M.R. Bache

A review of dwell sensitive fatigue in titanium alloys: the role of microstructure, texture and operating conditions

Abstract The phenomenon of ambient temperature dwell sensitive fatigue in titanium alloys has been a concern to the gas turbine industries for over three decades. However, largely due to economic constraints, it is unlikely that the titanium alloys will be replaced by novel substitutes in the foreseeable future. Therefore, with the use of the near α and α/β variants for safety critical components in the low and high pressure compressor sections of the engine, the potential effects of “cold dwell” continue to compromise design. A significant amount of research has been conducted during the intervening years to understand the fundamental deformation mechanisms controlling dwell behaviour. The present paper will explore these findings and highlight the important issues for consideration during the design and service of future aerospace components and alternative high performance applications.

Dwell sensitive fatigue in a near alpha titanium alloy at ambient temperature

The ambient temperature fatigue performance of the near alpha titanium alloy IMI834 was evaluated using laboratory specimens manufactured from two material sources: rolled bar stock and an isothermally forged compressor disc. The effect of dwell periods imposed at peak stress and R value were assessed. Significant differences were defined between the two variants with respect to their sensitivity to dwell loading. Variations in microstructural form together with a localised texture within the disc material are considered responsible for controlling the dwell performance in each case. The findings are consistent with a previously proposed model for facet development in this class of material.

Impact of texture on mechanical properties in an advanced titanium alloy

Abstract The mechanical response of highly textured Ti 6/4 plate material is evaluated under monotonic and cyclic loading. Significant variations in yield and ultimate tensile strength, bend ductility and total fatigue life to failure are all noted, depending on the orientation of the principal stress axis with respect to the dominant basal plane texture. These effects are discussed with reference to the inherent, anisotropic mechanical response of α + β and near α titanium alloys that results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.

Electron back scattered diffraction (EBSD) analysis of quasi-cleavage and hydrogen induced fractures under cyclic and dwell loading in titanium alloys

Evidence for sub-surface fatigue crack initiation is often reported for near alpha titanium alloys such as the coarse grained IMI685 and the fine duplex structured IMI834. In such materials with a typical as received hydrogen concentration of 40–60 ppm the initiation site is invariably characterized by quasi-cleavage facetting. Similar facetting is also associated with the low temperature dwell sensitive fatigue response in the same alloys. For IMI685, it is reported that this failure mechanism is replaced by α/β interface cracking when the alloy contains a relatively high concentration of interstitial hydrogen. The present paper characterises the local grain orientation and microstructural conditions associated with these various forms of failure through the use of a microtextural analysis technique based upon electron back scattered diffraction (EBSD) measurements. The observations are related to an existing model to account for facet formation based upon the pile-up of dislocations at grain-boundaries. The implications for further use of this technique with titanium alloys are discussed.

Processing titanium alloys for optimum fatigue performance

Abstract It is clear that titanium alloy developments over the past half century have been instrumental to the advancement of aero gas turbine technology. New ‘designer’ alloys and composites are currently under evaluation as potential replacement materials for selected fan and compressor applications. However, it is recognised that in the current economic climate, where cost is a major factor to be considered at the design stage, conventional alloy systems will remain an essential constituent for the foreseeable future. This implies that titanium alloys will have to be employed more efficiently if they are to continue to meet future requirements. With the implementation of ‘total life’ management procedures this highlights the need for a better understanding of deformation and crack development at the grain to grain level since this stage dominates overall life. The paper will address such issues and discuss the role of processing in the optimisation of fatigue performance.

The effects of texture in titanium alloys for engineering components under fatigue

Abstract The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.

