The anisotropy of deformation and fracture in a directionally solidified Ni/Ni3Al-Ni3Cb lamellar eutectic alloy

The orientation dependence of deformation and fracture modes was investigated for a directionally-solidified Ni−Ni₃Al−Ni₃Cb lamellar eutectic alloy (Ni-20 wt pct Cb-2.5 wt pct al-6.0 wt pct Cr) using optical and transmission microscopy to examine tensile and compression specimens tested at temperatures below the softening point of the δ (Ni₃Cb) reinforcing phase (∼1050 K). In this temperature range there is a large difference between longitudinal and transverse tensile ductibility (>5 pct longitudinalvs<1 pct transverse). No single preferred fracture path (such as interfacial delamination) could be found to account for the low transverse tensile ductility. Analysis of the δ twinning geometry, however, indicated that the twinning strains for twins of the type {211}, which operate copiously in longitudinal tension, are negative in most transverse orientations, with Schmid factors being very low (<0.013) in the limited range of transverse orientations where {211} twin strains are positive. Examination of transverse tension test specimens broken at 1033 K confirm the absence of {211} twins, with only limited {011} twinning being found in selected grains, leading to the conclusion that the relatively low transverse tensile ductility of the eutectic results from the very limited number of deformation systems which operate in the δ reinforcing phase below the softening temperature.     

Microscopy and tensile behavior of melt-spun Ni-Al-Fe alloys

Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered fee γ-Ni, which contains precipitates of γ′-Ni₃Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains. The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the high ductility.     

Plastic deformation and fracture of binary TiAl-base alloys

The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α ₂ lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.     

Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys

Directionally solidified (DS) β + (γ + γ′) Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q ) of 30.4 MPa √m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.     

Ductility enhancement in NiAl (B2)-base alloys by microstructural control

An attempt to improve ductility of NiAl (B2)-base alloys has been made by the addition of alloying elements and the control of microstructure. It has been found that a small amount of fccγ phase formed by the addition of Fe, Co, and Cr has a drastic effect not only on the hot workability but also on the tensile ductility at room temperature. The enhancement in ductility is mainly due to the modification of Β-phase grains by the coexistence ofγ phase. The effect of alloying elements on the hot forming ability is strongly related to the phase equilibria and partition behavior amongγ,γ′ (L1₂ structure), and Β phases in the Ni-Al-X alloy systems. The ductility-enhancement method shows promise for expanding the practical application of nickel aluminide.     

Deformation and fracture behavior of a directionally solidified β/γ′ Ni-30 At. pct Al alloy

The effect of a ductile γ′-Ni₃Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni₃Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately 850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties.     

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.     

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part II. calculation of flow curves

The role of metastable retained austenite(γ _R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of ‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived in Part I of this paper. It has been found that for a given volume fraction ofγ _R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ _R does not contribute either to strength or ductility of the steel, highly unstableγ _R which causes an increase in the strength is detrimental to ductility. Aγ _R which is moderately stable and undergoesγ _R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ _R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ _R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels.     

The influence of cobalt,tantalum, and tungsten on the elevated temperature mechanical properties of single crystal nickel-base superalloys

The influence of composition on the tensile and creep strength of [001] oriented nickel-base superalloy single crystals at temperatures near 1000 °C was investigated. Cobalt, tantalum, and tungsten concentrations were varied according to a matrix of compositions based on the single crystal version of MAR-M247.* For alloys with the baseline refractory metal level of 3 wt pct Ta and 10 wt pct W, decreases in Co level from 10 to 0 wt pct resulted in increased tensile and creep strength. Substitution of 2 wt pct W for 3 wt pct Ta resulted in decreased creep life at high stresses, but improved life at low stresses. Substitution of Ni for Ta caused large reductions in tensile strength and creep resistance, and corresponding increases in ductility. For these alloys with low Ta plus W totals, strength was independent of Co level. The effects of composition on properties were related to the microstructural features of the alloys. In general, high creep strength was associated with high levels ofγ′ volume fraction,γ-γ′ lattice mismatch, and solid solution hardening.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Environmental effect on room- temperature ductility of isothermally forged tial- base alloys

