Phase Diagram and Phase Transformation
Fig. 1 shows (FCC) Nickel-(BCC) Chromium binary phase diagram. The eutectic reaction takes place at 1345°C and the composition is 51% Cr and 49% Ni. The solid solubility of Cr in Ni is extensive which is evident from the phase diagram. Phase transformations of two Ni-Cr alloys are explained below. One is Ni40Cr60 (Line 2) which is closer to eutectic composition and contains greater % Cr and the other alloy is Ni68Cr32 (Line 1) which contains greater % Ni.
Figure 1: Ni-Cr phase Diagram [1]
When Ni40Cr60 is cooled below 1450°C, Cr2Ni (orthorhombic structure) grains start precipitating from liquid solution. When the alloy is quenched from 1450°C to 1200°C the grain size increases. When temperature is reduced below 1200°C, Cr2Ni grains get fragmented. Further decrease in temperature results in greater fragmentation. At 550°C, Cr2Ni grains are uniformly distributed in the matrix. Below 550°C, Ni2Cr3 (tetragonal structure) particles start to precipitate. As temperature is lowered further, Ni2Cr3 grains start coarsening and fragmentation of Cr2Ni continues. At room temperature, both Ni2Cr3 and Cr2Ni grains exist in the solid solution.
When Ni68Cr32 is cooled from 1450°C, clusters of Cr2Ni are precipitated as shown in Fig. 2 (even though %Cr is …show more content…
Greater the chromium content, greater is the oxidation resistance. Alloys with relatively high chromium content form thin film of Cr2O3 and (Cr, Ni)3O4 on the surface when exposed to moist air at high temperature and this film provides excellent oxidation resistance. Ni-Cr alloys with chromium content greater than 16% provides excellent corrosion resistance through formation of passive film on surface. When Mn and Mo are added to Ni-Cr alloys, pitting resistance is improved. In sulphur environments, Cr2S3 is formed on the surface and protects the Ni. Thus high Cr alloys provide better resistance in sulphur
Fig. 1 shows (FCC) Nickel-(BCC) Chromium binary phase diagram. The eutectic reaction takes place at 1345°C and the composition is 51% Cr and 49% Ni. The solid solubility of Cr in Ni is extensive which is evident from the phase diagram. Phase transformations of two Ni-Cr alloys are explained below. One is Ni40Cr60 (Line 2) which is closer to eutectic composition and contains greater % Cr and the other alloy is Ni68Cr32 (Line 1) which contains greater % Ni.
Figure 1: Ni-Cr phase Diagram [1]
When Ni40Cr60 is cooled below 1450°C, Cr2Ni (orthorhombic structure) grains start precipitating from liquid solution. When the alloy is quenched from 1450°C to 1200°C the grain size increases. When temperature is reduced below 1200°C, Cr2Ni grains get fragmented. Further decrease in temperature results in greater fragmentation. At 550°C, Cr2Ni grains are uniformly distributed in the matrix. Below 550°C, Ni2Cr3 (tetragonal structure) particles start to precipitate. As temperature is lowered further, Ni2Cr3 grains start coarsening and fragmentation of Cr2Ni continues. At room temperature, both Ni2Cr3 and Cr2Ni grains exist in the solid solution.
When Ni68Cr32 is cooled from 1450°C, clusters of Cr2Ni are precipitated as shown in Fig. 2 (even though %Cr is …show more content…
Greater the chromium content, greater is the oxidation resistance. Alloys with relatively high chromium content form thin film of Cr2O3 and (Cr, Ni)3O4 on the surface when exposed to moist air at high temperature and this film provides excellent oxidation resistance. Ni-Cr alloys with chromium content greater than 16% provides excellent corrosion resistance through formation of passive film on surface. When Mn and Mo are added to Ni-Cr alloys, pitting resistance is improved. In sulphur environments, Cr2S3 is formed on the surface and protects the Ni. Thus high Cr alloys provide better resistance in sulphur