After the first report for the characterization of the $Ti_2AlNb$ phase (orthorhombic structure, O-phase) in the Ti-Al alloy system, $Ti_2AlNb$-phase alloys have been studied as a potential material for high temperature applications. The $Ti_2AlNb$-based alloys having two phases (O and bcc phases) are known to have better mechanical properties than conventional alpha-2 based alloys. The creep properties and creep deformation mechanisms of $Ti_2AlNb$ based intermetallic alloy and particulate composite are investigated in this study.
The compositions of the test materials are Ti-22Al-27Nb, Ti-22Al-20Nb-2W (at.%) and Ti-22Al-27Nb-6.5TiB (mass%). The former and the latter alloys are hereafter referred to as a reference and W-modified alloy and the last material will be called as a composite, respectively. These alloys and composite were made by powder metallurgy process. The powders were prepared by gas atomizing method using cast ingots. Elemental powders were filled in a stainless can and hot pressed under the following conditions: 1100℃/200MPa/3hrs. After hot pressing, the consequently formed bar was rolled and solution treated at 1250℃/1hr followed by controlled cooling at a rate of 0.03℃/s. Subsequently, stabilization annealing at 850℃/33hrs was followed by air-cooling. Constant tensile stress creep tests were carried out at 600~800℃/200~250MPa using a creep machine equipped with a constant Andrade-Chalmers arm. A linear variable differential transformer (LVDT) and a LVDT signal conditioner were used to measure the creep strain and the displacement measurement accuracy was $1＼times 10^{-5}m$.
Two different creep deformation mechanisms are investigated in the temperature range of 600~800℃. Below 700℃, dislocation climb by pipe-diffusion may control the creep deformation rate and the creep resistance of test materials and composite are similar. Because the activity and density of dislocations in all test materials are similar in this temperature range. However, the creep behaviors are quite different above 700℃. In the high temperature regime, abnormal acceleration of creep rate is observed in the reference alloy. In this temperature regime, the density and activity of dislocations are higher than those below 700℃. Additionally, a multiplication of slip systems are occurred. However, this multiplication of slip system is unusual phenomenon. During creep deformation above 700℃, only the density of prismatic is rapidely increased. These numerous prismatic dislocations are generated by the bcc→O phase transformation during creep deformation. The temperature range of the abnormal acceleration of creep rate is same with that of the generation of prismatic dislocations by the phase transformation. When the creep test of reference alloy is carried out under vacuum condition, the generation of prismatic dislocations is restricted due to the limited oxygen level and the creep resistance under vacuum environment is increased. Because the limited oxygen level during creep deformation above 700℃ can keep the bcc phase stable. Therefore, the prismatic dislocations cannot supported during creep deformation at vacuum condition and the creep rate is decreased. The activation energy of creep above 700℃ is measured to be 250kJ/mol. The bcc to O phase transformation energy is also mesured to be around 250kJ/mol by DTA analysis. Considering all analyzing results, the creep deformation mechanism of $Ti_2AlNb$ based alloy above 700℃ may be the bcc to O phase transformation with generating prismatic dislocations.
The creep resistance of W-modified alloy is remarkably enhanced above 700℃. Because the tungsten is very strong beta stabilizer, the bcc phase of W-modified alloy can be stable during creep deformation without any transformation. Therefore, the creep resistance can be rightly improved. The restriction of bcc to O phase transformation is very useful way to improve creep deformation by the limited support of prismatic dislocations. A surface coating is also investigated to improve the creep resistance of the reference alloy. After surface coating, the bcc phase can be stable during creep deformation by the formation of oxygen diffusion barrier and the creep resistance is improved.
Although the prismatic dislocations are generated during creep deformation, the creep resistance can be improved by the restriction of dislocation movement. Fine micro-structued $Ti_2AlNb$ based alloy made by additional heat treatment and TiB particulated composite have better creep resistance than that of the reference alloy. Fine O phase laths and TiB reinforcements can block the movement of prismatic dislocations generated by the phase transformation.
A new alloy design concept of $Ti_2AlNb$ based alloy can be suggested with referring the creep deformation mechanism investigated in this study. The bcc phase should be stable during creep deformation to suppress the generation of prismatic dislocations and the movement of prismatic dislocations should be restricted. Additionally, the Nb composition should keep at least 15 at.% to maintain orthorhombic structure and the weight of $Ti_2AlNb$ based alloy should be decreased than that of the reference alloy. Vanadium, chromium and molybdenum can be considered as alloying elements. Those elements are well known beta-stabilizer. The density of suggested elements is lower than that of tungsten. The crystal structure of the elements is same with that of niobium and atomic radius of those are similar with that of niobium. Therefore, the suggested elements can exchange the position of Nb in the orthorhombic crystal cell accompanying with maintaining stable bcc phase.
