Magnetically stimulated transient creep processes in a heterogeneous aluminum alloy with ferromagnetic inclusions

Abstract

This study analyzes the creep processes of a heterogeneous aluminum alloy containing ferromagnetic inclusions with an average size of 3-5 μm, and the influence of preliminary magnetic exposure (in a constant magnetic field (MF) with induction B < 0.75 T) on these processes. Experimental investigations were carried out to determine the characteristic times of the transient stages of short-term creep, which do not exceed approximately 25 ms. We analyzed the transient deformation behavior of the aluminum-based alloy and consequently determined the elastic moduli of the material, along with the influence of preliminary magnetic exposure on them. It was established that the MF has the most pronounced effect on the "long-term" elastic modulus H (defined as the ratio of stress to the relative strain of the material after sustained application of a constant load). We propose that the observed linear decrease in H with increasing B is associated with the magnetostriction of the inclusions during preliminary magnetic exposure. An increase in magnetic induction enhances local stresses at the matrix-inclusion interfaces, which in turn leads to a rise in dislocation density. These microstructural changes influence the subsequent deformation behavior, including both transient responses under loading and stress relaxation during unloading. We conclude that the elastic modulus H is the most sensitive parameter to the influence of magnetic fields, indicating a significant impact of external MF on the creep dynamics of the structurally heterogeneous aluminum alloy under investigation.

Keywords: Aluminum alloy; Creep; Elastic moduli, relaxation time; Iron-based ferromagnetic inclusions; Transient processes.

Fig. 1

Family of creep curves with various magnitudes of tensile stresses: σ₁ < σ₂ < σ₃. OA ― creep stage I (transition process); AB ― creep stage II (steady creep mode); BC ― stage III (high strain rate mode, ends either with a brittle fracture near point B, or with ductile fracture with formation of a neck).

Fig. 2

Sample deformation under creep conditions at stages I and II (a) and stage III (b, c). The Kelvin viscoelastic model (d).

Fig. 3

Scanning electron microscopy image of the sample surface (a) with color distribution of element concentrations (b) on the section surface: O (red) + Fe (green) + Al (blue).

Fig. 4

Mechanical stress applied to the sample versus time (a), and creep dynamics of the sample-relative deformation under conditions of stepwise loading/unloading (b).

Fig. 5

Dependence of creep curves ε(t) of aluminum alloy samples with ferromagnetic inclusions after application of load σ₀ = 125 MPa (at time t = 0) and unloading (at time t₂ = 515 s, σ(t > t₂) = 0). Immediately before the creep tests, the samples had been exposed for 30 min at room temperature to a uniform magnetic field (MF) with induction B: 1 ― 0 T (witness sample), 2 ― 0.3 T, 3 ― 0.5 T, and 4 ― 0.75 T.

Fig. 6

Family of creep curves ε(t) of aluminum alloy samples after application of a load (σ = σ₀ = 125 MPa) at time t = 0 (a) and after removal of the load σ₀ at time t = 0 σ(t > 0) = 0 (b) after exposing the samples to a constant MF with induction: 1 ― 0 Т (witness sample), 2 ― 0.3 Т, 3 ― 0.5 Т, and 4 ― 0.75 T. Solid lines indicate approximation according to Eq. (12). The approximation results are presented in Table 1. Inset in Fig. 6a. Dependence of modulus H on magnitude of the constant MF induction. Immediately before creep testing, the samples had been exposed to a constant MF for 30 min at room temperature.