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Niobium and molybdenum alloys are used in structural parts of space power systems where stress, time, temperature and environmental conditions preclude the use of iron, nickel and cobalt-based alloys. The advantages of niobium and molybdenum alloys include high creep resistance and their microstructural stability during thousands of hours of operation in high vacuum space environments and at high temperatures. Furthermore, niobium alloys have excellent low-temperature plasticity and processability. Now, niobium alloys are being considered for space power systems in a multi-hundred-watt radioisotope turbine generator. Requires long-term high vacuum operation at a temperature of 700-1025°C. Based on the corresponding temperatures of niobium alloys, this temperature range is equivalent to 0.36 to 0.47Tm, where Tm is the absolute temperature of melting. Likewise, for molybdenum alloys, applications can be found in the same temperature range (700-1025°C), equivalent to a temperature range of 0.34 to 0.45Tm. Previous experimental results on tantalum alloys have shown that long-term aging near 0.4Tm will result in brittleness near room temperature and increased susceptibility to hydrogen embrittlement.
The main purpose of this study was to determine whether age brittleness occurs in several niobium alloys and a molybdenum alloy intended for use under space power system conditions, and in particular whether age brittleness depends on the composition of the niobium alloy. For age embrittlement in niobium alloys, alloy composition is the controlling mechanism. Four alloys were included in the current study. Research into two niobium alloys is being considered for use in the construction of a multi-hundred-watt radioisotope turbine power system. The system is widely used in space research as a power source, including long-life missions of 5-10 years. The main alloy is C-103 (Nb-10Hf-1Ti) and the backup alloy Cb-1Zr. Cb-752 (Nb-10W-3Zr) is also included in the current plan. The niobium alloy is assumed to exhibit age brittleness based on its critical constituents of tungsten and zirconium, a state of brittleness similar to that of tantalum alloys containing tungsten and hafnium. The fourth alloy studied was the widely used molybdenum-based alloy, Mo-TZM (Mo-0.5Ti-0.1Zr). The combination of a molybdenum matrix and the active element zirconium is considered possible in this alloy.

Four alloys will be aged at 700-1025°C for 1000 hours. This temperature range encompasses the corresponding temperature range where age embrittlement and increased susceptibility to hydrogen embrittlement have been observed in tantalum alloys. The alloys were evaluated by bending tests, optical microscopy and scanning electron microscopy after 1000 hours of aging treatment. After aging, some samples were doped with hydrogen and then evaluated.

Material
The resulting alloy is a sheet approximately 0.75 mm thick. Longitudinal bending specimens, 6.4 mm × 25.4 mm, were cut from sheets of each alloy. Each niobium alloy underwent a standard annealing process at 1345°C (0.59Tm) for 1 hour under a vacuum of 0.13 μN/m2. The Mo-TZM alloy was vacuum annealed at a corresponding temperature, 1425°C.
aging
Aging is to heat the bent sample at 700°-1025°C for 1000 hours under a vacuum of 0.13μN/m2. Five aging temperatures used for heating. The specimens were weighed before and after aging. The purpose is to analyze the interstitial impurity content after aging in order to determine whether contamination occurred during the aging treatment. As a result of 1000 hours of vacuum aging treatment, there is no significant change in weight and interstitial impurity content.
Doped with hydrogen
For each alloy, three annealed specimens were taken and four aged specimens from each of the five aging treatments were hydrogen doped. Hydrogen doping was attempted by heating the sample to a temperature of 825°C in an evacuated furnace. At this time, hydrogen is introduced into the furnace at a pressure of 13KN/m2. The specimens were maintained in this hydrogen condition for 10 minutes and then cooled to room temperature in a helium atmosphere.
evaluate
Bending test
Bending tests are used as the primary means of determining the effects of aging on niobium and molybdenum alloys. The tests were conducted on a screw-driven testing machine with a punch speed of approximately 25 mm/min. A bending radius of 2t, where t is the specimen thickness, and a maximum total bending angle of approximately 140° were used in all tests. The test was conducted in the temperature range of -196C-1000°C. A controlled liquid nitrogen nebulizer was used for testing below room temperature. For experiments above room temperature, a clamshell infrared furnace was used. The bending-to-brittle transition temperatures described in this study are determined as the lowest temperature at which a specimen of each alloy under annealed, aged and hydrogen-doped conditions can successfully fully achieve a bend angle of 140°.
Metallography
Specimens were obtained from each alloy in the annealed and aged condition and the effects of aging on grain size and general structure were examined by standard optical microscopy techniques. Fracture charts from selected specimens of each alloy were characterized by scanning electron microscopy.

In conclusion
Based on the study of long-term (1000 hours) aging of three niobium alloys and one molybdenum alloy in the temperature range from 700°C to 1025°C, the following conclusions are derived.

1. C-103, Cb-1Zr and Mo-TZM alloys are not sensitive to age brittleness. Specimens aged for 1000 hours in the temperature range of 725°C to 1025°C are plastic when bent at -196°C.
2. Aging brittleness occurs in Cb-752 alloy aged at 900 ℃ (0.43Tm) for a long time. After this aging treatment, the ductile-to-brittle transition temperature (DBTT) increases from less than -196°C to approximately -150°C. When increasing the aging time, age brittleness will occur at lower aging temperatures.
3. The aging brittleness produced by a critical component of tungsten in Cb-752 is similar to that produced in niobium-based alloys containing tungsten and additives. It is credible that tungsten promotes grain boundary segregation of errors during the aging of Cb-752 alloy. Subsequent age brittleness is characterized by grain boundary failure below the plastic transition temperature.
4. After aging, hydrogen embrittlement is the most significant in Cb-752 alloy. For samples with approximately 40 ppm hydrogen doping caused by annealing or aging, the plastic-brittle transition temperature increases by at least 200°C. The plastic-brittle transition temperature in aged Cb-1Zr samples doped with about 50 ppm hydrogen increased by about 100°C. Hydrogen embrittlement does not occur in C-103 doped with 60 ppm hydrogen. Aged Mo-TZM cannot be doped by hydrogen.