Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr₂Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr₂Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr₂Nb particles.     

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.     

Ductile-phase toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites

A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack propagation behavior of a molybdenum disilicide intermetallicmatrix composite reinforced with 20 vol pct niobium spheres. Using disk-shaped compact DC(T) samples, only moderate improvements (∼24 pct) in fracture toughnessK _lcvalues were found for the composite compared to the unreinforced MoSi₂ matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct ofK _Ic, with growth rates displaying a high dependency (∼14) on the applied stress-intensity range. The lack of significant toughening due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi₂, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical factors for the successful development of tougher Nb/MoSi₂ structural composites. Formerly with McDonnell Formerly with McDonnell     

Microstructure, mechanical properties, and fracture mechanism of As-cast (Ti0.5Cu0.25Ni0.15Sn0.05Zr0.05)100−x Mo x composites

Copper mold cast cylinders of (Ti_0.5Cu_0.25Ni_0.15Sn_0.05Zr_0.05)_100−x Mo _x composites are prepared. Addition of Mo in the bulk glass-forming alloy induces the formation of a dendrite/matrix composite. For 3-mm-diameter cylinders, the matrix exhibits a homogenous ultrafine microstructure for Mo content of 2.5 at. pct, and a fine eutectic microstructure for 5 at. pct Mo. For 5-mm-diameter cylinders, the matrix exhibits a dendritic microstructure for 2.5 at. pct Mo, and exhibits a coarser eutectic microstructure for 5 at. pct Mo. Despite the formation of a dendrite/nanostructured matrix composite in the cylinders, the quenched surface layer with a nanoscale grain size dominates the deformation and fracture of the 3-mm-diameter cylinders. More than 56 vol pct quenched layer leads to a distensile fracture mode and the samples exhibit high fracture strength and high Young’s modulus but low ductility. For 5-mm-diameter cylinders, the composite microstructure becomes dominant due to its more than 64 vol pct volume fraction leading to a cone-shaped fracture surface. The samples exhibit lower yield strength and lower Young’s modulus but better ductility compared to the 3-mm-diameter cylinders. The mechanical behavior of the Mo-bearing composites strongly depends on the microstructural homogeneity and casting defects formed upon solidification.     

Microstructural and fracture characterization of Nb-Cr-Ti mechanically alloyed materials

Three materials containing Nb, Cr, and Ti were fabricated by consolidating powders made by mechanical alloying. The Nb/Ti ratio was maintained at about 1.3 and Cr was increased to form the intermetallic Cr₂Nb. X-ray diffraction, metallography, and transmission electron microscopy were used to thoroughly characterize the microstructure and substructure of the materials. Fatigue and fracture toughness properties were also evaluated at ambient temperature. The alloyed powders contained only small amounts of intermetallic, but during the consolidation heat treatment, two of the materials precipitated large volume fractions of Cr₂Nb. In the third material, Cr₂Nb was precipitated by heat treatment, although this was not expected from the composition based on the Nb-Cr-Ti phase diagram. Maximum fracture toughness of the composutes was ≈ 11 MPa √m. The low fracture toughness was attributed to the high plastic constraint of matrix deformation by the Cr₂Nb and compositional change in the matrix.     

The mechanical response of the Ni−Ni3Nb eutectic composite: Part II. Cyclic behavior

In an effort to more fully understand fatigue crack propagation in composite materials, tension-tension fatigue studies of the Ni?Ni₃Nb eutectic composite were conducted. This composite displayed excellent fatigue resistance in a notched configuration by exhibiting an endurance limit of 55 pct of the 108.7 ksi smooth bar tensile strength. Under high stress-low cycle fatigue conditions, the fatigue resistance of the composite was controlled by the {112} twinning and subsequent twin boundary fracture of the Ni₃Nb phase. However, under low stress-high cycle fatigue conditions, Stage I crack propagation was observed in the nickel matrix. It was concluded that the nickel matrix controls the high cycle fatigue resistance of the composite. The fatigue behavior of the Ni?Ni₃Nb eutectic composite was compared with that of the Al?Al₃Ni composite. The similarity in the operative crack growth mechanisms in these two composites suggests a trend which may apply to all composites.     