The effects of environment and internal oxygen on fatigue crack propagation in Ti-6Al-4V

Abstract The role of environmental and internal oxygen on fatigue crack propagation is assessed for the titanium alloy Ti-6Al-4V at laboratory temperature. Growth rates for part through corner cracks are demonstrated to be highly dependent on the amount of oxygen present. Compared to data for conventional plate material at atmospheric conditions, tests conducted under a vacuum of 10 −6 torr illustrate a marked reduction in the rates of growth for the facet dominated phase at low values of Δ K. Intermediate vacuum levels and partial pressures of argon or hydrogen lead to growth rates that are similar to or slightly lower than the conventional atmospheric baseline. Significant growth rate accelerations are observed, however, for 6/4 material containing internal oxygen concentrations in excess of the standard 1700–2300 ppm. This acceleration is enhanced by the application of a high load ratio, R = 0.5. The implications with respect to current ideas on environmental interactions are discussed.

Experimental and crystal plasticity studies of deformation and crack nucleation in a titanium alloy

This paper presents recent developments in crystal plasticity modelling to incorporate a non-local crack nucleation criterion and the comparison of experimental observations and model predictions for deformation and crack nucleation in a near-alpha titanium (Ti) alloy polycrystal. The model polycrystal was established so as to have grain morphology and crystallography nominally identical to that in the experiment, enabling direct interpretation and comparison of the results. The experimental measurements of strain fields were obtained using electronic speckle pattern interferometry on a large-grained polycrystalline sample. The crystal plasticity model predictions and experiments show strong agreement in a number of important features relating to crack nucleation: the location of crack nucleation, the direction changes in crack growth when the crack crosses two grain boundaries, and the important role of crystallographic orientation in nucleation site and in growth directions. There was also good agreement in the establishment of strain heterogeneity and in localization; both experiment and crystal plasticity predictions demonstrate that the heterogeneity is established early in the loading history and that the localization correlates closely with the crack nucleation site.

Characterisation of foreign object damage and fatigue strength in titanium based aerofoil alloys

Abstract Simulated foreign object damage has been inflicted at room temperature on two competing titanium based materials destined for compressor aerofoil applications in future gas turbine engine designs. Laboratory specimens, manufactured in the conventional titanium alloy Ti–6Al–4V and the γ titanium aluminide Ti–45Al–2Mn–2Nb, were subjected to impacts of various energy, representative of severe in-service damage. Damage sites were fully characterised prior to subsequent fatigue assessment at both room temperature and elevated temperatures typical of the envisaged maximum working temperatures for each alloy. In terms of absolute fatigue endurance levels the γ aluminide offers the weaker response at room temperature, however, this system offers clear advantages in terms of temperature capability.

Microstructure and Mechanical Properties of an Advanced Nickel-Based Superalloy in the as-HIP Form

This study evaluates the suitability of as-hot isostatically pressed (HIP) RR1000 for non-critical applications in aero-engine components. RR1000, an advanced powder nickel-based superalloy, was developed for disc rotor components in aero-engines. For these critical applications, the consolidated alloy powder particles are extruded to break down carbide and oxide networks, known as prior particle boundaries (PPBs), and to refine the structure into a fine grain size for isothermal forging. In this study, hot isostatically pressed compacts, made from two different powder particle size fractions have been assessed following heat treatments below and above the gamma prime solvus temperature. A microstructural evaluation shows a greater degree of PPB decoration occurs in the finer powder particle size fraction. Following a super-solvus heat treatment these PPBs pin grain boundaries of the fine powder particle size compacts, whilst the reduction of PPB decoration in coarse powder particle compacts allows significant grain growth. Tensile test results of as-HIP RR1000 show, good yield strengths, ultimate tensile strengths and ductility, which are comparable with extruded and isothermal forged RR1000 disc material. Dwell crack propagation tests show that finer powder particle size compacts, which have received a sub-solvus heat treatment, give the highest crack growth rates; whilst the remaining material conditions show markedly improved crack growth resistance. In conclusion, as-HIP RR1000 demonstrates clear potential for use in non-critical applications, employing either powder particle size fraction used in this study subject to the appropriate solution heat treatment.