The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

Low-Cycle fatigue properties of a SiC Whisker-reinforced 2124 aluminum alloy

Low-cycle fatigue microcracking leading to failure of smooth specimens of a powder metallurgy (PM) 2124 aluminum alloy reinforced with 20 vol pct SiC whiskers was studied. The crack size near the onset of unstable growth was inferred to be 50 to 70 μm in the stress amplitude range of the present study (400 to 600 MPa,R = −1) from observations of the fracture surfaces of the specimens. This corresponds to stress intensities between 1/3 to 1/2 typical values ofK _1c or 1/4 to 1/9 the critical length predicted fromK _1c values of 12 to 14 MPa√m. The microcrack size distributions and growth data were obtained from the low-cycle fatigue specimens at various stages of fatigue, using a surface replica technique. During continued cycling, microcracks formed and were lost through linkage with other cracks. At the same time, the fraction of small cracks (<5 μm) decreased, while that of larger cracks (>5 μm) increased. The total number of cracks increased with increasing numbers of cycles. Typical microcrack growth rates were determined to bedb/dn = (3.57 to 6.11) × 10⁻¹⁰ (Δ/K)^2.2to2.48 in the lateral direction of the crack, andda/dn = (5.83 to 13.0) × 10⁻¹¹ (ΔK)^{1.54 to 1.60} in the depth direction of the crack.     

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Constant-amplitude high-cycle fatigue tests (σ_max=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN<1 × 10⁻⁷ m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN>1 × 10⁻⁶ m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K _max ^tr ≈6.0 MPa √m for the present cast A356 alloy.     

Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate(e.g., †K) or the SiC content was increased. 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.     

Microstructural analysis of fracture toughness variation in 2XXX-series aluminum alloy composites reinforced with SiC whiskers

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition. Formerly Research Assistant, Department of Materials Science and Engineering Pohang University of Science and Technology, Pohang, 790-784 Korea     

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

Fundamental aspects of fatigue and fracture in a TiAl sheet alloy

The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

An FEM study of the plastic deformation process of whisker reinforced SiC/Al composites

The finite element method (FEM) was employed to investigate the matrix plastic flow in a whisker reinforced SiC/Al composite under external tensile load. It was found that the plastic zone induced by the plastic relaxation of thermal stresses expands under the external tensile load. The overall matrix plastic flow was characterized by the expansion and interconnection of the plastic zones around whiskers. This process can be divided into several characteristi stages, and related to the global stress-strain relationship. It also indicated that the strengthening was due mainly to the thermally induced matrix work hardening at the tip of the whisker. Finally, the initiation of plastic flow in an inhomogeneously reinforced composite was governed by the redistribution of the average stress and strength in the reinforcement clusters.     

Failure mechanisms in SiC-fiber reinforced 6061 aluminum alloy composites under monotonic and cyclic loading

Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061 alloy metal-matrix composite (SiC_f/Al-6061) during monotonie and cyclic loading are examined at room temperature, both for the longitudinal (0 deg or L-T) and transverse (90 deg or T-L) orientations. It is found that the composite is insensitive to the presence of notches in the L-T orientation under pure tension loading due to the weak fiber/matrix interface; notched failure strengths are ∼1500 MPa compared to 124 MPa for unreinforced 6061. However, behavior is strongly dependent on loading configuration, specimen geometry, and orientation. Specifically, properties in SiC_f/Al in the T-L orientation are inferior to unreinforced 6061, although the composite does exhibit increasing crack-growth resistance with crack extension (resistance-curve behavior) under monotonie loading; peak toughnesses of ∼16 MPa√m are achieved due to crack bridging by the continuous metal phase between fibers and residual plastic deformation in the crack wake. In contrast, such bridging is minimal under cyclic loading, as the ductile phase fails subcritically by fatigue such that the transverse fatigue crack-growth resistance is superior in the unreinforced alloy, particularly at high stress-intensity levels. Conversely, fatigue cracks are bridged by unbroken SiC fibers in the L-T orientation and exhibit marked crack deflection and branching; the fatigue crack-growth resistance in this orientation is clearly superior in the composite.     

