Effects of changes in test temperature on fatigue crack propagation of Al6090/SiCp-Al 6013 laminated metal composites

The effects of changes in test temperature from 298 to 148 K on the fatigue crack propagation of 6090/SiC/20p-6013 and 6090/SiC/25p-6013 laminated metal composites (LMCs) tested in the crack arrester orientation were investigated. The fatigue crack propagation behavior of similar monolithic and discontinuously reinforced aluminum (DRA) materials was additionally compared to that of the laminates. The fatigue behavior of the laminates was significantly different from that of the monolithic and DRA and was affected by changes in the test temperature as well as by a thermal cycle to low temperatures (e.g., 77 or 148 K) prior to testing at either 213 or 298 K. Fatigue tests conducted either at low temperature or after a low-temperature thermal cycle exhibited significant changes in crack velocity upon traversing the individual laminae. Crack growth in these samples was accompanied by crack bifurcation and interfacial separation at high ΔK. Attempts at modeling the effects of thermally-induced residual stresses on fatigue using a weight function approach provided reasonable estimates of the residual-stress-induced shielding in the different layers. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

Effects of ductile laminate thickness,volume fraction,and orientation on fatigue-crack propagation in Ti-Al<Subscript>3</Subscript>Ti metal-intermetallic laminate composites

Fatigue crack propagation (FCP) has been studied in a new class of materials termed metal-intermetallic laminate (MIL) composites (Ti-Al₃Ti). Due to ease of fabrication and control over layer makeup, these MIL composites can be tailored to optimize the constituent properties for structural and higher performance aerospace applications. Effects of ductile reinforcement (titanium alloy) type, thickness, and volume fraction were systematically investigated in both arrester and divider orientations. Stress intensity (K _max) values as large as 40 MPa√m were observed in the higher crack growth regime, indicating that the fracture toughness of the MIL composites is comparable to common structural metals. In both divider and arrester orientations, the overall fatigue crack growth rate showed an improvement with increasing Ti volume fraction and with increasing Ti thickness (at constant ductile-phase volume fraction). It is noted that the fatigue resistance of monolithic Al₃Ti was improved by an order of magnitude by incorporating just 20 vol pct ductile Ti. In the divider orientation, toughening is obtained through plastically stretching the intact ductile Ti ligaments that bridge the crack wake, thus reducing the crack driving force. By virtue of its morphology, the arrester orientation provides toughening by trapping the crack front entirely at the metallic-intermetallic interfaces, thus requiring the crack to renucleate at each interface. Results are compared with specific crack growth rates of conventional monolithic alloys and other composite systems such as TiNb/γ-TiAl and Nb/Nb₃Al. Owing to their low density (∼3.8 g/cc), Ti-Al MIL composites exhibited specific crack growth rates (da/dN vs ΔK/ρ) on par with tougher, but relatively denser, ductile metals such as Ti alloys and high-strength steels.     

层状金属复合材料的发展历程及现状

三十多年来,多种层状金属复合材料的制备方法应运而生,蓬勃发展,包括爆炸复合法、轧制复合法、热压扩散法和沉积复合法等。爆炸复合法在中厚板的制备上具有不可替代的优势,其产品广泛应用于军工、船舶、电力和化工等领域。轧制法可以批量生产大尺寸层压板,应用最为广泛,目前层压板已经广泛用于汽车、船舶和航空航天等领域。真空热压扩散法由于可以避免氧气等气体的污染,几年来在Ti/Al、Ti/TiAl和Ti6Al4V/TiAl层状复合材料的制备上备受关注。沉积复合法制备的层状金属复合材料在作为耐蚀、耐磨涂层,高强导线,人体植入材料方面表现出巨大的潜力。在综述层状金属复合材料发展历程的基础上,介绍了层状金属复合材料的制备方法及各自的优缺点,并对层状金属复合材料目前在国内外的研究现状进行了分析和介绍。      

