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이 논문의 연구 히스토리 (3)
2019
(The) Effect of Double Stage Solution Treatment on the Volume Fraction of Massive Phase (αm) and the Mechanical property of Ti-6Al-4V Alloy
Ghozali Sup
,
obo
첨단뿌리산업공학과
2019.01
학위논문
Thermal Decomposition of Massive Phase to Fine Lamellar α/β in Ti–6Al–4V Additively Manufactured Alloy by Directed Energy Deposition
Ghozali Sup
,
obo
Metals and Materials International
2019.01
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The effect of double stage solution treatment (DSST) on the volume fraction of massive phase (αm) and mechanical property of Ti-6Al-4V alloy were successfully clarified in the present study. The microstructure of Ti-6Al-4V alloy is dependent on the cooling rate during a cooling from the β phase region. The αm, which has a microstructure characteristic between α′ martensite and lamellar α+β diffusion, has been reported to occur by introducing a cooling rate between 20 °C/s and 410 °C/s. Even though αm was observed as a featureless area using optical microscopy (OM), a higher magnification of scanning electron microscopy revealed α sub-lamellar morphology with two distinguishable regions: the clear and the white-dot region (rich in V content). The volume fraction of αm was evaluated after DSST with a first annealing above the β-transus temperature, followed by a second annealing above martensite start temperature with varying holding times. The second solution treatment at 850 °C for 1800 s yielded the highest fraction of αm (22.4%), which was confirmed with OM and further supported by the phase maps in the electron backscatter diffraction results. The hardness property of αm was positioned between α′ martensite and lamellar α+β diffusion since its morphology resembles lamellar while containing a high dislocation density similar to α′ martensite. However, the tensile test result shows that an increase in the αm fraction during a DSST was accompanied with the drop of the ultimate tensile strength and increase in the elongation. It was occurred since the fraction of α′ martensite decreased with the increase in αm volume fraction.
목차
CHAPTER 1 INTRODUCTION 11.1 Research Background 11.2 Research Objective 3CHAPTER 2 LITERATURE REVIEW 52.1 Titanium Alloys 52.1.1 Ti-6A-4V alloys 72.2 Microstructural Design in α+β Titanium Alloy 82.2.1 Martensitic Transformation 112.2.2 Fully Diffusion Transformation 122.2.3 Massive Transformation 122.3 Double Stage Solution Treatment (DSST) 18CHAPTER 3 EXPERIMENTAL PROCEDURE 203.1 Experimental Procedure for Effect of DSST on the volume fraction of αm 203.1.1 Initial Specimen and Heat Treatment Procedures 203.1.2 Microstructure and Phase Observation 233.2 Experimental Procedure for Effect of DSST on the Mechanical Property of Ti-6Al-4V alloy. 253.2.1 Initial Specimen and Heat Treatment Procedure 253.2.2 Microstructural Observation 263.2.3 Mechanical Testing 26CHAPTER 4 RESULT 284.1 Effect of DSST on the Volume Fraction of αm 284.1.1 Massive Phase Formation after DSST with Different Temperature 284.1.2 Massive Formation after DSST with different holding times at 850 °C 314.2 Effect of DSST on the Mechanical Property of Ti-6Al-4V 404.2.1 Microstructural Observation 404.2.2 Hardness Vickers Result 434.2.3 Tensile Test Result 45CHAPTER 5 DISCUSSION 485.1 Effect of DSST on the Volume Fraction of αm 485.1.1 Effect of Cooling Rates and Massive Phase Transformation Temperature Verification 485.1.2 Formation of αm during Holding at 850 °C 515.1.3 Kinetic for αm phase transformation 545.1.4 Morphology Alteration and Elemental Partitioning during Holding at 850 °C 555.2 Effect of DSST on the Mechanical Property 56CHAPTER 6 CONCLUSION 59CHAPTER 7 FUTURE RESEARCH IMPROVEMENT 61CHAPTER 8 REFERENCES 62CHAPTER 9 ATTENDED CONFERENCES 69CHAPTER 10 LIST OF PUBLICATIONS 71Korean Abstract 72
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