The influence of testing methods on the ductile brittle transition of Mo-14Re alloy

【Objective】 Molybdenum is a typical metal exhibiting a ductile-brittle transition phenomenon, with its ductile-brittle transition temperature generally ranging from 200 ℃ to 300 ℃. Therefore, it is prone to unforeseen brittle fracture during service at room temperature. The addition of rhenium (Re) to molybdenum can significantly reduce its ductile-brittle transition temperature while improving the strength of molybdenum alloys. There are various testing methods for ductile-brittle transition temperature, and the results obtained by different methods may vary. The variability in testing poses challenges for cross-comparisons, material selection, and engineering applications of molybdenum alloy ductile-brittle transition temperatures. This paper investigates the ductile-brittle transition phenomenon of Mo-14Re alloy under different testing conditions, providing a reference for the design and application of Mo-Re alloys.
【Method】 Mo-14Re sintered billets were obtained by cold isostatic pressing and high temperature hydrogen sintering at 2 100 ℃. Mo-14Re bars with a diameter of ϕ20 mm were obtained by multi pass high temperature forging of sintered billets at high temperature. Mo-14Re bars were sampled along the axial direction after vacuum heat treatment at 900 ℃/1 h. The ductile-brittle transition temperature of Mo-14Re alloy was measured by three-point bending method, impact toughness method and tensile method. The test temperatures are -120 ℃, -140 ℃, -160 ℃, -180 ℃ and -196 ℃ respectively, and at least three samples are tested under each condition. The low temperature environment required for the test is obtained by the mixture of liquid nitrogen and alcohol. After the test, the tensile and impact fracture surfaces were taken and the fracture morphology was observed by SEM.
【Result】 During the three-point bending test, the bending strength of the alloy increases gradually with the test temperature decreasing from -140 ℃ to -196 ℃, and brittle fracture occurs at -196 ℃. At -196 ℃, the fracture mode of the alloy changes from toughness to brittleness with the increase of loading rate from 0.5 mm/min to 2.0 mm/min. During the tensile process, the tensile strength of the alloy increases from 942 MPa to 1055 MPa with the test temperature decreasing from -120 ℃ to -196 ℃. The elongation after fracture decreases sharply to 1.6% at -196 ℃, and there is almost no plastic deformation before fracture. The fracture morphology shows that the alloy has ductile-brittle transition at -180~-196 ℃. During the impact test, the impact toughness of the alloy decreased with the decrease of the test temperature. At -196 ℃, the impact toughness decreased sharply to 3.05 J/cm², which obviously deviated from the linear trend of the impact toughness with the decrease of temperature in the range of -140~-180 ℃.
【Conclusion】 The ductile-brittle transition temperatures of Mo-14Re alloy bars tested by three-point bending method, tensile method and impact method are all in the range of -180~-196 ℃. With the decrease of temperature, the bending strength and tensile strength of Mo-14Re alloy increase, while the impact toughness and plastic deformation ability decrease. The low temperature results in the decrease of dislocation mobility and the decrease of crystal planes for sliding, which increases the strength and decreases the plasticity of Mo-14Re alloy, resulting in ductile-brittle transition. At the same test temperature, the change of strain rate is one of the key factors affecting the ductile-brittle transition temperature of Mo-14Re alloy.