Intercritically austempered ductile iron (IADI) with a matrix microstructure of ferrite plus metastable austenite has an excellent combination of strength and toughness. The high strength and good ductility of this material is due to the transformation of metastable austenite to martensite during deformation. In the present study, the transformation of austenite to martensite for intercritically austempered ductile irons of varying alloy chemistry (varying amounts of nickel and/or manganese) were examined using in-situ neutron diffraction under strain-controlled loading at VULCAN at the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL). Both diffraction and tensile data were collected and synced using the VDRIVE software (a proprietary ORNL software package). The single peak fit method was employed in the analysis of the diffraction data. In this thesis, the stress and strain for the start of the transformation of metastable austenite to martensite were determined. The development of residual stresses during deformation and the elastic diffraction constants for both the ferrite and austenite phases were also determined. The material was characterized using optical microscopy, backscattered imaging in the scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy. / Master of Science