Interfacial fatigue in a fiber reinforced metal matrix composite

An experimental investigation of interface fatigue in a fiber reinforced metal matrix composite has been conducted. For this purpose, the cyclic traction law (the relationship between the fiber stress and the pullout displacement) was measured using fiber pullout tests. On the first loading cycle, the traction law was found to be parabolic, in accord with predictions of a micromechanical model based on a constant interface sliding stress. Upon subsequent unloading and re-loading, the relationship changed, following trends which suggest that the sliding resistance degrades with cyclic sliding. Such effects have been confirmed through SEM examinations of the fiber coatings following fatigue testing. Furthermore, the degradation was found to be greatest near the plane of the matrix crack. The results are consistent with the notion that the degradation in sliding stress occurs most rapidly in regions where the relative sliding distance (fiber/matrix) is greatest. A phenomenological model incorporating such degradation is presented and compared with the experimental measurements.     

An investigation of the fatigue and fracture behavior of mn-containing gamma titanium aluminides

The fatigue and fracture mechanisms in Ti-48Al-xMn (x = 1.4 to 2.0 at. pct) gamma-based titanium aluminide alloys are elucidated. Unlike most gamma alloys, which fail predominantly by transgranular fracture at room temperature, fracture in ternary Ti-48Al-xMn alloys is shown to occur mainly by intergranular failure. The incidence of intergranular failure increased with increasing annealing duration and temperature. Intergranular fracture is shown to occur as a result of the segregation of Mn to equiaxed and interlamellar boundaries. Annealing either above or below the eutectoid temperature results in the precipitation of α₂ particles. The reduction in the strength and toughness of ternary Mn-containing alloys is attributed to the combined effects of segregation and α₂ precipitation. A micromechanics framework is presented for the assessment of twin toughening mechanisms under monotonie and cyclic loading. Formerly Staff Scientist with General Electric Research and Development, Schenectady, NY 12301 Formerly Undergraduate Student, Department of Materials Science and Engineering, The Ohio State University     

An investigation of contact deformation,fracture, and fatigue behavior in bulk metallic glasses

This article presents the results of a study of the contact-induced deformation, fracture/resistance-curve behavior, and fatigue-crack-growth behavior of two bulk metallic glasses (BMGs), one in the fully amorphous condition and the other containing a dispersion of microscale crystallites. Hertzian contact indentation experiments were performed on both materials under monotonic loading conditions. The contact-induced deformation characteristics observed in the Hertzian experiments are then compared with the stress distribution determined from finite-element analyses. In addition, the cracking patterns associated with resistance-curve behavior in single-edge notched bend (SENB) specimens are incorporated into a fracture-mechanics framework for the estimation of toughening due to microcrack shielding. The predicted steady-state toughness values are shown to be comparable to the measured values obtained from resistance-curve experiments. Subsequently, fatigue-crack-growth rare data obtained at stress ratios of 0.1 and 0.5 are presented and analyzed using a crack-tip opening displacement (CTOD) model. The implications of the results are then discussed.     

Mode I fatigue cracking in a fiber reinforced metal matrix composite

The mode I fatigue crack growth behavior of a fiber reinforced metal matrix composite with weak interfaces is examined. In the longitudinal orientation, matrix cracks initially grow with minimal fiber failure. The tractions exerted by the intact fibers shield the crack tip from the applied stress and reduce the rate of crack growth relative to that in the unreinforced matrix alloy. In some instances, further growth is accompanied by fiber failure and a concomitant loss in crack tip shielding. The measurements are compared with model predictions, incorporating the intrinsic fatigue properties of the matrix and the shielding contributions derived from the intact fibers. The magnitude of the interface sliding stress inferred from the comparisons between experiment and theory is found to be in broad agreement with values measured using alternate techniques. The results also indicate that the interface sliding stress degrades with cyclic sliding, an effect yet to be incorporated in the model. In contrast, the transverse fatigue properties are found to be inferior to those of the monolithic matrix alloy, a consequence of the poor fatigue resistance of the fiber/matrix interface.     

An investigation of strain hardening and creep in an AI-6061/AI2O3 metal matrix composite

Experiments were conducted to compare the influence of temperature on the flow and strain-hardening characteristics of an Al-6061 metal matrix composite, reinforced with ∼20 vol pct of Al₂O₃-based microspheres, with the unreinforced monolithic alloy. At room temperature, the yield stresses and the strain-hardening rates are higher in the composite material in the asquenched condition and after aging at 448 K for periods of time up to 300 hours. The 0.2 pct proof stress and the strain-hardening rate decrease with increasing temperature in both materials, but the rate of decrease is faster in the composite so that the unreinforced monolithic alloy exhibits higher yield stresses and strain-hardening rates at temperatures in the vicinity of 600 K. Under conditions of constant stress at high temperatures, the composite exhibits both a higher creep strength than the monolithic alloy and higher values for the stress exponents for creep. Formerly Visiting Scholar, Kyushu University, is Associate Professor, Department of Metallurgy, Xian Institute of Metallurgy and Construction Engineering, Xian 710055, People’s Republic of China.     

