Analysing the tension/compression asymmetry in creep deformed Co‐base superalloys using electron microscopy
Authors: Lenz Malte , Eggeler Yolita , Schweizer Peter , Zenk Christopher , Neumeier Steffen , Göken Mathias , Wollgramm Philip , Eggeler Gunther , Spiecker Erdmann 
Published: December 2016

Co‐based superalloys with a composition of Co‐9Al‐9W exhibit a stable γ/γ′ microstructure at 900°C [1], well known from Ni‐based superalloys. They are used for high temperature applications, for example first stage turbine blades. Shifting the base element of alloys from nickel to cobalt has promising benefits, such as higher melting points as well as a positive lattice misfit between the γ‐ and γ′‐phase.

In the present work a new single crystalline Co‐based superalloy, referred to as ERBOCo‐1, was cast, heat treated and subsequently creep deformed in tension and compression. The alloy contains high amounts of Co (44.5 at%), Ni (32 at%), Al (8 at%), Cr (6 at%), W (5 at%) and four minor alloying elements (Ti, Ta, Si and Hf). After heat treatment ERBOCo‐1 exhibits cuboidal γ′‐precipitates coherently embedded in a solid solution γ‐matrix (fcc). The precipitates have an average edge length of 610±125 nm and a projected area fraction of 84 %. Series of creep experiments were conducted under 400 MPa along the [001]‐direction at 850 °C and were interrupted at 0.3 % and 5 % plastic strain. The creep experiments show a pronounced tension/compression asymmetry concerning the total creep times, the minimal creep rates and the trend of the creep rates towards higher strains (see graphs in figures 1 and 2).

Complementary electron microscopy techniques (TEM, STEM, SEM) were applied to study the defect generation and evolution during tensile and compressive creep, which lead to the asymmetric material response. In tension a/2<112>‐shear is found to be the predominant mechanism governing early creep deformation. Two partial dislocations with Burgers vectors a/3<112> and a/6<112> cut through the γ′‐phase leaving behind a defect configuration where superlattice intrinsic stacking faults (SISF) are fully enclosed by an anti‐phase boundary (APB) (see figure 1, center and schematic) [2]. In contrast, early creep in compressively deformed specimens is goverened by glide of a/2<110>‐type matrix dislocations until a channel dislocation network is established (see figure 2, center and schematic). In addition, stacking faults (SFs) in γ′‐precipitates and occasionally extending over γ‐channels and precipitates are observed indicating glide of partial dislocations (see figure 2, center).

The altered microstructure of the 5 % strained samples shows a high SISF/SESF density and a pronounced matrix channel dislocation network for tensile (see figure 1, right) and compressive testing (see figure 2, right). Interestingly, directional coarsening of the precipitates, i.e. rafting, only occurs during tensile deformation.

In addition to the CTEM analysis further investigations of dislocations and planar faults by application of techniques like HRTEM and LACBED are presented for a better understanding of the underlying mechanisms leading to the asymmetric nature of creep deformation in single crystalline Co‐base superalloys.