Non-Equilibrium Time-Temperature-Transformation Diagram for Enhancing Magnetostriction of Fe-Ga Alloys

Coherent nanoprecipitates formed at early-stage decomposition in a number of body-centered-cubic (BCC) Fe-based alloys with nonmagnetic solute have been found to strengthen their magnetostriction significantly. Despite that the differences in structure and properties between equilibrium and metastable states have been well recognized in an extensively-studied system Fe-Ga, however, a non-equilibrium time-temperature-transformation (TTT) diagram for manipulating the intermediate nanoprecipitates towards further enhancing magnetostriction is still lacking. By systemically investigating the time- and temperature-dependent early-stage phase transformations of the magnetostriction-peak composition alloy Fe₇₃Ga₂₇, a non-equilibrium TTT diagram and the corresponding time-temperature-property (TTP) relation were successfully determined in this work. Defined by the shortest time to obtain peak magnetostriction, a nose temperature of ~400 ℃ to produce coherent face-centered-tetragonal (FCT) L6₀ nanoprecipitates was determined in the diagram. Above the nose temperature,&nbsp;the L6₀ nanoprecipitates grow much faster and become incoherent rapidly, characterized by their enlarged tetragonality <i>c/a</i> towards the equilibrium face-centered-cubic (FCC) L1₂ phase. Below the nose temperature, the L6₀ nanoprecipitates grow much slower and keep coherent with the bcc matrix over a wide aging time range, but the low atomic diffusion rate and the coherent elastic energy produce extra hexagonal omega nanoprecipitates at the phase transformation front. Based on the non-equilibrium TTT diagram, the optimally-aged random polycrystalline alloy with coherent, dense and fine L6₀ nanoprecipitates can exhibit magnetostriction as large as 180 ppm, nearly 3 times of that of the solution-treated counterpart. Consequently, this work not only provides a processing base for enhancing magnetostriction of Fe-Ga alloys, but also may shed important guidance to tailor the microstructure of other nanoprecipitates-bearing alloys with similar diffusion-controlled phase transformation.