Effect of Zn Content on the Microstructure and Properties of Super-High Strength Al-Zn-Mg-Cu Alloys

The microstructure and properties of three different Al-Zn-Mg-Cu alloys with high Zn content (9 wt pct, 10 wt pct, and 11 wt pct, marked as 9Zn, 10Zn, and 11Zn, respectively) were investigated. The strength of alloys increases as the Zn content increases from 9 wt pct to 10 wt pct, while it does not increase any more as the Zn content increases continuously from 10 wt pct to 11 wt pct. The stress-corrosion cracking (SCC) resistance decreases as the Zn content increases from 9 wt pct to 10 wt pct, while it changes unobviously as the Zn content increases continuously from 10 wt pct to 11 wt pct. The elongation and fracture toughness of alloys decrease as the Zn content increases in these Al-Zn-Mg-Cu alloys. The Zn content has little effect on the precipitation reaction of Al-Zn-Mg-Cu alloys that contain the mixture of GP zones, and η′ are the main Matrix Precipitates (MPt) in the peak-aging state, and the mixture of η′ and η are the main MPt in the over-aging state. The amount of MPt and coarse T (AlZnMgCu) phases are shown to increase with the increasing Zn content in Al-Zn-Mg-Cu alloys. The coarse T phases hardly dissolve into the matrix and are the source for the crack initiation, which may be the responsibility for the negative effect on the properties of high Zn content Al-Zn-Mg-Cu alloys.     

The microstructural,mechanical, and fracture properties of austenitic stainless steel alloyed with gallium

The mechanical and fracture properties of austenitic stainless steels (SSs) alloyed with gallium require assessment in order to determine the likelihood of premature storage-container failure following Ga uptake. AISI 304 L SS was cast with 1, 3, 6, 9, and 12 wt pct Ga. Increased Ga concentration promoted duplex microstructure formation with the ferritic phase having a nearly identical composition to the austenitic phase. Room-temperature tests indicated that small additions of Ga (less than 3 wt pct) were beneficial to the mechanical behavior of 304 L SS but that 12 wt pct Ga resulted in a 95 pct loss in ductility. Small additions of Ga are beneficial to the cracking resistance of stainless steel. Elastic-plastic fracture mechanics analysis indicated that 3 wt pct Ga alloys showed the greatest resistance to crack initiation and propagation as measured by fatigue crack growth rate, fracture toughness, and tearing modulus. The 12 wt pct Ga alloys were least resistant to crack initiation and propagation and these alloys primarily failed by transgranular cleavage. It is hypothesized that Ga metal embrittlement is partially responsible for increased embrittlement.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

The effects of test temperature,temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated. Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the 2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor (v ₀), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation energy for stage II crack growth.     

Effect of antimony on the creep fracture of stainless steel

Stainless steel samples doped with various contents of Sb were crept under the same conditions. With respect to the base material, a low content of Sb (0.1 wt pct or 0.2 wt pct) is found to improve creep strength and creep ductility of the alloys whereas the addition of 1 wt pct Sb decreases the mechanical properties. The microstructure of the samples was examined quantitatively by scanning electron microscopy. The improvement of mechanical properties is attributed to a reduction of the nucleation rate and of the growth rate of cavities. In order to predict the strain to fracture, a criterion on the fraction of grain boundary area cavitated is proposed.     

The effects of test temperature,temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated. Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the 2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor (v ₀), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation energy for stage II crack growth. An erratum to this article is available at .     

Microcracking in ferrous martensites

The relative susceptibility to plate microcracking of Fe-1.0 to 1.8 wt pct C alloys containing 0 to 5 wt pct Mn, 0 to 8 wt pct Cr, and 0 to 15 wt pct Ni has been investigated. In the binary Fe-C alloys, the crack area per unit volume of martensite increases with increasing carbon content in the range from 1.0 to 1.3 wt pct in agreement with previous work. However, above 1.4 wt pct C, the specific crack area decreases as the carbon content increases due to a decrease in martensite plate length which in turn arises mostly from a habit plane transition from {225}γ to {259}γ. Indeed, when results from all alloys are considered, it is found that the dominant variable affecting microcracking is the martensite plate length. A direct, but less important, influence of carbon content is also found but any effects resulting directly from other solutes (Ni, Cr, and Mn) are negligible. The impingement model for microcracking suggested by Marder, Benscoter, and Krauss⁸ is examined in further detail. In this way, a rationale is developed for our observations which are in accord with fracture behavior of macroscopic samples.     

