Coarsening and Hardening Behaviors of Cu-Rich Precipitates in Super304H Austenitic Steel
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THE fine Cu-rich precipitates formed during thermal aging make a major contribution to the increase of strength in the Cu-bearing alloys and steels.[1–4] Up to now, the most researches have focused on the precipitation and coarsening behaviors of the Cu-rich phase in ferritic Fe-Cu alloys.[5–11] It has been well recognized that the small Cu-rich precipitates in ferritic matrix first are formed with a coherent bcc structure, while large overaged precipitates are formed with a fcc structure bearing a Kurdjumov–Sachs (K–S) orientation relationship to the ferritic matrix.[5–7] Further investigations suggested that the phase transformation from bcc Cu clusters to fcc Cu is more complex, and the twinned 9R Cu precipitates with a space group of R3m are formed after coherent bcc Cu clusters have been formed and sequentially are transformed into the more stable 3R Cu within the same space group with the increasing aging time.[8,9] Here, the twinned 9R structure can be considered as a close-packed structure with stacking faults occurring in every third close-packed plane, while the formation of a 3R structure from a 9R structure can occur simply by the elimination of the regular stacking PING OU and XUANLI WANG, Ph.D. Students, HUI XING, Assistant Professor, and JIAN SUN, Professor and Principal Researcher, are with the Shanghai Key Laboratory of Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] ZHENGQIANG CUI and CHANGSHUN YANG, Engineers, are with the Shanghai Power Equipment Research Institute, Shanghai 200240, P.R. China. Manuscript submitted December 17, 2014. Article published online June 12, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
shifts occurring on every third close-packed basal plane.[9] The coarsening behavior of Cu-rich particles in ferritic alloys has been investigated by Monzen et al.[10,11] They found that the average radii of the spherical fcc and fct Cu particles with no specific relationship to the recrystallized ferritic grain and of the rod-shaped Cu particles with a K–S orientation relationship to the ferritic matrix increase with the increasing aging time as t1/3, as deduced from the Lifshitz–Slyozov–Wagner (LSW) theory.[12,13] Meanwhile, the value of activation energy Q agrees well with that of the volume diffusion of copper atoms in a-Fe, which implies that the coarsening of Cu-rich particles is controlled mainly by the volume diffusion of copper atoms in the ferritic matrix. In addition, the hardening behavior of the Cu-rich phase in ferritic Fe-Cu alloys also has been investigated by Osamura et al.[2] They found that the decrease of strength after reaching the peak strength is attributed to the loss of coherency of Curich phase with the matrix, where the structure of Cu-rich precipitates changes from bcc to fcc during aging. The fine Cu-rich phase is regarded to be of the most significant precipitation strengthening in the Super304H steel. Many stu
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