Dendrite Bending during Directional Solidification

Abstract

In order to ascertain the origin of deformation and sub-grains in single crystal castings, an in situ, time-resolved X-ray radiographic imaging experiment was performed at the BM05 beam line of the European Synchrotron Radiation Facility (ESRF). This provided direct experimental evidence of the origin of crystallographic misorientation during the directional solidification of the nickel-based superalloy CMSX4. Solidification monitoring revealed that dendrites deform by bending and possibly torsion, and with deformation occurring early during dendrite growth, influencing the establishment of the dendritic array. These observations were related with electron backscattered diffraction data from ex situ samples. The bending and torsion contributions to deformation were extracted from the electron backscattered diffraction data using a data analysis scheme capable of isolating the deformation modes. In the component tested, bending was identified as the predominant deformation mode in sub-grain formation, but torsion effects were also seen to be significant.

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

“…A good overview on crystal mosaicity in SX has been provided in reference [30]. Mosaicity is caused by the deformation of dendrites during solidification [37][38][39][40][41][42][43]. As each dendrite is expected to grow along its crystallographically preferred direction [44], the preparation of a perfect single crystal requires that all dendrites grow exactly parallel.…”

“…Dendrite deformation results in small crystal lattice rotations and thus in non-parallel growth, which gives rise to mosaicity. Possible reasons for dendrite deformation have been discussed in [29,41,42,[45][46][47][48][49][50][51]. Doherty [29] suggested two categories of dendrite deformation processes, namely, morphological and mechanical bending.…”

“…In contrast, mechanical bending is related to mechanical stresses that cause crystal lattice rotations. It has been documented in the literature [41,42] by combined in situ solidification and radiography experiments using small volume samples that dendrites are mainly deformed by bending. This is in line with findings observed by post-mortem electron back scatter diffraction (EBSD) [37][38][39]42,52].…”

2D single crystal Bragg-dip mapping by time-of-flight energy-resolved neutron imaging on IMAT@ISIS

“…Consequently, these methods elucidate little information regarding the influence of bulk variations in the solidification environment on resultant (001) misorientation 17 . Synchrotron topography studies have directly observed metallic deformation during solidification 18 , 19 . However, low X-ray penetration depth within metals has restricted investigation to samples with volumes orders of magnitude smaller than typical single crystal turbine blades.…”

Characterization of Tilt and Twist Low Angle Grain Boundaries and Their Effects on Intermediate-Temperature Creep Deformation Behaviour

“…Single crystal (SX) Ni-based superalloys are widely used as blade materials in advanced aeroengines and industrial gas turbines (IGTs) because of their excellent mechanical properties. However, when casting these SX components especially large-size IGT blades, there is a high probability of inducing LAGBs due to the intersection or distortion of primary dendrites during directional solidification [1,2]. An elastically distorted region is generated around LAGBs, and the resulting local strain field interacts with the moving dislocations during deformation, which may lead to premature damage.…”