Mechanical behavior of cast single phase alloys solidified from undercooled melts

Several ingots (0.0254 m in diam × 0.10 m long) of nickel-30 wt pct copper, nickel-10 wt pct cobalt and iron-25 wt pct nickel were solidified with various undercoolings up to about 200 K, prior to nucleation of the solid. The materials were mechanically tested in the ascast condition. In nickel-30 wt pct copper and iron-25 wt pct nickel alloys the 0.2 pct offset yield strength, ductility and fatigue strength increased with undercooling. A linear relationship was established between 0.2 pct offset yield strength and the square root of secondary dendrite arm spacing in dendritic alloys (undercooled less than 170 K) or that of grain diameter in nondendritic alloys (undercooled more than 170 K). In iron-25 wt pct nickel limited testing indicated improvements in Charpy V-notch impact strength and in fracture toughness with undercooling. No improvement of tensile properties with undercooling was observed in nickel-10 wt pct cobalt, an alloy which solidified normally with very low microsegregation.     

Effects of alkali-metal impurities on fracture toughness of 2090 Al-Li-Cu extrusions

The effects of alkali-metal impurity (AMI) content, temperature, and crack-mouth-opening displacement (CMOD) rate on the fracture toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for short-transverse (S-L) orientations. Decreasing AMI content resulted in increasing room-temperature fracture toughness, especially for underaged S-L and T-L specimens. Unlike most Al-Li based alloys, material with very low (<2 wt. ppm) AMIs produced by vacuum refining had a high S-L fracture toughness (up to 38 MPa√m for proof strengths ∼440 MPa) as well as high toughness in other orientations. The increase in room-temperature fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergranular and cleavagelike islands, and a corresponding increase in the proportion of high energy dimpled fracture modes, on fracture surfaces. Both the present and previous studies indicate that the brittle islands result from liquid-metal embrittlement due to the presence of discrete sodium-potassium rich liquid phases. For medium to high AMI contents (5 to 37 wt ppm), S-L fracture toughness increased with decreasing temperature due to solidification of these phases and a consequent decrease in the mobility of embrittling atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so that CMOD rate influenced fracture toughness. The grain structure (degree of recrystallization) appeared to be another important parameter affecting fracture toughness.     

Effect of strontium modification on near- threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast com-mercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt pct drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the mi-crostructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10 to 20 pct higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1. Formerly Research Project Engineer, Briggs & Stratton Corporation, Milwaukee, WI 53222     

Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

In order to study the influence of Cu-alloying on the susceptibility of Be to cleavage along the basal planes, a series of single phase alloys containing up to 5 at. pct Cu in Be were prepared and tested at room temperature by the DCB technique. Copper additions were found to influence strongly the plastic events induced by the shear stresses acting at the crack tip. The crack propagation energy increases with Cu content up to about 1.2 at. pct Cu and decreases with further additions. The maximum value attained at 1.2 at. pct Cu is about seven times that of pure Be. This remarkable initial increase in the crack propagation energy can be accounted for on the basis of the higher energy needed to activate basal and prism slip due to solid solution hardening. In alloys containing more than about 1.2 at. pct Cu, twinning is found to be the most prominent deformation mechanism. The propensity to twin is observed to increase with increasing Cu content, and it is assumed that this reflects a corresponding decrease in the twinning energy. Since twin boundaries are highly susceptible to cleavage cracking, a decrease in the crack propagation energy results which has the effect of increasing the tendency of Be to fracture in a brittle manner in the alloys with higher Cu contents.     

Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

In order to study the influence of Cu-alloying on the susceptibility of Be to cleavage along the basal planes, a series of single phase alloys containing up to 5 at. pct Cu in Be were prepared and tested at room temperature by the DCB technique. Copper additions were found to influence strongly the plastic events induced by the shear stresses acting at the crack tip. The crack propagation energy increases with Cu content up to about 1.2 at. pct Cu and decreases with further additions. The maximum value attained at 1.2 at. pct Cu is about seven times that of pure Be. This remarkable initial increase in the crack propagation energy can be accounted for on the basis of the higher energy needed to activate basal and prism slip due to solid solution hardening. In alloys containing more than about 1.2 at. pct Cu, twinning is found to be the most prominent deformation mechanism. The propensity to twin is observed to increase with increasing Cu content, and it is assumed that this reflects a corresponding decrease in the twinning energy. Since twin boundaries are highly susceptible to cleavage cracking, a decrease in the crack propagation energy results which has the effect of increasing the tendency of Be to fracture in a brittle manner in the alloys with higher Cu contents.     

