Ti-6Al-7Nb顯微組織對腐蝕與機械性質之影響研究

Abstract

本研究係針對Ti-6Al-7Nb鈦合金，探討試片經過不同溫度固溶與冷卻速率後之顯微組織變化，並選擇部分試片於乳酸林格氏與0.9 wt. % NaCl溶液進行極化試驗。由於鈦合金試片表面之緻密氧化層，測試結果皆有大範圍鈍態區，故試片之腐蝕性評估係以鈍態電流密度做為指標，此值越低抗蝕性越佳。此外，本實驗亦針對腐蝕性質較佳之熱處理試片進行拉伸與缺口拉伸測試，而後評估不同顯微組織對腐蝕與機械性質之影響。 Ti-6Al-7Nb為α + β雙相鈦合金，其在1600 ˚F至1800 ˚F間熱處理時，β 相含量隨著溫度上升而遞增，水淬後完全變態為 α' 麻田散體，空冷後轉變為細小魏德曼組織，爐冷試片之 β 相則主要集中於晶界。若固溶溫度超過 Tβ 溫度( 1864 ˚F )，水淬、空冷與爐冷後之試片顯微組織則分別為α' 相、細小與粗大魏德曼組織。在 Tβ 溫度附近固溶之爐冷試片，可觀察到α 與 β 兩相間，因體積限制與成分梯度而形成之FCC雙晶結構過渡相。此界面相與 α 、 β相之方位關係與Ti-6Al-4V類似，即有 [-111]β

[110] FCC

[11-20]α與 (01-1)β

(11-1)FCC

(0002) α的方位關係。 極化試驗結果顯示：0.9 wt. % NaCl溶液含有較高濃度之 [ Cl- ]，易破壞表面之氧化層，故較乳酸林格氏溶液有更高的鈍態電流密度。水淬試片之α' 相增加會使腐蝕性質變差；成分組成差異較大之爐冷α / β界面依固溶溫度上升而遞增，使其抗蝕性下降；空冷試片較無優選腐蝕情形，抗蝕性隨 α 相減少而有略微增加的趨勢。總體而言，水淬者之抗蝕性最差，爐冷次之，空冷者最佳。拉伸試驗結果指出，試片於雙相區固溶後空冷，其抗拉強度與延性隨著固溶處理溫度上升而遞減。低溫固溶之1600 A試片晶粒細微，故有較佳之機械性質；超過 Tβ 溫度固溶之試片，晶粒粗大而使延性下降。

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

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.