Strain rate and temperature effects on dynamic properties of high-strength weldable aluminum-scandium alloy
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The dynamic mechanical behavior, fracture characteristics, and microstructural evolution of high-strength weldable aluminum-scandium (Al–Sc) alloy were investigated using a compressive split Hopkinson pressure bar at strain rates of 1.3 103, 3.6 103, and 5.9 103 s1, respectively, and temperatures of 100, 25, and 300 C. The results showed that the flow stress, work hardening rate, and strain rate sensitivity increase with increasing strain rate but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Moreover, the fracture strain decreases with increasing strain rate and decreasing temperature. It was shown that the Zerilli–Armstrong face-centered-cubic (fcc) constitutive equation provides accurate predictions of the mechanical response of the Al–Sc alloy under the considered strain rate and temperature conditions. Scanning electron microscopy observations revealed that the fracture surfaces of the impacted specimens are characterized by transgranular dimpled features, which are indicative of a ductile failure mode. Moreover, transmission electron microscopy observations indicated the presence of both fine and coarse randomly dispersed precipitates within the matrix and at the grain boundaries. It was found that an increasing strain rate reduces the size of the dislocation cells within the impacted Al–Sc microstructure and therefore increases the dislocation density. However, at higher temperatures, the dislocations are annihilated, leading to a reduction in the dislocation density and a corresponding increase in the dislocation cell size. The variations observed in the size and density of the dislocation cells were found to be consistent with the dynamic tendencies noted in the stress–strain response of the Al–Sc alloy.
I. INTRODUCTION
The strength of age-hardened weldable aluminum (Al) alloys is generally enhanced via the addition of suitable alloying elements such as Sc, Zr, and Mn, for example. However, the resulting Al alloys are highly susceptible to hot-cracking, and thus their practical applications are severely curtailed. It has been shown that the addition of small quantities of scandium (Sc) improves the resistance of Al alloys to recrystallization under high-temperature conditions and prompts a grain refinement effect, thereby enhancing the mechanical strength of the alloy.1,2 The improved recrystallization resistance and mechanical strength of Al–Sc alloys is the result of the precipitation of Al3Sc (L12) phases during the solidification process. These precipitates are thermally stable and fully coherent with the Al alloy matrix. The fine dispersion of these a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0017
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 1, Jan 2009 Downloaded: 01 Apr 2015
precipitates throughout the matrix prevents grain boundary movement, thereby suppressing the recrystallization effect, and i
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