Enhanced tensile properties and intersecting nanotwins formation mechanism of Ti44Al48Nb8 alloy via electromagnetic levitation processing

Combining the electromagnetic levitation (EML) method with materials processing techniques (pressure die-casting and copper mold solidification), bulk Ti44Al48Nb8 alloy samples with different shapes were prepared, and their microstructure evolution, tensile properties and deformation behavior were systematically investigated. Assisted with a double aberration corrected transmission electron microscope (TEM) and molecular dynamics (MD) simulation, three types of intersecting nanotwins were observed and the formation mechanism of step-like and intersecting nanotwins under both tensile loading and at highly undercooled state were discussed. For the 35 g cone-shaped Ti44Al48Nb8 sample by EML, the microstructure consists of lamellar alpha 2Ti3Al+gamma-TiAl matrix, Nb-rich and Al-rich region, showing a typical peritectic morphology. The average grain size has been significantly reduced after levitation die-casting and copper mold casting, due to the relatively high cooling rate and external stress. The rapidly solidified microstructure of a 0.6 g sample undercooled at 61 K has the similar microstructure with large samples. With the increase of bulk undercooling at 300 K, elemental segregation has been effectively inhibited and the intersecting nanotwins were formed, which shows a unique hierarchical structure. The samples processed by copper-mold casting have good tensile properties, which can be attributed to the refined microstructure, the generation of dislocation and stacking faults, and the formation of intersecting nanotwins. Based on the results of MD simulation and atomic-scale quasi-in-situ TEM observation, the analogous dissociation of dislocation dominated by pole mechanism occurs, resulting in the formation of 1/6<112> Shockley partial dislocation, 1/ 6<110> stair-rod dislocation and 1/3<111> Frank partial dislocation. When samples were highly undercooled, the generated internal stress can facilitate twinning at a much lower stress level, which can be beneficial for the formation of nanotwins at undercooled state.

Automated summary

By combining the electromagnetic levitation method with materials processing techniques, bulk Ti44Al48Nb8 alloy samples with different shapes were prepared and systematically investigated for their microstructure evolution, tensile properties, and deformation behavior. Three types of intersecting nanotwins were observed under highly undercooled state, and samples processed by copper mold casting showed improved tensile properties due to refined microstructure and the formation of intersecting nanotwins.