Low temperature brittleness of Fe-Si alloy crystals was investigated in connection with the twin formation. In tensile tests at 77K, most of Fe-Si alloy crystals used exhibited the brittle fracture of {100} cleavage type except for crystals oriented near a ‹111› axis where the stress factor for twinning is rather small. The values of resolved shear stress on the twinning system at brittle fracture were in good agreement with those of twinning stress which were obtained from the compression tests at 77K. After the brittle fracture, deformation twins were observed on the side and fracture surfaces of crystals. SEM observation on the brittle fracture surfaces showed that the site of crack initiation exists on the ‹011› type intersection of deformation twins. To improve the low temperature ductility of Fe-Si crystals, dislocation substructures such as the sub-boundary and mobile dislocations were introduced into the crystals by the pre-deformation of two stages at 1160K and room temperature. The pre-treated crystals deformed in tension at 77K up to the strain larger than 10%, followed by ductile fracture. Computer simulation for the initiation and growth of a crack showed that the plastic relaxation around a twin-crack system is enhanced by lowering the twin growth rate and by increasing the mobile dislocation density as well as dislocation mobility so as to inhibit the crack extension.
