Fatigue crack growth in Ti-matrix composites with spatially varied interfaces

Spatially varied interfaces (SVIs) is a design concept for composite materials where the interface mechanical properties are varied along the length and circumference of the fiber/matrix interface. These engineered interfaces can be used to modify critical titanium matrix composite properties such as transverse tensile strength and fatigue crack growth resistance in ways that produce a balanced set of properties. The SVI approach may also be used to probe interface failure mechanisms for the purpose of understanding complex mechanical phenomena. Single lamina Ti-6Al-4V matrix composites containing strongly bonded SiC fibers were fabricated both in the as-received condition and with a weak longitudinal stripe along the sides of the fibers. The striped SVI composites exhibited an increase in the overall fatigue crack growth life of the specimens compared to the unmodified specimens. This improvement was caused by an increased extent of debonding and crack bridging in SVI composites. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Fiber-Matrix interactions in brittle matrix composites

Brittle matrix composites, including carbon-carbon (C-C) and ceramic matrix, offer a new dimension in the area of high-temperature structural materials. Fiber-matrix interactions determine the mechanism of the load transfer between the fiber and matrix and resulting mechanical properties. Composites studied in this work include a C-C composite densified with a chemical vapor infiltration (CVI) pyrolytic carbon, silicon carbide fiber-silicon carbide matrix composite, and carbon fiber-silicon carbide matrix composites densified by the CVI technique. The type of the interfacial carbon in C-C composites was found to control their mechanical properties. The presence of the compressive stress exerted by the matrix on the carbon fibers was attributed to an increase in flexural strength. The transverse matrix cracking in C/SiC composites was believed to cause a lowering in the flexural strength value. Brittle fracture behavior of SiC/SiC composites was correlated with the presence of an amorphous silica layer at the fiber-matrix interface. This invited paper is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presented as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

Effects of interfacial strength on fatigue crack growth in a fiberreinforced Ti-alloy composite

The fatigue crack growth behavior of a Ti-6A1-4V composite with boron fibers was previously studied in the as-received and thermally exposed conditions. Fracture strengths of the composite, fiber, and interface were characterized together with fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fiber properties as input, a recently proposed model was exercised to elucidate the effects of interfacial strength on crack growth rates in the composite. Comparison of experimental results with model calculations revealed that a weak fiber/matrix interface combined with a strong, high-modulus fiber led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Granular product in a high strength,low alloy containing molybdenum and niobium

The formation and microstructure of the granular product and its effect on the mechanical properties of a high-strength, low alloy steel containing molybdenum and niobium have been investigated. It was found that the granular product “islands” are composed of both twinned martensite and dislocated martensite. The effect of the granular “islands” on the strength at room temperature and at 400 °C has been determined. The results showed that the strength increased and both the impact and fracture toughness decreased as the volume fraction of granular “islands” was increased.In situ fracture studies indicated that the three stages of the microfracture process of the specimen containing granular “islands” are the initiation of voids at interfaces between the granular “islands” and the bainitic ferrite matrix, followed by void growth and finally, coalescence by shear.     

Characteristics of Mg-based composites synthesized using a novel mechanical disintegration and deposition technique

In the present study, for the first time, a new technique termed “mechanical disintegration and deposition” was conceptualized and used for synthesis of Mg-based composites. Microstructural characterization studies conducted on the extruded composite samples using optical and scanning electron microscopy revealed a minimal level of porosity, uniform distribution of SiC particulates, recrystallized grain morphology, and good interfacial integrity between the metallic matrix and the reinforcement. Mechanical properties characterization conducted using a free-free beam method and servohydraulic tensile testing machine revealed that with an increasing percentage of reinforcement, there was an increase in dynamic elastic modulus and 0.2 pct yield strength (YS), a decrease in ductility, and no effect on the ultimate tensile strength (UTS). The results of the mechanical properties were then rationalized in terms of the microstructural characteristics associated with the composite samples.     

