Mechanical Behavior and Processing of DS and Single Crystal Superalloys

Abstract

This article examines mechanical anisotropy of single crystals, cold work induced surface recrystallization on directionally solidified (DS) materials, and the effect of temperature gradient in a DS furnace on the fatigue strength of single crystals. It draws attention to the highly anisotropic creep behavior of some modern single crystal alloys showing, in particular, extremely poor creep resistance in the <111> orientation. Effects of surface recrystallization on the creep strength are evaluated. The present work incites further investigation on heat treatments and alloy chemistry modifications in order to reduce the effect of mechanical anisotropy. Great care should be taken during the "mechanical" handling of DS or single crystal components to avoid surface recrystallization. HIP'ing or high gradient solidification are shown to be two possible ways for enhancing the durability and the fatigue strength of single crystal superalloys. In certain liquid fuel rocket engine applications, where hydrogen embrittlement of single crystal turbopump blades can be of concern, both these techniques can be useful.

Plastic Strain Localization in Superalloy Single Crystals.

“…These fatigue lives were comparable to those reported elsewhere for other superalloy single crystals (Ref. 24,28). The evidence indicates that other single crystal alloys originate fatigue cracks by a similiar mechanism.…”

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

“…This is due to the so-called thin wall effect. [3] The specimens with ''thin wall'' are more sensitive to defects. For the same crack length, the stress carried by the remaining material is much higher in the sample with thinner gage thickness.…”

“…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.…”

“…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.…”

Dynamic Recrystallization Behavior of a Directionally Solidified Superalloy

“…When DS components are deformed by the improper operations such as severe grit-blating or polisihing and subsequently heated above the static recrystallization temperature, the static recrystallization occurs. [5][6][7][8] The static recrystallization temperature of DS nickel-base superalloy, dominated by the solution behavior of gamma prime (c′), is close to the dissolution temperature of c′ and about 0.8 ∼ 0.9 times the absolute melting temperature. [9][10] In addition, DS components may be dynamically recrystallized because of the rigorous service conditions even if the operation temperature is slightly lower than that of the static recrystallization.…”