Forging cracks in NiCr superalloy: Precipitation and formation kinetics mechanism of AlN

Cracks and fractures that occur in NiCr electric heating alloys during forging constitutes a primary challenge affecting the yield rate and service performance. In this work, we investigated the distribution and precipitation mechanism of AlN in the forging cracking process of NiCr alloys through microscopic analysis and nucleation kinetics. Key findings reveal that cracks in NiCr alloys during forging primarily originate from the second-phase particles (metal oxides and AlN compounds). Al first co-precipitates as the γ' transition phase. When [Al]>2.5%, the γ' strengthening phase becomes excessively high, causing AlN phases to accumulate extensively at the cracks in the ingot and precipitate in irregular shapes, leading to grain boundary embrittlement and ingot cracking. When [Al] [N] exceeds the equilibrium concentration product for AlN precipitation, AlN begins to appear in the equilibrium precipitation phase and remain stable. The initial precipitation temperature of the AlN is 1320°C, and the maximum molar fraction of precipitation is 1.8 × 10-3. The critical nucleation energy for grain boundary nucleation in AlN is lower than that for dislocation nucleation and homogeneous nucleation, with the highest temperature at which precipitation occurs occurring at the fastest rate. This work provides critical guidance for improving the mechanical properties and increasing manufacturing efficiency and yield in Ni-based superalloys.