The interactive role of inclusions and SiC reinforcement on the high-cycle fatigue resistance of particle reinforced metal matrix composites

The effect of intermetallic inclusions on the fatigue crack initiation and growth in 2080 Al alloy and 2080/SiC _p composites was investigated. Using surface replication, it was determined that, in the highcycle fatigue region, life is dominated by the initiation process. It was also determined that the majority of initiation sites were associated with intermetallic inclusions. While 2080/SiC/20 _p showed a definitive relationship between inclusion size and fatigue life, i.e., a higher inclusion size resulted in lower fatigue life, there was no correlation in 2080/SiC/30 _p . This was attributed to more of the load being shared by the higher volume fraction of SiC particles and smaller average inclusion sizes in the latter composite. A conceptual model is proposed that accounts for these observations and qualitatively shows the effect of reinforcement on stress enhancement in near-surface inclusions. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Fatigue behavior of a 2XXX series aluminum alloy reinforced with 15 vol Pct SiCp

The fatigue behavior of a naturally aged powder metallurgy 2xxx series aluminum alloy (Alcoa MB85) and a composite made of this alloy with 15 vol pct SiC_p, has been investigated. Fatigue lives were determined using load-controlled axial testing of unnotched cylindrical samples. The influence of mean stress was determined at stress ratios of −1, 0.1, and 0.7. Mean stress had a significant influence on fatigue life, and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. When considered on a strain-life basis, however, the composite materials had a somewhat inferior resistance to fatigue. Fatigue cracks initiated from several different microstructural features or defect types, but fatigue life did not vary significantly with the specific initiation site. As the fatigue crack advanced away from the fatigue crack initiation site, increasing numbers of SiC particles were fractured, in agreement with crack-tip process zone models. Formerly Graduate Student, Department of Materials Science and Engineering, The University of Michigan     

Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiC p composite

The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.     

Fatigue crack initiation and early propagation in Al 2219-T851

Fatigue crack initiation in Al 2219-T851 for fully reversed loading(R = σ/σ_max =−1) parallel to the material rolling direction is found to occur at intermetallic inclusions at the specimen surface. The inclusions are not involved in crack initiation for fatigue perpendicular to the rolling direction, and for this orientation crack initiation is at grain boundaries and specimens have an increased fatigue life. Except for fatigue at low peak stress, multiple numbers of microcracks are formed and for selected failed specimens the number of cracks has been determined as a function of crack length. Such crack length distribution measurements show that there is significant retardation of microcracks by interaction with grain boundaries. Furthermore it is found that the coalescence of microcracks provides a mechanism for cracking to “jump“ grain boundaries and reduce fatigue lifetime. The effect of relative humidity on this process is to increase the observed mean crack length, and decrease the number of crack initiations apparently due to weakening of the matrix-intermetallic interface at potential initiation sites. The overall result is that no significant dependence of fatigue life on relative humidity is found. Formerly with the Science Center, Rock-well International     

The influence of matrix microstructure

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and artificially aged 2219/TiC/15_p and unreinforced 2219 Al were investigated utilizing plastic strain-controlled and stress-controlled testing. The cyclic response of both the reinforced and un-reinforced materials was similar for all plastic strain amplitudes tested except that the saturation stress level for the composite was always greater than that of the unreinforced material. The cyclic response of the naturally aged materials exhibited cyclic hardening and, in some cases, cyclic softening, while the cyclic response for the artificially aged materials showed no evidence of either cyclic hardening or softening. The higher ductility of the unreinforced material made it more resistant to fatigue failure at high strains, and thus, at a given plastic strain, it had longer fatigue life. It should be noted that the tensile ductilities of the 2219/TiC/15_p were significantly higher than those previously reported for 2XXX-series composites. During stress-controlled test-ing at stresses below 220 MPa, the presence of TiC particles lead to an improvement in fatigue life. Above 220 MPa, no influence of TiC reinforcement on fatigue life could be detected. In both the composite and unreinforced materials, the low-cycle and high-cycle fatigue lives were found to be virtually independent of matrix microstructure. G.M. VYLETEL, formerly Graduate Student, Department of Materials Science and Engineering, The University of Michigan D.C. VAN AKEN, formerly Assistant Professor, Department of Materials Science and Engineering, The University of Michigan     

