J.L. Wright

Tensile properties and fracture toughness of TiAl alloys with controlled microstructures

Abstract The objective of this study is to improve the mechanical properties by careful control of both microstructure and alloy additions in two-phase TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temperatures above Tα produces refined lamellar structures, whose microstructural features can be further controlled by subsequent heat-treatment at and above 900 °C. The mechanical properties of the alloys with lamellar structures depend on three factors: colony size, interlamellar spacing, and alloying additions. The tensile elongation at room temperature is strongly dependent on lamellar colony size, showing increasing ductility with decreased colony size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing increasing strength with decreased colony spacing. The fracture toughness at room temperature can be substantially improved by heat-treatment at 1320 and 1350 °C. The tungsten addition at a level of 0.2% improves the tensile strength, whereas the silicon addition at a level of 0.3% reduces the castability of the TiAl alloys. The TiAl materials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments.

Localized Corrosion Behavior of a Zirconium-Based Bulk Metallic Glass Relative to Its Crystalline State

Abstract To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na 2 SO 4 solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na 2 SO 4 solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.

Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass

Abstract In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional high-strength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum.

Effects of B and W alloying additions on the formation and stability of lamellar structures in two-phase γ-TiAl

Abstract The effects of additions of 140 appm B and 0.5 at.% W on the formation and stability of fully-lamellar structures in a Ti-47Al base alloy were studied. The fully-lamellar structure was formed by heat-treatment at 1400°C for 1 h (in vacuum) followed by furnace cooling. Stability was studied by aging these alloys for 168 h at 800, 1000 and 1200°C and making quantitative measurements of lamellar structure parameters (i.e. average lamellar spacing, α2-α2 spacing) using transmission electron microscopy (TEM) and hardness measurements. The B addition alone refines the lamellar structure relative to the binary TiAl alloy, but causes fragmented and discontinuous α2 lamellae. By contrast, W+B addition refines the lamellar structure and produces more uniform and continuous α2 lamellae. The as-heat-treated Ti-47Al+W+B alloy is harder than the other two alloys. Aging for 168 h at 800°C does not change the fine initial lamellar microstructure of any of these alloys. Aging at 1000°C significantly coarsens the Ti-47Al alloy, causes some coarsening in the Ti-47Al+B alloy, but has little effect on the lamellar structure of the Ti-47Al+W+B alloy. Aging at 1200°C causes discontinuous coarsening and complete loss of the initial, fine, lamellar microstructure in all three alloys. Aging in vacuum produces a subsurface ‘damage region’ consisting of an α(Ti3Al) surface layer with another layer of coarsened γ microstructure underneath. At 1000°C, this layer is greatest in the binary alloy and least in the Ti-47Al+W+B alloy. Specimens were 5.5% cold-forged and then aged at 1000°C to evaluate the effects of subsurface deformation caused during cutting of the specimens. Prior cold-deformation caused considerable recrystallization in the binary alloy, but almost none in the Ti-47Al+W+B alloy. The additions of W and B to a Ti-47Al alloy refine the fully-lamellar structure obtained by heat-treatment, and make it more resistant to lamellar coarsening (and recrystallization if cold deformed) during aging at 800–1000°C.

High strength, ductility, and impact toughness at room temperature in hot-extruded FeAl alloys

Abstract Ingots and powders of FeAl alloys (Fe-36Al-0.5Mo-0.05Zr-0.13C (at%), base composition) were hot extruded at 900–1100 °C to produce fine or ultrafine grained materials. Ingot material ( I M ) FeAl alloys with (100 or 210appm) and without added boron extruded at 900 °C had fully recrystallized, fine-grained (35–50 μm) microstructures and 8–10% tensile ductility in air. Boron microalloying also changed the fracture mode from intergranular to a more ductile transgranular quasicleavage mode. Charpy impact toughness tests measured an absorbed energy of 25 J in the boron-free I M FeAl, and a much higher impact energy of 63 J in the boron-doped I M FeAl alloy. Intragranular microstructures of all these I M FeAl alloys consisted of dislocation networks with some dislocation loops and no fine precipitates. Powder material ( P M ) FeAl alloys were directly consolidated by hot extrusion at temperatures of 950, 1000, and 1100 °C. This processing produced complex microstructures of deformed remnant oxide ‘envelopes’ (950, 1000 °C) or stringers (1100 °C) structures related to the prior particle interfaces, and ultrafine recrystallized grain structures within those particles. P M FeAl alloys extruded at 950 °C and 1000 °C had an ultrafine 2–4 μm grain size, 12.5–15% elongation in air (21–29% in oxygen), and ductile-dimple fracture within the prior powder particles. Despite their ductility, these P M FeAl alloys were also much stronger than I M FeAl alloys (yield strength (YS) of 600–670 MPa compared to 410–435 Mpa, respectively). These P M FeAl alloys also had high Charpy impact energies of 85–105 J. By contrast, P M FeAl extruded at 1100 °C had slightly coarser grain size (9–13 μm), had 9% elongation in air (11 % in oxygen), had a more brittle mixture of intergranular fracture and transgranular cleavage, was weaker (YS of 500–520 MPa), and had lower impact energy (25J). The ultrafine-grained P M FeAl alloys also have subtle differences in intragranular precipitate (ZrC) and dislocation (loops) components of the microstructure which may play a role in their excellent mechanical properties.

