Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α₂) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.     

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.     

航空用新型低成本钛合金显微组织与损伤容限性能关系研究

利用扫描电镜对某新型航空用Ti—A1-Mo—Cr—Zr系低成本钛合金的双态组织、片层组织及网篮组织3种典型显微组织特征和裂纹扩展过程进行了观察和分析,并对具有不同类型显微组织的合金进行了拉伸、断裂性能和疲劳性能的检测。结果表明：该新型Ti—A1-Mo—Cr—Zr系高性能低成本钛合金在不同显微组织下均具有良好的强度-塑性-韧性-疲劳性能的匹配。其中,双态组织的该合金具有最高的强度和塑性,但损伤容限性能较低（断裂韧性稍低,疲劳裂纹扩展速率高）;网篮组织的该合金具有良好的断裂韧性和疲劳强度,疲劳裂纹扩展速率与双态组织的水平相当;片层组织的该合金具有最为优异的损伤容限性能（最低的疲劳裂纹扩展速率和最高的断裂韧性）,但疲劳极限、强度和塑性稍低于双态组织和网篮组织的该合金。     

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. S.J. BALSONE, formerly with the United States Air Force, Wright Laboratory, Materials Directorate     

损伤容限型钛合金TA15 ELI 43 mm厚板组织与性能

研究了TA15 ELI合金43mm厚板双态组织和片层组织的室温拉伸性能、断裂韧性KIC以及疲劳裂纹扩展速率da／dN等损伤容限性能。讨论了显微组织对该合金损伤容限性能的影响。结果表明，相对双态组织而言，该合金片层组织在较少损失拉伸强度的前提下，提高了合金的断裂韧性，降低了合金的疲劳裂纹扩展速率，具有更好的损伤容限性能。     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

Fatigue crack growth rates and fracture toughness of rapidly solidified Al-8.5 pct Fe-1.2 pct V-1.7 pct Si alloys

The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes. For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition. The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy of the fracture surfaces.     

Rate and environmental effects on fracture of a two-phase TiAl-alloy

The influence of strain rate and environment on the fracture behavior of a two-phase TiAl-alloy, Ti-47Al-2.6Nb-2(Cr + V), heat-treated to a nearly fully lamellar microstructure has been studied by performing conventional tensile, compression, and fracture toughness tests in air, argon, and vacuum at 25 °C and 800 °C. Both tensile and compression tests were conducted at strain rates of 1 × 10⁻³ and 1 × 10⁻⁵ s⁻¹, and fracture toughness tests were performed under displacement rates of 0.25 to 2.5 mm/min. In addition,in situ fracture toughness tests were conducted at slow rates both in vacuum and in air. The results indicated that both strain rate and environment affected the tensile stress-strain behavior and ductility and the fracture resistance of the TiAl-alloy at 800 °C. In contrast, neither the tensile ductility nor the fracture toughness was significantly affected by the environment at ambient temperature. For compression in air, the stress-strain behavior was insensitive to both strain rate and test temperature within the conditions tested. Studies of fracture surfaces revealed that low tensile ductility in this alloy at ambient temperature is associated with the tendency to delaminate alongγ/γ andγ/α ₂ interfaces. formerly with Metcut-Materials Research Group, Wright-Patterson AFB, Dayton, OH 45433-0511     

The effect of microstructure on fracture toughness and fatigue crack growth behavior in γ-titanium aluminide based intermetallics

Ambient-temperature fracture toughness and fatigue crack propagation behavior are investigated in a wide range of (γ+α ₂) TiAl microstructures, including single-phase γ, duplex, coarse lamellar (1 to 2 mm colony size (D) and 2.0 μm lamellar spacing (λ)), fine lamellar (D ∼ 150 μm, λ=1.3 to 2.0 μm), and a powder metallurgy (P/M) lamellar microstructure (D=65 μm, λ=0.2 μm). The influences of colony size, lamellar spacing, and volume fraction of equiaxed γ grains are analyzed in terms of their effects on resistance to the growth of large (>5 mm) cracks. Specifically, coarse lamellar microstructures are found to exhibit the best cyclic and monotonic crack-growth properties, while duplex and single-phase γ microstructures exhibit the worst, trends which are rationalized in terms of the salient micromechanisms affecting growth. These mechanisms primarily involve cracktip shielding processes and include crack closure and uncracked ligament bridging. However, since the potency of these mechanisms is severely restricted for cracks with limited wake, in the presence of small (<300 μm) cracks, the distinction in the fatigue crack growth resistance of the lamellar and duplex microstructures becomes far less significant.     

