<i>In situ</i> x-ray diffraction of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>α</mml:mi> <mml:mtext>−</mml:mtext> <mml:mi>ɛ</mml:mi> </mml:mrow> </mml:math> phase transition in iron at intermediate strain rates

The <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>α</a:mi> </a:math> (bcc)- <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>ɛ</b:mi> </b:math> (hcp) phase transition in polycrystalline Fe has been investigated under fast quasihydrostatic compression using piezoelectric actuator driven dynamic diamond anvil cells. X-ray diffraction data from Fe under intermediate strain rate compression were collected at megahertz repetition rates. Our results demonstrate, for the first time, the full evolution of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>α</c:mi> <c:mtext>−</c:mtext> <c:mi>ɛ</c:mi> </c:mrow> </c:math> phase transition in powder and foil samples at strain rates of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mo>∼</d:mo> <d:msup> <d:mrow> <d:mn>10</d:mn> </d:mrow> <d:mn>2</d:mn> </d:msup> <d:mspace width="0.16em"/> <d:msup> <d:mrow> <d:mi mathvariant="normal">s</d:mi> </d:mrow> <d:mrow> <d:mo>−</d:mo> <d:mn>1</d:mn> </d:mrow> </d:msup> </d:mrow> </d:math> . Under fast compression, the majority of the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mi>α</g:mi> </g:math> phase transforms to <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mi>ɛ</h:mi> </h:math> -Fe in a timescale of tens of microseconds, with a corresponding coexistence pressure of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:mo>∼</i:mo> <i:mn>2</i:mn> <i:mspace width="0.16em"/> <i:mi>GPa</i:mi> </i:mrow> </i:math> . The observed onset phase transition pressure of 13.4–14.4 GPa is in good agreement with the results of previous quasihydrostatic compression studies, suggesting that the transition is not influenced by compression rates up to <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mo>∼</k:mo> <k:mn>80</k:mn> <k:mspace width="0.16em"/> <k:mi>TPa</k:mi> <k:mspace width="0.16em"/> <k:msup> <k:mrow> <k:mi mathvariant="normal">s</k:mi> </k:mrow> <k:mrow> <k:mo>−</k:mo> <k:mn>1</k:mn> </k:mrow> </k:msup> </k:mrow> </k:math> .