Origins of misorientation defects in single crystal castings: A time resolved in situ synchrotron X-ray radiography study

Abstract

Abstract. The presence of grain boundaries in single crystal castings is intolerable owing to the detrimental impact on creep and fatigue behaviour. Whilst the origins of many defects such as freckles have been understood since the 1970s, other defects such as slivers, or indeed the small mosaicity observed in many castings have eluded comprehensive treatments. In the present work, in situ X-ray imaging has been used to examine the origin of misorientation defects that arise during solidification. Dendrite deformation was observed, which impacted growth characteristics and led to a permanent misoriention. Digital image correlation analysis showed the dendrites becoming increasingly bent as solidification progressed. In order to probe the deformation modes further, a method was devised to convert standard EBSD data into measurements of bending and torsion angle. It was demonstrated that such defects form as the result of bending moments arising from differential thermal contraction and gravity.

Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades

“…Other strains can be induced in the base metal due to the microscale plasticity which is caused by differential thermal contraction of metal, mould, and core [47][48][49] . It was estimated that when the mould and core do not crush or deform suf ciently or crack the metallic components can impose a strain value of about 2 3% 46) , which is over the critical plastic strain (0.2%) for the recrystallization of turbine blades 47) . These processes can lead to extensive plastic deformation being imparted to an as-cast turbine blade at high temperature and this can be responsible for strain relaxation through recrystallization during subsequent heat treatments.…”

On Crystal Mosaicity in Single Crystal Ni-Based Superalloys

“…Since then, various studies on mosaicity have been conducted considering different types of samples, including cylindrical laboratory specimens [11,32,33], complex superalloy turbine blades [38][39][40][41] and other geometries [42][43][44][45][46]. It has been suggested that SX mosaicity is caused by the deformation of dendrites in the mushy zone during solidification [38,42,43,[46][47][48][49]. In a directional solidification process SXs form by the growth of dendritic arrays.…”

“…In several publications, thermal and shrinkage stresses were invoked to explain the deformation of dendrites [38,[50][51][52][53]. Other explanations, such as convective forces [54][55][56], precipitation-related phenomena [45], interactions with the mold wall [47][48][49], gravity [49] and asymmetric distributions of the solute around dendrite stems [46] have also been put forward. [15,[28][29][30]) affect performance and how they can be controlled, there is one aspect which has received insufficient attention.…”

“…Although it is widely accepted that dendrite bending represents the main origin of mosaicity in SXs [38,42,43,[46][47][48][49], experimental evidence which supports this claim is scarce. Most previous Although it is widely accepted that dendrite bending represents the main origin of mosaicity in SXs [38,42,43,[46][47][48][49], experimental evidence which supports this claim is scarce.…”

“…Although it is widely accepted that dendrite bending represents the main origin of mosaicity in SXs [38,42,43,[46][47][48][49], experimental evidence which supports this claim is scarce. Most previous Although it is widely accepted that dendrite bending represents the main origin of mosaicity in SXs [38,42,43,[46][47][48][49], experimental evidence which supports this claim is scarce. Most previous studies used diffraction-based methods, such as the Laue method [33], (multi-axial) X-ray diffraction analysis [34,[57][58][59], orientation imaging in the scanning electron microscope (SEM) by electron back scatter diffraction (EBSD) [38,42,43,48,53], X-ray topography [39][40][41]44,46,47,49,57,60] and γ-ray characterization [44,45].…”