Development of high creep-resistant TiAl-base alloys for directional solidification and measurements of their creep resistance using large-sized specimens

使用Ti-43Al-3Si种子合金可以生长出具有排列层状组织的Ti-47Al合金锭。与Ti-47Al合金类似，如果合金化不会对基体合金的凝固路径造成任何显著变化，那么基于Ti-47Al合金的多组分合金应该可以进行播种。确定基体Ti-47Al合金的合金元素和各元素添加量后，对基体Al含量进行调整，使α/β初生区边界的相对位置与二元体系保持一致。新合金C_< ai>的Al含量可由下式计算。c_<Al>=47 + aX_<eq> + bY_<eq> + cZ_<eq> +…式中，a、b、c和X_<eq>、Y_<eq>、Z_<eq>分别为合金元素X、Y、Z的量和al当量。当46 < c_<Al> < 48时，Ti-43Al-3Si种子合金以10mm/h的生长速率沿生长方向排列。采用长30 mm、直径6 mm的定向凝固钢锭试样，在750℃、240MPa的空气中进行蠕变试验。结果表明，沿加载轴方向调整片层边界不仅可以降低合金的二次蠕变速率，还可以降低合金的一次蠕变应变。二元Ti-47Al合金在蠕变条件下的二次蠕变速率为2.07 × 10^<-9> × 10^< -1>，与高合金化高级钛基多晶合金相当或低于，远高于四元合金和五元合金。值得注意的是，添加少量Si和Mo、W、Re等过渡金属可显著提高tial基合金定向凝固锭的抗蠕变性能。Ti-46Al-0.5Si-0.5W和Ti-46Al-0.5Si-0.5Mo合金定向凝固锭的抗氧化性可与Ti-48Al-2Cr-2Nb相比较，Ti-48Al-2Cr-2Nb是最常见的钛基合金之一。然而，添加3% Nb可显著提高抗氧化性能，而不会引起机械性能的显著变化。少

Ingots of Ti-47Al alloy with an aligned lamellar microstructure can be grown using a seed of the Ti-43Al-3Si seed alloy. Similarly to the Ti-47Al alloy, seeding should be possible for multi-component alloys based on this alloy if alloying may not cause any significant change in the solidification path of the base alloy. When the alloying elements and the amount of each element to be added to the base Ti-47Al alloy are determined, the Al content of the base alloy is modified to let the relative position of the α/β primary region boundary remain as that is in the binary system. The Al content of the new alloy, C_<AI> can be calculated using the following equation. c_<Al>=47 + aX_<eq> + bY_<eq> + cZ_<eq> +..., where a, b, c and X_<eq>, Y_<eq>, Z_<eq> are the amount and the Al-equivalent of alloying elements X, Y, Z, respectively. When approximately 46 < c_<Al> < 48, the lamellar microstructure can be aligned along the growth direction using a seed of the Ti-43Al-3Si seed alloy at a growth … More rate of 10mm/h.Creep tests were performed in air at 750℃ and 240MPa using specimens with a gauge size of 30 mm in length and 6 mm in diameter cut from directionally solidified ingots. It was found that not only the secondary creep rate but also primary creep strain were reduced by aligning the lamellar boundaries along the loading axis. The secondary creep rate of the binary Ti-47Al alloy under the creep condition is 2.07 x 10^<-9>s^<-1>, which is still comparable to or lower than that reported for the highly alloyed advanced TiAl-base polycrystalline alloys, is much higher than those for the quaternary and quinary alloys. It is of interest to note that the addition of a small amount of Si and transition metals such as Mo, W and Re dramatically increases creep resistance of directionally solidified ingots of TiAl-base alloys. The oxidation resistance of the directionally solidified ingots of Ti-46Al-0.5Si-0.5W and Ti-46Al-0.5Si-0.5Mo alloys is comparable to that of Ti-48Al-2Cr-2Nb, which is one of the most common TiAl-base alloys. However, adding 3% Nb significantly improves oxidation resistance without giving rise to any significant change in the mechanical properties. Less