The origin of microtwinning in HAYNES® 244® superalloy

Abstract Superalloys exhibit varying deformation mechanisms at differing temperatures and strain rates. The HAYNES ® 244 ® superalloy stands out due to its consistent mechanism of planar fault formation. This distinctive behavior is attributed to the presence of the Ni 2 (Cr, Mo, W) $${\gamma }^{{\prime\prime} {\prime} }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>″</mml:mo> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> intermetallic phase, wherein stacking faults form by partial dislocations and subsequently thicken into microtwins via the transmission of partials on adjacent planes. Here we find that, in contrast to conventional $${\gamma }^{{\prime} }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> -strengthened superalloys where deformation begins in the γ matrix, twinning in the 244 alloy initiates at the γ – $${\gamma }^{{\prime\prime} {\prime} }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>″</mml:mo> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> interface within the $${\gamma }^{{\prime\prime} {\prime} }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>″</mml:mo> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> precipitates and then extends outward into the matrix. Our study supports previous hypotheses on twin formation using advanced techniques such as high-resolution scanning transmission electron microscopy, in-situ transmission electron microscopy, and high-strain-rate testing. Contrary to conventional literature, where twinning is often considered detrimental, our work highlights twinning as a unique and significant behavior across temperatures up to the precipitate dissolution temperature and strain rates as high as 500 s −1 . In-depth analysis of this alloy at the onset of plasticity and characterization of the γ – $${\gamma }^{{\prime\prime} {\prime} }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>″</mml:mo> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> interface highlights a new and additional structural driving force for the stability of the deformation twinning mechanism.