Dwell fatigue in two Ti alloys: an integrated crystal plasticity and discrete dislocation study
File(s)
Published version
Author(s)
Zheng, Z
Balint, D
Dunne, F
Type
Journal Article
Abstract
It is a well known and important problem in the aircraft engine industry that alloy Ti-6242 shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is included in the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for the differing dwell debit of these alloys has remained elusive for decades. In this work, crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for both Ti alloys based on independent experimental data. This then allows the markedly different cold creep responses of the two alloys to be captured accurately and demonstrates why the observed near-identical rate sensitivity under non-dwell loading is entirely consistent with the dwell behaviour. The activation energies determined are then utilised within a recently developed thermally-activated discrete dislocation plasticity model to predict the strain rate sensitivities of the two alloys associated with nano-indentation into basal and prism planes. It is shown that Ti-6242 experiences a strong crystallographic orientation-dependent rate sensitivity while Ti-6246 does not which is shown to agree with recently published independent measurements; the dependence of rate sensitivity on indentation slip plane is also well captured. The thermally-activated discrete dislocation plasticity model shows that the incorporation of a stress dwell in fatigue loading leads to remarkable stress redistribution from soft to hard grains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but that no such load shedding occurs in alloy Ti-6246. The key property controlling the behaviour is the time constant of the thermal activation process relative to that of the loading. This work provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debit but Ti-6246 does not.
Date Issued
2016-11-01
Date Acceptance
2016-08-12
Citation
Journal of the Mechanics and Physics of Solids, 2016, 96 (1), pp.411-427
ISSN
0022-5096
Publisher
Elsevier
Start Page
411
End Page
427
Journal / Book Title
Journal of the Mechanics and Physics of Solids
Volume
96
Issue
1
Copyright Statement
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC
BY license (http://creativecommons.org/licenses/by/4.0/).
BY license (http://creativecommons.org/licenses/by/4.0/).
License URL
Sponsor
Engineering & Physical Science Research Council (EPSRC)
Royal Academy Of Engineering
EPSRC
Identifier
https://www.sciencedirect.com/science/article/pii/S002250961630223X
Grant Number
EP/K034332/1
MMRE_P54661
EP/K034332/1
Subjects
Science & Technology
Technology
Physical Sciences
Materials Science, Multidisciplinary
Mechanics
Physics, Condensed Matter
Materials Science
Physics
Discrete dislocation plasticity
Dwell fatigue
Load shedding
HCP crystals
Nano-indentation
STRAIN-RATE SENSITIVITY
ROOM-TEMPERATURE CREEP
CRACK NUCLEATION
TITANIUM-ALLOY
DEFORMATION
MECHANISMS
MICROSTRUCTURE
INDENTATION
ORIENTATION
PHASE
Mechanical Engineering & Transports
01 Mathematical Sciences
02 Physical Sciences
09 Engineering
Publication Status
Published
Date Publish Online
2016-08-21