High-strength steel development for pipelines: A brazilian perspective

The production of American Petroleum Institute (API) class steels using the traditional controlled rolling route rather than the process involving accelerated cooling necessitates a careful adjustment of steel composition associated with the optimization of the rolling schedule for the deformation and phase transformation characteristics of these modified alloys. The current work presents a study of two, NbCr and NbCrMo, steel systems. The microstructure obtained is correlated not only with the resulting mechanical properties, but also with the weldability and resistance to damage in the aggressive environments to which the materials are exposed. The evaluation of the steels was undertaken at two stages along the production route, sampling the material as plate and as tubular product, according to the API 5L 2000 standard. Tensile testing, Charpy-V impact testing, and hardness measurements were used to determine the mechanical properties, and microstructural characterization was performed by optical and scanning electron microscopy. The results showed that it was possible to obtain good impact properties, for both steels, in plate and tube formats. The Charpy-V impact energy, measured at −20 °C from 100 to 250 J corresponds to a toughness level above that required by the API 5L 2000 standard, which specifies 68 to 101 J at 0 °C. The yield strength (YS) to ultimate tensile strength (UTS) ratio was determined to be 0.8, the API standard establishing a maximum limit of 0.93. Both of the alloys investigated exhibited a bainitic microstructure and were successfully processed to fabricate tubular products by the “UOE” (bending in “U”, closing in “O,” and expanding “E”) route. with regard to weldability, the two experimental steels exhibited a heat-affected zone (HAZ) for which toughness levels (using the temperature associated with a 100 J impact energy as a base for comparison) were higher than those for both the base metal (BM) and the weld metal (WM) itself. In order to perform the evaluation of the behavior of the steels in an aggressive environment, more specifically their resistance to the deleterious effects of H₂S, slow strain rate tests (SSRTs) were carried out, immersing the samples in a sodium thiosulfate solution during the tests. Though no secondary cracking was observed in the test samples, the ductility levels measured were lower than those for the same materials tested in air. Constant load tests were also conducted according to the standard NACE conditions. Despite the more aggressive nature of the test solution in these cases, no samples of either steel suffered failure.     

The effect of matrix microstructure on the tensile and fatigue behavior of SiC particle-reinforced 2080 Al matrix composites

The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of S′ precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed S′ precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within “rogue” inclusions was quantified using a finite-element analysis (FEA) unit-cell model.     

Influence of interfacial reactions on the fiber-matrix interfacial shear strength in sapphire fiber-reinforced Nial(Yb) composites

The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM com-posites after directional solidification (DS). The fiber-pushout stress-displacement behavior con-sisted of an initial “pseudoelastic” region, wherein the stress increased linearly with displacement, followed by an “inelastic” region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a “fric-tional” stress. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed that the inter-facial region in the PM NiAl(Yb) composites was comprised of Yb₂O₃,O-rich NiAl and some spinel oxide (Yb₃Al₅O₁₂), whereas the interfacial region in the HT and DS composites was comprised mainly of Yb₃Al₅O₁₂. A reaction mechanism has been proposed to explain the pres-ence of interfacial species observed in the sapphire-NiAl(Yb) composite. The extent of inter-facial chemical reactions and severity of fiber surface degradation increased progressively in this order: PM < HT < DS. Chemical interactions between the fiber and the NiAl(Yb) matrix resulted in chemical bonding and higher interfacial shear strength compared to sapphire-NiAl composites without Yb. Unlike the sapphire-NiAl system, the frictional shear stress in the sap-phire-NiAl(Yb) composites was strongly dependent on the processing conditions. Formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

The effect of matrix reinforcement reaction on fracture in Ti-6Ai-4V-base composites

Samples of Ti-6A1-4V containing 10 vol pct of either TiC or SiC have been tested in tension at temperatures up to 760 °C, and the mechanical properties have been compared with those of the unreinforced matrix alloy. It has been found that the yield stress and the tensile strength of the TiC-containing composite are superior to those of the SiC composite at room temperature but that this behavior is reversed at the higher temperatures. The ductility of the TiC composite is about 2 pct at room temperature and increases with increase of temperature. No ductility is found for the SiC composite at room temperature, but some ductility is observed at the higher temperatures. These observations are interpreted in terms of the extent and nature of the reaction zones between the matrix alloy and the reinforcement and in terms of the failure mechanisms observed using scanning (SEM) and transmission electron microscopy (TEM). D.G. KONITZER, formerly with the Aluminum Company of America, Alcoa Center, PA. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Simulation of deformation and failure process in bimodal Al alloys

Recent scientific interest in nanostructured materials stems from reports of attractive physical and mechanical properties. It has been proposed that the presence of multiple-length scales in a “nanostructured” matrix can alleviate the problem of low ductility by facilitating dislocation activity. One example of the concept of multiple-length scales is illustrated by a “bimodal” microstructure, e.g., containing a mixture of nanostructured and submicron grains. The present work reports on a numerical study of the tensile deformation and fracture of a nanostructured Al alloy with a bimodal microstructure. In the theoretical framework used in the present study, the elastic-plastic behavior, deformation, and fracture processes are approximated by the Ramberg-Osgood formula and finite-element method, respectively. The numerical results are found to be in reasonable agreement with the experimental behavior.     

