Characterization of Ti-6%Al-4%V and VascoMax C-350

Sunwoo, A J

Lawrence Livermore National Laboratory

关键词: Tensile Properties;    Lawrence Livermore National Laboratory;    Alloys;    Irradiation;    Solution Heat;   

DOI  :  10.2172/15015902
RP-ID  :  UCRL-TR-211573
RP-ID  :  W-7405-ENG-48
RP-ID  :  15015902

美国|英语

来源: UNT Digital Library

【 摘 要 】

The {alpha}-{beta} Ti-6% Al-4% V (Ti64) alloy can be heat treated to meet the specified requirements of the applications. The as-received material from SLAC was given a solution heat treatment (SHT) to have a good strength and ductility combination. The SHT was done at 200 C below the Beta transus of 990 C for 15 min and air-cooled to 20 C. The designed microstructure consists of {beta} phase precipitates within the {alpha} phase matrix. The characterization of the as-received Ti64 alloy sheet microstructure reveals equiaxed, 10 {micro}m-sized grains on the flat surface and finer, 8 {micro}m-sized grains in the through thickness. Figures 1 and 2 show the microstructure of the alloy. The typical Ti64 microstructure is lamellar structure, consisting of alternating {alpha} and {beta} phases. In order for the alloy to have the micron sized, equiaxed grains, it had to undergo extensive wrought processing. The Vicker's microhardness numbers (VHN) showed that the slightly larger grained flat surface had a higher averaged value than the through thickness; 33 kg/mm{sup 2} vs. 30 kg/mm{sup 2}. The residual effect of wrought processing is still present even after the SHT to cause the small difference in the hardness values. The results of tensile tests conducted at LLNL and at BNL are given in Tables 2 and 1 in Appendices 1 and 2, respectively. The effects of the irradiation dosage damage on the tensile properties of the Ti64 are presented in Appendix 2. The as-received tensile specimens are not the standard specimens for testing. As shown in Attachment, Figure 1, only the 6 mm length is used in the reduced gage section of the specimens. As a result, a small change in the gage length will translate to a higher percentage change in elongation, giving higher elongation values than using the 30 mm length of the specimen. Since most of the deformation is concentrated in the reduced gage section, the present results are more accurate measurement of ductility. The Ti64 specimens failed in the center of the gage section. The Ti64 alloy contains extra low interstitials (ELI). There are an advantage and a disadvantage to having ELI. The advantage is that the alloy can exhibit good ductility since there is no effective deformation hindrance to mitigate the deformation twinning. The strength of the alloy is predominantly obtained from fine, equiaxed a grains. Given only the SHT, the alloy exhibits an adequate strength and ductility combination. The disadvantage of ELI is that the Young's modulus is sensitive to composition and heat treatment. The present alloy displays slightly lower Young's modulus values than the reported value of 108 GPa with the comparable SHT [1]. If needed, higher Young's modulus and strength can be obtained with subsequent aging treatment. The fracture surfaces of the specimens suggest ductile, dimple failure. Figure 3 shows the representative fracture characteristics of SHT Ti64. The cross-sectional view of the broken specimen has the appearance of extensive deformation prior to fracture (see Fig. 3a). The contribution of microstructure is clearly seen in the fracture surface, where the larger dimples represent the {alpha} phase since the dimple sizes are equivalent to the grain size. The smaller dimples are surmised to be {beta} precipitates. In addition to dimples, the fracture surface contained some sheared grains, indicated by the arrow in Figure 3b. It is inferred that since the deformation mechanism is twinning, it could be the twin interface separation.