Analysis of grain size effect on tensile properties of Ni3Al - based intermetallic strips

The results of investigation of grain size effect on mechanical properties of Ni3Al – based intermetallic strips measured in a tensile test at room temperature in air have been shown in the paper. Thin intermetallic strips with average grain size of 1, 5, 7, 26, 43 and 83 micrometers prepared by cold rolling and heat treatment at parameters chosen on the basis of our earlier completed recrystallization maps have been used for tensile tests. It has been stated that increasing of basic γ’ phase grain refinement caused intensive increase of tensile yield strength (TYS) and lower increase of ultimate tensile strength (UTS). The highest effectiveness of grain boundary strengthening has been found for average grain size up to 10 micrometers and was connected with a strong reduction of intermetallic material plasticity as well with increasing of its elasticity finding expression in TYS/UTS ratio. However, even for extremely fine-grained Ni3Al sample (average grain size approximately 1 micrometer) a value of ultimate tensile elongation obtained at room temperature in air (“hard conditions” for intermetallic alloys sensitive to environmentally influenced brittleness) was still high – approximately 30% at tensile yield strength above 1200 MPa. The results obtained (for recrystallized Ni3Al) were much better then those for appropriate alloys described in literature. In the analyzed grain size range, a classic exponential dependence of yield strength on average grain size was confirmed. Tension test results were in good correlation with fracture structure. It has been stated that fractures of samples with different grain size were quite similar and mixture-type. There was cleavage fracture via micro-volume of each grain, related to ductile fracture via microregions along grain boundaries. For all tested range of grain sizes a fraction of ductile fracture microregions was high, but higher for coarse-grained samples with ultimate elongation ran into 70%. A dependence of a phase structure (a fraction of γ disordered phase) and degree of γ’ phase ordering on mechanical properties and fracture in a tensile test at room temperature in air was also discussed in the paper.