role of crack tip shielding in the initiation and growth of long and small fatigue cracks in composite microstructures

The role of crack tip shielding in retarding the initiation and growth of fatigue cracks has been examined in metallic composite microstructures (consisting of hard and soft phases), with the objective of achieving maximum resistance to fatigue. Specifically, duplex ferritic-martensitic structures have been developed in AISI 1008 and 1015 mild steels to promote shielding without loss in strength. The shielding is developed primarily from crack deflection and resultant crack closure, such that unusually high long crack propagation resistance is obtained. It is found that the fatigue threshold ΔK _TH in AISI 1008 can be increased by more than 100 Pct to over 20 MPa Vm, without sacrifice in strength, representing the highest ambient temperature threshold reported for a metallic alloy to date. Similar but smaller increases are found in AISI 1015. The effect of the dual-phase microstructures on crack initiation and small crack (10 to 1000 ώm) growth, however, is markedly different, characteristic of behavior influenced by the mutual comPctition of intrinsic and extrinsic (shielding) “toughening” mechanisms. Accordingly, the composite microstructures which appear to show the highest resistance to the growth of long cracks, show the lowest resistance to crack initiation and small crack growth. In general, dual-phase steels are found to display remarkable fatigue properties, with fatigue limits as high as 58 Pct of the tensile strengths and fatigue thresholds in the range of 13 to 20 MPaVm. Formerly with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720, is with National Semiconductor Corporation, Santa Clara, CA 95051.     

Effect of alternative aging process on the fracture and interfacial properties of particulate Al2O3-reinforced Al (6061) metal matrix composite

The effects of different aging processes on ductility, fracture, and interfacial properties in particulate Al₂O₃-reinforced Al (6061) metal-matrix composite (MMCs) were studied. Tensile tests based on relevant ASTM standards were performed to investigate the mechanical responses of specimens under different heat-treatment conditions. Scanning electron microscopy (SEM), field-emission SEM (FESEM), and transmission electron microscopy (TEM) studies were carried out to correlate fracture mechanism(s) with microstructural features. Based on the experimental results, the overall effect of heat treatment on tensile properties is similar to that in the monolithic alloy, however, the rate of recovery in fracture-related properties, such as elongation to fracture, is lower in the overaged condition for the MMC samples. To explain this “low” recovery behavior of overaged MMCs, the following observations have been taken into account: (1) a shift of the materials’ behavior from particle fracture to interface (or near-interface) debonding fracture, when moving from the underaged to overaged regime, and (2) more frequent observations of interfacial reaction products (spinel) on the fracture surface of overaged specimens. The presence/formation of spinel phase at the interface was recognized as the main cause of this behavior. Although spinel products mainly form during material processing, they may continue to form in the solid state as well. As a result, the surface morphology of the spinel phase in the underaged specimens is different from that in the overaged specimens.     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700°C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. 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.     

Processing-property-microstructure relationships in TiAl-based alloys

The influence of thermomechanical processing on the microstructure of a range of TiAl-based alloys has been assessed using optical and electron microscopy, and the room-temperature mechanical properties have been determined. Long-term exposure at high temperatures has been used to assess the thermal stability of some of the structures generated through the different processing routes, and it has been found that the (gamma and alpha 2) lamellar structures, in some of the alloys, are unstable at 700 °C, a likely operating temperature. Addition of boron increases the stability of the lamellar structure. The influence of the difficulty of slip transfer between gamma and alpha 2 has been assessed as one of the factors limiting ductility in samples with this lamellar structure. In addition to the alloys produced via the ingot route, some atomized material has been produced and the microstructure and properties of hot-isostatically pressed “hipped” material assessed. Regions, high in titanium, are present in all atomized powders that have been examined, and these regions are found to initiate fracture at very low strains. These results are briefly discussed in terms of the factors that control the room-temperature strength and fracture behavior of TiAl-based alloys. 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.     