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     

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.     

Finite-element method simulation of effects of microstructure, stress state, and interface strength on flow localization and constraint development in Nb/Cr2Nb in situ composites

The effects of volume fraction of particles, stress state, and interface strength on the yield strength, flow localization, plastic constraint, and damage development in Nb/Cr₂Nb in situ composites were investigated by the finite-element method (FEM). The microstructure of the in situ composite was represented in terms of a unit rectangular or square cell containing Cr₂Nb particles embedded within a solid-solution-alloy matrix. The hard particles were considered to be elastic and isotropic, while the matrix was elastic-plastic, obeying the Ramberg-Osgood constitutive relation. The FEM model was utilized to compute the composite strength, local hydrostatic stress, and plastic strain distributions as functions of volume fraction of particles, stress state, and interface strength. The results were used to elucidate the influence of volume fracture of particles, stress state, and interface property on the development of plastic constraint and damage in Nb/Cr₂Nb composites.     

Microstructure/processing relationships in reaction-synthesized titanium aluminide intermetallic matrix composites

Near-γ TiAl- and Al₃Ti-based intermetallic matrix composites have been produced using in-situ reaction-synthesis techniques. The intermetallic matrices have been reinforced with relatively high loadings (e.g., 20 to 50 vol pct) of dispersed TiB₂ particulates. It is shown that the as-synthesized TiB₂ size is strongly dependent on the specific alloy formulation; specifically, the TiB₂ size tends to increase as the nominal volume percent of TiB₂ in the composite increases. The observed size effect is determined to be associated with the temperature that is attained during the synthesis event, which is established primarily by the net exothermicity of the participating synthesis reaction(s). The exothermicity of the reactions can be assessed through the calculation of a formulations’s adiabatic temperature, which is found to increase with the percentage of TiB₂ over the range of approximately 10 to 60 vol pct. The coupling of a composite’s characteristic adiabatic temperature with the resulting reinforcement size provides direct links among composition, processing, and mechanical performance, since the size of a reinforcing particle is influential in establishing the interparticle spacing, which, in turn, establishes the strengthening potency of the dispersed phase within the composite.     

Preparation and casting of metal-particulate non-metal composites

A new process for the preparation and casting of metal-particulate non-metal composites is described. Particulate composites of ceramic oxides and carbides and an Al-5 pet Si-2 pct Fe matrix were successfully prepared. From 10 to 30 wt pct of A1₂O₃, SiC, and up to 21 wt pct glass particles, ranging in size from 14 to 340 ώ were uniformly distributed in the liquid matrix of a 0.4 to 0.45 fraction solid slurry of the alloy. Initially, the non-wetted ceramic particles are mechanically entrapped, dispersed and prevented from settling, floating, or agglomerating by the fact that the alloy is already partially solid. With increasing mixing times, after addition, interaction between the ceramic particles and the liquid matrix promotes bonding. Efforts to mix the non-wetted particles into the liquid alloy above its liquidus temperature were unsuccessful. The composite can then be cast either when the metal alloy is partially solid or after reheating to above the liquidus temperature of the alloy. End-chilled plates and cylindrical slugs of the composites were sand cast from above the liquidus temperature of the alloy. The cylindrical slugs were again reheated and used as starting material for die casting. Some of the reheated composites possessed “thixotropy.” Distribution of the ceramic particles in the alloy matrix was uniform in all the castings except for some settling of the coarse, 340ώ in size, particles in the end-chilled cast plates.     

The effect of the carbide phases on the crack grow of 0.5 pct Mo-B steels under impact,cyclic, and monotonically increasing loading

