The precipitation of epsilon-carbide in twinned martensite

Tempering of martensite has been investigated by means of thin foil electron microscopy in a high carbon steel, a high nickel steel, and a silicon steel. ε carbide has been unambiguously identified in each steel. It was found that the carbide was precipitated with the Jack orientation relationship: $$begin{gathered} left( {0001} right)_varepsilon parallel left( {011} right)_{alpha '} hfill left( {10bar 10} right)_varepsilon parallel left( {2bar 11} right)_{alpha '} hfill end{gathered} $$ In the silicon steel the ε carbide precipitated in the form of needles which grew with a (left[ {01bar 10} right]_varepsilon ) close to (left[ {21bar 1} right]_{alpha '} ) . This growth direction minimizes the surface energy of the needles, yet allows growth in a direction of low mismatch.