Isothermally forged TiAl-base alloys (Al-rich, Mn-containing, and Cr-containing TiAl) were heat-treated in various conditions, and equiaxed grain structures consisting of γ and α₂ or Β phases were obtained. The heat-treated alloys were tensile tested in vacuum and air at room temperature, and the environmental effect on tensile elongation was studied. The ductility of the alloys consisting of equiaxed γ grains and a large amount of α₂ grains was not largely affected by laboratory air, and a decrease in the amount of α₂ grains resulted in a large reduction of ductility in air. The Β phase in the Cr-containing alloy improved the ductility in vacuum, but it resulted in a large reduction of ductility in air. Formerly with Kougakuin University, Shinjyuku-ku, Tokyo, Formerly with National Research Institute for Metals, Meguro-ku, Tokyo,     

Influence of the temperature on the plastic deformation in TiAl

A new alloy, Ti-48.6Al-1.9Cr-1.9Nb-1B, with a near-equiaxed γ microstructure and with a lamellar microstructure is investigated by compression tests between 20 and 800 °C and transmission electron microscopy (TEM). The yield stress exhibits no anomaly as a function of the temperature, while an anomaly, related to strain hardening, is found at 400 °C in the hardening rate and the activation volume for both the lamellar and nonlamellar structure. Above 700 °C, a change in the deformation mechanism occurs and the material becomes remarkably softer. The TEM micrographs highlight the importance of ordinary dislocation motion for both structures at all temperatures. The comparison with previously reported TEM observations on single-phase TiAl alloys shows definitely that the density of ordinary dislocations is higher in the investigated two-phase TiAl alloy deformed at room temperature. Also, the presence of the lamellar interfaces drastically changes the mechanical properties of the alloy and the deformation mechanism. In contrast to the nonlamellar samples, superdislocations are rare, and twinning is very frequent in the lamellar structure.     

Deformation characteristics in Β phase Ti-Nb alloys

Deformation modes in Β-phase Ti-Nb alloys containing from 36 to 52 wt pct Nb have been investigated as a function of composition, crystallographic orientation, or deformation temperature using both two surface trace analysis and transmission electron microscopy. 332 (113) twinning occurs in the low niobium content alloys independent of orientation and temperature. With increasing niobium content, 332<113> twinning or (111) slip occurs dependent on orientation and temperature. The operative twinning systems are explained by considering both the polarization of twinning shear and the Schmid factor. The 332 (113) twinning is suppressed in the high niobium content alloys and also suppressed by aging or oxygen addition in the low niobium content alloy. These characteristics have previously been observed in Β phase Ti-V and Ti-Mo alloys. The occurrence of 332 (113) twinning in Β-phase Ti alloys is related to the instability of Β phase. Formerly Graduate Student of Tohoku University     

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     

Transmission electron microscopy investigation of < c + a > dislocations in Mg and α-solid solution Mg-Li alloys

The ductility of Mg alloys is limited due to a shortage of independent slip systems. In particular, c-axis compression cannot be accomodated by any of the easy slip or twinning modes. Basaltextured samples of pure Mg and Mg-15 at. pct Li were examined for the presence of <c+a> dislocations by post-mortem transmission electron microscopy (TEM) after a small deformation, which forced the majority of grains to compress nearly parallel to their c-axes. A higher density and more uniform distribution of <c+a> dislocations is found in the Li-containing alloy. Because the pyramidal slip mode coffers five independent slip systems, it provides a satisfying explanation for the enhanced ductility of α-solid solution Mg-Li alloys as compared to pure Mg. The issue of <c+a> dislocation dissocation and decomposition remains open from an experimental point of view. Theoretically, the most feasible configuration is a collinear dissociation into two 1/2 <c+a> partial dislocations, with an intervening stacking fault on the glide plane. It is speculated that Li additions may lower the fault’s energy and, thereby, increase the stability of this glissile configuration. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committe, Joint Nuclear Materials Committee, and Titanium Committee.     

