ANNOTATION Internal damage accumulation of Ti-6Al-4 V specimens subjected to controlled levels of low cycle fatigue (LCF) and high cycle fatigue (HCF) is being studied. The material was processed into a forged plate with a duplex microstructure representing engine components. The purpose of the controlled-damage samples is to create calibration standards for damage characterization and quantification for the novel nondestructive technique of nonlinear acoustics (NLA). Nonlinear acoustics provides real-time, in-situ monitoring of internal damage during mechanical· testing. The knowledge gained from transmission electron microscopy (TEM) is being used to develop a new characterization method for fatigue damage accumulation by the NLA technique in the crack precursor stage, which is prior to c~ack nucleation. The desired outcome is a methodology to predict the remaining life in fatigue loaded samples using the nondestructive, nonlinear acoustic technology.

In the present study titanium alloys were synthesized by the blended elemental press-andsinter powder metallurgy approach using hydrogenated titanium powder. Experimental investigation and modeling of the homogenization processes during synthesis were used to analyze peculiarities of mass transfer and factors affecting diffusion. Processes of alloying elements redistribution during chemical homogenization of powder blends are shown to be strongly dependent on the chemical composition of the initial powders. Optimization of the processing parameters allows to synthesize uniform, nearly-dense material with reduced grain size, at relatively low temperatures and short time. This will provide improvement of mechanical properties simultaneously with better cost-effectiveness of the process.

In an effort to produce an ultra-fine alpha titanium equiaxed grain structure, suitable for superplastic deformation processes, Armstrong-Process CP Ti powder, was consolidated into compacts with grain-size on the order of 2 to 3 microns. This powder has very fine dendritic-shaped particles with an inherent sub-micron grain-structure. In order to preserve as much as possible the fine powder microstructural scale, the compaction was accomplished by rapid-heating and short-hold VHP, using a procedure derived from a processing technique originally developed at the University of Dayton for producing nano-phase hard permanent magnets. It was modified to suit the titanium powder, and a range of parameters was experimented to produce a variation of microstructures. One set of compaction conditions resulted in the desired microstructure, and subsequent tensile testing demonstrated strength and ductility exceeding CP Ti Grade 3, due to the ultra-fine equiaxed alpha grain structure. The paper will discuss the various microstructures and the potential applications.

Car weight can be reduced more by utilizing commercially pure titanium(CPTi) and titanium(Ti) alloys. This provides a good opportunity for titanium industries to develop a more efficient mass production process,i.e.,continuous casting and hot direct rolling process(CC-HDR). This process enables mass production having homogeneous quality with low price as was realized in the steel industries. In this paper, we discuss the possibility of the introduction of CC-HDR process, based on the studies of hot workability and thermomechanical treatment. The characteristics of strength and ductility at elevated temperature were examined for most of CP Ti as well as Ti alloys. The Gleeble machine was quite useful tool to characterize the hot workability of these titanium alloys as has been found previously for steels. Embrittlement in Ti alloys exists in the temperature range between Tβ and Tβ-200K in α, near α,and α+β alloys, which is related to the transformation from β phase into α phase and the difference of the strength between two phases. This embrittlement can be suppressed by getting fine grain size and rapid cooling from β region, but it is intrinsic in nature. On the other hand, hot ductility in the CPTi, near βand βalloys shows more than 60% in reduction of area for a wide range of temperature ranging from T m to 700K although the strength becomes higher below 1200K compared with carbon steels. Thus, it appears possible to apply continuous casting such as a strip casting or straight type cast strand CC and HDR in the temperature range above 1200K to get sound products without any cracking problems. © Institute of Materials Engineering Australasia Ltd.

