Abstract
The authors previously reported that the tensile strength and elongation of dental casting Ni-Cr alloys for crown and bridge were greatly affected by the casting method.The Present study was undertaken to find out the causes of the differences in tensile properties among the different casting methods.A close correlation was found between tensile strength and Vickers hardness of the castings of five commercial Ni-Cr alloys only when they were prepared with a casting machine composed of induction melting in Ar gas atmosphere and vacuum-Ar gas pressure.Thus, the tensile properties of the castings made by the other casting methods might have been markedly affected by some casting defects.This was confirmed by the fractographic cbservations of the specimens.On the fracture surfaces of the castings showing lowered tensile properties, dendritic patterns were very often observed together with or without intergranular facet patterns.The dendritic patterns were regarded as those of the inherent gas porosity surfaces for the alloys containing lower Cr, Si and Mn contents and of the hot tear surfaces for the one containing extremely higher C content.
