Effects of Cyclic Solution Treatment on the Microstructures and Mechanical Properties of Alloy 718

Effects of solution treatment on the microstructure and mechanical properties in wrought Alloy 718 were investigated. For the improvement of tensile and fatigue properties of wrought Alloy 718, a cyclic solution treatment, heating at 1000 degrees C for 3 minutes followed by furnace cooling at the rate of 3 degrees C/minute and holding at 985 degrees C for 8 minutes, was proposed. The cyclic treatment was performed repeatedly 3 times and the samples were air-cooled to room temperature. After the cyclic solution heat treatment, Alloy 718 was subject to the standard aging treatment, holding at 718 degrees C for 8 hours followed by cooling at the rate of 55 degrees C/hour and holding at 620 degrees C for 8 hours, and finally air cooled to room temperature. For the purpose of comparison, a standard heat treatment was also performed by solution treatment at 955 degrees C followed by aging. The microstructures of the heat-treated samples were analyzed by optical microscopy and scanning electron microscope (SEM). Cyclic solution treatment resulted in the formation of small spherical shaped delta-phases without grain growth. However, the delta-phases of the standard heat-treated specimen showed needle-like morphologies. Tensile and low cycle fatigue tests were performed on both cyclic heat-treated and standard heat-treated specimens at room temperature, 350 degrees C and 650 degrees C. Low cycle fatigue tests on the cyclic treated specimens showed promising results without reduction of strength.