Elastic and Anelastic Properties of a Copper-Precipited Alloy Steel

Published

October 16, 2008

Author(s)

H M. Ledbetter, Sung Kim, H Ogi

Abstract

We studied the elastic and anelastic properties of an alloy steel containing 1.1 mass % copper subjected to various isothermal agings. The steel's hardness increases with aging time; after reaching a maximum, it decreases. Peak hardness was 17% higher than in the nonaged material. Using acoustic-resonance spectroscopy, we determined the complete elastic-stiffness tensor C_ij and the associated internal friction Q^-1_ij. In the shear stiffnesses C_ij, the materials show an elastic anisotropy of about 3 %. With increasing annealing time, all the usual elastic stiffnesses (B, E, G, C₁) increase and then decrease slightly after reaching optimum aging (maximum hardness). Within measurement error, the Poisson ratio v remains unchanged. All the elastic stiffnesses correlate linearly with hardness. Mass-density measurements suggest a three-stage aging process, proposed originally by Hornbogen and confirmed by the Young-modulus measurements of Lahiri and coworkers. Internal friction Q^-1 decreases continuously with increasing annealing time. This C-Q^-1 signature suggests that most of Q^-1 arises from dislocations, which we verified by measuring strain-amplitude effects. As dislocations are pinned, stiffness increases and Q^-1 decreases, following the Koehler-Granato-Lucke stretched-vibrating-string theory. The well-known equation of motion for this theory is Ay + By + Ky = bς_oexp(iωt). From our measurement results, we deduced A, B, K, and other properties such as the relaxation time τ, the dislocation-segment length l, and the maximum frequency in the Q^-1 frequency curve f_m. The striking correlation among the B(t), K(t), τ (t), l(t), and hardness (t) curves leaves little doubt that the internal friction reflects the hardening mechanism: diffusion of copper atoms from b.c.c. lattice sites to form b.c.c. copper-rich clusters, which become noncoherent and transform to an f.c.c. phase after reaching a certain size.

Citation

Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science

Pub Type

Journals

Keywords

clusters, coefficients, elastic-stiffness, ferritic steel, hardness, high-strength low-alloy steel, internal friction, Koehler-Granato-Lucke therory, precipitates

Citation

Ledbetter, H. , Kim, S. and Ogi, H. (2008), Elastic and Anelastic Properties of a Copper-Precipited Alloy Steel, Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science (Accessed April 26, 2024)