New, cold-resistant, metal alloys tolerate extreme temperatures up to −253°C.
Researchers tested two niobium-based refractory alloys at about 20 K and found their strength and fracture toughness stayed nearly level as temperatures fell, with no apparent shift to brittle behavior.
Metals with a body-centered cubic crystal structure, including many steels and refractory alloys, usually turn brittle below a certain temperature. The authors of a new Nature paper report that two alloys in this family avoided that pattern when tested at about 20 K, which is roughly −253°C.
The alloys were Nb45Ta25Ti15Hf15 and Nb70Ti30. Both are made mostly of group IV and group V elements with similar electronic structures. The team ran tensile tests and fracture toughness tests, then examined the broken samples with electron microscopy to see how the metal deformed.
In Nb45Ta25Ti15Hf15, yield strength was about 938.9 MPa at 20 K, close to 936.6 MPa at 77 K. Nb70Ti30 showed a similar plateau, at about 839.9 MPa at 20 K and 825.8 MPa at 77 K. Crack-initiation toughness in Nb45Ta25Ti15Hf15 was about 96.5 MPa√m at 20 K. In Nb70Ti30 it was 170 MPa√m at 20 K, compared with 188 MPa√m at 77 K and 219 MPa√m at room temperature (298 K).
According to the authors, the cause is a sequence of deformation mechanisms that do not depend much on temperature. Twinning and a transformation to a structure called the ω-phase start the yielding. Where the twins reach interfaces such as grain boundaries, they create stress concentrations that generate dislocations rather than cracks. The researchers say this keeps the plastic zone at a crack tip from shrinking as temperatures drop.
The paper also reports limits. At 20 K, Nb45Ta25Ti15Hf15 lost about 33% of its uniform ductility and about 50% of its elongation to fracture compared with 77 K. Near a crack, where stresses are higher, fracture was partly intergranular and showed some brittle features. For Nb45Ta25Ti15Hf15, the yield strength figures come from three tensile tests and the toughness figures from two fracture tests, and the work was done in the laboratory.
Note: The page tested the alloys at 20 K, which is about −253°C. It describes them as alloys already studied at higher temperatures (Nb45Ta25Ti15Hf15 at 77 K and 298 K), so the new part is the 20 K testing and the mechanism explanation.
At a Glance
- Tests were run at about 20 K (roughly −253°C) on Nb45Ta25Ti15Hf15 and Nb70Ti30 alloys.
- Nb45Ta25Ti15Hf15 yield strength was about 938.9 MPa at 20 K and 936.6 MPa at 77 K.
- Nb70Ti30 crack-initiation toughness was 170 MPa√m at 20 K, versus 219 MPa√m at 298 K.
- Nb45Ta25Ti15Hf15 had crack-initiation toughness of about 96.5 MPa√m and crack-growth toughness of about 118 MPa√m at 20 K.
Sources
This brief was written by Brightside Report from the reporting linked above. Spot an error? Request a correction.