Slip transfer and dislocation nucleation processes in multiphase ordered Ni-Fe-Al alloys

Directionally solidified (DS) alloys with the nominal composition Ni-30 at. pct Fe-20 at. pct Al having eutectic microstructures were used to study slip transfer across interphase boundaries and dislocation nucleation at the interfacial steps. The slip transfer from the ductile second phase, γ(fcc) containing ordered γ′(L1₂) precipitates, to the ordered β(B2) phase and the generation of dislocations at the interface steps were interpreted using the mechanisms proposed for similar processes involving grain boundaries in polycrystalline single-phase materials. The criteria for predicting the slip systems activated as a result of slip transfer across grain boundaries were found to be applicable for interphase boundaries in the multiphase ordered Ni-Fe-Al alloys. The potential of tailoring the microstructures and interfaces to promote slip transfer and thereby enhance the intrinsic ductility of dislocation-density-limited intermetallic alloys is discussed.     

Microstructure evolution through the α→γ phase transformation in a Ti-48 At. pct Al alloy

The α→γ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α ₂/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α ₂/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α ₂ → γ phase transformation in the fine α ₂/γ lamellae. 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 Transformation Committees.     

Microstructural stability of fine-grained fully lamellar XD TiAl alloys by step aging

XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8 vol pct TiB₂) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of α ₂ to γ, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in α ₂ volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the α ₂ lamellae become finer due to dissolution, whereas most of the γ lamellae coarsen. The dissolution of α ₂ involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of γ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in α ₂/γ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.     

Development of ultrafine lamellar structures in two-phase γ-TiAl alloys

Processing of two-phase γ-TiAl alloys (Ti-47Al-2Cr-2Nb, or minor modifications thereof) above the α-transus temperature (T _α ) produced unique refined-colony/ultrafine lamellar structures in both powder-and ingot-metallurgy (PM and IM, respectively) alloys. These ultrafine lamellar structures consist of fine laths of the γ and α ₂ phases, with average interlamellar spacings (λ _L ) of 100 to 200 nm and α ₂-α ₂ spacings (λ _α ) of 200 to 500 nm, and are dominated by γ/α ₂ interfaces. This characteristic microstructure forms by extruding PM Ti-47Al-2Cr-2Nb alloys at 1400 °C and also forms with finer colony size but slightly coarser, fully lamellar structures by hot-extruding similar IM alloys. Alloying additions of B and W refine λ _L and λ _α in both IM Ti-47Al (cast and heat treated at 1400 °C) and IM Ti-47Al-2Cr-2Nb alloys (extruded at 1400 °C). The ultrafine lamellar structure in the PM alloy remains stable during heat treatment at 900 °C for 2 hours but becomes unstable after 4 hours at 982 °C; the ultrafine lamellar structure remains relatively stable after aging for >5000 hours at 800 °C. Additions of B+W dramatically improve the coarsening resistance of λ _L and λ _α in the IM Ti-47Al alloys aged for 168 hours at 1000 °C. In both the PM and IM Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine lamellar structures correlate with high strength and good ductility at room temperature, and very good strength at high temperatures. While refining the colony size improves the room-temperature ductility, alloys with finer λ _L are stronger at both room and high temperatures. Additions of B + W produce finer as-processed λ _L and λ _α in IM TiAl alloys and stabilize such structures during heat treatment or aging. 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.     

Room-temperature deformation behavior of directionally solidified multiphase Ni−Fe−Al alloys

Directionally solidified (DS) β+(γ+γ′) Ni−Fe−Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar+proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q) of 30.4 MPa were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer. A. MISRA, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan is Research Associate