Heat generation during the fatigue of a cellular Al alloy

The heat generation from a notch during the compression-compression fatigue of a cellular Al alloy has been measured and compared with a model. The measurements indicate that heat is generated because of hysteresis occurring in narrow cyclic plastic zones outside the notch. This process continues until the notch closes. At closure, a brief period of heat generation arises because of friction along the notch faces. A plasticity model based on the Dugdale zone is shown to provide a reasonably accurate characterization of the heat generated, with the proviso that an “ineffective” zone be transposed onto the notch tip. It is found that the temperatures generated are too small to cause fatigue by thermal softening. A fatigue mechanism based on either geometric softening of the cells or crack growth in the cell walls is implied.     

The influence of SiC particulates on fatigue crack propagation in a rapidly solidified Al-Fe-V-Si alloy

The fatigue crack propagation properties of a rapidly solidified aluminum alloy are compared with those of a metal matrix composite (MMC) made of the same base alloy with the addition of 11.5 vol pct SiC particulate. The high-temperature base material, alloy 8009 produced by Allied-Signal, Inc. (Morristown, NJ), is solidified and processed using powder metallurgy techniques; these techniques yield a fine-grained, nonequilibrium microstructure. A direct comparison between the fatigue crack propagation properties of the reinforced and unreinforced materials is possible, because alloy 8009 requires no postprocessing heat treatment. As a consequence, this comparison reflects the influence of the SiC particulate and not differences in microstructure that could arise during processing and aging. The experimental data demonstrate that the SiC-reinforced material exhibits modestly superior fatigue crack propagation properties: slower crack growth rates for a given ΔK, at near-threshold crack growth rates. Even when the data are corrected for crack closure using an effective stress intensity factor, ΔK_eff, the composite exhibits lower crack propagation rates than the unreinforced matrix alloy. Microscopic evidence shows a rougher fracture surface and a more tortuous crack path in the composite than in the base alloy. It is argued that the lower crack growth rates and higher intrinsic threshold stress intensity factor observed in the composite are associated with crack deflection around SiC particles. Formerly Graduate Research Assistant, University of California-Davis     

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.     

Mechanical and fatigue properties of self-piercing riveted j oints in high-strength steel and aluminium alloy

Static tensile and fatigue tests were performed on shear and tensile self-piercing riveted aluminium-steel structures to evaluate their mechanical and fatigue properties.The influences of the thickness and the strength of the high-strength steel on mechanical and fatigue performances were investigated based on the tensile and F-N curves of the joints.The results show that mechanical and fatigue properties of the shear self-piercing riveted joints are much better than those of the tensile self-piercing riveted joints.Mechanical and fatigue performances of the two joints were significantly influenced by the thickness and strength of the steel sheet, and were markedly improved when the thickness of steel sheet increased.The steel strength showed significantly different effects on shear and tensile riveted structures, i.e., when the steel strength increased, the strength of the shear structure greatly increased while the tensile structure just had a slight increase in the strength.Fatigue failure generally occurred in the sheet materials and the fa-tigue crack location changed with increasing the sheet thickness and the sheet strength.     

Crack initiation and propagation during high-temperature fatigue of oxide dispersion-strengthened superalloys

The mechanisms of crack initiation and propagation have been investigated in two oxide dispersion-strengthened (ODS) Ni-base superalloys under conditions of symmetric low-cycle fatigue (LCF) and creep-fatigue. The behavior of both ODS alloys is compared with that of conventional alloys of otherwise similar composition. While the improvement in fatigue resistance previously reported for ODS metals and alloys is confirmed by the present study for temperatures below about0.6T _m, the potential advantage of dispersion strengthening is not being exploited by the current generation of ODS superalloys at higher temperatures; crack initiation is found to occur prematurely due to the presence of recrystallization defects in the form of fine grains. The mechanism of crack initiation at fine grains is creep-type cavitation on boundaries transverse to the applied stress. Experimental results indicating the influence of temperature, loading frequency, and waveshape on the crack initiation rate are presented and discussed in detail. A qualitative correlation between waveshape and creep-fatigue life is suggested based on the macroscopic inelastic strain rate which is determined by the waveform and limits in turn the rate at which cavity growth can be accommodated. Formerly with Max-Planck-Institut für Metallforschung     

铝合金熔炼镁的变化和影响

介绍了铸造铝合金中，镁含量的变化对铝合金机械性能的影响以及铝合金在熔过程中，镁的损耗主要是由除气精炼，变质处理，保温三个阶段所构成。