Low cycle fatigue behavior of Ni-Nr lamellar eutectic composites at elevated temperatures

Low cycle fatigue properties of unidirectionally solidified lamellar eutectic Ni-51 Cr alloy were determined and compared with those of the cast microstructure in the temperature range of 300° to 760°C. Both materials exhibited an initial cyclic strain hardening followed by saturation over most of the temperature range. The rate and the amount of cyclic work-hardening decreased with temperature above 600°C. Rapid softening due to macro-crack propagation occurred at later stages of the fatigue process, which occupied an increasing portion of the fatigue life in the lamellar material as the strain amplitude was decreased. At Δ∈_T = 0.0190, the lamellar material exhibited longer fatigue life over the entire temperature range which has been related to the ability of Cr-rich lamellae to deflect fatigue cracks. At 625°C, the fatigue life (Nf) of both materials was related to the plastic strain range ( Δ∈_P) through the relationship (Δ∈_P/2 =K(2N_f)^c wherec andK are -0.39 and 0.068 for the lamellar, and -0.45 and 0.074 for the cast structure, respectively. At this temperature with decreasing strain amplitude lamellar material became more resistant to fatigue than as-cast structure, which has been related to the more efficient deflection of fatigue cracks by Cr-rich lamellae at lower strain amplitudes . Formerly with the Dept. of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pa. Formerly Visiting Scientist, Department of Metallurgical and Materials Engineering, University of Pittsburgh Formerly Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Pittsburgh     

A Fatigue Life Model for Predicting Crack Nucleation at Inclusions in Ni-Based Superalloys

Engineering alloys such as Ni-based alloys, Al-alloys, and steels often contain non-metallic inclusions in their microstructures. These inclusions, which include oxide particles, carbides, and intermetallic particles, are introduced during component manufacturing processes such as casting, powder-metallurgy, or additive manufacturing methods. The presence of inclusions in the microstructure can promote fatigue crack nucleation by competing against slipband nucleation and reduce fatigue life performance of an engineering component. While it has been reported in many occasions, the competition between fatigue crack nucleation at inclusions and slipbands is still not well understood. In this article, the conditions for the concurrent occurrence of fatigue crack nucleation at inclusions and slipbands are analyzed theoretically. The analysis indicates that there exists a critical inclusion size (diameter) below which there is no fatigue life debit due to crack initiation at inclusions and above which a transition from slip-induced to inclusion-induced crack nucleation occurs. The low-cycle fatigue life model is applied to Ni-based superalloys and the model predictions are compared against experimental data from the literature to assess the dependence of the critical inclusion size on the slip morphology, grain size of the matrix, and the shear modulus of the inclusion.     

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm² when σ^a is 90 pct of the yield strength, to less than I/mm² when σ^a is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al₇Cu₂Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks. Formerly Graduate Students in the Department of Metallurgical Engineering at Wayne State University During 1982–83 he is on leave as Associate Director of the Metallurgy Program, Division of Materials Research, National Science Foundation, Washington, DC 20550.     

The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part I. room temperature behavior

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and overaged 2219/TiC/15p and unreinforced 2219 Al were investigated using plastic strain-controlled and stress-controlled testing. In addition, the influence of grain size on the particle-reinforced materials was examined. In both reinforced and unreinforced materials, the naturally aged conditions were cyclically unstable, exhibiting an initial hardening behavior followed by an extended region of cyclic stability and ultimately a softening region. The overaged reinforced material was cyclically stable for the plastic strains examined, while the overaged unreinforced material exhibited cyclic hardening at plastic strains greater than 2.5 × 10⁻⁴. Decreasing grain size of particle-reinforced materials modestly increased the cyclic flow stress of both naturally aged and overaged materials. Reinforced and unreinforced materials exhibited similar fatigue life behaviors; however, the reinforced and unreinforced naturally aged materials had superior fatigue lives in comparison to the overaged materials. Grain size had no effect on the fatigue life behavior of the particle-reinforced materials. The fatigue lives were strongly influenced by the presence of clusters of TiC particles and exogenous Al₃Ti intermetallics. formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 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.     

