Peculiar phase transformation and change in mechanical properties during room-temperature aging in bcc Ti alloys

2019 Fiscal Year Final Research Report

• Project/Area Number: 17H03414
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Structural/Functional materials
• Research Institution: Osaka University
• Principal Investigator: Tane Masakazu 大阪大学, 産業科学研究所, 准教授 (80379099)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: 相転移 / 弾性論 / チタン合金

Outline of Final Research Achievements

It was clarified that the formation of diffuse ω structures during aging near room temperature, which leads to an increase in elastic modulus with the aging, is caused by a diffusionless isothermal ω transformation, which is a new type of ω transformation. Furthermore, it was found that the transformation rate of the diffusionless isothermal ω transformation is dominated by two activation processes: a dynamical collapse of {111} atomic plane pairs, corresponding to the elementary process of the phase transition, and a nucleation process of ω phase. In addition, it was clarified that the diffusionless isothermal ω transformation is a peculiar local phase transition, caused by quenched-in statistical compositional fluctuations.

Free Research Field

弾性論、相転移、マイクロメカニックス

Academic Significance and Societal Importance of the Research Achievements

従来から知られている拡散型の等温(熱的)ω変態および無拡散の非熱的(非等温)ω変態とは異なり、室温近傍での時効によって無拡散で時々刻々と変態が乗じる「無拡散等温ω変態」と名付けた新たな相転移を発見した。さらに、無拡散等温ω変態は、熱ゆらぎによって生じる統計的な合金組成のゆらぎが室温で凍結された凍結合金組成ゆらぎによって引き起こされる特異な相転移であることも明らかにした。加えて、無拡散等温ω変態の変態速度における熱活性化過程の詳細を明らかにした。すなわち、本研究成果の最も重要な学術的意義は、新たな相転移を発見し、その相転移のメカニズム(相転移の熱力学と速度論)を解明したことである。