Crystal structure of fine age-precipitates in microalloyed ultra-low carbon Cu-bearing steel

A high resolution electron microscope (HREM) was used for morphology observation and crystal structure identification of fine age-precipitates in a practical ultra-low carbon Cu-bearing steel complex-microalloyed with Nb,Cr,Mo and Ni. A new approach to effectively apply HREM for microstructure study of engineering materials was explored. Fine dispersed single age-precipitated particle of 10 nm with twin structure and transient structure formed in local micro-areas was observed on the HREM at magnification of 3×10⁶. The crystal structure of three types of age-precipitates except ε-Cu was identified by lattice streaks on HREM images of the single particles. The results show that Cu, Nb, Cr, Mo and Ni carbide (carbonitride) particles are precipitated during aging and produce age-strengthening with ε-Cu together. The experimental method for identifying the crystal structure of fine dispersive particles using HREM images is proposed.