Dissertation > Excellent graduate degree dissertation topics show

Study on Formation and Evolution of Nanostructure and Mechanical Properties in Cold Deformed Ti-15-3 Alloy

Author: GuoQiang
Tutor: SunDongLi
School: Harbin Institute of Technology
Course: Materials Science
Keywords: Cold rolling deformation nanostructure apparent activation energy for grain growth orientation relationship mechanical properties
CLC: TG146.23
Type: Master's thesis
Year: 2008
Downloads: 37
Quote: 1
Read: Download Dissertation

Abstract

The Microstructure evolution in cold rolling of Ti-15-3 alloy and the effect of heat trentment on the microstructure and property was investigated in the paper.0.6mm cold-rolled sheet of Ti-15-3 alloy was obtained through five different cold rolling processes(40%/pass-3 passes Single direction cold rolling, 30%/pass-5 passes Single direction cold rolling, 10%/pass-11 passes Single direction cold rolling, 30%/pass-5 passes cross cold rolling, 10%/pass-9 passes cross cold rolling). It can be found that fibrous microstructure in which grains were refined to nanoscale was formed. The size of nanograins in the alloy by three Single direction cold rolling treatment was below 50nm, the electron diffraction pattern were continuous polycrystal circles. However, nanograins in the alloy by two cross cold rolling treatment were larger than that by three Single direction cold rolling treatments, electron diffraction pattern were uncontinuous polycrystal circles. It was shown from XRD of simples after five different cold rolling processes that the widths of diffraction peak were broaden, which further testify the existence of nanograin. For five different cold rolling processes, tensile strength increased immediately at the beginning and decreased in the end with increase of cold rolling pass.It was shown that the formation of nanostructure during colding rolling included there steps: first, partial shear bands due to Elastic instability; second, the crossing and refinement of paralleled shear bands of different direction within the banded structure; third, the formation of wide-region fibrous microstructure containing completely refined grains.For the alloy after the 40%/pass-3 passes cold rolling treatment , The grain growth kinetics curves of the nanograins between 450℃～800℃were obtained after heat treatment. The apparent activation energy for grain growth at single phase region and two phase region was calculated through Arrhenius equation, for the heat treatment from 450℃to 650℃,Q1=79.3KJ/mol, for the heat treatment from 750℃to 800℃, Q2=200.8KJ/mol.The variation ofαprecipitate phase and orientation relationship of two phase in the alloy after the 40%/pass-3 passes cold rolling treatment and heat treatment from 450℃to 650℃were systematically studied in the paper. For the non-nanostructure region, needlelikeαprecipitates formed at 450℃; they grew from needle to lath-shaped during heating at 550℃and finally became thin-plate when the heating temperature rose up 650℃. For the nanostructure region,αprecipitate phase was thin-needle during the heating from 450℃to 650℃. The orientation relationship betweenβphase andαphase was obtained from electron diffraction pattern: <110>α‖<111>β,(001)α‖{110}β. Andαphase had twelve possible orientation relationship withβphase, the habit planes of variants were {111}β.After 40%/pass-3 passes cold rolled and heated at 450℃for 4h , hardness of the alloy got the peak(HV532.8). When heated at 550℃, the time of peak was 30min, but peak hardness decreased sharply. It was shown that the hardness of the alloy was below as-cold rolled at all times when heating at 650℃.Variantion of strength during the different heat treatment was simlar to the variantion of hardness. After 40%/pass -3 passes cold rolled and heated at 450℃for 4h , strength of the alloy can be increased to 1562MPa , which was about 830MPa higher than that after solid-solution treatment. However , the elongation was only 1.8%. Variantion of these properties are related to the morphology, dispersity and volume ofαprecipitate phase during heating.

Related Dissertations

CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Titanium
© 2012 www.DissertationTopic.Net Mobile