Fatigue-crack propagation behavior of ductile/brittle laminated composites

A study has been made of the fatigue-crack propagation properties of a series of laminated Nb-reinforced Nb₃Al intermetallic-matrix composites with varying microstructural scale but nominally identical reinforcement volume fraction (20 pct Nb). It was found that resistance to fatigue-crack growth improved with increasing metallic layer thickness (in the range 50 to 250 μm) both in the crack-divider and crack-arrester orientations. For a given layer thickness, however, the properties in the crack-arrester orientation were superior to the crack-divider orientation. Indeed, the fatigue resistance of the crack arrester laminates was better than the fatigue properties of unreinforced Nb₃Al and pure Nb; both laminate orientations had significantly better fatigue properties than Nb-particulate reinforced Nb₃Al composites. Such enhanced fatigue performance was found to result from extrisic toughening in the form of bridging metal ligaments in the crack wake, which shielded the crack tip from the applied (far-field) driving force. Unlike particulate-reinforced composites, such bridging was quite resilient under cyclic loading conditions. The superior crack-growth resistance of the crack-arrester laminates was found to result from additional intrinsic toughening, specifically involving trapping of the entire crack front by the Nb layer, which necessitated crack renucleation across the layer.     

Synthesis and properties of Ti-Al laminated composites with an intermetallic layer

Metal-intermetallic laminated composites are fabricated upon reaction sintering of titanium and aluminum foils of various thicknesses. The mechanical properties of the composites with various metallic and intermetallic component contents are estimated using static and dynamic tests. The mechanical properties of the laminated composites are found to be anisotropic during static and dynamic loading.     

On the particle-size dependence of fatigue-crack propagation thresholds in SiC-particulate-reinforced aluminum-alloy composites: Role of crack closure and crack trapping

A study has been made of ambient-temperature fatigue-crack propagation behavior in P/M Al-Zn-Mg-Cu metal-matrix composites reinforced with either 15 or 20 vol.% silicon-carbide particulate, with specific emphasis on the role of SiC-particle size on the fatigue-crack growth threshold condition. It is found that measured threshold stress-intensity levels ΔK_TH, are a function of both SiC-particle size and volume fraction; however, whereas coarse-particle distribution results in higher ΔK_TH values at low load ratios, fine particles give higher threshold at high load ratios. Such behavior is analyzed in terms of the interaction of SiC particles with the crack path, both in terms of the promotion of (roughness-induced) crack closure at low load ratios and by crack trapping by particles. Consideration of the latter mechanism yields a limiting requirement for the intrinsic threshold condition in these materials that the maximum plastic-zone size must exceed the effective mean particle size; this implies that for near-threshold crack advance, the tensile stress in the matrix must exceed the yield strength of the material beyond the particle.     

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr_41.2Ti_13.8Cu_12.5 Ni₁₀Be_22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.     

The effect of interfacial reaction layer thickness on fracture of titanium-SiC particulate composites

Composites of commercial-purity Ti reinforced with 10 vol pct of SiC particles have been produced by cospraying and by powder blending and extrusion. Interfacial reaction layers have been studied by electron and optical microscopy and by Auger electron spectroscopy (AES) of fracture surfaces. The work of fracture has been measured as a function of reaction layer thickness for extruded and heat-treated composites. Material with very thin reaction layers (<∼0.1 μn) can be produced by cospraying, but porosity levels are relatively high (∼5 to 10 pct). Extruded material has been produced with a thin reaction layer (∼0.2 μm) and low porosity (<1 pct). It appears that the rate of reaction conforms with published parabolic rate constant data over a wide range of time and temperature. The reaction layer always consists of TiC and Ti₅Si₃, but the TiC grains tend to be larger than those of Ti₅Si₃. As the reaction layer thickness becomes greater than about 1 μm, the work of fracture falls sharply and the cracking pattern changes from one involving fracture of SiC particles to one in which cracking between the particles and adjacent reaction zones becomes predominant. It is suggested that the volume contraction accompanying this reaction, calculated at about 4.6 pct from density data, has a significant effect in promoting crack formation in these locations by generating radial tensile stresses across the interface. Thus, for this particular composite system, the important effect of a thicker reaction layer may be that it promotes the formation of an interfacial crackvia an effect on the local stress state, rather than itself constituting a larger flaw in the form of a through-thickness crack assumed to be present.     

The influence of prestrain and ageing on near-threshold fatigue-crack propagation in as-rolled and heat-treated dual-phase steels

The influence of prestrain and ageing on near-threshold fatigue-crack propagation (FCP) in as-rolled and heat-treated dual-phase steels (DPS) was investigated over a wide range of 10^?10 to 10^?7 mm/cycle in laboratory air under a load ratio of R = 0. It was found that the fatigue-crack propagation threshold value increases with increasing grain size and decreasing yield stress, and that a combination of 10% prestraining with 175°C/30 min ageing showed almost no effect on the threshold level of as-rolled dual-phase steel, but decreased that of heat-treated dual-phase steels more than 37%. This different behaviour was suggested to result from the remarkable differences in grain size and volume fraction of martensite (Vm) in these two kinds of dual-phase steels. An estimation of the plastic zone size of fatigue-crack tip supported this suggestion.     

