If you’ve ever baked a cake, you would know how the careful combination of distinct ingredients like eggs, butter, sugar and flour can result in something more delicious than the sum of their parts. In a somewhat similar fashion, metal alloys combine two or more elements with distinct physical properties in exact ratios to enhance the end product's performance. For example, stainless steel, derived from the addition of chromium to steel, boasts improved properties like heat and corrosion-resistance and is used in everything from surgical tools and cutlery to trains and airplanes.
Nonetheless, one environment that alloys still struggle in is the cold. In chilly regions, metal components for constructing spacecraft or processing plants are typically subjected to extreme mechanical stress. For such applications, the cobalt-chromium-nickel (CoCrNi) alloy is presently an attractive choice as it has high printability and durability even under ultra-cold conditions. Another unique and intriguing characteristic of CoCrNi is that it comprises an equal proportion of each of the three elements—a trait that is not commonly seen in other alloys.
While more studies are performed to elucidate the properties and applications of laser aided additive manufactured (LAAMed) CoCrNi, materials scientists have revealed that chromium oxidation can limit the alloy's strength.
“Oxidation is almost inevitable during the LAAM process,” said first author Fei Weng, a Research Scientist at A*STAR's Singapore Institute of Manufacturing Technology (SIMTech). “However, the effect of oxides on the mechanical properties of CoCrNi, especially in a cryogenic environment, remains unknown.”
To create stronger CoCrNi alloys that can withstand cold conditions, the research team set out to tweak established LAAM processes to optimise CoCrNi’s oxide content. The team, led by SIMTech’s Youxiang Chew and Guijun Bi, first investigated the amounts of inert gas—used in LAAM to minimise alloy oxidation—influenced CoCrNi's mechanical properties. In the process, the team generated two distinct classes of CoCrNi alloys with either low or high oxide content.
Next, the team analysed how oxide content affected the strength of the alloy. To do so, the group performed a test that stretched and pulled the alloys under cold and ambient temperatures to measure how much force had to be applied before they deformed.
CoCrNi with higher amounts of oxide was found to be weaker, stretching apart under lower stress levels and elongation at room temperature. During tensile deformation, tiny structural gaps called microvoids form between the alloy matrix and oxides.
“With a higher oxide content, micro-voids easily form and coalesce at the oxide matrix interface during deformation, resulting in premature fracture and lower elongation. Interestingly, the highly oxidised CoCrNi alloy showed comparable elongation at room temperature and -130°C, which is attributed to the compensated effect from more deformation twinning formed at -130°C.” explained Weng.
This discovery illustrates how LAAM can be modified to create alloys with specific properties, such as corrosion resistance and resilience against stretching forces at low temperatures. “By adjusting the process parameters, we can tune the microstructure and oxide levels of CoCrNi alloys to improve their mechanical properties at room or low temperatures,” commented Weng.
Moving forward, the team plans to perform more studies to enhance the mechanical properties of the CoCrNi alloy fabricated using the LAAM technique. This work would pave the way for new opportunities in the energy, aerospace, manufacturing industries and beyond.
The A*STAR researchers contributing to this work are from the Singapore Institute of Manufacturing Technology (SIMTech).