The Effects of Re, W and Ru on Microsegregation Behaviour in Single Crystal Superalloy Systems

Abstract

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.

Creep Rupture Strength of Re and Ru Containing Experimental Nickel-Base Superalloys

“…As known from literature, refractory elements in general, and Re in particular, feature a high segregation tendency to the dendrite core during solidification [12,[24][25][26]. Since the dendrite is the first solid formed, the high dendritic refractory fraction accordingly causes high T L .…”

Design of Solutionizing Heat Treatments for an Experimental Single Crystal Superalloys

“…The alloy was cast with a withdrawal rate of 5.65×10 -5 m/s (8.0 in/hr) into SX bars of 16 mm diameter and 200mm length in a Bridgman furnace using PWA 1484 SX starter seeds [4]. The SX bars with the longitudinal axes along <001> were sectioned in the transverse direction to obtain discs of approximately 6 mm thickness.…”

“…The SX bars with the longitudinal axes along <001> were sectioned in the transverse direction to obtain discs of approximately 6 mm thickness. [4]. To track the microstructural evolution during the stepwise heat treatments specimens were quenched in water after each step.…”

“…It has been established that a sharp compositional gradient exists in the as-cast alloy from the interdendritic eutectic to dendrite core region along [011] type direction [4]. Hence, studying the variation of this composition profile after each heat treatment step gives an estimate of degree of homogenization and hence the effectiveness of the heat treatment step.…”

“…To increase the high temperature capabilities, modern single crystal (SX) superalloys are alloyed with the refractory elements, W, Ta, and Re [3]. As a consequence, the microsegregation of these alloys is being more pronounced leading to higher eutectic fractions [4] and chemical instabilities [5]. Hence, heat treatment of modern SX alloys is becoming an increasingly challenging, time consuming and expensive process [6].…”