The effects of BaF2-CaF2 particle morphology on National Aeronautics and Space Administration (NASA) PS304 feedstock powder flowability were investigated, BaF2-CaF2 eutectic powders were fabricated by comminution (producing an angular morphology) and by gas atomization (producing a spherical morphology). The fluoride powders were added incrementally to the other powder constituents of the NASA PS304 feedstock, (Ni-Cr, Cr2O3, and Ag powders). A linear relationship between flow time and concentration of the BaF2-CaF2 powder was found. The flow of the powder blend with spherical BaF2-CaF2 was better than that with angular BaF2-CaF2. The flowability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. However, the flow of the powder blend with spherical fluorides was independent of fluoride concentration. The results suggest that for this material blend, particle morphology plays a significant role in flow behavior, offering potential methods to improve powder flowability and enhance the commercial potential. These findings may be applicable to other difficult-to-flow powders such as cohesive ceramics.

The extraordinary mechanical properties of high strength aluminum alloys such as AA7075-T6 are caused by coherent nanoprecipitations. These nanoprecipitations generate local stress fields and interact with moving dislocations and propagating microcracks. In this paper, image correlation techniques are used to determine the local strain and stress field in the vicinity of fatigue crack tips during the loading of compact tension (CT) specimen. The fatigue crack tip was sharpened with decreasing fatigue loading after fatigue cracks initial appearance. Images of the crack tip were taken using atomic force microscopy/ultrasonic force microscopy (AFM/UFM) and white light interference microscopy (WLIM) before and after mechanical loading of the specimen. Both techniques are applicable for measuring the out-of-plane displacement during the loading process. In addition, image correlation techniques can be used to determine the in-plane displacement resulting from mechanical loading. This information is used to calculate the local stress intensity factor in the vicinity of the crack tips.

Maintenance and reliability of aircraft is a major safety concern and economical factor. Many civilian and military aircraft have been in service for 35 years or more. Aircrafts that were originally designed for a service life of 20 years are currently considered for life extensions of up to 80 years. The cost of corrosion and fatigue related maintenance on these aging structures has increased dramatically. A study conducted in 1998 showed that the direct costs of corrosion maintenance to the United States Air Force were $775 million/year [1] . These costs continue to rise in spite of Air Force structure changes resulting in a 20% reduction in the overall fleet. In an effort to reduce these maintenance costs, several programs have been initiated for the development of methods to manage and control corrosion and fatigue damage in aging aircraft.

Due to its excellent combination of a high strength/weight ratio and good corrosion behavior, Ti–6Al–4V alloys are ranked among the most important advanced materials for a variety of aerospace, chemical engineering, biomaterials, marine and commercial applications. However, in many of these technological applications, this alloy is exposed to environments which can act as sources of hydrogen, and severe problems may arise based on its susceptibility to hydrogen embrittlement. Even small hydrogen concentrations might lead to failure. Consequently, a comprehensive knowledge of hydrogen–trapping interactions is necessary to better understand the trapping mechanisms, the types of the trap sites, the trapped hydrogen content, in order to determine the safe service conditions of this alloy in the aerospace industry. The objective of this paper is to investigate the role of microstructure on hydrogen absorption/desorption behavior in Ti–6Al–4V alloy, with specific emphasis on the nature of the interaction between microstructural traps and hydrogen atoms. The effect of low fugacity hydrogen on the microstructure is studied using X-ray diffraction (XRD), and electron microscopy (SEM and TEM), while the absorption and desorption characteristics are determined by means of a hydrogen determinator and thermal desorption spectroscopy (TDS), respectively. The role of microstructure on hydrogen absorption and desorption behavior is discussed in detail.

7075 T651 aluminum alloy is frequently used in aircraft applications for its high strength to weight ratio. However, aircraft parts made of this alloy have been plagued by stress corrosion cracking (SCC). Retrogression and re-aging (RRA) is a post T651 two-stage heat treatment that provides improved SCC resistance with minimal loss in tensile strength. In this study, various forms of microscopy and mechanical testing are used to investigate how the RRA process affects the microstructure. The microscopic observations in this paper show that the precipitates in the aluminum alloy coarsen and that the grain boundary regions are depleted of copper and magnesium. The mechanical testing performed shows that the aluminum alloy decreases in strength and increases in conductivity when exposed to longer retrogression times prior to re-aging.

