On the orientation dependence of ion-induced phase transformations in austenitic stainless steel

This study characterises the effects of crystal orientation on the evolution of austenite decomposition to ferrite/martensite in AISI-304 stainless steel, induced by exposure to gallium ions with a focused ion beam. Samples were exposed to the beam multiple times and imaged by electron backscatter diffraction before and after successive exposures, with the data aggregated by orientation clustering to derive insights into the effects of orientation on phase change processes. Propensity to decomposition and produced surface morphologies were observed to have a strong dependence on orientation. Three distinct orientation-based transformation behaviours were observed: grains with beam orientations close to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>100</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>γ</mml:mi></mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>110</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>γ</mml:mi></mml:mrow> </mml:math> axes remained untransformed or become amorphised; grains with beam orientations close to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>111</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>γ</mml:mi></mml:mrow> </mml:math> axis were partially transformed to non-contiguous products at various absolute orientations with beam orientations close to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>110</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>α</mml:mi></mml:mrow> </mml:math> axis; and grains with beam orientations not close to any of the three principal axes were fully transformed to a single contiguous product with beam orientations close to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>100</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>α</mml:mi></mml:mrow> </mml:math> axis. Grains were not produced close to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>⟨</mml:mo> <mml:mn>111</mml:mn> <mml:mo>⟩</mml:mo> <mml:mi>α</mml:mi></mml:mrow> </mml:math> axis in any case.