Titanium Alloys: Processing and Properties

The invited review paper to accompany this special issue, authored by Williams and Boyer [...

Corrosion fatigue and damage tolerance in the nickel-based superalloy RR1000 subjected to SO2 environments

When exposed to a high temperature corrosive environment nickel based superalloys may experience surface pitting and sulphide diffusion, which will influence concurrent or subsequent fatigue behaviour. Sulphur, pre-existing in the environment or as a bi-product of burning fossil fuels, reacts with sodium (as an atmospheric pollutant) creating molten sodium sulphate deposits on the metal surface. Combined with sodium chloride, these deposits attack the protective oxide layer allowing sulphides to migrate along grain boundaries. Continued sulphide diffusion promotes a weakened subsurface layer, inducing grain dropout and fatigue crack initiation. The present investigation focussed upon the subsequent effects of exposure to SO2 containing atmospheres on low cycle fatigue performance, together with the impact of an intermediate cleaning process. Damage tolerance data suggest that exposure to a SO2 environment fails to affect fatigue crack growth threshold or stage II growth behaviours when compared with standard laboratory air

Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography

To enhance the understanding of matrix cracking and damage progression on the macroscopic scale, within a 0/90° fibre reinforced SiCf/SiC ceramic matrix composite (CMC), X-ray computed tomography (XCT) imaging and analysis have been performed in conjunction with a commercially available in-situ mechanical loading device. CMC test coupons were subjected to tensile cyclic loads and inspected using XCT without removal from the tensile loading device. Attempts to measure and quantify the resulting damage using volumetric image analysis techniques are presented, by characterising the crack network from XCT images acquired at both the maximum and minimum load condition during selected fatigue cycles. The XCT detection of significant crack development within the first loading half-cycle shows good agreement with cumulative acoustic emission energy data recorded under similar test conditions. The results are seen as an important step towards correlating the damage behaviour detected via different NDE and health monitoring techniques

Novel Experimentation for the Validation of Mechanistic Models to Describe Cold Dwell Sensitivity in Titanium Alloys

Previous mechanistic models, proposed to explain the process of damage accumulation and stress redistribution between strong and weak regions inherent within the microstructure of α/β and near α titanium alloys, are validated through a matrix of experiments employing a non-standard variant of the alloy Ti 685. The grain size of the model material was deliberately processed to offer grains up to 20 mm in diameter, to facilitate constitutive measurements within individual grains. A range of experiments were performed under static and cyclic loading, with the fatigue cycle conducted under either strain or load control. Data will be reported to demonstrate significant variations in elastic and plastic properties between grains and emphasise the role of time dependent strain accumulation. Implications for the “dwell sensitive fatigue” or “cold creep” response of conventional titanium alloys will be discussed

Fatigue assessment of Ti-45Al-2Mn-2NbXD sub-element specimens

A novel test technique is described for the sub-element fatigue assessment of dovetail geometric features commonly employed as a fixture for blade/disc assemblies in steam and gas turbines. Once developed, the test method was employed to assess the performance of a model dovetail geometry employing the gamma titanium aluminide Ti-45Al-2Mn-2NbXD. In addition to generating information describing fatigue crack initiation, crack growth and ultimate failure, essentially via a trans-lamellar failure process, the associated wear processes were also characterised

The contribution of small punch testing towards the development of materials for aero-engine applications

This paper, invited for presentation at the 33rd Meeting of the Spanish Group on Fracture and Structural Integrity, March 2016 in San Sebastian, Spain, reviews the recent work carried out in the authors’ laboratory, addressing the elucidation of tensile and creep characteristics of materials for aero engine components. Two specific applications of the Small Punch (SP) test assessment technology were identified, the first of these takes on board the unique potential of the SP test for testing small quantities of materials which are either in development or through their directional structure cannot easily be produced in quantities which would allow conventional mechanical testing. This goal also required the development and procurement of new SP test facilities capable of operation up to 1150 °C. The examples given in this paper are TiAl intermetallic alloys and nickel based single crystals, all studied utilising the Code of Practice for SP Creep Testing. The second application illustrates the use of SP testing to assess both the tensile and creep properties of additive layer manufactured (ALM) alloys such as IN718 and Ti-6Al-4V using the Code of Practice for SP Tensile and Fracture Testing. Due to the unavailability of sufficient material to facilitate conventional testing for comparison of materials property data, SP testing is unable to provide absolute data for all of these applications, nevertheless the ranking capabilities of SP testing are demonstrably proven

Microstructural Control of Fatigue Behaviour in a Novel α + β Titanium Alloy

The novel titanium alloy TIMETAL® 407 (Ti-407) has been developed as an alternative to Ti-6Al-4V (Ti-6-4), for applications that demand relatively high ductility and energy absorption. Demonstrating a combination of lower strength and greater ductility, the alloy introduces a variety of cost reduction opportunities, including improved machinability. Thermo-mechanical processing and its effects on microstructure and subsequent mechanical performance are characterised, including a detailed assessment of the fatigue and crack propagation properties. Demonstrating relatively strong behaviour under high-cycle fatigue loading, Ti-407 is nevertheless susceptible to time-dependent fatigue effects. Its sensitivity to dwell loading is quantified, and the associated deformation and fracture mechanisms responsible for controlling fatigue life are explored. The intimate relationship between thermo-mechanical processing, micro-texture and fatigue crack initiation through the generation of quasi-cleavage facets is highlighted. Consistent fatigue crack growth kinetics are demonstrated, independent of local microstructure

Devolution of power, revolution in public health?

No abstract available