Correlation between microstructure evolution and deformation behavior during isothermal compression

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Organization of the thesis

As shown above, although the studies on the displacive characters of the β to α transformation have greatly advanced our knowledge on the formation of specific microstructural features of the α phase in titanium alloys, investigations on the constituents of individual α precipitates and their correlation with the transformation strain are still limited. In addition, despite the numerous theoretical and experimental investigations, when the α variant selection happened during thermomechanical processing, the possible interplay between the imposed external deformation and the internal transformation lattice deformation is still not clearly addressed. The reported selection mechanisms stay applicable for individual situations. Moreover, the hot deformation of the metastable β titanium alloys with an initial single β phase in the α+β two phase region has seldom been reported, and the influence of the transformation, especially the transformation associated lattice deformation, on the mechanical response of the alloy has not yet been addressed.
In order to control the microstructure, to understand processing microstructure-property relationships, and thus to tailor manufacturing conditions to obtain specific mechanical properties through thermomechanical processing, it is of significant importance to investigate phase transformation behavior and variant selection mechanisms for its occurrence at different scales. Thus, the objective of the current work is to investigate phase transformation behavior and the α variant selection during the thermomechanical processing. The scale of studies on variant selection is varied from the individual parent grain to the whole polycrystalline β sample. Meanwhile, the correlation between the hot deformation behavior and the microstructure evolution was made. In the present work, a metastable β titanium alloy, Ti-7Mo-3Nb-3Cr-3Al (wt. %, Ti-7333), was chose to be the subject material.
sub-structures, was conducted in the Ti-7333 alloy under heat treatment condition. To obtain the α precipitates produced by the displacive process of the transformation that happens at the very beginning of the formation of α phase, the specimens were aged very shortly after the over β transus solution treatment.
In Chapter 4, a thorough investigation with statistical significance on phase transformation and variant selection in the Ti-7333 alloy under uniaxial isothermal compression was conducted at two temperatures (700 oC and 600 oC). Special attention was paid to the interplay between the transformation strain and the imposed strain and the applied load.
In Chapter 5, a study on the correlation between the microstructure evolution and the hot compression stress-strain behavior was conducted. The effort was made to resolve the underlying mechanisms of the hot deformation behavior of the alloy. Special attention was paid to the contribution of the β to α phase transformation at different deformation stages.
The final conclusions and perspective on some future directions that would extend the current work are presented in Chapter 6.

Materials preparation

The material used in the present work is a metastable β-Ti alloy, Ti-7Mo-3Nb-3Cr-3Al (wt. %, Ti-7333). This alloy was prepared first by multiple vacuum arc-melting and then by forging in the β and the α+β phase region. The composition was analyzed by capacity chemical analysis method and is given in Table 2.1. The β transus temperature of this alloy measured by the metallographic method is approximately 850 ˚C.
Cylindrical specimens with 15 mm in height and 10 mm in diameter were cut out of the center part of the forged Ti-7333 bar (150 mm in diameter). All the specimens in the present studies were first solution-treated at 900 °C in the β phase region for 30 min followed by water quenching to obtain a homogeneous single β microstructure. All the specimens in the work in Chapter 3, 4 and 5 were further treated with this initial microstructure.

Experimental details

Thermal mechanical processing

In the present work, the thermal mechanical processing constituted two parts, the one being the heat treatment and the other the isothermal hot compression. For the heat treatment, the as-solution-treated specimens were aged at 700 °C for 5 minutes and quenched in ice water to allow one part of the β phase transforming to α phase. This experiment provided the specimens for the analysis constituting the work in Chapter 3. For the hot compression process, the as-solution-treated specimens were compressed using a Gleeble-3500 thermo-mechanical simulator under vacuum. The specimen was first heated to the compression temperature at a rate of 25˚C /s and held for 5 seconds to homogenize the temperature. Then they were compressed at a strain rate of 10-3 s-1 to a defined reduction and quenched in ice water to preserve the deformed microstructure. The compression temperatures were 700 °C and 600 °C , respectively, in the present work (for Chapter 4 and 5). The deformation (in reduction) was designed from a true strain of 0.015 to 0.35 (0.015, 0.025, 0.03, 0.04, 0.10 and 0.35). During the hot compression, a thermocouple was welded at the mid span of the specimens to measure the temperature. Two pieces of thin tantalum sheets were placed between the specimen and the compressive die to reduce the friction and to maintain a uniform deformation. A schematic chart of the isothermal treatment and the isothermal compression processes is shown in Fig. 2.1.

Determination of the lattice constants

The lattice constants of the constituent phases at 700°C were measured in-situ by neutron diffraction. The through-volume measurements were performed with the neutron diffractometer STRESS-SPEC located at a thermal beam port of FRM-II in Garching, Germany. The Ge (311) monochromator was selected to produce neutrons with a wavelength of 1.618 Å. The bulk specimen with dimensions of Φ 5 x 15 mm was inserted into a vanadium crucible and immersed in the neutron beam with a size of 5×10×10 mm3 under vacuum to prevent oxidation of the specimens at elevated temperatures. The specimen was heated to 700 °C and isothermally held for 50 min, and then cooled at a rate of 12 °C/min. A thermocouple was inserted from the top of the crucible to record the temperature of the specimen. Neutron diffraction patterns were collected in-situ during the isothermal holding and the cooling at each 30 seconds. The (110)β and (100)α; (002)α; (101)α diffraction peaks were captured at the detector position 2θ=41° with a detector window of 15°. The instrument parameters were fitted by the measurement of Si powder. The software StressTextureCalculator (STeCa) [107] was used to extract diffraction patterns. The measured lattice constants of the constituent phases will be used for analyses in Chapter 3, 4 and 5.

