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Conventional electron microscopy was done on thermally-aged Fe-0.6 wt.% Cu alloy to study the precipitation behavior of copper in α-iron. The alloy was aged at 500°C for 1000 h after water-quenching at 850°C. Transmission electron microscopy observations showed that the disk-shaped copper precipitate of a fcc-structure and ∼0.4 nm in thickness was formed with water-quenching, where there was a Kurdjumov-Sachs orientation relation between the precipitate and the matrix. Aging was proved to reduce the density and diameter of the precipitates and to increase their thickness. Peak hardness was achieved at 100 h-aging. The precipitate was identified to be still disk-shaped (∼2 nm in thickness and ∼7 nm in diameter) and coherent with the matrix after 1000 h-aging.
In addition to copper precipitates, the small-sized particles of 1 to 2 nm in diameter were recognized to be uniformly distributed through the matrix, where their density was estimated to be on the order of 1023/m3. The particle was considered to reasonably be Fe3O4 from the analysis of the selected area diffraction patterns and the compositions of the alloy. Fe3O4 was believed to have formed during alloying and to have been less effective for age-hardening.
By taking account of the interfacial energy between the matrix and copper precipitate (chemical hardening mechanism), an estimation was tentatively made to explain the hardening behavior with aging of the alloy. Assuming the interfacial energy of 1000 erg/cm2, the results showed that the flow stress was increased by ∼29 kg/mm2 due to copper precipitation at 100 h-aging, followed by softening with further aging. The increase in the thickness of copper precipitates was concluded to be the most important factor governing the strength of the alloy during aging.
Fe-Cu alloy, pressure vessel steel, hardening, embrittlement, aging, copper precipitate, chemical hardening mechanism, electron microscopy
Research Associate, University of Tokyo, Tokyo,
Senior Engineer, Japan Atomic Energy Research Institute, Ibaraki-ken,