Researchers have found a way to simultaneously increase the strength and ductility of an alloy by introducing tiny precipitates into its matrix and tuning their size and spacing. The precipitates are solids that separate from the metal mixture as the alloy cools.
Ductility is a measure of a material’s ability to undergo permanent deformation without breaking. It determines, among other things, how much a material can elongate before fracturing and whether that fracturing will be graceful or catastrophic. The higher the strength and ductility, the tougher the material. If structural materials could become stronger and more ductile, components of cars, planes, power plants, buildings, and bridges could be built using less material. Lighter-weight vehicles would be more energy-efficient to make and operate and tougher infrastructure would be more resilient.
Model alloys were made with the special ability to undergo a phase transformation from a face-centered cubic (FCC) to a body-centered cubic (BCC) crystal structure, driven by changes in either temperature or stress. Nanoprecipitates were placed in a transformable matrix and their attributes were controlled, which in turn controlled when and how the matrix transformed.
The alloy contains four major elements — iron, nickel, aluminum, and titanium — that form the matrix and precipitates, and three minor elements — carbon, zirconium, and boron — that limit the size of grains or individual metallic crystals. The researchers kept the composition of the matrix and the total amount of nanoprecipitates the same in different samples but varied precipitate sizes and spacings by adjusting the processing temperature and time. For comparison, a reference alloy without precipitates but having the same composition as the matrix of the precipitate-containing alloy was also prepared and tested.
While nanoprecipitates in conventional alloys can make them super strong, they also make the alloys very brittle. The new alloy avoids this brittleness because the precipitates perform a second useful function: by spatially constraining the matrix, they prevent it from transforming during a thermal quench — a quick immersion in water that cools the alloy to room temperature. Consequently, the matrix remains in a metastable FCC state. When the alloy is then stretched (strained), it progressively transforms from metastable FCC to stable BCC. This phase transformation during straining increases strength while maintaining adequate ductility.
In contrast, the alloy without precipitates transforms fully to stable FCC during the thermal quench, which precludes further transformation during straining. As a result, it is both weaker and more brittle than the alloy with precipitates. Together, the complementary mechanisms of conventional precipitation strengthening and deformation-induced transformation increased strength by 20-90 percent and elongation by 300 percent.
Next, the team will investigate additional factors and deformation mechanisms to identify combinations that could further improve mechanical properties.