GRADAR: orientation-map-driven detection and ranking of grains for targeted two-beam electron channeling contrast imaging

Abstract

Electron channeling contrast imaging (ECCI) resolves individual dislocations in bulk samples, but only when the imaged grain is held in a two-beam diffraction condition to well under half a degree. In a polycrystal it must be re-established grain by grain, and every published workflow we are aware of chooses the grain first and solves the stage for that orientation. This paper treats the inverse problem: nominate one reflection family and a stage envelope, and determine which grains of an EBSD-mapped polycrystal can reach that family's two-beam condition, a question posed in 2019 and open since. At fixed tilt $t$, stage rotation sweeps the incident beam around a cone of half-angle $t$ in each grain's crystal frame, and a family with Bragg angle $\theta_B$ is reachable exactly when one of its plane normals lies within $(t + \theta_B)$ of the specimen surface. On a measured 163-grain austenitic stainless steel map at 20 kV, {111} reachability rises from 38.7% at 7 deg tilt to 77.3% at 15 deg; {220} and {311} already reach 79.8% and 91.4% at 7 deg. Ranking survivors by predicted darkness alone is wrong: same-family candidates are equivalent with respect to their own band at exact Bragg incidence, and the darkest tend to lie closest to rival-band Bragg conditions. GRADAR therefore ranks darkest-clean, gating every candidate on a minimum clearance from all rivals, and returns a dial-ready stage move per selected grain. Because the beam moves only ~0.12 deg per degree of stage rotation at 7 deg tilt, an ordinary rotation stage sets the selected condition to better than 0.1 deg. The predicted sweep is checked against the AstroECP dataset's published silicon precession series: simulated and measured traces agree at r = 0.84, and the measured intensity is dark at all 16 predicted two-beam edges. Per-grain polycrystal validation is deferred to a companion study.