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Micromechanisms governing plastic instability in Al–Li based alloys
Citation Link: https://doi.org/10.15480/882.1267
Publikationstyp
Doctoral Thesis
Date Issued
2015
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2015-11-11
TORE-DOI
The investigation of the underlying microscopic mechanisms that govern plastic in-
stability in solution strengthened and precipitation strengthened Al alloys has been the
subject of several studies. These studies are largely motivated by the need to devise strategies to mitigate the undesirable effects, such as reduction in ductility and formation of surface striations, which occur in alloys that exhibit this phenomenon. While the microscopic origin of plastic instability in solution strengthened alloys Al alloys is fairly well established, there is yet no convincing model that is consistent with experimental observations and gives a clear mechanistic description of the origin of the phenomenon in precipitation strengthened Al alloys.
In this work, detailed experimental investigations of several tempers of a multi–
component Al–Li based alloy, AA2198, have been carried out. Both mechanical and mi-
crostructural characterization techniques were employed in order to correlate microstruc-
tural characteristics to global and local mechanical behaviour. Specifically, high resolution
nanoindentation and micro–tensile testing were used for mechanical testing, while trans-
mission electron microscopy based methods – including in situ TEM tensile straining,
along with high energy x–ray diffraction and atom probe tomography were used to inves-
tigate the relevant microstructural characteristics.
The experimental results clearly showed that dynamic strain aging of temporarily
trapped mobile dislocations by Li atoms, widely accepted as the underlying mechanism
for plastic instability in Al–Li based alloys, cannot sufficiently account for the occurrence
of plastic instability in AA2198. Moreover, theoretical analyses of strengthening mecha-
nisms in the investigated tempers showed that only the overaged temper, which is also the
only temper that displayed plastic instability, is governed by order hardening.
In light of the wealth of experimental results, a mechanistic model describing the
microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li
based alloy systems was developed. It is proposed that the phenomenon is governed by an altogether different mechanism than what has so far been considered, namely a diffusion controlled pseudo–locking mechanism that accompanies order hardening at low strain rates. The applicability of the model to other Al–Li alloy based systems was also examined. It was demonstrated, by critical examination of the instability behaviour of a number of binary and multi–component Al–Li based alloy systems reported in literature, that plastic instability only occurs in these alloy systems when strength is governed by order hardening.
stability in solution strengthened and precipitation strengthened Al alloys has been the
subject of several studies. These studies are largely motivated by the need to devise strategies to mitigate the undesirable effects, such as reduction in ductility and formation of surface striations, which occur in alloys that exhibit this phenomenon. While the microscopic origin of plastic instability in solution strengthened alloys Al alloys is fairly well established, there is yet no convincing model that is consistent with experimental observations and gives a clear mechanistic description of the origin of the phenomenon in precipitation strengthened Al alloys.
In this work, detailed experimental investigations of several tempers of a multi–
component Al–Li based alloy, AA2198, have been carried out. Both mechanical and mi-
crostructural characterization techniques were employed in order to correlate microstruc-
tural characteristics to global and local mechanical behaviour. Specifically, high resolution
nanoindentation and micro–tensile testing were used for mechanical testing, while trans-
mission electron microscopy based methods – including in situ TEM tensile straining,
along with high energy x–ray diffraction and atom probe tomography were used to inves-
tigate the relevant microstructural characteristics.
The experimental results clearly showed that dynamic strain aging of temporarily
trapped mobile dislocations by Li atoms, widely accepted as the underlying mechanism
for plastic instability in Al–Li based alloys, cannot sufficiently account for the occurrence
of plastic instability in AA2198. Moreover, theoretical analyses of strengthening mecha-
nisms in the investigated tempers showed that only the overaged temper, which is also the
only temper that displayed plastic instability, is governed by order hardening.
In light of the wealth of experimental results, a mechanistic model describing the
microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li
based alloy systems was developed. It is proposed that the phenomenon is governed by an altogether different mechanism than what has so far been considered, namely a diffusion controlled pseudo–locking mechanism that accompanies order hardening at low strain rates. The applicability of the model to other Al–Li alloy based systems was also examined. It was demonstrated, by critical examination of the instability behaviour of a number of binary and multi–component Al–Li based alloy systems reported in literature, that plastic instability only occurs in these alloy systems when strength is governed by order hardening.
Subjects
Plastic instability, Al-Li alloys, strengthening mechanisms, dynamic strain aging, pseudo locking mechanism
DDC Class
600: Technik
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