Processing and alloying effects on tensile and impact properties of FeAl alloys

Abstract Two series of iron aluminide alloys [Fe-36–38Al (at.%)] were produced by ingot metallurgy or powder metallurgy methods, to examine the effects of processing and of slight changes in chemical composition on the microstructure and mechanical properties. For the first series of alloys, the powder metallurgy materials developed a very fine grain size when extruded at 950 or 1000°C, much finer than the initial powder particle size. Extrusion at 1100°C resulted in a coarser grain size. The fine-grained powder alloys showed excellent strength and ductility, and high levels of energy absorption in impact tests. The high levels of energy absorption were caused by extensive crack deflection along the remnant oxide layers from the prior particle boundaries; these boundaries had been elongated and oriented perpendicular to the notch by the extrusion process. The coarse-grained material, which had larger, more isolated oxide particles, had slightly lower ductility, but had much poorer impact properties, with no crack deflection. Ingot metallurgy alloys of similar composition, extruded at 900°C, had lower strength and ductility than the powder materials. The addition of boron (0.021 at.%) resulted in a change of fracture mode in the impact tests from intergranular to transgranular, which significantly increased the energy absorption. A second series of cast FeAl alloys examined the effect of changes in the levels of Zr, C, and B, as compared to the first series of alloys. Increases in the Zr and C resulted in increases in the strength and slight decreases in the ductility. Numerous flaws, apparently created during machining of the cast material, were observed on the surfaces of the impact specimens, which still showed some energy absorption when tested. Different fracture modes, including cleavage, intergranular, and ductile tearing, were observed on the fracture surfaces of various impact specimens.

Microstructures and properties of a hot-extruded TiAl containing no Cr

Abstract Microstructures and tensile properties of the TiAl alloy CTI-8, with the composition of Ti–47Al–(2–4)Nb–(0.5–1.0)W–(0.08–0.18)B (at.%) were studied by hot extrusion at various temperatures below and above T α (=1315xa0°C). Both extrusion temperature and boron concentration are important in controlling lamellar structures in the alloy. Uniform grain structures were formed in the alloy with >0.1% B, whereas abnormal grain growth was observed in the alloy containing less B and hot extruded at temperatures above 1350xa0°C. Electron microprobe studies revealed a non-uniform distribution of W and Nb in the extruded alloy, presumably due to the initiation of phase transformations involving the redistribution of these alloying elements in the α and γ phases during cooling from the extrusion temperatures. Tensile properties at room and elevated temperatures are sensitive to microstructure and boron addition. The post-extrusion heat treatment at 1315xa0°C moderately lowers the yield strength but substantially increases the tensile ductility of CTI-8. Grain size is the key parameter in controlling the room-temperature ductility of TiAl alloys with and without Cr additions. Boron does not affect the yield strength but lowers the ductility of CTI-8 containing >0.10% B. In comparison, hot-extruded CTI-8 is substantially stronger and much more ductile than other advanced TiAl alloys with lamellar and duplex structures.

Low temperature embrittlement of Ni3(Si, Ti) alloys

Abstract Three ternary L12Ni3(Si,Ti) alloys were tensile tested in various environments at room temperature to evaluate the effect of stoichiometry on the severity of hydrogen embrittlement. In addition, three quaternary alloys, each containing 1 at% Cr were also similarly tested to verify previously reported beneficial effects of the Cr addition. The embrittlement sensitivity to hydrogen partial pressure was studied. The results of this study showed that the severity of embrittlement is strongly dependent on alloy stoichiometry, that Cr addition does not provide any direct benefit, that embrittlement is a function of the hydrogen partial pressure in the environment, and that molecular hydrogen dissociation is more critical than water vapor dissociation. Based on the data, it is argued that hydrogen ‘wetting’ of the crack tip and hydrogen penetration play a role in embrittling the alloys, the relative importance being stoichiometry-dependent.

Key microstructures controlling the mechanical properties of two-phase TiAl alloys with lamellar structures

TiAl alloys with the base composition of Ti-47Al-2Cr-2Nb (at.%) were prepared by arc melting and drop casting, followed by hot extrusion above the {alpha}-transus temperature, T{sub {alpha}}. The hot extruded materials were then heat treated above and below T{sub {alpha}} in order to control microstructural features in these lamellar structures. Mechanical properties of these alloys were determined by tensile testing at temperatures to 1000 C. Tensile elongation at room temperature (RT) is strongly dependent on grain size, showing increased ductility with decreasing grain size. Strength at RT and elevated temperatures is sensitive to interlamellar spacing, showing increased strength with decreasing lamellar spacing. Hall-Petch relations hold well for yield strength at RT and elevated temperatures and for tensile elongation at RT. Tensile elongations of about 5% and yield strengths around 900 MPa are achieved by controlling both colony size and interlamellar spacing. Mechanical properties of the TiAl alloys with controlled lamellar structures produced directly by hot extrusion are much superior to those produced by conventional thermomechanical treatments.

Effect of Zr and C Additions on Environmental Embrittlement of Fe-28Al-5Cr Aluminide

The objective of this study is to try to understand the beneficial effect of 0.5Zr + 0.05C on environmental embrittlement in the Fe-28Al-5Cr alloy at room temperature. The microstructure of the alloys with and without 0.5Zr + 0.05C was controlled by heat treatment, and their tensile properties were determined in different test environments. Attempts were made to separate the beneficial effect of alloying elements themselves with their associated change in the microstructures of the aluminide alloys.