HREM Observation of Age-Precipitated Particles in Practical Cu-bearing Ultra-Low Carbon Steels

Morphology, distribution and crystal structures of age-precipitated particles in practical Cu-bearing steels with ultra-low carbon were studied by high resolution electron microscopy (HREM). The age-precipitation behavior was explored when poly-microalloying elements Cu, Nb, Cr, Mo and Ni were used and ɛ-Cu particles coexisted with carbides (carbonitrides) in steels. The fine dispersed ɛ-Cu particles and carbides of microalloying element Nb, Cr, Mo being about 10 nm only in size were observed in peak aging specimens and over-aging specimens. The twin structures and stacking faults in the particles and the transient structures formed in the early stage of the particle precipitation were also observed. The crystal structure of three types of age-precipitation particles was firstly identified by lattice streaks in HREM images of the single particles. The experimental method identifying crystal structure of fine and dispersed particles using HREM images of single particle was proposed.