment than in air. Damage processes included matrix cracking, fiber/matrix interface damage, matrix embrittlement by interstitials, and oxide scale formation at specimen surfaces and, in some cases, at matrix/fiber interfaces. Fiber orientations which allowed rapid ingress of oxygen lead to greater matrix embrittlement and resulted in more pronounced reductions in strength. Formerly with the Materials Directorate, Wright Laboratory, Wright Patterson AFB, Dayton, OH 45433     

Fracture behavior of stainless steel-toughened NiAl composite plate

Analysis of the tensile and fracture behavior of a composite system consisting of boron carbide particulate-reinforced NiAl with continuous 304 stainless steel toughening regions was performed. The composite was fabricated by extrusion, with the toughening regions extending along the length of the plate in the extrusion direction. Mechanical properties were determined as a function of orientation. Tensile testing revealed that the composite modulus varied only slightly as a function of testing direction, the strength was approximately 25 pct greater in the longitudinal relative to the transverse orientation, and the transverse failure strain was only 0.3 pct compared to values in excess of 10 pct for longitudinal testing. Notched Charpy impact testing indicated that the energy absorption values varied significantly as a function of specimen location and crack growth direction, ranging from 2 to 40 Joules. In addition,K _IC values measured on subsize compact tension samples were found to range from 17 to 27 MPa ⋅ m^1/2. It was also established that theK _max values determined from the maximum load measured during compact tension testing were similar to theK _Q values calculated from instrumented notched Charpy impact testing. Finally, the fatigue crack growth characteristics of the composite were determined as a function of orientation.     

Transformation superplasticity of iron and Fe/TiC metal matrix composites

Unreinforced iron was thermally cycled around the α/γ phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the α/γ phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.     

The thermal expansion of the directionally solidified AI-CuAI2 eutectic

Alloys of Al-CuAl₂ eutectic composition were prepared from 99.999 pct pure materials and directionally solidified in a temperature gradient of about 45 °C/cm at different growth ratesR. The λ²R= constant relation was verified and lamellar spacings of 7.5, 3.5, 2.6, 1.8 and 1.4 μm were obtained. Dilatometer specimens were machined with axes aligned in the principal lamellae coordinate directions. Thermal expansion was measured by standard dilatometry (Cu standard) using a set point program cycling between room temperature and 500 °C. Thermal expansion of the directionally solidified Al-CuAl eutectic is greatest in the growth direction (in the plane of the lamellae), least in the tranverse direction (orthogonal to the growth direction in the plane of the lamellae) and intermediate in the direction normal to the lamellae. The most significant finding of the study is that the thermal expansion increases with decreasing lamellar spacing between limits defined approximately by the thermal expansion of the CuAl₂ phase alone and the predicted thermal expansion of an isotropic elastic model of the composite.     

Transformation superplasticity of zirconium

A tensile strain of 270 pct was achieved for coarse-grained zirconium subjected to transformation superplasticity conditions, where strain increments are accumulated upon repeated thermal cycling around the allotropic transformation temperature under the biasing effect of a uniaxial tensile stress. The strain increment per cycle was found to consist of two equal contributions from transformations on heating and cooling and to increase linearly with the applied stress. The measured strain increments are in good quantitative agreement with predictions based on the average internal stress during the transformation, which was determined independently from experimental transformation times. As the cycling frequency is raised, the average strain rate increases (a maximum value of 1.3·10⁻⁴ s⁻¹ was measured), but the strain increment per cycle decreases above a critical cycling frequency, for which the sample gage section undergoes only a partial phase transformation. The resulting reduction in internal mismatch and increase in internal stress are modeled using the experimental observation that β-Zr deforms by a mixture of diffusional and dislocation creep in the stress range of interest.