A study of the influence of carbide phases on the cracking resistance of as-quenched and of quenched and tempered 0.5 pct Mo-B steels was made using notched or notched and precracked specimens that were subjected to impact, cyclic, and monotonically increasing loading. The carbide influence on fracture, while limited in extent, was found to increase as load, loading rate, volume fraction, and particle size increase. The results for the asquenched condition showed that the susceptibility of these steels to crack initiation under impact loading at temperatures below - 100°F is greater when even a small amount of titanium carbide (less than 0.2 vol pet of 1 to 5 μm particles) is present than when none is present. At room temperature, this same carbide concentration has no influence on impact properties, fatigue-crack initiation (in the presence of a notch), fatigue-crack growth rate, or the ductile fracture resistance under monotonically increasing loading at slow strain rate. In the case of the quenched and tempered materials, the alloy containing a large amount of M₂₃C₆ (2 vol pct of 1 to 10 μm particles) exhibited behavior similar to that observed in the as-quenched materials containing titanium carbide. That is, the presence of M₂₃C₆ was associated with increased susceptibility to crack initiation for impact loading at low temperature. In addition, at room temperature this alloy had a reduced impact energy for crack propagation. For monotonically increasing loading at slow strain rate, this same carbide distribution had no influence on the net section stresses required to initiate stable or unstable crack growth. These stresses fall closely in line with, respectively, the yield stress and tensile strength of the material. The alloy containing M₂₃C₆ required less crack opening for a given crack extension—an effect most pronounced after maximum load. Finally, some attention is directed to the use of Charpy test data to assess fracture resistance for modes of loading other than impact.     

Intermetallic-reinforced light-metal matrix in-situ composites

Transition-metal trialuminide intermetallics such as Al₃Zr and Al₃Ti, having low densities and high elastic moduli, are good candidates for the in-situ reinforcement of light-metal matrices based on Al and Mg alloys. In this work, in-situ composites based on Al and Al-Mg matrices reinforced with an Al₃Zr intermetallic were successfully processed by conventional ingot metallurgy. The microstructural studies showed that “needle” or “feathery”-like particles of Al₃Zr phase, whose volume fraction increased with increasing concentration of Zr, were formed in the Al matrix in the investigated range of Zr contents from 0.9 to 11.6 at. pct. Properties of Al-Zr alloys were investigated as a function of volume fraction of Al₃Zr. It is shown that the density, hardness, and yield strength of the in-situ Al/Al₃Zr composites can be quite adequately described by the composite rule-of-mixtures (ROM) behavior. Alloying of a binary Al-2.4 at. pct Zr alloy with Mg up to ∼25 at. pct reduces profoundly its density and, additionally, strengthens the matrix by a Mg solid-solution strengthening mechanism.     

The mechanical response of the Ni−Ni3Nb eutectic composite: Part I. Monotonic behavior

To understand the mechanical behavior of the Ni?Ni₃Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni₃Nb intermetallic phase. It was found that Ni₃Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni?Ni₃Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni?Ni₃Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni₃Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively.     

The effects of reinforcement additions and heat treatment on the evolution of the poisson ratio during straining of discontinuously reinforced aluminum alloys

The effects of reinforcement additions and heat treatment on the evolution of the Poisson ratio were determined for a 7xxx aluminum alloy reinforced with 15 vol pct SiC_p, a 2xxx alloy with 20 pct SiC_p, and a 2014 alloy with 15 pct A1₂O₃. The Poisson ratio of the monolithic alloy was 0.31 to 0.32 in the elastic regime. At the onset of the plastic regime, the Poisson ratio of the monolithic materials rose rapidly to about 0.45 and then gradually increased to 0.47 by 3.5 pct strain. For discontinuously reinforced aluminum (DRA) materials, the Poisson ratio in the elastic regime was considerably lower than that exhibited by the matrix alloy, while the magnitude of the difference was dependent upon the type, volume fraction, and elastic properties of the reinforcement. In addition, th evolution of the Poisson ratio for DRA material depends upon heat treatment and level of strain due to damage evolution(e.g., SiC_p cracking, matrix failure,etc.) which accompanies straining in these materials. Both the magnitude and extent of change in the Poisson ratio with increasing strain in the composite is rationalized by the accumulation of damage which accompanies increasing strain.     