Fatigue crack initiation and early propagation in Al 2219-T851

Fatigue crack initiation in Al 2219-T851 for fully reversed loading(R = σ/σ_max =?1) parallel to the material rolling direction is found to occur at intermetallic inclusions at the specimen surface. The inclusions are not involved in crack initiation for fatigue perpendicular to the rolling direction, and for this orientation crack initiation is at grain boundaries and specimens have an increased fatigue life. Except for fatigue at low peak stress, multiple numbers of microcracks are formed and for selected failed specimens the number of cracks has been determined as a function of crack length. Such crack length distribution measurements show that there is significant retardation of microcracks by interaction with grain boundaries. Furthermore it is found that the coalescence of microcracks provides a mechanism for cracking to “jump“ grain boundaries and reduce fatigue lifetime. The effect of relative humidity on this process is to increase the observed mean crack length, and decrease the number of crack initiations apparently due to weakening of the matrix-intermetallic interface at potential initiation sites. The overall result is that no significant dependence of fatigue life on relative humidity is found.     

Loading rate and test temperature effects on fracture ofIn Situ niobium silicide-niobium composites

Arc cast, extruded, and heat-treatedin situ composites of niobium suicide (Nb₅Si₃) intermetallic with niobium phases (primary—Nb_p and secondary—Nb_s) exhibited high fracture resistance in comparison to monolithic Nb₅Si₃. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb₅Si₃ and plastic deformation of the Nb_p and Nb_s phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500μm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nb_p. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nb_p, while the Nb_s, always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nb_p phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models. Formerly Graduate Student, Department of Materials Science and Engineering, the Case School of Engineering, Case Western Reserve University, and Postdoctoral Research Associate, the Materials Department, Oxford University, Oxford, 0X1 3PH England     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Fatigue crack growth behavior of 2124/SiC/10p functionally graded materials

Powder metallurgy processing involving cold pressing and hot extrusion has been used to fabricate bulk functionally graded materials (FGMs) based on the 2124/SiC/10p composite system. Two forms of single-core bulk FGMs with circular cross section were fabricated. One form (designated 10SiC-2124) had a central core of unreinforced Al-2124 alloy that was surrounded by a 2124/SiC/10p reinforced surface layer: the other (designated 2124-10SiC) had a composite core and an alloy surface layer. These forms enabled the effect of the radial graded core on fatigue to be investigated with fatigue crack propagation from either (1) a ductile core to a more brittle region or (2) a brittle core to a ductile region of the FGM. The fatigue crack growth rate was measured using a constant applied stress intensity factor range (δK=7 MPa ) technique. Two main fatigue crack growth rates were distinguished corresponding to growth in the core and in the surface layer. The results show that FGMs may exhibit good fatigue crack propagation resistance. For example, when the crack propagated from the brittle core to the tough surface layer, the average fatigue crack growth rate in the Al-2124 core (3.9×10⁻⁶ mm/cycle) was significantly lower than for the Al-2124 alloy (1.5×10⁻⁵ mm/cycle) at a similar δK value (7 MPa ), due to the highly tortuous crack path in the 2124/SiC/10p brittle layer. The 2124/SiC/10p brittle layer had a lower fatigue crack growth rate (6.6×10⁻⁶ mm/cycle) than the 2124/SiC/10p conventional composite (7.5×10⁻⁶ mm/cycle) because of the compressive residual stresses in the surface layer. Thus, FGMs could be more acceptable for critical applications than their conventional composite counterparts.     

Nucleation of solid aluminum on inclusion particles injected into Al-Si-Fe alloys

Systematic inoculation experiments were carried out to study the influence of various inclusions on the nucleation of the α-Al phase in Al-Si-Fe alloys at different cooling rates. The results showed that in dilute alloys, containing less than 1.5 pct Si+Fe, almost all the inclusion types have high percentages of occurrence within the α-Al phase, indicating that nucleation can be promoted on the surface of such inclusions. In a hypoeutectic Al-Si alloy containing 6.3 pct Si, the inclusion particles of MgO, TiB₂, TiC, α-Al₂O₃, and SiC become mostly inactive nucleants and are pushed to the interdendritic regions because of the dominating poisoning effect of Si. The current results were used successfully to explain the efficiency differences between the commercial grain refiners in the hypoeutectic Al-Si alloys. Silicon is observed to preferentially segregate to the liquid-Al/inclusion interfaces so as to lower the free energy of such interfaces. A theoretical analysis of the poisoning effect of Si showed that Si segregation to the liquid/nucleant interface alters the interfacial energy balance so that the catalytic efficiency of the nucleant particles is dramatically reduced. Careful analysis showed that the poisoning effect of Si in the hypoeutectic alloy is overcome when the nucleant particles have active surface characteristics, as represented by the high catalytic potencies of γ-Al₂O₃, CaO, and Al₄C₃ particles in nucleating the α-Al phase of the hypoeutectic Al-Si alloy. Although some inclusions have comparable or higher occurrence levels than TiB₂ in the α-Al phase, they cannot be used as efficient nucleants because of either their poor wettability with liquid aluminum or their chemical reactivity, which can change the alloy chemistry.     

