A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-Based Single Crystal Superalloys

Evolution of local misorientations in the γ/γ'-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ'-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ'-neighborhoods, on a size scale above characteristic γ/γ'-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. RESEARCH HIGHLIGHTS: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ' phases, with accuracy of 0.01°. Separation of γ/γ' phases using experimental post-processing of raw EBSD data.

Effect of Blade Geometry on γ' Lattice Parameter and Primary Orientation of SX Cored Turbine Blades (I)

The γ' lattice parameter a_γ' and the α angle defining the primary crystal orientation of the single-crystalline cored turbine blades made of CMSX-4 superalloy were measured in the areas located near the selector situated asymmetrically, considering the top view of the blade. The distributions of the a_γ' and the α angle were determined along the lines parallel to the vertical blade axis Z using X-ray diffraction methods. The relations between changes in the a_γ'(Z) and α(Z) were analyzed on the Z levels where the shape of the blade's cross-section changes. For the first time, the local increase in a_γ'(Z) was found near the root-airfoil connection level and near certain other root levels, which is related to the change in blade section shapes on such levels. The local extremes in α(Z), representing the dendrite bend, were observed at these levels. The increase in the a_γ'(Z) with the local bending of dendrites was discussed concerning the local redistribution of alloying elements and local residual stresses of the γ-dendrites. For the first time, a method of analyzing the local bending of the dendrites was proposed by studying the behavior of the α(Z). The presented results concern the first stage of the research covering areas relatively close to the selector, considering the top view of the blades. The second stage will include the analysis of the areas of the blade localized at a longer distance from the selector.

The Cooling Rate and Residual Stresses in an AISI 310 Laser Weld: A Meso-Scale Approach

A three-dimensional coupled temperature-displacement finite element model was developed to generate values of temperature distribution, cooling rate, and residual stresses at the meso-scale level in a thick sheet AISI 310 laser welding test sample. High cooling rates (cooling time from liquid-to-solid temperatures) ranging from 960 °C/s to 2400 °C/s were observed when the calculations were made at the meso-scale level. These high cooling rates that arise during the formation of the weld pool originate the highest observed residual stresses that evolve throughout the weld during the entire heating and cooling cycles. An ABAQUS CAE meso model with dimensions of 10 × 5 × 1 mm (element size 100 µ) constructed from a global macro model of 40 × 10 × 75 mm (element size 1 mm) via the submodeling technique is presented in the present paper. In both analyses, macro and meso, the C3D8T thermally coupled brick, trilinear displacement and temperature elements were used. To mesh the entire plate with elements of regular size 100 × 100 × 100 µ, a total of 30 million elements are necessary. With the present approach, 1 macro mesh of 30 thousand elements (1 × 1 × 1 mm) and a meso mesh of 50 thousand elements (100 × 100 × 100 µ) were enough to simulate the weld problem at the meso-scale level.

Correlation between the Dendritic Structure and Lattice Parameter of γ′-Phase in Single-Crystalline Turbine Blades Made of Superalloys

The dendritic structure and the distribution of the γ′-phase lattice parameter (a_γ′) along selected lines of the longitudinal section in a model single-crystalline blade made of CMSX-4^® nickel-based superalloy were studied. It was established that there is a correlation between the value of the a_γ′ and the predomination of initial or ending fragments of the secondary dendrite arms. It is most noticed for the areas where the dendrite growth conditions are similar to steady. They are located in the center and near the root’s selector extension (SE) area. The correlation has been related to the dendritic segregation mechanism. It was shown that in the single-crystalline blades obtained by the directional crystallization using a spiral selector, the “walls” of the primary dendrite arms that grow at a low angle to the blade axis are created. It was found for the first time that the value of the lattice parameter a_γ′ is decreased near such “walls”. Additionally, it was found that competitive growth of the dendrites may occur at a distance of even several millimeters from the bottom surface of the root. The first-time applied X-ray diffraction measurements of a_γ′ made in a single-pass along the line allow the analysis of the dendritic segregation in the whole blade cast.