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Isothermal compression under in-situ neutron diffraction

The isothermal compression was also performed in-situ under neutron diffraction measurements at 700 °C to the as-solution-treated specimens with the dimensions of Φ5mm x 10mm, using the rotatable multifunctional (tension/compression/torsion) loading frame incorporated in the STRESS-SPEC instrument. The loading frame was also integrated with a clip-on extensometer and a home-made inductive heating system. The specimens were heated in air to 700 °C at a rate of 50 °C /s, then isothermally compressed to a true strain of 0.35 (30% in reduction) and finally cooled with compressed air. The neutron diffraction measurements were performed using the neutron diffractometer STRESS-SPEC located at a thermal beam port of FRM-II (Garching, Germany). The Ge (311) monochromator was selected to reflect neutrons with a wavelength of 1.7076 Å. The {110}β diffraction peak (corresponding to β phase) and the {0002}α and {101̅1}α diffraction peaks (corresponding to α phase) were recorded at the detector position of 2θ=42° with a window of 15°. The diffraction patterns were collected in-situ during the heating, isothermal compression and cooling processes each at every 20 seconds. The {110}β pole figure of the un-deformed specimen (as-solution treated) and the {110}β and {0002}α pole figures of the same specimen after the deformation were measured ex-situ to find out the orientation evolution.. The gauge volume of the specimens for the pole figure measurements was Φ5 × 10 mm. The software StressTextureCalculator (STeCa) [107] was used to extract the diffraction patterns. This experiment is corresponding to the work in Chapter 5.

Microstructural and crystallographic characterization

The microstructural and crystallographic features of the specimens were examined by post mortem characterization techniques from the macroscopic scale to the microscopic scale to ensure a statistical relevance of the results. For the phase constituents and the morphological features, the specimens were examined using a Jeol JMF6500-F SEM and Supra 40 Zeiss SEM. For the crystallographic features, the specimens were examined using a JEOL 6500F SEM equipped with an EBSD acquisition camera and the Aztech online acquisition software package (Oxford Instruments). The data were processed using Channel 5 (Oxford Instruments) and ATEX software [108]. For the deformed specimens, the examined total area covers the whole homogeneous deformation region in the cross section. To achieve the surface quality for SEM observations and the EBSD measurements, the specimens were first mechanically polished and then electrolytically polished with a solution of 10% per-chloric acid in methanol at 35 V for 5 seconds at a temperature lower than 5°C. The EBSD measurements were conducted both automatically and manually. The automatic measurements were performed under the beam controlled mode with a step size of 2μm for the global measurements and with a step size of 70 nm for the local fine measurements. The manual measurements were conducted for the determination of the crystallographic features of fine α constituents.

Table of contents :

Chapter 1 Literature review
1.1 General introduction
1.2 Crystal structure and classification of titanium alloys
1.2.1 Crystal structure of pure titanium
1.2.2 Classification of titanium alloy
1.3 Phase transformation under thermal or thermomechanical processing in metastable β titanium alloys
1.3.1 β to α phase transformation
1.3.2 Variant selection during β to α phase transformation
1.4 Hot deformation behavior of metastable β titanium alloy
1.4.1 Hot deformation in single β phase region
1.4.2 Hot deformation in α+β phase region
1.4 Organization of the thesis
Chapter 2 Materials, experimental details and basic crystallographic calculations
2.1 Materials preparation
2.2 Experimental details
2.2.1 Thermal mechanical processing
2.2.2 Determination of the lattice constants
2.2.3 Isothermal compression under in-situ neutron diffraction
2.2.4 Microstructural and crystallographic characterization
2.3 Basic crystallographic calculation
2.3.1 Coordinate system setting
2.3.2 Coordinate transformation
2.3.3 Stereographic projection
2.3.4 Misorientation
2.3.5 Trace analysis method
2.3.6 Deformation gradient tensor
Chapter 3 Microstructure evolution and phase transformation under heat treatment 
3.1 Introduction
3.2 Experimental
3.3 Microstructure of the initial β phase and lattice constant information
3.4 Phase transformation under heat treatment
3.4.1 Microstructure characteristics of α precipitates
3.4.2 Sub-structures of intragranular α precipitates
3.5 Formation mechanisms of sub-structures of intragranular α precipitates
3.5.1 Phase transformation lattice strain characters
3.5.2 Formation mechanism of interface α and local variant selection of major α
3.5.3 Formation mechanism of non BOR α domains
3.6 Summary
Chapter 4 Phase transformation and α variant selection mechanism during isothermal compression
4.1 Introduction
4.2 Experimental
4.3 Microstructure of the initial β phase
4.4 Phase transformation and α variant selection during 700 oC compression
4.4.1 Phase transformation and α variant selection in stress-free state
4.4.2 Phase transformation and α variant selection under 700 °C isothermal compression
4.4.3 Variant selection mechanisms
4.5 Phase transformation and α variant selection during 600oC compression
4.5.1 Phase transformation and α variant selection during heat treatment
4.5.2 Phase transformation and α variant selection during 600 °C isothermal compression
4.6 Summary
Chapter 5 Correlation between microstructure evolution and deformation behavior during isothermal compression
5.1 Introduction
5.2 Experimental
5.3 Microstructure of the initial β phase
5.4 Correlation between microstructure evolution and hot deformation behavior during 700oC compression
5.4.1 Mechanical behavior
5.4.2 Correlation between microstructure and deformation behavior
5.5 Correlation between microstructure evolution and hot deformation behavior during 600oC compression
5.5.1 Mechanical behavior
5.5.2 Correlation between microstructure and deformation behavior
5.6 Summary
Chapter 6 Conclusions and Perspectives
6.1 Conclusions
6.2 Perspectives

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