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Deformation Behaviour, Microstructure and Texture Evolution of CP Ti Deformed at Elevated Temperatures
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Mechanical Metallurgy. (Mechanical Metallurgy)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , iv, ii, 57 p.
Keyword [en]
Commercially pure titanium; Zener–Hollomon parameter; Dynamic recovery; Dynamic recrystallization; Recrystallized volume fraction; Grain size; Flow stress; Constitutive equation; Texture; High angle grain boundary; Low angle grain boundary; Deformation twinning; Schmid factor; Misorientation; Hot compression; Slip system; Subgrain; EBSD
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-10404ISBN: 978-91-7415-305-7 (print)OAI: oai:DiVA.org:kth-10404DiVA: diva2:216744
Public defence
2009-06-04, B2, KTH, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-11 Last updated: 2010-08-19Bibliographically approved
List of papers
1. Deformation behaviour of commercially pure titanium during simple hot compression
Open this publication in new window or tab >>Deformation behaviour of commercially pure titanium during simple hot compression
2009 (English)In: Materials and Design, ISSN 0261-3069, Vol. 30, no 8, 3105-3111 p.Article in journal (Refereed) Published
Abstract [en]

Commercially pure titanium (CP Ti), grade II, is subjected to hot compression at temperatures ranging from 673 to 973 K with 50 K intervals and strain rates of 0.001, 0.01, 0.1 and 1/s up to 60% height reduction. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. By plotting the data in a T vs. log(strain rate) diagram, the present and previous data fall into three distinct domains which can be separated by two distinct values of the Zener–Hollomon parameter. The microstructure after deformation is characterized by optical microscopy and electron back scattered diffraction. The formation of {10-11} twins is related to the Zener–Hollomon parameter. Geometric dynamic recrystallization seems most appropriate when describing the grain refinement process of CP Ti during hot compression.

Keyword
Commercially pure titanium, Zener–Hollomon parameter, High angle grain boundary
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10413 (URN)10.1016/j.matdes.2008.12.002 (DOI)000267746800034 ()2-s2.0-67349114922 (Scopus ID)
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-12 Last updated: 2010-12-07Bibliographically approved
2. Modeling the flow stress for single peak dynamic recrystallization
Open this publication in new window or tab >>Modeling the flow stress for single peak dynamic recrystallization
2009 (English)In: MATERIALS & DESIGN, ISSN 0264-1275, Vol. 30, no 6, 1939-1943 p.Article in journal (Refereed) Published
Abstract [en]

A model is developed to predict the flow stress for single peak dynamic recrystallization during hot working based on the analysis of the mechanism of the process. The model reveals the dependence of flow stress on strain, strain rate, temperature and microstructure. The flow stress in the recrystallized zone is derived by an integral with the recrystallized volume fraction as the variable. The correlation between the microscopic variable and flow stress is investigated with the model. The performance of the model is evaluated through application on magnesium alloy AZ31D. The mean error of flow stress between the experimental and predicted results is examined. Good agreement between the predicted and experimental data is achieved. All mean errors are between −5.9% and 6.7%.

Keyword
Dynamic recrystallization; Recrystallized volume fraction; Grain size
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10414 (URN)10.1016/j.matdes.2008.09.004 (DOI)000265669100010 ()2-s2.0-63049113698 (Scopus ID)
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-12 Last updated: 2010-12-07Bibliographically approved
3. Constitutive equations for pure titanium at elevated temperatures
Open this publication in new window or tab >>Constitutive equations for pure titanium at elevated temperatures
2009 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 505, no 1-2, 116-119 p.Article in journal (Refereed) Published
Abstract [en]

Isothermal compression tests were conducted on pure titanium using a Gleeble 1500 thermal simulator under constant strain rates of 0.001, 0.01, 0.1 and 1.0/s and at deformation temperatures ranging from 673 to 973 K up to a 60% height reduction of the sample. The high temperature deformation behaviour of pure titanium was characterized based on an analysis of the stress–strain curves. A set of constitutive equations for pure titanium were proposed by employing an Arrhenius-type equation. Material constants, A, β and activation energy Q, were found to be functions of strain. The equations revealed the dependence of flow stress on strain, strain rate and temperature. In order to evaluate the accuracy of the deformation constitutive equations, the mean errors of flow stress between the experimental data and predicted results were plotted. The results show that there is a close agreement between the predicted and experimental stress–strain curves.

Keyword
Pure titanium, Flow stress, Constitutive equation, Hot working
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10415 (URN)10.1016/j.msea.2008.11.017 (DOI)000264364100018 ()2-s2.0-59649095191 (Scopus ID)
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-12 Last updated: 2017-12-13Bibliographically approved
4. Microstructure and texture evolution of commercial pure titanium deformed at elevated temperatures
Open this publication in new window or tab >>Microstructure and texture evolution of commercial pure titanium deformed at elevated temperatures
2009 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 513, 83-90 p.Article in journal (Refereed) Published
Abstract [en]

Microstructure and texture evolution of commercial pure titanium were investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. The basal planes of both the fine and coarse grains in the deformed samples tend to rotate from the initial orientations, perpendicular to the compression axis, to an inclination of 45°. The Schmid factor is used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. After deformation, the distribution frequency of the misorientation angles shows that the low angle grain boundaries increased dramatically while the high angle grain boundaries decreased. In particular, after deformation at 723 K and 0.1/s, a peak around 50–60° in the misorientation frequency-distribution is found, which is due to {10-11} twinning. The analysis of the deformed microstructure indicates that dynamic recovery is the dominant deformation mechanism for commercial pure titanium when subjected to the investigated deformation conditions.

Keyword
Commercial pure titanium; Texture; Schmid factor; Misorientation; Hot compression; Slip system
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10416 (URN)10.1016/j.msea.2009.01.065 (DOI)000266577400012 ()2-s2.0-65349171501 (Scopus ID)
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-12 Last updated: 2017-12-13Bibliographically approved
5. The effects of deformation conditions on microstructure and texture of Commercially Pure Ti
Open this publication in new window or tab >>The effects of deformation conditions on microstructure and texture of Commercially Pure Ti
2009 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 57, no 19, 5822-5833 p.Article in journal (Refereed) Published
Abstract [en]

Electron backscattered diffraction is employed to explore the dependence of deformation reduction, strain rate and temperature on microstructure and texture evolution of CP Ti after hot compression tests. It is found that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10-11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. The data for the stable subgrain size and amount of {10-11} twins are separated into two groups at logZ = 13. Above this value, twinning is observed and the subgrain size is much finer, < 3μm. It is concluded that the presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. After deformation, three (0001) texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD.

Keyword
CP Ti; Texture; Misorientation; Hot compression; EBSD
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-10417 (URN)10.1016/j.actamat.2009.08.016 (DOI)000271668200018 ()2-s2.0-70349774364 (Scopus ID)
Note
QC 20100819Available from: 2009-05-18 Created: 2009-05-12 Last updated: 2017-12-13Bibliographically approved

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