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The major advancements in mechanical and thermal properties of the most recently developed single crystal (SX) superalloys can be attributed to the addition of specific refractory elements into base alloy compositions. The present study investigates the effect of modifying refractory addition levels on the solidification behaviour of SX superalloys systems. Specifically, a series of six Ni-base alloy compositions are set in a controlled manner, such that the chemical microsegregation effects of Re, W, and Ru can be independently assessed. Fabrication of grain-free SX bars from each alloy composition is achieved by utilizing a modified Bridgman casting process, with subsequent compositional analysis of the solidification structures via electron microprobe analysis (EPMA) methods. Further validation of these EPMA microsegregation results are supported by means of eutectic phase fraction analysis and differential scanning calorimetry (DSC) methods. Qualitative partitioning results indicate typical SX alloy segregation behavior with elements such as Cr, Co, Re, Mo, and W all segregating towards the dendrite core regions, while the forming elements of Al, Ti, and Ta partition to the interdendritic-eutectic regions. Both Ni and Ru exhibit ideal segregation behaviour with no favorable partitioning to either liquid or solid phase. Quantitative EPMA results indicate that as the nominal Re level increases, the severity of microsegregation to the dendrite core regions rises dramatically for Mo, Cr, and Re. Evidence is presented that demonstrates the role that Ru plays in counteracting the microsegregation effects of both increased Re and higher overall total refractory levels. In addition to experimental trials, chemical partitioning predictions are also presented for the alloy system, utilizing a solid-liquid phase equilibria model generated using a customized chemical thermodynamic database. Using this CALPHAD approach, a comparison of the computational predictions and the actual experimental segregation results is also provided for discussion.

XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn ϩ 0.8 vol pct TiB 2 ) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of ␣ 2 to ␥, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in ␣ 2 volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the ␣ 2 lamellae become finer due to dissolution, whereas most of the ␥ lamellae coarsen. The dissolution of ␣ 2 involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of ␥ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in ␣ 2 /␥ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.

A new modified heat treatment has been developed for Inconel 718.

The microstructures and creep properties at 760 °C and 276 MPa of three powder metallurgy TiAl alloys (Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2Nb+0.5W, and Ti-48Al-2Cr-2Nb+1W (atomic percent)) are presented. The results indicate that the addition of W to the base composition, the use of a solution heat treatment combined with controlled cooling (to generate a fully lamellar microstructure), and the use of an aging heat treatment (to generate precipitate particles at the lamellar interfaces) improve creep properties dramatically. The solution heat treated and aged Ti-48Al-2Cr-2Nb+1W alloy has a time to 0.5% strain of 8.3 hours, a time to 1% strain of 46.4 hours, and a creep life of 412 hours with a rupture ductility of 16.9%.

Three kinds of microstructures with different grain boundary morphologies and their creep properties of a Ti-47Al-2Nb-2Mn+0.8 vol%TiB 2 alloy are investigated. Tensile creep tests and microstructural examinations indicate that a stabilized fine-grained fully lamellar (FGFL) microstructure with relatively smooth grain boundaries shows inferior creep resistance. A stabilized fully lamellar (FL) microstructure with well-interlocked grain boundaries and wider lamellar spacing yields reduced minimum strain rate and extended creep rupture life. Furthermore, a nearly lamellar microstructure (NL) with well-interlocked grain boundaries exhibits better creep resistance than the stabilized FGFL microstructure though it has four times wider lamellar spacing and 15 vol% equiaxed grains at the grain boundaries, but worse creep resistance than the stabilized FL microstructure. Examinations to the deformed microstructures show that grain boundary instability involving spheroidization of the lamellae is a major microstructural degradation process, resulting in fine globular regions at the grain boundaries. Voids develop along the grain boundaries, particularly in the fine globular regions, leading to intergranual fracture. It is suggested that grain boundary sliding (GBS) is operating in the stabilized FGFL microstructure, and promotes mutually with the grain boundary instability during subsequent creep deformation, resulting in increased minimum strain rate and shortened tertiary stage. The well-interlocked grain boundary inhibits the onset of GBS and enhances the grain boundary stability effectively. These results demonstrated that the grain boundary stability has a great effect on creep behavior of TiAl Alloys.

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn 1 0.8 vol pct TiB 2 ) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed g grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed g grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.

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