The Effects of Tempory Hydrogenation on Microstructural Changes and on Superplastic Deformation Behaviors of Ti-6Al-6V-2Sn Alloys

• Main Authors: Lin, Sang-Chaur, 林聖朝
• Other Authors: Wang Wen-Hsiung
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/75779755227417562901

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 85 === At the beginning of this work, the relations between microstructural changes occurring during superplastic deformation and superplastic deformation behaviors of an annealed Ti-6Al-6V-2Sn rolled plate with 2mm in thickness were investigated. Although the initial microstructure of test material is characterized by mixed long, narrow and equiaxed alpha grains, modification of grain shape was happen to occur during high temperature annealing or during superplastic deformation. Under the effect of thermal and/or mechanical strain energy, long and narrow alpha grains were activated to undergo recovery and to separate into several equiaxed grains by the precipitations of beta phase through alpha subgrain boundaries. It resulted in the achievement of high superplastic deformation capability of test material. Increasing the concentration of hydrogen in titanium alloy by hydrogenation is able to effectively increase the volume fraction of beta phase under fixed temperature condition, but is not necessary to increase the superplasticity of the alloy at the same time.The hydrogenation behavior of Ti-6Al-6V-2Sn alloy with starting microstructures of coarse equiaxed alpha grains and of coarse Widmanstatten alpha grains were evaluated. The effects of hydrogenation on alpha grain transformation can be concluded into two different ways : (1) In the initial peroid of hydrogenation, hydrides precipitated from the alpha grain boundaries and then from the interior of alpha grains. The hydrides were kept going on transformation into beta phase during hydrogenation. That is, the increasing of hydrogen content of alloys initiated the transformation of α(H)＋γ(H)→ β(H) to occur. (2) After the volume fractions of alpha and beta phases achieved equilibrium with the hydrogenating atmosphere, the transformation of α(H)＋γ(H) into β(H) no more occurred. In the following peroid of hydrogenating, the alpha phase was oversaturated with hydrogen, and it resulted in hydride precipitations remained stable within the alpha grain. In additions, hydrogenating specimens at different hydrogen supply rate resulted in different microstructures obtained. Original alpha grains were refined as small alpha particles in hydrogen- stabilizing beta phase matrix for specimens being hydrogenated at high hydrogen supply rate. Indistinguishable grain structure was observed for specimens being hydrogenated at low hydrogen supply rate.Dehydrogenation caused the escape of hydrogen from the hydrogenated alloy and then the dissolution of hydrides. It resulted in highly defective microstructure with high density of dislocations and stacking faults. Dehydrogenation also leads to the formation of new recrystallized alpha grains and the precipitation of fine beta phase from the locations of retained hydride plates. Both are the mechanisms that contribute to the refinement of alpha grains.