Cold Creep of Titanium: Analysis of Stress Relaxation Using Synchrotron Diffraction and Crystal Plasticity Simulations

Abstract

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in 'soft' grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring 'hard' grain * Corresponding author E-mail address: yi.xiong@materials.ox.ac.uk (Yi Xiong) have a significant effect on stress redistribution to hard grain orientations during cold dwell fatigue. Author ContributionsThis work formed part of a larger testing campaign proposed by AJW and carried out at Diamond Light Source by YX, PK, CTN, DMC, CMM & AJW. PK oversaw preparation of test pieces and preliminary characterisation undertaken by YX, CMM and PK. CTN and PK setup and guided use of the ETMT system. YX, PK, DMC and AJW developed data analysis procedures. YX carried out crystal plasticity simulations supervised by ET who supplied the UMAT code. YX drafted the initial manuscript, with all authors contributing to and agreeing the final version.

Slip Activity During Low-Stress Cold Creep Deformation in a Near-Α Titanium Alloy

“…With increasing stress levels, but still below σ0.2%, both microstructure conditions have shown that prismatic <a> slip starts to become the dominant slip mode. This is in good agreement with previous studies where deformation studies well beyond the proof stress have always shown dominance of prismatic <a> slip [29,91]. It is noticeable that the sample with the finer transformation product exhibits a higher frequency of pyramidal slip at all load steps, with most of it being localised in the transformed β phase.…”

“…However, the reported critical resolved shear stress (CRSS) values for basal <a> and prismatic <a> slip in Ti-alloys cover a wide range and in some studies basal <a> slip is said to be more difficult, i.e. CRSS(Ba)>CRSS(Pr) [20,[23][24][25][26][27][28][29] and in others easier i.e. CRSS(Ba)<CRSS(Pr) [18,19,[30][31][32][33][34].…”

Quantifying the effect of macrozones on the cold-dwell fatigue response of UD-rolled Ti-6Al-4V using high-energy X-ray diffraction

“…Synchrotron diffraction offers the advantage of being able to examine grain-scale response through measurement of lattice strains in-situ while probing a statistically significant number of grains. In the last decade, synchrotron diffraction has been used more widely to characterise the behaviour of titanium alloys, including of forged Ti64 under tensile loading [22], mill-annealed Ti64 [23], CP Ti [24,25], Ti-7Al [26] and equiaxed Ti64 [27,28].…”

“…While some of the previous work pays attention to stress relaxation effects during extended hold periods in CP Ti [25], there are no studies that examine the lattice strain response of dwell fatigue over a sustained number of cycles.…”

Nanoindentation in multi-modal map combinations: a correlative approach to local mechanical property assessment

“…Over the past decades, significant advances have been made in techniques used to determine material properties on an ever finer length scale, allowing for a better understanding of basic material property-microstructure relationships [1][2][3]. These developments have provided crucial insight into fundamental materials science, facilitating more accurate multiscale modelling [4][5][6][7][8][9][10], as well as the necessary property assessment for modern manufacturing processes with small features [11][12][13][14][15][16].…”