Mixed-mode, high-cycle fatigue-crack-growth thresholds in Ti-6Al-4V: Role of bimodal and lamellar microstructures

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti-6Al-4V turbine blade alloy in both lamellar and bimodal microstructural conditions. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles (β=tan ⁻¹ (ΔK _II/ΔK _I) from 0 to 62 deg (ΔK _II/ΔK _I∼0 to 1.9), is examined at a load ratio (ratio of minimum to maximum loads) of R = 0.1 and a cyclic loading frequency of 1000 Hz in ambient-temperature air. When the mixed-mode, crack-driving force is characterized in terms of the strain-energy release rate (ΔG), incorporating contributions from both the applied tensile and shear loading, the threshold for fatigue-crack growth is observed to increase significantly with the applied mode-mixity (ΔK _II/ΔK _I) for both microstructures, an effect attributed to enhanced crack-tip shielding. The pure mode I threshold, in terms of ΔG _TH, is observed to be a lower bound (worst case) with respect to mixed-mode (I + II) behavior. For large crack sizes, the threshold fatigue-crack growth resistance of the lamellar structure is observed to be superior to that of the bimodal material for all phase angles investigated. Consideration of mode I fatigue-crack growth thresholds for small cracks in these same microstructures suggests that this rank ordering of mixed-mode fatigue resistance may not hold for crack lengths that are comparable to microstructural size scales. Examination of the fatigue-crack wake indicates that, for the lamellar microstructure, the path of crack extension is significantly influenced by the local microstructure over length scales on the order of the relatively coarse lamellar colonies (∼500 μm). Comparatively, the crack path in the bimodal material is more strongly influenced by the applied crack-driving force. This disparity in behavior is attributed primarily to the relatively heterogeneous crack-growth resistance of the coarse lamellar microstructure.     

Variability of light perception thresholds in brain-injured children.

"Magnitude and intra-individual variability of absolute light perception and apparent movement thresholds of 10 children with pyramidal tract damage were compared with the threshold and variability of 10 non-brain injured handicapped children of comparable age, IQ, and sex distribution. The results show that the brain-injured Ss displayed significantly higher thresholds than the control Ss, thus supporting the hypothesis that visual efficiency may be lowered demonstrably in brain-injured Ss with presumably nonoccipital lesions." (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

The low-cycle fatigue and fatigue-crack-growth behavior of HAYNES® HR-120 alloy

The low-cycle fatigue and fatigue-crack-growth behavior of the HAYNES HR-120 alloy was investigated over the temperature range of 24°C to 980°C in laboratory air. The result showed that increasing the temperature usually led to a substantial decrease in the low-cycle fatigue life. The reduction of fatigue life could be attributed to oxidation and dynamic strain-aging (DSA) processes. The strain vs fatigue-life data obtained at different temperatures were analyzed. It was also found that the fatigue-crack-growth rate per cycle generally increased with increasing temperature and R ratio (R=σ _min/σ _max, where σ _min and σ _max are the applied minimum and maximum stresses, respectively). The relationship between the stress-intensity-factor range and fatigue-crack-growth rate was determined. Scanning-electron-microscopy (SEM) examinations of the fracture surfaces revealed that the fatigue cracks initiated and propagated predominantly in a transgranular mode.     

Atomic ordering and structural transformation in the V-Co-Ni ternary alloys

The transformation behavior of the close-packed ordered nine-layered hexagonal structure (κ phase) existing in the V-Co-Ni ternary alloys was investigated by various metallurgical methods. Both neutron and X-ray diffraction indicate that atomic ordering and structural transformation take place separately. The isothermal aging at subcritical temperatures resulted first in the formation of cubic long-range-ordered α' from the fcc α matrix and subsequently transformation of the α' to hexagonal κ. The α' to κ transformation proceeds by nucleation and growth; however, both mode and morphology of transition vary with reaction temperature. These observation have been discussed in terms of the temperature dependence of nucleation and growth processes. The structural transformation produces massive substructures, which harden the alloy substantially.     

