The dislocation behaviour and GND development in a nickel based superalloy during creep

Abstract

In the current study, dislocation activity and storage during creep deformation in a nickel based superalloy (Waspaloy) was investigated, focussing on the storage of geometrically necessary (GND) and statistically stored (SSD) dislocations. Two methods of GND density calculations were used, namely; EBSD Hough Transformation and HR-EBSD Cross Correlation based methods. The storage of dislocations, including SSDs, was investigated by means of TEM imaging. Here, the concept of GND accumulation in soft and hard grains and the effect of neighbouring grain orientation on total dislocation density was examined. Furthermore, the influence of applied stress (below and above Waspaloy yield stress) during creep on deformation micro-mechanism and dislocation density was studied. It was demonstrated that soft grains provided pure shear conditions at least on two octahedral (111) slips for easy dislocation movement reaching the grain boundary without significant geometrically necessary accumulation in the centre of the grain. Hence, the majority of the soft grains appeared to have minimum GND density in the centre of the grain with high GND accumulation in the vicinity of the grain boundaries. However, the values and width of accumulated GND depended on the surrounding grain orientations. Furthermore, it was shown that the hard grains were not favourably oriented for octahedral slip system activation leading to a grain rotation in order to activate any of the available slip systems. Eventually, (i) the hard grain resistance to deformation and (ii) neighbouring grain resistance for the hard grain reorientation caused high GND density on a number of octahedral (111) slip systems. The results also showed that during creep below the yield stress of Waspaloy (500 MPa/700C), the GND accumulation was relatively low due to insufficient microscopic stress level. However, the regions near grain boundaries showed high GND density.Whereas, in addition to the movement of pre-existing dislocations (SSD and GND) at higher mobility rate under 800 MPa/700C above yield creep condition, large numbers of dislocations were generated and moved toward the grain boundaries. This resulted in much higher GND density but narrower width of high intensity GND near the grain boundaries. It is concluded that although GND measurement by means of EBSD can provide a great insight of dislocation accumulation and its behaviour, it is critical however to consider SSD type which is also contributes to the strain hardening of the materials.

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

“…The GND and slip trace mapping were calculated using in-house python code. The principles and details of the calculations were reported previously in (Liu et al, 2018;Birosca et al, 2014Birosca et al, , 2019. Furthermore, specific site extraction close to initiation sites and beneath fatigue striations for the dislocation analyses were prepared using Focus Ion Beam (FIB) and characterised by means of FEI-Talos 200F utilising Scanning Transmission Electron Microscopy (STEM)-High Angle Annular Dark field (HAADF).…”

How hard metal becomes soft: crystallographic analysis on the mechanical behavior of ultra-coarse cemented carbide

“…As proposed in the literature (Calcagnotto et al, 2010;Jiang et al, 2013;Ruggles et al, 2016;Birosca et al, 2019), the dislocation density can be considered proportional to the misorientation angle between the neighboring WC crystals. Thus, the dislocation density in WC grains can be evaluated by EBSD analysis, as shown in Fig.…”

Texture Evolution and Dislocation Behavior in a Nickel‐Based Superalloy during Hot Compression

“…This finding is likely to be resulted from reducing an internal stress induced by hot compression during the DRX process, as ⟨001⟩ direction in a Ni‐based superalloy exhibits lower elastic modulus . A recent report displayed that the slip system activation is dependent on the original texture and individual grain orientations …”