Effect of prolonged isothermal exposure on elevated-temperature,time-dependent fatigue-crack propagation in INCONEL alloy 783

The effect of isothermal exposure on the elevated-temperature, time-dependent fatigue-crack propagation (FCP) in INCONEL Alloy 783 is investigated. Commercially produced Alloy 783 was annealed and aged following the standard heat-treatment procedure. One set of specimens was then isothermally exposed at 500 °C for 3000 hours. All specimens were subjected to FCP tests with various hold-time periods and sustained-loading crack-growth tests at 538 °C and 650 °C in a laboratory-air environment. Without a hold time, the as-produced and isothermally exposed materials had comparable FCP rates at both test temperatures. With hold times of 100 and 300 seconds, the as-produced and isothermally exposed specimens had comparable FCP rates at 538 °C. Hold-time testing of the as-produced material at 650 °C showed abnormal time-dependent FCP and sustained-loading crack-growth retardation. However, hold-time testing of isothermally exposed material at 650 °C showed the steady sustained-loading crack growth and fully time-dependent FCP typically observed in many superalloys. Comparison with Alloy 718 data from the literature shows that FCP rates of as-produced Alloy 718 and isothermally exposed Alloy 783 are comparable at 650 °C. A fully time-dependent FCP model based on the damage-zone concept and a thermal-activation equation is proposed to characterize the FCP behaviors.     

Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites

The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J _Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.     

Damping and acoustic harmonics in cracked laminated composites

This article reports on a novel experimental technique conceived and used to study damping and generation of acoustic harmonics in a cracked laminated composite. The cracked jaws of a double cantilever specimen with an interlaminar crack were vibrated symmetrically at 22.5 kHz; damping in the specimen and acoustic harmonics in the vicinity of the crack tip were measured as functions of amplitude of vibration. At low amplitudes, the damping was linear(i.e., independent of amplitude) with no detectable acoustic harmonics. At high amplitudes, the damping was nonlinear with generation of acoustic harmonics up to the fifth. Among known models for harmonic generation, the best choice to explain these data seems to be smooth motion of dislocations in graphite vibrating within a potential well of small asymmetry. Formerly Graduate Student, Columbia University, New York, NY 10027. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee.     

吸收层及缓冲层厚度对Cu3BiS3太阳能电池的性能影响

三元硫化物半导体Cu₃BiS₃的组成元素在地壳中含量丰富、无毒,且具有优异的光电性能,被认为是一种极具潜力的太阳能电池吸收层材料。目前关于Cu₃BiS₃太阳能电池器件的研究报道还非常少,在器件结构设计与制作工艺方面有一系列亟待解决的关键科学问题。文章分别通过溶液旋涂的层数及化学浴沉积时间来调控Cu₃BiS₃吸收层与CdS缓冲层薄膜厚度,详细分析了吸收层及缓冲层厚度对太阳能电池器件的影响规律及机制。结果表明,吸收层厚度的增加能够使光的吸收增强,使短路电流密度J_SC增大,进而提高光电转换效率;然而吸收层厚度过高,会造成器件效率的下降。缓冲层厚度的增加,有利于提高器件的开路电压V_OC;缓冲层过厚,同样造成器件短路电流密度的减小以及效率的下降。实验中器件的较优光电转换效率为0.288%,对应的开路电压、短路电流密度、填充因子分别为0.215 V,2.292 mA/cm2,48.049%。      

Microscopic characteristics of fatigue crack propagation in aluminum alloy based particulate reinforced metal matrix composites

Microscopic characteristics of fatigue crack propagation in two aluminum alloy (A356 and 6061) based particulate reinforced metal matrix composites (MMCs) were investigated by carrying out three point bending fatigue tests. The impedance offered by the reinforcing particles against fatigue crack propagation has been studied by plotting the nominal and actual crack lengths vs number of cycles. Surface observation shows that fatigue cracks tend to develop along the particle-matrix interface. In the case of Al (A356) MMCs, stronger interaction of fatigue crack with Si particles, as compared to SiC particles, was evident. In both MMC materials, particle debonding was more prominent as compared to particle cracking. The attempted application of Davidson's model to calculate ΔK_th indicated that for cast MMCs the matrix grain including the surrounding reinforcing particles has to be considered as a large “hard particle”, and the grain boundary particles themselves behave like an hard “egg-shell” to strengthen the material.     

