Influence of Recrystallization on the High-Temperature Properties of a Directionally Solidified Ni-Base Superalloy

Abstract

The effect of recrystallization (RX) on the high-temperature tensile and creep properties of a directionally solidified (DS) Ni-base superalloy was studied. The tensile properties were not apparently influenced by RX, the depth of which is up to 18 pct of the overall gage thickness. However, an almost linear reduction of the creep rupture life was observed with the increase of the local RX depth. The failure mechanisms of the specimens were discussed based on the microstructural observations.Recrystallization (RX) induced by residual strains in directionally solidified (DS) Ni-base superalloys is a well-known issue in the investment casting industry. [1] It is generally accepted that the newly formed RX grain boundaries, particularly the transverse grain boundaries, are weak sites during service and may degrade the mechanical properties of the DS alloys. Therefore, strict specifications of RX grains have been proved in foundry manufacturing of the DS components. On the other hand, however, the data concerning the effect of RX on the mechanical properties of DS alloys is surprisingly limited in the open literature. Studies on different alloy systems under different conditions have not yet produced conclusive results. [2][3][4] The purpose of the present investigation was to study the effect of local RX on the high-temperature tensile and creep rupture properties of an experimental DS Nibase superalloy and to explore the role of the RX based on the microstructural observation.The nominal composition of the DS alloy studied is 9Cr, 10Co, 7W, 2Mo, 5Al, 3.5Ti, 4Ta, 0.1C, 0.01B, and balance Ni, in wt pct. The alloy was directionally solidified into a plate with the size of 220 · 70 · 16 mm using a Bridgman furnace. Details of the DS process were reported elsewhere. [5] The resulted DS slab has a primary dendrite arm spacing around 300 lm.Plates were cut by electrodischarge machining and indented on one side using a Brinell hardness tester. After full heat treatment (1220°C/2 h + 1080°C/ 4 h + 900°C/16 h) during which the RX occurred, the indentation was carefully removed by grinding. Based on the relationship between the depth of the local RX and the applied load obtained from our previous work, [6,7] specimens with different depths of local RX were obtained by applying different indentation loads and controlling the subsequent grinding process. (The depth of RX was also cross-checked by cutting and measuring some of the machined specimens after full heat treatment.) The plates were subsequently machined on the opposite side to the final dimension (35 · 8 · 5 mm gage section for tensile tests, and 15 · 5 · 2 mm gage section for creep rupture tests). To check the effect of sample thickness, two specimens with 15 · 5 · 0.8 mm gage section (with and without local RX) were also prepared for creep tests. After macroetching the machined specimens, six to eight grains were found for the 2-mm specimens and three to four grains for the 0.8-mm specimens within the gage section. As for specimens with a different number of local R...

Simulation and Experimental Study of Recrystallization Kinetics of Nickel Based Single Crystal Superalloys

“…The disadvantage is that the blades from single crystal superalloys become very more difficult to cast, and great care should be taken to prevent defects such as freckles [1], recrystallization [2] and so on. Recrystallization can introduces high-angle grain boundaries and degrades the creep [3][4][5] and fatigue [6] properties significantly.…”

A study on the effect of composition, and the mechanisms of recrystallisation in single crystal Ni-based superalloys

“…There is considerable evidence that grain boundaries, even at low density, are detrimental for the creep and fatigue properties of the alloy [2][3][4]. Recrystallisation accounts for a conservative 4% of the casting defects [5].…”

“…Several aspects of recrystallisation in Ni-based superalloys have been studied, such as the effect of strain, deformation temperature, annealing conditions (for example [2][3][4][6][7][8]) and the interaction with secondary phases (for example [9][10][11]). There is, however, a lack of understanding for the effect of alloy composition on the recrystallisation of superalloys, and this is more so because the alloys vary in all alloying elements.…”

Intermediate temperature creep of directionally solidified Ni-based superalloy containing local recrystallization

“…It is believed that RX may result in the reduction of the mechanical properties of DS superalloys [11][12][13][14][15][16][17][18]. For example, only ∼5-50% of the creep rupture life can be achieved in DZ22 alloy if a surface RX layer was introduced by shot peening.…”