Tensile properties and fracture toughness of a Ti-45Al-1.6Mn alloy at loading velocities of up to 12 m/s

A γ-base TiAl alloy with duplex microstructure of lamellar colonies and equiaxed γ grains was prepared with a reactive sintering method. Tensile tests and fracture toughness tests at loading velocities up to 12 m/s (strain rate for tensile tests up to 3.2×10²/s) were carried out. The micro-structure of the alloy before and after tensile deformation was carefully examined with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile strength (UTS) and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited similar strain rate sensitivity. The strain rate sensitivity exponent at strain rates lower than 10/s is about 1.5×10⁻² and at higher strain rates is almost zero. In this study, fracture toughness was found to be less sensitive to the loading velocity, having values of around 25 MPa √m, which is believed to be attributed to the high strain rate experienced at the crack tip. The predominant deformation mechanism for the strain rates used in this study was found to be twinning. However, in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated by twinning. The interaction between deformation twinning and grain boundaries resulted in intergranular fracture in the γ grains and delamination of α ₂/γ interfaces in the lamellar colonies.     

Superplastic behavior of two-phase titanium aluminides

A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.     

Microstructure and fracture toughness of the aged ,β-Ti Alloy Ti-10V-2Fe-M

Fracture mechanics and tensile tests have been performed on the metastable β-Ti alloy Ti-IOV-2Fe-3AI. A variety of microstructures was established by several combinations of forging and heat treatment resulting in different types, morphologies, and volume fractions of the a-phase which precipitates from the matrix-β phase. Both fracture toughness and ductility are strongly reduced by increasing hardening by the secondary a-phase. An elongated primary a-phase (α _p ) shows higher toughness compared to a globular α _p -phase. A thick, continuous subgrain boundary a-film lowers the toughness significantly. For microstructures without primary a a grain boundary α-film does not affect the toughness, while the ductility is drastically reduced. Very attractive combinations of fracture toughness and ductility were found for a microstructure without primary a and without grain boundary α. The results are discussed based on the fractographic observations, and a model is proposed which includes the effect of microstructure and slip distribution on the crack nucleation, the crack growth path, and the crack deviation.     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.     

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

To establish correlations between microstructure and mechanical properties for the Ti-ll alloy, twelve different combinations of hot die forging and heat treatment, in the a + 8 and Β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566‡C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior. Formerly a National Research Council Associate, Air Force Materials Laboratory Formerly with AFML     

The fracture resistance of a binary TiAl alloy

The fracture resistance of a binary Ti-47Al (in at. pct) alloy has been investigated. The binary alloy was cast, forged, and heat treated to a fully lamellar microstructure with a colony size of either 640 or 1425 μm. Fracture toughness tests were performed in a scanning electron microscope (SEM) equipped with a loading stage. Direct observations of the fracture process indicated that crack extension commenced at a stress intensity level of 1.2 to 4 MPa√m. The crack path was primarily interlamellar and crack extension across an individual colony or across similarly oriented colonies was relatively easy. In contrast, crack arrest was prevalent when the crack encountered the boundaries of unfavorably oriented colonies. To extend into an unfavorably oriented neighboring colony, the K level of the approaching crack had to be increased significantly to renucleate a microcrack at a location away from the crack tip, resulting in the formation of an interconnecting ligament that must be fractured to further crack growth. This interaction between the crack and the microstructure led to a large variation in the slope of the K _R curves. Comparison of the K _R curves for the binary Ti-47Al alloy against published data for quinary Ti-47Al-xNb-yCr-zV alloys indicates that the initiation toughness of the quinary alloys is higher by a factor of 5 to 10, implying the existence of a significant beneficial effect of alloying additions on the initiation toughness.     

Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.     

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.     

Fundamental aspects of fatigue and fracture in a TiAl sheet alloy

The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading. 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.     

择优取向层片组织TiAl合金的室温疲劳行为

采用旋转弯曲加载方式,评价了择优取向层片组织Ti-47.5Al-2.5V-1.0Cr-0.2Zr(原子数分数,%)合金的室温高周疲劳性能,并采用扫描电镜对疲劳断口进行了观察和分析。结果显示,实验合金的应力-寿命(S-N)曲线呈现平直形态,符合Basquin方程;其条件疲劳极限为477MPa,相当于其抗拉强度的83%。断口观察发现,疲劳试样以穿层片解理方式发生断裂。疲劳裂纹主要沿位于试样表面层、与外加应力成30°~90°的层片界面萌生,之后以穿层片方式发生扩展。在同一应力水平下,疲劳寿命随疲劳源尺寸增加而减少,疲劳源尺寸波动是导致疲劳寿命大幅分散的主要原因。