Over the past decade, environmental concerns and safety regulation have led to increasing demand for vehicles with higher passenger safety and fuel economy. This has spurred intensive research on advanced high strength steels (AHSS). The quench and partitioning (Q&P) heat treatment is a novel approach that has led to development of one group of third generation AHSS alloys. In recent years most of the studies on the Q&P process were dedicated to the effect of the heat treatment parameters on microstructural evolution and mechanical properties. However, micromechanical deformation behavior of constituent phases and damage evolution in Q&P steels are not fully understood. In this study, damage micromechanisms in a commercial QP980 were investigated with the aid of in-situ tensile tests under a scanning electron microscope (SEM) followed by local strain mapping using microscopic digital image correlation (µ-DIC) analysis so as to quantify the microstructural deformation of constituent phases. Nano-hardness measurements were conducted to correlate the amount of plastic deformation of each phases to its strength. Ex-situ tensile tests coupled with electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) were conducted to study the influence of transformation induced plasticity (TRIP) of the retained austenite phase on microstructural damage and deformation. It was found that average local true strain in ferrite was approximately two times and three times greater than that of martensite and blocky retained austenite respectively, which was with the good agreement with nano-hardness measurements showing that retained austenite blocks was three times and two times harder than martensite and ferrite respectively. Damage in both ferrite and martensite starts at the same total strain; however, damage growth is faster in martensite leading to the formation of large cavities. The average local true strain ratio of ferrite to martensite decreases after total true strains higher than 0.1 and the reduction is more pronounced in regions with higher martensite volume fraction. EBSD results showed that at total true strain of 0.07 some of the retained austenite blocks located at the ferrite and martensite interfaces were almost fully transformed to martensite. According to XRD results at the point of necking 57% of retained austenite transformed to martensite. There is evidence of brittle cracking of large blocky retained austenite in regions with strain localization starting at relatively low strains but appear to have little impact on the final failure process. The good deformation ability of QP980 is attributed primarily to co-deformation of ferrite and martensite and secondarily to the TRIP effect. / Thesis / Master of Applied Science (MASc)