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Four alloys TMS-75 (Base), Base+B, Base+C and TMD-103 (Base+B+C) with varying levels of B, C were directionally solidified (DS), and their weldability was investigated by bead-onplate electron beam welding.It was found that cracking occurred in the weld-metal, as well as in the heat affected zone (HAZ).Both γ/γ and/or γ/carbide eutectic contributed to the weld-metal cracking, and the γ/γ eutectic was found to be mainly responsible for cracking in HAZ However, constitutional liquation of carbides seems to have made some contribution also.The B addition had a detrimental effect on HAZ cracking susceptibility, while C addition improved it appreciably even in the presence of B.In addition, no obvious differences were observed in cracking susceptibility of weld-metal in the four alloys.The cracking modes and mechanisms underlying the effect of B and C addition on weldability could be explained on the basis of microstructures of the four alloys.