An analysis of non-propagating fatigue cracks

An analysis for the formation of nonpropagating fatigue cracks at (the base of V-shaped) notch roots, based on the considerations of the extent of the critically stressed region ahead of a notch or a crack tip, and the resulting volumetric strength effect, is developed. Assuming that the minimum local cyclic stress required for crack initiation from a notch root is equal to the unnotched fatigue limit, σ_e, and that the minimum local cyclic stress required for the propagation of the crack is equal to the theoretical strength of the material, σ_e, a model of notch fatigue limit is proposed that shows that nonpropagating cracks should form at the notch base if ρ≤ ρ⁰, a critical root radius, provided the notch is sufficiently deep,i.e. d ≥ ρ₀. The radius ρ₀ is a material constant and can be estimated from known material properties. The estimated values of ρ₀ are in fairly good agreement with available experimental values for steels and pure copper. For stresses near the notch fatigue limit it is suggested that p₀ be regarded as a radius above which notch fatigue limit is essentially initiation controlled and below which essentially propagation controlled. The notch fatigue limit based on complete fracture can then be estimated more accurately with mild as well as sharp notches. D. N. LAL, formerly a Graduate Assistant in Materials Science, Syracuse University     

Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel

The microstructural parameter(s) controlling the critical cleavage fracture stress, σ_F, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,S _p, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased with decreasingS _p, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and notched-bar geometry independent values for σ_F, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values for σ_F over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than the predicted peak stress location. The calculated values for σ_F were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence does not necessarily change σ_F in fully pearlitic microstructures. formerly Graduate Student, Carnegie Mellon University     

Comparisons of experimental measurements and a theoretical model for specimen self-heating during fatigue of type 316 LN stainless steel

The Type 316 stainless steel is being considered as a candidate target-container material for the spallation neutron source (SNS) being built at the Oak Ridge National Laboratory. Satisfactory behavior under fatigue loading is a requirement for the target container. Stress-controlled fatigue experiments were performed on the 316 stainless steel at 0.2 and 10 Hz with an R ratio of −1, where R=σ _min./σ _max.; σ _min. and σ _max. are the minimum and maximum applied stresses, respectively. At R=−1, a large specimen-temperature increase at 10 Hz was observed, which approached approximately 350 °C at a stress amplitude of 263 MPa, and affected fatigue lives. The specimen temperature at 0.2 Hz was about room temperature. The fatigue lives at 10 Hz were found to be shorter than those at 0.2 Hz. Different specimen temperatures were achieved by varying test frequencies. Significant differences in fatigue lives as a function of test frequency were observed with shorter fatigue lives at higher frequencies. The higher specimen temperature at 10 than at 0.2 Hz reduced the fatigue life at 10 Hz. A model based on the dissipation energy of the specimen during fatigue tests was developed to explain the fatigue-life result and predict the specimen-temperature evolution. The present research is sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, United States Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.     

Fatigue crack initiation and microcrack growth in 4140 steel

Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions: as-quenched, tempered at 400°C, and tempered at 650°C. Micro-scopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an x-y micrometer base mounted directly on an MTS machine. Following Barsom and McNicol, the cycles to fatigue crack initiationN _i were plottedvs ΔK/√p and threshhold values of gDK√p were determined. The data of logN _i vs log [ΔK/√p-ΔK/√p|_th] fit on a straight line. Microcracks grew most rapidly in as-quenched specimens and least rapidly in 650°C tempered specimens at the same ΔK/√p. In as-quenched specimens, fatigue cracks initiated at grain boundaries but in the 400 and 650°C tempered specimens they initiated at intrusions-extrusions. They are also associated with Northwestern’s Materials Research Center.