Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Abstract

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.

Appendix

General creep equation:

$$ \dot{\varepsilon } = A\left( {{\frac{\sigma }{E}}} \right)^{n} \exp \left( { - \frac{Q}{RT}} \right) $$

(A1)

where A is the material constant, σ is the flow stress, n is the stress exponent, E is the temperature-compensated Young’s modulus, Q is the activation energy for deformation, and R is the universal gas constant.

Diffusional flow creep:

$$ \dot{\varepsilon } = k_{1} \left( {{\frac{{D_{L} }}{{d^{2} }}}} \right)\left( {{\frac{{Eb^{3} }}{kT}}} \right)\left( {{\frac{\sigma }{E}}} \right)\quad \left( {{\text{lattice}}\,{\text{diffusion}}} \right) $$

(A2)

$$ \dot{\varepsilon } = k_{2} \left( {{\frac{{D_{gb} }}{{d^{2} }}}} \right)\left( {{\frac{{Eb^{3} }}{kT}}} \right)\left( {{\frac{\sigma }{E}}} \right)\quad \left( {{\text{grain}}\,{\text{boundary}}\,{\text{diffusion}}} \right) $$

(A3)

SD creep:

$$ \dot{\varepsilon } = k_{3} \left( {{\frac{{D_{s} }}{{{{b}}^{2} }}}} \right)\left( {{\frac{\sigma }{E}}} \right)^{3} $$

(A4)

GBS:

$$ \dot{\varepsilon } = k_{4} \left( {{\frac{{D_{L} }}{{d^{2} }}}} \right)\left( {{\frac{\sigma }{E}}} \right)^{2} \quad \left( {{\text{lattice}}\,{\text{diffusion}}\,{\text{controlled}}} \right) $$

(A5)

$$ \dot{\varepsilon } = k_{5} \left( {{\frac{{D_{gb} {{b}}}}{{d^{3} }}}} \right)\left( {{\frac{\sigma }{E}}} \right)^{2} \quad \left( {{\text{boundary}}\,{\text{diffusion}}\,{\text{controlled}}} \right) $$

(A6)

For Eqs. [A2] through [A6], D _L is the lattice diffusion coefficient, D _gb is the GB diffusion coefficient and D _s is the solute atom diffusion coefficient, b is the Burgers vector, d is the grain size, E is the temperature compensated Young’s modulus, k is the Boltzmann’s constant, and k ₁ through k ₅ are material constants.