【作者】 肖伟；

【导师】 郭景杰； 李新中；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 硕士

【摘要】 本文以Ti-Al合金为研究对象,选取Ti-50at.%Al,Ti-52at.%Al两组合金成分,在1μm/s~35μm/s的凝固速度区间内开展了定向凝固实验研究。通过光学金相,电子显微分析技术对从初始过渡到稳态凝固组织的演化规律及其对后续固态相变形成的片层结构取向的关系进行了研究。研究结果表明:随着生长速度的加大,固液界面形态由平到胞、枝再到等轴晶变化;对于Ti-50at.%Al合金,在低速条件下以α单相凝固,随着生长速度的加大,α胞、枝之间出现少量的γ相,α相在凝固过程中始终占优;对于Ti-52at.%Al合金,其凝固组织由α和γ两相组成,随着生长速度的增大,凝固组织由平面α和岛状γ相转变成α和γ胞状共生,再转变成枝晶α和枝晶间γ相,γ相的体积含量逐渐增加,主要由液相直接凝固得到;在较高的生长速度条件下,α初生相以枝晶形态生长,α相与液相的接触面积加大,促进了L +α→γ包晶反应,α枝晶臂被消耗形成细小的α晶粒。α相在后续的固态相变过程中转变成α2/γ片层组织。在通常情况下,由于α母相与片层组织中γ相确定的位向关系,得到片层取向与生长方向垂直的α2/γ片层组织。但是当α相以铸态晶粒为形核衬底时,α2/γ片层保留了原始铸态晶粒内的片层取向。同时在高的生长速度条件下,定向凝固过程中形成了大量枝晶主干偏离热流方向的α枝晶,当熔体前方新形成的枝晶以之为形核衬底时,保留了原有枝晶内的片层取向。因此可以利用母体合金本身“自引晶”来控制片层取向。更多还原

【Abstract】 In this paper, Ti-Al alloys with two compositions of Ti-50at.%Al and Ti-52at.%Al were studied with growth rate range from 1μm/s to 35μm/s. The solidification structure evolution from initial transition region to steady state region and its influence on theα2/γlamellar orientation were analyzed.The results show that the interface with planar front evolves to cell, dendrite and equiaxed dendrite with the increase of growth rate. Ti-50at.%Al alloy solidifies as singleαphase at low growth rate. As the growth rate increases, there forms a littleγat intercells or interdendrites.αphase dominates during the whole solidification process. Ti-52at.%Al alloy solidifies asαandγphases. With the increase of growth rate, the planarαand islandγphase evolves toαandγcellular coupled growth, then toαdendrites and interdendriteγ.γphase is mainly obtained by directly solidified liquid and its volume fraction increases as growth rate increases. At high growth rate, primaryαphase grows as dendrites and contacts more liquid, which contributes to the L +α→γperitectic reaction. During the process,αdendrites are consumed to form fineαgrains.αphase transforms toα2/γlamellar in subsequent solid phase transformation. In general, the lamellar orientation is perpendicular to growth direction because of the definite orientation betweenαandγphases. Whenαphase nucleates on the cast grains, the lamellar orientation within cast grains is maintained. At high growth rate, there forms manyαgrains whose growth direction deviate from heat flow. When new dendrites nucleate on those grains, the lamellar orientation is maintained. Thus, it is possible to develop“self seeding”technology to control the lamellar orientation.更多还原