P. Bowen

Thermal exposure induced α2+γ→B2(ω) and α2→B2(ω) phase transformations in a high Nb fully lamellar TiAl alloy

As-HIPped Ti–44A1–8Nb–1B was exposed to 700 °C air for up to 3000 h. It has been found that the β phase removed by hot isostatic pressing (HIPping) reforms as ordered B2 with ω phase during parallel decomposition of α2 lamellae to α2+γ lamellae and perpendicular decomposition of single α2 laths into α2 short sections, causing phase transformations of α2+γ→B2(ω) and α2→B2(ω).

Fatigue performance of a cast aluminium alloy Al-7Si-Mg with surface defects

Fatigue performance has been studied in a cast Al-7Si-Mg alloy with regard to various casting defects, particularly surface or subsurface defects. Fatigue tests were carried out under four-point bending to investigate the stress-life relationship and fatigue fracture characteristics in order to understand crack initiation and growth behaviour in conjunction with the examination of surface roughness and porosity. Surface hollows were found to control crack initiation of as-cast specimens. The fracture surfaces of polished specimens revealed that surface or subsurface shrinkage pores replaced the hollows to act as crack initiators when the rough surface was removed. The importance of oxide films in crack initiation was also demonstrated. The effects of all these casting defects on fatigue life are discussed.

Small and long fatigue crack growth behaviour of a PM Ni-based superalloy, Udimet 720

Abstract The fatigue crack growth of long and small surface cracks has been investigated in air under four-point bending conditions at a load ratio of 0.1 for a PM Ni-based superalloy, Udimet 720. Cracks were found to initiate from surface pores despite an increased level of inclusions introduced by doping. Attention has been focused on small fatigue crack growth at maximum stresses beyond the 0.2% proof stress of the material. Typical small fatigue crack growth behaviour was found, i.e. growth rates for small fatigue cracks were faster than for long fatigue cracks at all fatigue stress intensity factor ranges investigated. A faceted–striated transition on the fatigue fracture surfaces has been found for both long and small fatigue crack growth. For long fatigue cracks, this transition appears to occur when the reversed plastic zone size is approximately equal to the grain size. For small fatigue cracks, the transition crack depth, d 0 , was found to be 10–20 times the grain size and independent of stress range employed.

Cyclic crack growth in titanium aluminides

Abstract Micromechanisms of fatigue crack growth have been studied in specific Ti 3 Al-based aluminides and TiAl-based aluminides. Effects of test temperature, environment and microstructure on fatigue crack growth resistance are considered. For both intermetallic systems brittle mechanisms of crack extension are observed at room temperature, and such mechanisms persist in the TiAl-based aluminides considered to a test temperature of 800 °C. The implications of such observations for the engineering application of these materials are addressed briefly.

The effect of lamella thickness on the creep behaviour of Ti-48Al-2Nb-2Mn

The work reported in this paper forms part of a larger investigation of the effects of microstructure on creep in a series of two phase {gamma}--TiAl based alloys. Here the authors examine a fully lamellar Ti-48Al-2Mn-2Nb alloy. They have attempted to quantify the microstructure in terms of a set of parameters, the most important of which are considered to be: colony size; perfection of the lamellar structure; lamella thickness; proportion of {alpha}{sub 2}; how serrated the colony boundaries are; and nature of the lamella interfaces.

Fatigue crack growth in a fibre-reinforced titanium MMC at ambient and elevated temperatures

Abstract Fatigue crack growth rates in an SCS6 fibre-reinforced Ti-15-3 based metal-matrix composite have been measured at a number of different temperatures, frequencies and load ratios. In specimens fatigued under a constant load range, crack growth rates decrease with increasing crack length, despite an increase in the nominal applied stress intensity range ( δK ). At room temperature the crack arrests after growing for MPa m 1 2 , whereas at elevated temperatures the deceleration of crack growth is interrupted by discrete events, which lead to instantaneous rises in propagation rate of an order of magnitude or greater. These events are associated with the failure of fibres, and crack arrest does not occur at elevated temperatures (200–500°C). In general, fatigue cracks grow faster at higher temperatures, higher load ratios and lower frequencies.

In situ SEM observations of the contrasting effects of an overload on small fatigue crack growth at two different load ratios in 2024-T351 aluminium alloy

In situ SEM observations and measurements were made of near-tip COD and crack growth at small, thumb-nail cracks subjected to a single overload during fatigue at constant load amplitude in 2024-T351 aluminium alloy. A delayed retardation in crack growth was observed at R = 0.05, but not at R = −1. During retarded growth at R = 0.05, near-tip COD decreased below that measured before overload and the level of crack closure increased. These events were not evident at R = −1. An elastic/plastic finite element modelling of the growth of similar small cracks predicted enhanced residual compressive stresses ahead of the crack tip for R = 0, but not for R = −1. The experimental and modelling work are consistent in linking differences in the crack growth profiles at the two R ratios with differences in COD, crack closure and residual compressive stress ahead of the crack tip following overload.

A probabilistic methodology for fatigue life prediction

This paper describes efforts in the probabilistic modelling of fatigue. The methodology proposed deals with crack nucleation from surface defects within a fracture mechanics framework. The methodology provides a quantitative understanding of the effects of bulk defects and their size distribution on the variation of fatigue life. It also considers the contributions of the variations of fatigue crack nucleation life and of crack growth resistance to the variation of fatigue life. A PM Ni-based superalloy Udimet 720 was used to evaluate the probabilistic methodology by an analytical solution and a Monte Carlo simulation. The model predictions were found to be in good agreement with the experimental results. However, it is found that Monte Carlo simulation provides a closer agreement with the experimental life distribution than the analytical solution. This is because the Monte Carlo simulation removes some untrue assumptions which have to be assumed to obtain the analytical solution.

Elevated temperature crack growth resistance of TiAl under monotonic and cyclic loading

Abstract Effects of microstructure and environment on elevated temperature fatigue crack growth rates have been studied in Ti48Al2Mn2Nb (at.%). Microstructures containing predominantly α2/γ two phase lamellar regions and small amounts of allotriomorphic γ grains possess much higher fracture toughness and increased crack growth resistance than those containing predominantly twin-related and allotriomorphic γ grains. Such γ grains may fail by transgranular cleavage at ambient temperature and by intergranular decohesion at temperatures of 700 and 800 °C, under both fatigue loading and during unstable fast fracture. Such “static” failure modes increase the dependence of crack growth rate on the alternating stress intensity factor and microstructures containing significant amounts of γ grains should not be subjected to fatigue loading at elevated temperatures.

High temperature fatigue of friction welded joints in dissimilar nickel based superalloys

Abstract The high temperature fatigue performance of two dissimilar material joints made by inertia friction welding and linear friction welding is assessed in the present study. The fatigue strength of the welded joints is found to be comparable to that of the weaker parent material for the plainsided specimens. However, the fatigue crack growth resistance within the weld zone of both material systems is found to be reduced compared to that of parent material. This effect is more pronounced for high temperature Ni alloys than for the lower temperature nickel alloy. Environmental attack is confirmed to be the controlling failure mechanism for acceleration of crack growth, but the extent to which it is observed is sensitive to local microstructure, i.e. to grain size, redistribution of the main strengthening phase γ′, and to the grain boundary carbide morphology. Such local microstructural variations are the result of welding and subsequent post-weld heat treatment procedures.