Superplastic silicon nitride sintered body

A superplastic, sintered silicon nitride body consisting of .beta.-Si.sub.3 N.sub.4, unavoidable minor components selected from the group consisting of SiO.sub.2 and Si.sub.3 N.sub.4, and at least one sintering assistant selected from the group consisting of a liquid phase forming agent and a melting point reducing agent, said liquid phase forming agent, which is Al.sub.2 O.sub.3, MgO, Sc.sub.2 O.sub.3, oxides of yttrium or oxides of a lanthanide metal, reacting with said Si.sub.3 N.sub.4 and SiO.sub.2 as a surface oxidized phase, thereby forming a liquid phase, and said melting point agent, which is CaO, SrO, MgO, BaO or Al.sub.2 O.sub.3, lowering the temperature at which said liquid phase forms, said body having a grain boundary phase in an amount of 2-20 volume % and a relative density of at least 98%, said body formed of silicon nitride grains having an average grain size of at most 0.3 .mu.m, the amount of grains having grain sizes within the range of the average grain size .+-.0.2 .mu.m being at least 85 volume %, wherein at least 90% of the silicon nitride grains have an aspect ratio of less than 3 and the amount of grains having grain sizes exceeding 0.5 .mu.m being at most 3 volume % and at least 90 volume % of the silicon nitride being in .beta.-form, and wherein the superplasticity of the sintered silicon nitride body is such that when a compression or tensile stress of from 30-2000 kg/cm.sup.2 is applied thereto at a temperature within the range of from 1350.degree.-1650.degree. C., said sintered body deforming at a deformation rate of from 10.sup.-4 /sec to 10.sup.-1 /sec.