Influence of fabrication technique on the fiber pushout behavior in a sapphire-reinforced NiAl matrix composite

Directional solidification (DS) of “powder-cloth” (PC) processed sapphire-NiAl composites was carried out to examine the influence of fabrication technique on the fiber-matrix interfacial shear strength, measured using a fiber-pushout technique. The DS process replaced the fine, equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parallel to the fiber axis, with fibers either completely engulfed within the NiAl grains or anchored at one to three grain boundaries. The load-displacement behavior during the pushout test exhibited an initial “pseudoelastic” response, followed by an “inelastic” response, and finally a “frictional” sliding response. The fiber-matrix interfacial shear strength and the fracture behavior during fiber pushout were investigated using an interrupted pushout test and fractography, as functions of specimen thickness (240 to 730 μm) and fabrication technique. The composites fabricated using the PC and the DS techniques had different matrix and interface structures and appreciably different interfacial shear strengths. In the DS composites, where the fiber-matrix interfaces were identical for all the fibers, the interfacial debond shear stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundaries. The matrix grain boundaries coincident on sapphire fibers were observed to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larger for the DS composites compared to the PC composites. The study highlights the potential of the DS technique to grow single-crystal NiAl matrix composites reinforced with sapphire fibers, with fiber-matrix interfacial shear strength appreciably greater than that attainable by the current solid-state fabrication techniques.     

Interface-controlled fatigue cracking of SCS-6/Ti-22Al-23Nb “orthorhombic” titanium aluminide composite

The effect of aging at elevated temperature on interfacial stability and fatigue behavior of a SCS-6/Ti-22Al-23Nb “orthorhombic” (O) titanium aluminide composite is investigated. The composite was heat treated in vacuum at 900 °C for up to 250 hours to change the microstructural characteristics. The stability of the matrix alloy and interfacial reaction zone after extended thermal exposure was analyzed. The effect of interface on fatigue behavior, including stiffness degradation, evolution of fatigue damage, and crack growth rates, was characterized. Finally, a modified shear-lag model was used to predict the saturated matrix crack spacing in the composite under fatigue loading. The results demonstrate that aging at elevated temperature affects the stability of the interfacial reaction zone, which, in turn, degrades the fatigue properties of the composite. However, fatigue crack will not develop from the ruptured interfacial reaction layer until the thickness of the reaction zone or the maximum applied stress exceeds a critical value.     

Strengthening of wrought aluminum alloys by fractional melting

A new process has been developed and successfully employed to produce wrought aluminum alloys of high strength and ductility by a new process, “Strengthening by Fractional Melting” (SFM). In this process, cast ingots containing a larger amount of solute than the final desired amount are held at moderate pressure over a ceramic filter and simultaneously heated to above the solidus. As partial melting occurs, the liquid is forced out of the interdendritic spaces and through the filter. The solid material remaining above the filter comprises the “billet” used for subsequent working and heat treatment. 7000-series alloys produced by the “SFM” process have comparable or superior combinations of ultimate strength, yield strength, and elongation compared to similar alloys produced by any other process. For example, properties were obtained as high as 692 MPa (99 Ksi) ultimate strength, 643 MPa (92 Ksi) yield strength, and 10 pct elongation. The high strengths and ductility result from a homogeneous microstructure with high solute content and low residual second phase.     

Damage mechanisms and fiber orientation effects on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost ceramic composite

In this article, the fiber orientation effect on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost composite was experimentally investigated, and damage mechanisms were analyzed. Ultrasonic nondestructive evaluation (NDE) was performed to interrogate the composite samples before testing. A four-point bending test was conducted to study the mechanical behavior. Scanning electron microscopy (SEM) fractography and SEM energy-dispersive X-ray spectroscopy (EDXS) along with SEM/EDXS elemental mapping were employed to characterize damage mechanisms, microstructures, and the microchemical distribution of elements following mechanical tests. A mechanistic understanding of the fracture behavior of the NEXTEL/BLACKGLAS ceramic composite was provided. 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.     

Damage mechanisms and fiber orientation effects on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost ceramic composite

In this article, the fiber orientation effect on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost composite was experimentally investigated, and damage mechanisms were analyzed. Ultrasonic nondestructive evaluation (NDE) was performed to interrogate the composite samples before testing. A four-point bending test was conducted to study the mechanical behavior. Scanning electron microscopy (SEM) fractography and SEM energy-dispersive X-ray spectroscopy (EDXS) along with SEM/EDXS elemental mapping were employed to characterize damage mechanisms, microstructures, and the microchemical distribution of elements following mechanical tests. A mechanistic understanding of the fracture behavior of the NEXTEL/BLACKGLAS ceramic composite was provided. 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.