Hi

Enhanced  re-crystallization of copper is produced here by addition of  thermal re-crystallization and quantum electronic activation. Last point probably caused by some chemical recombination energy release (CRER) , intimately associated to the original copper material surface and bulk at shallow depth. CRER  of higher rate and of higher recombination energy is first activating the copper electrons from the Fermi level  to higher conducting level ( before producing heat) and firs step  can enhance  atomic mobility of copper atoms  at  micro-crystalline Cu grain surfaces what is here in favor of  re-crystallization.

This effect is in fact similar to what is enhancing the diamond crystal growth (with the recombination of the dissociated hydrogen, and the enhanced nucleation favored, with adsorption of higher energy and higher rate. and  observed with quite uniform  diamond mono-crystal growth on crystalline  Molybdenum in a continuous process where carbon has been diffusing into Mo first.

In present case, it is suspected copper contamination with the residual vacuum of the system (which at about  10-3 mbar is mainly composed by water vapor and HCx  which can be dissociated in atomic hydrogen and atomic carbon  which both of them can diffuse into the bulk of the hot copper material (the cold trap can pump a lot, but  wall of the vacuum system can in present case desorb more, when contaminated by  water vapor and  molecular retro-diffusion from the primary pump.  Small size Boron atoms deposited on a hot  Cu surface will diffuse into  Cu material and will recombine with the dissolved carbon, or the dissolved atomic hydrogen, releasing high  B-C, H-H bonding energy , what can be added to the high interface BN( ~ 5 eV) or C-C (7eV) bonding energy,  all together in favor of stronger re-crystallization, in so far that considered atoms are small and will not significantly disturb the Cu crystal lattice.

Note that depositing BN and Carbon on the Cu substrate surface, where strong BN and/or C-C will immediately be reformed, will not enhance the Cu re-crystallization, in spite of the  high chemical energy release, because in such a case the CRER effect will not be intimately associated to the Cu bulk and will not surround the existing Cu micro-crysals.

Evident solution:

Use  better  and cleaner secondary vacuum with  10-7 mbar end vacuum residual contamination.

I recomand reading following paper  and book 

  -S.Neuville SCT Vol206 N°4(2011)703 Quantum Electronic Mechanism of atomic rearrangements during growth of hard carbon films    

  -S.Neuville (Book) Lap Academic Publishing . July 2014 (Carbon Structure Analysis by  new  revisited and extended Raman spectroscopy fundamentals) 

All by best.