Fracture toughness and R-Curve behavior of laminated brittle-matrix composites

The fracture toughness and resistance-curve behavior of relatively coarse-scale, niobium/niobium aluminide (Nb/Nb₃Al) laminated composites have been examined and compared to other Nb/Nb₃Al composites with (in situ) Nb particulate or microlaminate reinforcements. The addition of high aspectratio Nb reinforcements, in the form of 20 vol. pct of 50- to 250-μm-thick layers, was seen to improve the toughness of the Nb₃Al intermetallic matrix by well over an order of magnitude, with the toughness increasing with Nb layer thickness. The orientation of the laminate had a small effect on crack-growth resistance with optimal properties being found in the crack arrester, as compared to the crack divider, orientation. The high fracture toughness of these laminates was primarily attributed to large (∼1- to 6-mm) crack-bridging zones formed by intact Nb layers in the crack wake; these zones were of sufficient size that large-scale bridging (LSB) conditions generally prevailed in the samples tested. Resistance-curve modeling using weight function methods permitted the determination of simple approximations for the bridging tractions, which were then used to make smallscale bridging (SSB) predictions for the steady-state toughness of each laminate.     

Effects of plasticity on the crack propagation resistance of a metal/ceramic interface

Fracture experiments have been conducted on a gold/sapphire interface. The interface is found to fail by interface separation in a nominally “brittle” manner with a critical strain energy release rate, G_c ≈ 50 Jm⁻², substantially larger than the work of adhesion, W_ad ≈ 0.5 Jm⁻². Evidence of plastic deformation on the gold fracture surface, such as blunting steps and slip steps, suggest that plastic dissipation is the primary contribution to the measured G_c. Calculations suggest that the majority effect occurs in the plastic zone through the crack wake. The interface is also found to be susceptible to slow crack growth.     

Fracture of Ti-Al<Subscript>3</Subscript>Ti metal-intermetallic laminate composites: Effects of lamination on resistance-curve behavior

The fracture toughness and resistance-curve (R-curve) behavior of Ti-Al₃Ti metal-intermetallic laminate (MIL) composites have been studied in the crack-divider orientation, by examining the effect of ductile-laminate-layer thickness and volume fraction. The MIL composites were fabricated in open air by a novel one-step process, and the final structure consists of alternating layers of ductile Ti and brittle Al₃Ti. Such a laminate architecture in conjunction with a relatively low volume fraction of tougher Ti (18 to 40 pct) was seen to augment the fracture toughness of the inherently brittle intermetallic by over an order of magnitude. Additionally, as a result of their low density, MIL composites exhibit a specific fracture toughness (K/ρ) on par with tougher but relatively denser ductile metals such as high-strength steel. Such vast improvements may be rationalized through the toughening provided by the ductile Ti layers. Specifically, toughening was obtained through plastically stretching the intact ductile Ti layers that formed bridging zones in the crack wake, thus reducing the crack driving force. Such toughening resulted in R-curve behavior, and the toughness values increased with an increase in the volume percentage of Ti. Weight-function methods were used to model the bridging behavior, and they indicated that large bridging zones (∼2 to 3 mm) were responsible for the observed increase in toughness. The role of large-scale bridging (LSB) conditions on the resistance curves is explored, and steady-state toughness (K _SS ) values are estimated using small-scale bridging (SSB) approximations. A new approach to gage the potential of laminate composites in terms of their true fracture-toughness values as determined from a cyclic crack-growth fatigue test is proposed, wherein small-scale specimens can be utilized to obtain fracture-toughness values.     

Fracture and fatigue-crack growth behavior in ductile-phase toughened molybdenum disilicide: Effects of niobium wirevs particulate reinforcements

A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pct Nb-wire mesh reinforcements (Nb _m /MoSi₂); results are compared with monolithic MoSi₂ and MoSi₂ reinforced with 20 vol pct spherical Nb particles (Nb _p /MoSi₂). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi₂, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb _m /MoSi₂ composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa , compared to unstable fracture atK _c values below 5 MPa in unreinforced MoSi₂ or Nb _p /MoSi₂. Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix, which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be far superior in the wire-reinforced composite over pure MoSi₂ and Nb _p /MoSi₂. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of the Nb _m /MoSi₂ composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along the (Nb,Mo)₅Si₃ reaction layer interface.