An appraisal of dwell sensitive fatigue in Ti-6Al-4V and the governing role of inhomogeneous micro-texture

Highlights

• Characterisation of a proprietary Ti-6Al-4V fan disc forging has illustrated the presence of “macrozones” or “microstructurally textured regions (MTRs)”

• LCF tests have illustrated a debit in fatigue strength if a hold of two minutes is imposed at the peak of the loading waveform.

• EBSD can be applied to metallographic polished sections or directly from the fracture surface of failed fatigue specimens to identify MTRs.

• Ti-6Al-4V should be considered as “dwell sensitive” when containing MTRs of the scale investigated during the present study.

Abstract

The fact that the α/β titanium alloy Ti-6Al-4V is sensitive to cold dwell loading regimes has recently been endorsed by a combination of evidence gathered from fatigue studies performed on laboratory scale specimens and the findings of a failure investigation into an in-service fan disc failure. This work confirms that Ti-6Al-4V can demonstrate dwell characteristics under fatigue loading, subject to the microstructure and micro-texture developed during thermo-mechanical processing. Additional fatigue data are presented to support this conclusion, together with detailed metallographic and crystallographic characterisation to identify the factors controlling inhomogeneous strength, stress distribution and the crack initiation process on the micro to macro scale.

Introduction

The phenomenon of room temperature, dwell sensitive fatigue in titanium alloys was first considered in the early 1970s, a consequence of investigations into well publicised in-service fan disc failures suffered by early variants of the RB-211 engine powering Lockheed Tristar aircraft [1]. As a direct outcome of those investigations a seminal publication by Evans and Gostelow [2], based at the Royal Aircraft Establishment at Farnborough UK, characterised the effects of “dwell fatigue” in the near-α titanium alloy Ti-685, the specific service alloy of concern at that time, utilising laboratory specimens extracted from disc forgings. They quantified a reduction in fatigue lives when the material was subjected to loading waveforms incorporating a hold at peak applied stress, compared to standard “cyclic” waveforms. The ability of titanium alloys in general to accumulate strain under static loading at room temperature had been recognised since the mid twentieth century [3]. Consequently, the “cold dwell” effect was attributed to the superposition of time dependent creep strain accumulating during the dwell period in addition to conventional fatigue damage.

Detailed inspections of model spin rig disc failures and fractographic studies of conventional laboratory test specimens indicated that dwell fractures often initiated at sub-surface locations from quasi-cleavage facets. Evans and Bache [4] proposed a model to describe facet formation as the result of inhomogeneous strain leading to stress redistribution between “weak” and “strong” orientated grains within the microstructure with respect to the loading direction, fundamentally controlled by the juxtaposition of hexagonal grains of different orientations. In support of the neighbouring grain model, early practitioners of electron back scattered diffraction (EBSD) were able to identify the crystallographic orientations of pertinent grains and it has been widely accepted since that the quasi-cleavage facets form on basal (or at least near basal) planes [5], [6], [7].

Ever since the early Farnborough research, all manufacturers of large civil aero-engines have employed the α/β alloy Ti-6Al-4V as the variant of choice for fan disc applications. Notably, different thermo-mechanical processes applied to Ti-6Al-4V can generate a wide variety of α/β microstructures. However, one of the mitigating factors thought to explain an apparently lower level of dwell sensitivity compared to the near alpha alloys was the typical fine grain size relative to near-α variants such as Ti-685, and Ti-829 [8]. The mechanism of quasi-cleavage faceting is encouraged by a large grain size allowing more extensive planar slip [9], [10].

Alternatively, the localised concentration of multiple hexagonal grains with a similar basal plane orientation has been recognised to promote dwell fatigue failures in near-α variants such as Ti-834 and Ti-6242 [11], [12], [13]. Such “macrozones” or “microstructurally textured regions (MTRs)” can be refined through optimised thermo-mechanical processing but this can be more challenging for large section parts [14]. MTRs increase the effective structural unit size in such finer grained alloys. This can be a particular problem when forming large through section engineering components.

The recognition of significant MTRs in full scale Ti-6Al-4V fan disc forgings and their potential role in dwell fatigue failure was highlighted by the failure investigation relating to an in-service disc failure which occurred in 2017, reported in 2020 [15]. Inspection of the fractured disc illustrated a region of contiguous quasi-cleavage facets at the sub-surface crack initiation site. Subsequent metallographic sectioning revealed the presence of significant MTRs immediately below the fracture plane, with the facets associated with underlying, primary α grains with a common basal plane orientation. Sometime before that failure investigation, Venkatesh et al had generated a comprehensive low cycle fatigue (LCF) database from laboratory scale specimens subjected to cyclic and dwell waveforms, eventually published in 2020 [16]. A major objective of their study was to correlate fatigue crack initiation mechanisms to the microstructure and micro-texture found within typical Ti-6Al-4V fan disc forgings [17]. Focussing on two nominal stress conditions, the research deliberately assessed the degree of scatter in measured fatigue lives and the LCF data ultimately confirmed the dwell sensitive nature of Ti-6Al-4V when containing MTRs.

Similar objectives were established during the present study, from which the current data will be presented. Cyclic and dwell LCF experiments were conducted employing a different fan disc forging of similar microstructure and mechanical properties. The current findings are compared to the research reported under [16] to establish the potential for performance variability between different components. Detailed characterisation is included to demonstrate the critical role of MTRs in the crack initiation process.