STUDY OF THE COARSENING AND HARDENING BEHAVIORS OF COHERENT g-Fe PARTICLES IN Cu-2.1Fe ALLOY
DONG Qiyi, SHEN Leinuo, CAO Feng, JIA Yanlin, WANG Mingpu()
School of Materials Science and Engineering, Central South University, Changsha 410083
Cite this article:
DONG Qiyi, SHEN Leinuo, CAO Feng, JIA Yanlin, WANG Mingpu. STUDY OF THE COARSENING AND HARDENING BEHAVIORS OF COHERENT g-Fe PARTICLES IN Cu-2.1Fe ALLOY. Acta Metall Sin, 2014, 50(10): 1224-1230.
As one of the most widely used integrated circuit (IC) lead frame materials, Cu-2.1Fe alloy (C19400) shows excellent comprehensive properties, such as 90° bend fatigue, 90° bend formability, corrosion-proof, solder ability and resistance of solder peeling off. As a successful medium-strength and high-conductivity copper alloy, the Cu-2.1Fe alloy is strengthened by precipitation hardening and work hardening. Metastable coherent g-Fe particles will precipitate from supersaturated copper matrix during aging. The effects of long-term aging at different temperatures on the g-Fe coarsening characteristics and the mechanical properties of Cu-2.1Fe alloy were investigated, by means of conventional TEM, SEM, hardness, tensile strength and electrical conductivity testing. The results show that solution-treated Cu-2.1Fe alloys can reach its peak hardness and maintain for a longer time when aging at 500 ℃. The maximum of strength occurred at a particle size of about 12 nm in mean diameter. The coarsening kinetics of g-Fe follows Lifshitz-Slyozov-Wagner (LSW) theory and the activation energy for growth is estimated to 222 kJ/mol. Furthermore, it is found that coherent Fe particles gradually evolve into semi-coherent and cubical particles after aging for a long time and at high temperatures. The aging strengthening effect of Fe particles is not significant, and the maximum increment of stress is about 100 MPa. The strengthening mechanism of undeformed Cu-Fe alloy is coherency strengthening during the under-aged stage and changes to Orowan mechanism during the over-aged stage. Experimental results are in agreement with theoretical predictions.
Fig.2 Hardness and electrical conductivity variations of the Cu-2.1Fe alloy with different aging temperatures for 1 h
Fig.3 Hardness (a) and electrical conductivity (b) variations of the Cu-2.1Fe alloy with aging temperatures of 400, 500, 600 and 700 ℃ for different times
Fig.4 TEM images of coherent g-Fe particles in Cu-2.1Fe alloy with aging temperatures of 500 ℃ (a), 600 ℃ (b) and 700 ℃ (c) for 1 h
Fig.5 TEM images of coherent and incoherent Fe particles in Cu-2.1Fe alloy (Insets at the top right corner show the corresponding SAED patterns)
Fig.6 Changes of Fe particle size with aging time at 500~700 ℃
Fig.7 Coarsening plot (a) and the calculation of coarsening active energy of Fe particles (b) in Cu-2.1Fe alloy aged at 500, 600 and 700 ℃
Fig.8 SEM images of fracture micrographs in Cu-2.1Fe alloy for solution treatment (a) and after aging at 500 ℃ for 1 h (b)
Sample
sm / MPa
s0.2 / MPa
Elongation / %
Solution treated
264.0
85.6
46.2
400 ℃, 1 h
270.8
109.2
34.0
500 ℃, 1 h
297.7
138.3
37.8
600 ℃, 1 h
316.0
126.5
38.7
700 ℃, 1 h
322.8
142.4
44.6
700 ℃, 8 h
299.0
116.5
42.4
Table 1 Mechanical properties of Cu-2.1Fe alloy for different aging treatments
Fig.9 Calculated strengthening vs precipitate radius for Cu-2.1Fe alloy (The maximum of tensile stress occurs at r≈b×24=6.12 nm, CS—coherency strengthing, OS—Orowan strengthing, r—average radius, b—Burgers vector, r0-inner cut-off distance for the dislocation line energy)
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