| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === This study investigated the effect of microstructure on the corrosion and mechanical properties of Ti-6Al-7Nb alloy. The microstructural evolution of Ti-6Al-7Nb was examined in various combinations of heat treating temperature and cooling rate. To evaluate corrosion behavior of the alloy, polarization tests in lactated Ringer’s and 0.9 wt. % NaCl solutions were conducted on selected specimens. The polarization curves exhibited a wide range of passive regions since the presence of oxides on the surfaces of these specimens. For this reason, passive current density was chosen as the most informative index to estimate the corrosion properties of Ti-6Al-7Nb. Lower passive current density indicates better corrosion properties. Some specimens with good corrosion resistance were also chosen to perform tensile and notch tensile tests. The effect of microstructure on corrosion and mechanical properties of the alloy was then assessed.
Ti-6Al-7Nb belongs to (α + β) type titanium alloy. When the solution temperature was in the range of 1600 to 1800˚F, the amount of β increased with the increasing temperature. The β phase was transformed completely into α′ (HCP martensite) after water quenching, and into a fine Widmanstatten structure after air cooling. For those furnace-cooled specimens which had more time for partitioning of alloying elements, the β phase was mainly located on the grain boundaries. If the samples were solutionized at a temperature higher than the β-transus temperature (1864˚F), the β phase was transformed into α′, fine Widmanstatten and coarse Widmanstatten after water quenching, air and furnace cooling, respectively. The interface phase (fcc), which contains internal twins, was also observed between the α and β phases in the specimen after furnace cooling from near β-transus temperature. The orientation relationships among the α, interface and β phases can be written as β || FCC || α and β || FCC || α , similar to those observed in Ti-6Al-4V.
Polarization test results indicated that 0.9 wt. % NaCl solution tended to destroy the oxide layer due to the existence of higher concentration of chloride ions. As a result, such specimens tested in 0.9 wt. % NaCl solution had a higher value of passive current density than in Ringer’s solution. The corrosion resistance of water-quenched specimens deteriorated as the amount of the α'' phase increased. Significant differences in the composition between the α and β phases were accounted for the deteriorated corrosion resistance of the furnace-cooled specimens. On the other hand, the air-cooled specimens did not show dissolution of specific phases in both solutions and exhibited low passive current densities. In general, the corrosion resistance of variously cooled specimens after heat treatment in the range of 1600 to 1800˚F could be sorted in descending order as follows: air-cooled, furnace-cooled, and water-quenched specimens. Tensile test results also demonstrated that tensile strength and ductility dropped as the heat-treatment temperature increased in the two-phase region, followed by cooling in air. The 1600A specimen, which was heat-treated at 1600˚F and then cooled in air, had better mechanical properties than others, possibly due to relatively smaller grains. In the case of the specimens heat treated at a temperature higher than the β-transus temperature, significant grain growth resulted in large grains that caused poor ductility of the material.
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