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

以Ti-43Al-3Si为晶种，可以生长出具有取向片层组织的Ti-47Al合金铸锭。与Ti-47Al合金类似，如果合金化不会导致基础合金的凝固路径发生任何重大变化，则基于该合金的多组分合金应该可以进行晶种处理。当确定合金元素和添加到基钛47Al合金中的每种元素的量时，修改基合金的Al含量以使α/β主区边界的相对位置保持在二元系中。新合金的铝含量可用下列公式计算。C_&lt；Al&gt；=47+ax_&lt；eq&gt；+by_&lt；eq&gt；+Cz_&lt；eq&gt；+...，其中a、b、c和X_&lt；eq&gt；、Y_&lt；eq&gt；、Z_&lt；eq&gt；分别是合金元素X、Y、Z的量和铝当量。当约为46&lt；c；&lt；Al&gt；&lt；48时，可以使用生长…处的Ti-43Al-3Si种子合金的种子沿生长方向对准片层组织蠕变试验以10 mm/h的速度进行。蠕变试验在750℃和240 MPa的空气中进行，试样的厚度尺寸为30 mm，直径为6 mm，从定向凝固的钢锭中切割而成。结果表明，沿加载轴方向排列片层边界，不仅降低了二次蠕变速率，而且降低了一次蠕变应变。二元系Ti-47Al合金在蠕变条件下的二次蠕变速率为2.07×10~(-9)~(-9)~(Gt)，与文献报道的高合金化先进TiAl基多晶合金的二次蠕变速率相当或更低，远高于四元系和五元系合金。值得注意的是，少量Si和过渡金属Mo、W、Re的加入显著提高了TiAl基合金定向凝固铸锭的蠕变抗力。定向凝固的Ti-46Al-0.5Si-0.5W和Ti-46Al-0.5Si-0.5Mo合金的抗氧化性与TiAl基合金中最常见的Ti-48Al-2Cr-2Nb相当。然而，添加3%的Nb显著提高了抗氧化性，而不会对力学性能产生任何显著的影响。较少

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