The grain boundary brittleness at intermediate temperature in a precipitation strengthened Ni-based polycrystalline alloy

The intermediate temperature brittleness (ITB), sudden fracture with insufficient plasticity in the range around 600-900 °C in precipitation strengthened polycrystalline alloys, plays a serious threat on the failure of structural hot-components. To reveal the relationship between microstructure and ITB, a typical precipitation strengthened alloy GH4151 has been fabricated and differently processed, including single crystal (SX) growth and thermal-mechanical processing and their mechanical properties have been tensile tested. The SX GH4151 alloy fractures in a ductile manner with elongation to fracture (EF) exceeding 10 % at all test temperatures. In contrast, all polycrystalline alloys exhibit some degree of brittleness (EFs less than 5%) at the temperature range of 700-900 °C. Microstructural characterization by using advanced microscopy techniques and a high-temperature mechanical testing system in a transmission electron microscope, dynamically reveal the origin of the ITB in situ and find that the ITB is primarily caused by oxidation accelerated GB fracture. Moreover, the microstructural effect, mainly γ′ volume fraction and grain size on the plasticity of precipitation strengthened alloys is established. Our findings might provide guidelines for overcoming ITB of precipitation-strengthened polycrystalline alloys.