The fretting fatigue tests of Ti-6Al-4V were, carried out using Cu-Ni plasma coated and as-received pads (un-coated pads). Fretting fatigue tests of Ti-6Al-4V, where Cu-Ni plasma coated and as-received pads were used, repeatedly were also carried out. The characterization of pads before and after testing were carried out in order to investigate the mechanism of fretting fatigue of Ti-6Al-4V in contact with the Cu-Ni plasma coated pad used. Cu-Ni plasma coating on the pads prolonged the fretting fatigue life of Ti-6Al-4V. The fretting fatigue life of Ti-6Al-4V decreased with repeated use of test when the Cu-Ni plasma coated pads. On the other hand, the opposite trend was observed when the as-received pads were repeatedly used. The surfaces of Cu-Ni plasma coated pads were smoother after testing than before testing due to the plastic deformation. On the other hand, the surface of as-received pads were rougher after testing than before testing.

In order to improve the observed low fracture toughness of β -rich α + β -type Ti-4.5Al-3V-2Mo-2Fe annealed at the temperature of 1123 K, a two-step cooling (TSC) after solution treatment at that temperature was proposed instead of air cooling. Solution treatment plus aging (STA) and slow furnace-cooling (SFC) treatments were also carried out on the same alloy for comparison. It was found that the relatively higher fracture toughness ( J IC ) is obtained by the TSC treatment, that is, by slow cooling the alloy at a cooling rate of 0.075 Ks −1 from a temperature of 1123 to 723 K and, subsequently, water quenching to room temperature. The J IC of the two-step-cooled alloy has the same value as that of the alloy annealed at 993 K after having first been annealed at 1123 K (duplex annealing (DA)), which has been previously observed to have high fracture toughness. The J IC value can also be improved by STA, but it is still lower than that of two-step-cooled alloy. The relatively lower J IC is obtained in slow-furnace-cooled alloys. The J IC of slow-furnace-cooled alloys decreases monotonously with decreasing cooling rate for cooling rates less than 0.075 Ks −1 . It is suggested that the factors responsible for increasing J IC> in two-step-cooled and solution treated-and-aged alloys are the plate-like α and secondary α , respectively, which appears in the β phase during cooling and aging, respectively. The presence of such kinds of transformation products increases the crackdeflection effect and, thus, increases fracture toughness.

The effects of BaF2-CaF2 particle size and size distribution on PS304 feedstock powder flowability have been investigated. Angular BaF2-CaF2 eutectic powders were produced by comminution and classified by screening to obtain 38 to 45 mum, 45 to 106 mum, 63 to 106 mum, 45 to 53 mum, 63 to 75 mum, and 90 to 106 gm particle size distributions. The fluorides were added incrementally from 0 to 10 wt% to the other powder constituents of the PS304 feedstock: nichrome, chromia, and silver powders. The flow rate of the powder blends decreased linearly with increasing concentration of the fluorides. Flow was degraded with decreasing BaF2-CaF2 particle size and with increasing BaF2-CaF2 particle size distribution. A semi-empirical relationship is offered to describe the PS304 powder blend flow behavior. The Hausner Ratio confirmed the funnel flow test results, but was slightly less sensitive to differences in BaF2-CaF2 particle size and size distribution. These findings may have applicability to other powders that do not flow easily, such as ceramic powders.

The stability of the beta phase at room temperature in various microstructures of a beta-rich alpha+beta type Ti-4.5Al-3V-2Mo-2Fe alloy and its relationship with the fracture toughness, hardness and tensile properties were investigated. A variety of microstructures were established by varying solution treatment temperatures in alpha+beta field, cooling rate after solution treatment and the condition of subsequent second-step annealing treatment after air-cooling treatment. These microstructures have beta phase with lattice parameters of beta phase ranging between 0.3244 nm and 0.3221 nm. The stability of beta phase, which is indicated by decreasing lattice parameter of beta phase, is increased by either lowering cooling rate or formation of diffusional transformation products (secondary phases) in the beta phase. The beta phase with lattice parameter of beta phase around 0.3242 nm is the minimal instability of unstable beta phase at room temperature for attaining deformation-induced martensite in tensile specimens. There exists a proper degree of beta phase stability for increasing the fracture toughness, J(IC). The relatively higher fracture toughness is obtained at low or high stability of beta phase. The high fracture toughness at low stability of beta phase (unstable beta) is mainly due to the deformation-induced martensite. While, the high fracture toughness at high stability of beta phase (stable beta) is mainly due to the secondary phase in the phase that produces a prominent crack deflection toughening mechanism. However, the relatively lower fracture toughness is obtained at high stability of beta phase when the beta phase contains small amount or no secondary phase, This leads to conclude that, if only the phase stability is taken into account for explaining fracture mechanism, the fracture toughness would decrease monotonously with increasing stability of beta phase. The Vickers hardness is nearly independent of stability of beta phase.