Crack closure and residual stress effects in fatigue of a particle-reinforced metal matrix composite

A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have been eliminated, has highlighted their rôle in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of ΔK, measured at R = 0.1, with threshold being displaced to a lower nominal ΔK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of ΔK_eff in the unstretched material, explaining the slower crack growth rates. Effective ΔK_th values are measured at 1.7 MPa√m, confirmed by constant K_max testing. In the absence of residual stress, closure levels of approximately 2.5 MPa√m are measured and this is attributed to a roughness mechanism.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Titanium alloy fatigue fracture facet investigation by selected area electron channeling

Flat regions (facets) found on fracture surfaces caused by initiation and propagation of fatigue cracks through the titanium alloys IMI-685 tested with and without a 5 min load dwell and Ti-11 have been examined using selected area electron channeling. The crystallographic planes of the facets have been identified as being near basal for the IMI-685 and more random for the Ti-11. None of the dwell specimens showed a pure basal facet orientation. The plastic zone size was also assessed and found to correlate approximately with stress intensity, allowing confirmation of crack initiation sites as determined by fractography. One case of beta to alpha-beta transformation crystallography is also examined. A partial channeling map for titanium is presented.     

A fiber fracture model for metal matrix composite monotape consolidation

The consolidation of plasma sprayed monotapes is emerging as a promising route for producing metal and intermetallic matrix composites reinforced with continuous ceramic fibers. Significant fiber fracture has been reported to accompany the consolidation of some fiber/matrix systems, particularly those with creep resistant matrices. Groves et al. [Acta metall. mater.42, 2089 (1994)] determined the predominant mechanism to be bending at monotape surface asperities and showed a strong dependence of damage upon process conditions. Here, a previous model for the densification of monotapes [Elzey and Wadley, Acta metall. mater.41, 2297 (1993)] has been used with a stochastic model of the fiber failure process to predict the evolution of fiber fracture during either hot isostatic or vacuum hot pressing. Using surface profilometer measured roughness data for the monotapes and handbook values for the mechanical properties of different matrices and fibers, this new model is used to elucidate the damage dependence on process conditions, monotape surface roughness, and the mechanical properties both of the fiber and matrix. The model is used to investigate the “processibility” of several currently important matrix and fiber systems and to identify the factors governing this. An example is also given of its use for the simulation of a representative consolidation process cycle. This approach to the analysis of a complex, nonlinear, time-varying process has resulted in a clear understanding of the causal relationships between damage and the many process, material and geometric variables of the problem and identified new strategies for its elimination.     

The fracture resistance of a model metal/ceramic interface

Crack propagation has been measured for the Al₂O₃/Au interface subject to conditions that exclude stress corrosion. Crack growth has been shown to occur with a rising resistance, governed by intact metal ligaments in the crack wake. The level of resistance also increases as the metal layer thickness increases. Crack extension occurs by a combination of plastic void growth and interface debonding. The fracture energies are much larger than the work of adhesion, but appreciably smaller than those expected for ductile interface fracture. The fracture energy is nevertheless dominated by plastic dissipation, which increases at larger metal layer thicknesses.     

Effect of thermal residual stresses on fatigue crack opening and propagation behavior in an Al/SiC p metal matrix composite

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle-reinforced aluminum alloy have been measured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements using neutron diffraction have shown that this introduces an approximately parabolic stress field into the plates, varying from compressive at the surfaces to tensile in the center. Long fatigue cracks were grown in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. Crack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. By monitoring the compliance of the specimens during fatigue cycling, the degree to which the residual stresses close the crack has been evaluated. formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front. Formerly , Postdoctorate Associate, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minn.     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front.     

Particle fracture, retention, and fluid flow in metal matrix composite friction joints

Friction joining of metal matrix composite (MMC)/MMC and MMC/AISI 304 stainless steel base materials is examined using a combination of experimental testing and numerical modeling. In particular, the fracture of reinforcing particles during the friction-joining operation is investigated. The particle diameter and interparticle spacing decrease and the area fraction of particles markedly increases in material immediately adjacent to the bondline. Smaller particles are observed in frictionwelded joints produced using high friction pressures. The principal effect of the forging operation is in decreasing the interparticle spacing. There was excellent correspondence between predicted fluid flow in A1/A1 joints and experimental test results examining the transfer of Al₂O₃ particles during the alloy 6061/alloy 6061 friction-joining operation. It is suggested that small-diameter particles formed due to fracture early in the friction-joining operation are retained at the bondline of MMC/MMC joints as a direct consequence of the flow of plasticized material and reinforcing particles in the contact zone. A combination of numerical modeling of fluid flow and direct experimental testing have confirmed that Al₂O₃ particles transfer from the stationary to the rotating boundary in MMC/MMC friction joints. Also, limit cycles embedded within the flow favor the retention of smalldiameter fractured particles at the bondline of MMC/MMC joints. A quite different situation exists in dissimilar MMC/AISI 304 stainless steel joining. In dissimilar joints, a dynamically quiescent region is formed immediately adjacent to the stainless steel boundary. It is suggested that the absence of flow of plasticized material promotes retention of fractured alumina particles in dissimilar joints.