Some structural characteristics of the binder phase of W-Ni-Fe alloys

Conclusions The structure of the binder phase of a W-Ni-Fe alloy depends on the conditions of heat treatment after liquid-phase sintering. The binder of such an alloy consists of nickel-base solid solution grains (colonies) which, depending on the conditions of cooling and annealing, may differ in size by as much as several orders and thus impoverish from a few to several hundreds of grains of the refractory phase. The fcc lattice parameter of the binder phase varies as a result of the temperature dependence of the solubility of tungsten and of a redistribution of tungsten between the main and boundary volumes of grains of the principal phase constituents.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 45–50, December, 1980.     

Computation of thermodynamic properties of liquid ferrous alloys from structural measurements

Relation between X-ray scattering intensities, mean square thermal fluctuations and thermodynamic properties. High temperature X-ray diffraction study of liquid Fe-Ni and Fe-Si alloys using reflection and transmission geometries. Calculation of the structure factor as a function of wave vector. Extrapolation to zero wave vector. Calculation of the concentration-concentration correlation function defined by A. B. Bhatia and D. E. Thorton. Computation of thermodynamic quantities of mixing ΔG, ΔH and ΔS for the binary alloys. Comparison with direct thermodynamic measurements reported in the literature.     

Hydrogen-induced phase and structural transformations in titanium alloys with β-eutectoid stabilizers

The mechanism of hydrogen-induced structure formation in titanium alloys with β-eutectoid stabilizers is analyzed. Additional hydrogen alloying of Ti-Cr alloys is shown to increase the stability of the β phase with respect to the α phase and to decrease its stability with respect to the intermetallic compound TiCr₂. Hydrogen dissolved in a commercial multicomponent TS6 alloy containing 11.3% Cr is found to favor the precipitation of an intermetallic compound Ti_xCr_y, which also contains other alloying elements present in the alloy. The mechanism and kinetics of the formation of this intermetallic compound during hydrogenation annealing and isothermal heat treatment are considered. Structures with the intermetallic compound Ti_xCr_y can be produced upon low-temperature vacuum annealing of a hydrogen-charged TS6 alloy.     

Electric-spark alloying of structural alloys with a composite based on TiCN-AIN

The mass-transfer kinetics, the mechanism of formation, the tribotechnical characteristics, and the resistance to high-temperature oxidation of a coating formed on the hard alloy WC — 6% Co and on the titanium alloy VT6 by electric-spark alloying with electrode material based on TiCN — AIN with an Fe — Cr binder have been investigated. The phase distribution of the components in the coating was shown to be the same for both alloys. Electric-spark alloying of WC — Co and VT6 was found to reduce their wear by 33 and 60%, respectively. Moreover, the working temperature of the coated WC — Co alloy increased by 160 deg compared to the original surface. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 21–29, May–June, 2000.     

Microstructure and properties of high strength aluminium alloys for structural applications

This article discusses the fundamental basis of high strength Al alloy design and describes the role of alloying elements, mechanical processing parameters and heat treatments toward the evolution of microstructure that controls the desired properties i.e. strength, fracture toughness, stress corrosion cracking (SCC) resistance, fatigue crack initiation and propagation resistance, and weldability in 7xxx series Al alloys. The beneficial effects of suitable micro/trace alloying elements, and deleterious effects of certain impurity elements on a variety of properties are further discussed within the present context.     

Modeling of structural and compositional homogenization of plutonium-1 weight percent gallium alloys

The microstructure of as-cast Pu-1 wt pct Ga alloys is characterized by extensive Ga microsegregation often referred to as “coring.” This process results in grains that consist of Ga-rich cores (∼1.6 wt pct) with Ga-poor (∼0.1 wt pct) edges. Cored grains can be homogenized at moderately high (i.e., >400 °C) temperatures, though the time required to achieve chemical homogeneity is not well constrained. In this article, we apply several analytical diffusion modeling techniques to characterize the kinetics of alloy homogenization as a function of time and temperature. We also review the experimental investigations that have used analytical tools such as X-ray diffraction, density, dilatometry, and electron microprobe analysis to characterize Pu-Ga alloy homogenization. Data from these studies are used as a basis of comparison with modeling results. In particular, Ga coring-profile modeling appears to be a powerful tool for predicting alloy homogenization.