It is well known that an unstable phase such as retained austenite in ferrous alloys, and retained b in titanium alloys can transform into martensite during deformation. This transformation process, which is commonly called as transformation induced-plasticity, TRIP, or deformation-induced transformation, results in the well-known tensile properties, i.e. low yield stress, excellent ductility and high work hardening rate. This phenomenon attracts a special interest due to certain reasons; such as the ability of the TRIP to control the behavior of shape-memory alloys. It is also expected to suppress initiation and propagation of cracks by relaxation of stress concentration due to the phase transformation. For titanium alloys, the occurrence of deformation-induced martensite, DIM, strongly depends strongly on the b phase stability of the alloys. The DIM can be observed in many less stable b alloys such as a1b type Ti–6Al–2Sn–4Zr–6Mo (Ti-6246) and metastable b type Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe (b-Cez) alloys. The main factor for the occurrence of DIM in the alloys is obviously the low stability of b phase. The unstable b phase resulting from a rapid cooling from the b field transforms easily to hexagonal a9 or orthorhombic a0 during loading (straining). While, the unstable b phase resulting from a rapid cooling from the a1b field is difficult to produce DIM because of the b phase is stabilized by the presence of primary a phase and the decrease of prior b grain size as pointed out recently by Grossdidier et al. They have explained that the presence of primary a phase tends to hinder the formation of martensite by increasing the stability of b phase and thus its martensite start temperature (Ms) is decreased. While, the decrease of prior b grain size also tends to stabilize b phase by making it more difficult to accommodate the shape change associated with the transformation. This indicates that the stability of b phase depends on the microstructural parameters of both a and b phases ISIJ International, Vol. 42 (2002), No. 2, pp. 191–199

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 107 cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ~0.3 for the Cu–Ni coated pad against an as-received specimen and ~0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.

The ingestion of debris into jet engines creates nicks and dents on the leading edges of blades and vanes. This is commonly known as foreign object damage (FOD). Such damage, which can often result in premature failure, was simulated in the laboratory using diamond cross-section axial fatigue samples that were impacted with 1 mm diameter glass beads at 305 m s−1 at either 0 or 30° angle of incidence. The samples had either a thin leading edge (LE) with a radius of 0.127 mm or a thick LE with a radius of 0.381 mm. Fatigue strength of impacted specimens showed degradation of 10–50% due to LE damage, regardless of the depth of the damage zone. FOD related impact notch depth, loss of material (LOM), shear, folds, embedded shattered glass, and microstructural damage were characterized by SEM. Fatigue strength degradation was found to be higher for the 30° impacts than for the 0° impacts. No clear correlation between notch depth or LE thickness and fatigue strength was found.

The effects of BaF2-CaF 2 particle morphology on PS304 feedstock powder flow ability have been investigated. BaF2-CaF2 eutectic powders were fabricated by comminution (angular) and by gas atomization (spherical). The fluoride powders were added incrementally to the other powder constituents of the PS304 feedstock: nichrome, chromia, and silver powders. A linear relationship between flow time and concentration of BaF2-CaF2 powder was found. Flow of the powder blend with spherical BaF2-CaF2 was better than the angular BaF2-CaF2. Flow ability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. Flow of the powder blend with spherical fluorides was independent of fluoride concentration. Results suggest that for this material blend, particle morphology plays a significant role in powder blend flow behavior, offering potential methods to improve powder flow ability and enhance the commercial potential. These findings may have applicability to other difficult-to-flow powders such as cohesive ceramics.