The asquenched microstructure and tempering behavior of rapidly solidified tungsten steels

Transmission electron microscopy, and microhardness testing were used to examine the as-quenched structure and mechanical properties of a series of rapidly solidified (RS) iron-tungsten-carbon-alloys ranging from 6 to 23 pct tungsten with a constant W:C atomic ratio of 2:1, and Tl high speed tool steel. The RS iron-tungsten-carbon alloys were found to exhibit a significant change in microstructure and hardness as the tungsten and carbon content was increased. The change in morphology was from lath martensite in the lower tungsten alloys to a solidification structure of δ-ferrite cells surrounded by austenite and M₆C carbide, in the higher tungsten alloys. A model is proposed to explain the morphologicl change. In addition, the tempering behaviors of RS Fe-6.2 wt pct W-0.21 wt pct C, Fe-23 wt pct W-0.75 wt pct C and Tl high speed tool steel were examined and compared to those observed for the conventional solution-treated and quenched alloys. A discussion is also included on the microstructural dependence upon cooling rate of RS high speed tool steels. formerly of School of Engineering and Applied Sciences, University of Sussex, Falmer, Brighton, England.     

Microstructure-property relationships of two AI-3Li-2Cu-0.2Zr-XCd alloys

The microstructure and tensile properties of two A1-3 wt pct Li-2 wt pct Cu-0.2 wt pct Zr alloys, one Cd-free and one containing 0.2 wt pct Cd, have been investigated. The Cd-free alloy remained unrecrystallized for all solutionizing treatments studied, whereas a special treatment had to be developed to prevent recrystallization during solutionizing of the 0.2 wt pct Cd alloy. In combination with cadmium, zirconium either enters into, or nucleates on, the course Al₇Cu₂Fe and T₂ phases during high temperature annealing. This reduces the volume fraction of small coherent Al₃Zr particles in the matrix which normally inhibits recrystallization. Consequently, a low temperature anneal to precipitate Al₃Zr is necessary prior to high temperature solutionizing in order to prevent recrystallization in the Cd-containing alloy. Unlike its effect in lower lithium, higher copper content aluminum alloys, cadmium does not significantly affect the nucleation of the strengthening precipitates. If anything, cadmium has a detrimental effect on the age hardening response of this alloy, since it increases the formation of coarse Al-Cu-Li equilibrium phases at grain and subgrain boundaries and thus removes some of the copper and lithium from participating in the formation of the strengthening precipitates T₁ and δ′. Subgrain boundary fracture occurred during tensile tests of both alloys in the unrecrystallized condition; however, transgranular fracture occurred in tests of the partially recrystallized 0.2 wt pct Cd alloy. Both types of fractures are believed due to a form of strain localization associated with precipitate free zones and shearable precipitates. Formerly with the Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA     

Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys. The work was carried out when the authors were with Chrysalis Technologies Inc., Richmond, VA. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys

High-Li alloys, with the composition Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr, were synthesized using a spray deposition technique (wt. pct, X=0∼1.5). The microstructure of the spray-deposited Al-Li alloys consisted of equiaxed grains with an average grain size in the range from 20 to 50 μm. The grain-boundary phases were fine and discrete. The spray-deposited and thermomechanically processed materials were isothermally heat treated at 150 °C and 170 °C to investigate the age-hardening kinetics. It was noted that the spray-deposited Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr alloys exhibited relatively sluggish aging behavior. The peak-aged condition was achieved at 170 °C in the range from 20 to 90 hours. It was noted that Cu increases the hardness of alloys during aging. Moreover, the influence of Cu on age-hardening kinetics is marginal. The mechanical properties of the spray-deposited and extruded Al-Li alloys were studied in the underaged, peak-aged, and overaged conditions. For example, the peak-aged yield strength, tensile strength, and ductility of Al-3.8Li-1.0Cu-1.0Mg-0.4Ge-0.2Zr are 455 MPa, 601 MPa, and 3.1 pct, respectively. Moreover, an increase in the Cu content of the alloy led to improvements in strength, with only slight changes in ductility, for Cu contents up to 1.0 wt pct. Beyond this range, an increase in Cu content led to decreases in both strength and ductility.     

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E _a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (<0.2 wt pct) are typically ranging from 80 to 85 kJ/mol, whereas for high-copper-containing alloys (>~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.     

Ductility and impact resistance of powder-metallurgical molybdenum-rhenium alloys

Mo-Re alloys containing between 5 and 47.5 wt pct Re were fabricated from Mo and Mo-Re powders by sintering and hot radial forging. The mechanical properties of as-forged, stress-relieved, and recrystallized specimens were examined. Up to a concentration of 41 wt pct Re, the Charpy ductile-to-brittle transition temperature decreased monotonically with increasing rhenium concentration. Consistent with this, bend angles for fracture at T=−100 °C increased monotonically with increasing rhenium concentration. Between 10 and 41 wt pct rhenium, the room-temperature tensile ductility of recrystallized Mo-Re remained nearly constant with values on the order of 35 to 45 pct. This result differs from the low ductility values observed previously by Lundberg (1997) for compositions on either side of Mo-13 wt pct Re.     

Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature

The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6 at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental to, both fracture toughness and fatigue crack resistance.