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 sic content on fatigue crack growth in

An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant K_max control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.     

Effects of gaseous hydrogen on fatigue crack growth in pipeline steel

The effects of hydrogen on crack growth rates in a moderate-strength pipeline steel subjected to cyclic loads were studied. Fatigue crack growth experiments were conducted in high-pressure hydrogen and nitrogen environments, and the influences of stress ratio, stress intensity, and cyclic loading frequency on hydrogen-accelerated fatigue crack growth were investigated. Hydrogen acceleration of intermediate-rate (Stage II) crack growth was greatest at low stress ratios and decreased to approximately zero at a stress ratio of about 0.5. However, hydrogen promoted the premature onset of accelerated (Stage III) crack growth. This appeared to be related to a hydrogen-induced reduction of fracture toughnessJ _IC.     

Mechanisms of ambient temperature fatigue crack growth in Ti-46.5Al-3Nb-2Cr-0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 °C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K _max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK _th , when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.     

Effects of environment on high-temperature fatigue crack growth in a superalloy

Fatigue crack growth rate measurements were made on Inconel alloy 718 samples at 650°C in fourteen different gaseous environments. Minor amounts of either oxygen or sulfur bearing species in the environment produced large increases in crack growth rates. Aggressive environments promoted intergranular crack growth. Kinetic factors rather than thermodynamic ones appear to be the variables dominating the effects of an environment on crack growth. Environments that markedly increased the crack growth rate did not produce significant corrosion attack on unstressed samples. Thus conventional high temperature corrosion tests may not be useful for predicting service performance of stressed components.     

Effects of temperature and environment on fatigue crack growth in ordered (Fe,Ni)3 V-type alloys

The effects of long range order, test temperature, and test environment on fatigue crack growth of two (Fe, Ni)₃ V-type ordered alloys have been determined. Long range order suppressed crack growth at low and intermediate δK, but had less effect at high δK. Crack growth resistance of the ordered alloys decreased at 600 ‡C, but still compared favorably to that of several commercial high temperature alloys. Crack growth resistance of ordered material was decreased in the presence of hydrogen (precharged or gaseous), accompanied by a change in fracture path from transgranular to intergranular. Disordered material was nearly unaffected by hydrogen. The effects of long range order and hydrogen exposure on crack growth rates are discussed in terms of the characteristic superlattice dislocations in ordered alloys.     

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m^1/2) in deaerated deionized water and 0.3 M Na₂SO₄ solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na₂SO₄ solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.     

Effects of temperature on the fatigue crack growth behavior of cast gamma-based titanium aluminides

This article reports the results of an experimental study of the effects of temperature (25 °C, 450 °C, and 700 °C) on the fatigue crack growth behavior of three near-commercial cast gamma titanium aluminide alloys (Ti-48Al-2Cr-2Nb, Ti-47Al-2Mn-2Nb+0.8 pct TiB₂, and Ti-45Al-2Mn-2Nb+0.8 pct TiB₂). The trends in the fatigue crack growth rate data are explained by considering the combined effects of crack-tip deformation mechanisms and oxide-induced crack closure. Faster fatigue crack growth rates at 450 °C are attributed to the high incidence of irreversible deformation-induced twinning, while slower crack growth rates at 700 °C are due to increased deformation by slip and the effects of oxide-induced crack closure.     

Effects of temper level on the dependence of fatigue crack growth threshold and crack closure on the prior austenitic grain size

An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of both the total threshold stress intensity range, ΔK _th , and the intrinsic threshold stress intensity range, ΔK _{eff, th} , on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK _th and ΔK _{eff, th} on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK _th and ΔK _{eff, th} on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation of ΔK_th and ΔK_{eff, th} with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive” to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size effects on fatigue thresholds and crack closure. K.S. RAVICHANDRAN, formerly Research Scholar, Department of Metallurgy, Indian Institute of Science     

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.     

Effects of oxidation on the fatigue crack growth resistance and the interfacial properties of a Ti-MMC laminate composite

Fatigue crack growth rates in a [0/90]_2s Ti-6Al-4V/SCS-6 cross-ply laminate, correlated with push-out tests, have been measured to assess the effects of varying test temperature, environment, load ratio (R), and initial stress intensity factor range (ΔK). The fatigue crack growth resistance is degraded in tests at 450 °C in air, but tests carried out at test temperatures of up to 450 °C under vacuum, both at R=0.1 and R=0.5, have shown crack arrest/catastrophic failure transitions (CA/CF), which are similar to those observed for specimens studied at room temperature and at 300 °C in air. Moreover, for such [0/90] composites, the critical role of intact 0 deg fibers bridging in the crack wake, in promoting fatigue crack growth resistance, has been confirmed. Sudden increases of fatigue crack growth rate can be attributed to individual fiber failure(s), which were detected by acoustic emission techniques. The effect of the experimental conditions (environment, test temperature, and duration) on the mechanical behavior (fatigue crack growth rate, push-out tests, and broken fibers pull-out lengths) of this laminate may be explained by the modification of the interfacial zone (decrease in the carbon layer thickness due to oxidation and formation of TiO₂).     

The influence of crack length on fatigue crack growth in deep sharp notches

The fatigue crack growth rateda/dN of short cracks and the transition to long crack behavior were investigated for ARMCO-iron. Deep notched specimens with very small notch radius (between 1.5 and 4 μm) were used. The experiments were performed with constant stress intensity ranges for various stress ratios; the fatigue crack growth rate was measured as a function of the crack length. The results permit a discussion of the mechanisms responsible for the different behavior of “short” and “long” cracks.     

Influence of environment on fatigue crack growth in the threshold region

Low fatigue crack growth rates (down to 4 × 10⁻¹⁴m/cycle) were produced using a high frequency 20 kHz ultrasonic fatigue testing machine. The influence of non corrosive (silicone oil) and corrosive (3.5% sodium chloride solution) environments was compared. Down to crack propagation rates of some 10⁻¹⁰ m/cycle which corresponds to a crack growth rate of one lattice space per cycle no difference of crack growth rates was found. However, below this rate there seems to exist for non corrosive environments a threshold cyclic stress intensity, below which crack growth becomes diminishingly small, whereas no threshold was found for the corrosive environment. In the first case crack propagation is controlled by plastic deformation processes, in the second case these processes are markedly restricted. For this region, a transition in fracture mode from ductile transcrystalline to intergranular cracking was found.     

Fatigue and fracture of damage-tolerant In Situ titanium matrix composites

In an effort to engineer damage-tolerant ingot metallurgy (IM) in situ titanium matrix composites with attractive mechanical properties, the fatigue and fracture properties of a range of high-modulus titanium alloys reinforced with TiB whiskers were examined. The strengthening effects due to elastic whisker reinforcement are quantified using shear lag and rule-of-mixture models. The effects of alloy composition and microstructure on the fatigue behavior of the in situ titanium composites will also be discussed.     

Effects of inert gases on fatigue crack growth and their transportation into subsurface regions in titanium

To clarify the effects of inert gases on the fatigue behavior of titanium, fatigue crack growth tests were carried out in pure inert gases and in vacuum. Fatigue crack growth rates increased, and the fracture surface appearance was changed in inert gases, as compared to those in vacuum. The transportation of inert gases into subsurface regions of fracture surfaces was confirmed using Auger electron spectroscopy. This transportation is considered to be due to the reverse slip of slip planes on which inert gas atoms have adsorbed.