The bending grain boundary structure also significantly influences the fatigue and creep properties, extending fatigue life, increasing creep resistance, and improving creep ductility. For precision alloy materials at 940°C and 215 MPa, under standard heat treatment, the fatigue life is 65 hours, while under bending grain boundary treatment, it reaches 116.5 hours, approximately doubling the fatigue life.
Creep curves of precision alloy materials with bent grain boundaries and straight grain boundaries under conditions of 850°C and 550 MPa are shown. The relationship between creep rate and time is depicted. It is evident that compared to standard heat treatment, isothermal bending grain boundary treatment increases the time to creep fracture, with an average increase in creep fracture life of 22% to 26%, and enhanced fracture ductility. The creep rate is reduced by approximately half. Therefore, bending grain boundary treatment can simultaneously increase creep strength and ductility. Creep rate versus time curves for standard heat treatment and isothermal bending grain boundary heat treatment are presented.
The influence of bent grain boundaries in precision alloy materials on fatigue crack propagation rates depends on the testing frequency: at high frequencies, the effect is minimal, whereas at low frequencies, it significantly reduces the fatigue crack propagation rate. The impact of bent grain boundaries on fatigue crack propagation rates in precision alloy materials is substantial. Standard compact tension specimens with dimensions of thickness B = 12mm and width W = 32mm are used for testing, with a pre-crack length of a0 = 12mm. Tests are conducted on a closed-loop hydraulic servo fatigue testing machine using three-zone resistance heating, at a test temperature of 850°C ± 3°C, with an R ratio (Pmin/Pmax) of 0.25, using a triangular waveform. Test frequencies are 4.2Hz, 1Hz, and 0.1Hz respectively. Crack measurement is performed using the oxide-staining method along the crack lines.
In the fatigue crack propagation of precision alloy materials, the influence of bent grain boundaries depends on the testing frequency. At a frequency of 4.2Hz, there is no discernible effect. However, at frequencies below 1Hz, the bent grain boundaries exhibit superior resistance to fatigue crack propagation compared to the standard heat-treated straight grain structure. Metallographic examination of fracture surfaces of precision alloy materials shows that at 4.2Hz, both bent and straight grain boundary specimens exhibit transgranular fracture characteristics. At 1Hz, fatigue cracks in both types exhibit a mixed transgranular and intergranular mode. At 0.1Hz, cracks in both types are predominantly intergranular. Therefore, in fatigue crack propagation tests that induce intergranular fracture in precision alloy materials, bent grain boundaries consistently demonstrate their superiority.