The age-hardening characteristics of an AI-6061/AI2O3 metal matrix composite

Experiments were conducted to determine the age-hardening behavior of an Al-6061 metal matrix composite reinforced with ∼20 vol pct of A1₂O₃ microspheres and prepared using liquid metallurgy processing. The presence of alumina microspheres increases the dislocation density in the matrix of the composite in the as-quenched state and, by comparison with the monolithic alloy, this leads to a significant increase in the yield stress in the as-quenched and unaged condition. From measurements of the 0.2 pct proof stress, there is no evidence for any significant acceleration in the aging process in the composite material: both materials attain similar peak-aged conditions after essentially the same aging times. The microstructures of the composite and the monolithic alloy are similar in the peak-aged condition, with a high density of fine needlelike β″ precipitates and, in the over-aged condition, with a reasonably homogeneous distribution of the rod-shaped β′ phase. It is proposed that the aging behavior is better quantified by determining the yield stress rather than by taking hardness measurements. Formerly Visiting Scholar, Kyushu University,     

Environmentally assisted,sustained-load crack growth in powder metallurgy nickel-based superalloys

To examine the influence of niobium (Nb) on sustained-load crack growth (SLCG) in oxygen, three powder metallurgy (P/M) nickel-based superalloys, with nominal compositions similar to IN100, but with 0, 2.5, and 5 wt pct of Nb, are used. These alloys are gamma-prime (γ’) strengthened and have comparable volume fractions (53 vol pct) of γ’ precipitates. The SLCG experiments are conducted in high-purity oxygen and argon at 873, 923, and 973 K. The environmental cracking sensitivity (ECS) for the alloys with 2.5 and 5 wt pct of Nb is consistent with that of INCONEL 718 and supports the previously identified role of Nb-rich carbides in enhancing crack growth. The susceptibility of the Nb-free alloy to oxygen, however, is much greater than expected. The apparent activation energy for crack growth in oxygen was found to depend on stress-intensity-factor (K) levels for the Nb-containing alloys and ranged from about 320 to 260 kJ/mol for K levels of 35 to 60 MPa√m. It was nearly independent of K at about 250 kJ/mol for the Nb-free alloy. The results are discussed in terms of the rate-controlling process and of the mechanism for crack-growth enhancement.     

SiC particle cracking in powder metallurgy processed aluminum matrix composite materials

Particle cracking is one of the key elements in the fracture process of particulate-reinforced metal-matrix composite (MMC) materials. The present study quantitatively examined the amount of new surface area created by particle cracking and the number fraction of cracked particles in a series of SiC-reinforced aluminum-matrix composite materials. These composite materials were fabricated by liquid-phase sintering and contained 9 vol pct of 23, 63, or 142 μm SiC. The matrix properties were varied by heat treating to either an underaged or peak-aged condition. In general, the new surface area created by particle cracking (S _v ) and the number fraction of cracked particles (F_no) were linearly dependent on the local strain along the tensile specimen. Multiple cracks were frequently observed in the composites containing large particles. It was found that the new surface area created by particle cracking per unit strain was higher for the case of high-strength matrices and was not systematically affected by particle size within the range studied. The number fraction of cracked particles was affected by both particle size and matrix strength. A higher number fraction of particles cracked in the composites reinforced with large particles and with high matrix yield strengths. These results are interpreted in terms of the size of the particle defects, which is a function of particle size, and the critical flaw size necessary to crack a given particle, which is a function of the stress on the particle. The new surface area created by cracking and the fraction of cracked particles were related and are in good agreement for the large and medium sized particles.     

Correlation of microstructure with hardness and wear resistance of surface composites fabricated by high-energy electron beam irradiation of Fe-based metamorphic powders

In this study, surface composites were fabricated with Fe-based metamoprhic powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and wear resistance was investigated. Fe-based metamorphic powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders without flux to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powders deposited on the one-layered surface composite. The composite layers of 1.3∼1.9 mm in thickness were homogeneously formed without defects and contained a large amount (up to 48 vol pct) of hard and fine Cr₂B crystalline phases in the Cr_0.19Fe_0.7Ni_0.11 matrix. Since the hardness and wear resistance of the surface composite layers were directly influenced by hard Cr₂B phases, they were two to three times greater than those of the steel substrate. In particular, the two-layered surface composite showed a high hardness of ∼300 VHN even at 750 °C, as well as at room temperature, because Cr₂B phases and the Cr_0.19Fe_0.7Ni_0.11 matrix were hard and thermally stable.