Original ArticlesInfluences of strain rate and grain size on yield and serrated flow in commercial Al-Mg alloy 5086
Introduction
Much attention has been paid to the rather special stress-strain characteristics of Al-Mg alloys. Beginning with the pioneering work of Portevin and LeChatelier in 1923 [1], the focus has been on the appearance of tensile band-type deformation markings that are observed coincident with the appearance of serrated flow in stress-strain curves. The serrations are the result of dynamic strain aging (DSA). Cottrell’s solute-locking of dislocations, originally developed to explain the yield point phenomenon in steel [2], carries over to the consideration of solute diffusing to moving dislocations as the mechanism of DSA. It is also well known that the yield point behavior of steel exhibits a strong Hall-Petch (H-P) dependence.
Researchers have focused on the critical strain, ϵc, at which DSA serrations begin in materials such as steel, nickel and Al-Mg alloys 3, 4, 5, 6, 7, 8, 9. The value of ϵc depends on temperature, strain rate and grain size. Thomas [6] observed strain plateaus associated with the DSA serrations, whose appearance was strongly grain size dependent. He attributed the behavior to the successive propagation of Lüders fronts across the specimen even during straining well past the yield point. The flow stress values at ϵc were shown to follow a Hall-Petch dependence. Other model descriptions of DSA give an explanation of temperature and strain rate influences based only on consideration of dislocation density and solute concentration; grain size considerations are not explicitly involved 10, 11, 12. Thus, van den Beukel and Kocks [10] and McCormick [11] utilized a linear superposition of flow stresses to determine a negative strain rate sensitivity for DSA behavior. These models employ only a friction stress and an added stress for dislocation interactions.
A major influence of the DSA phenomenon is to produce both a higher flow stress and, very importantly, greater strain hardening at lower strain rates than for higher ones at which serrations do not appear. Both of these influences are seen in Figure 1 for a pair of relatively smooth curves of Al-Mg alloy 5083 reported at strain rates of 10−3 s−1 and 0.1 s−1 as superposed on a graph of very high strain rate measurements obtained in split Hopkinson pressure bar (SHPB) tests performed at the same and higher temperatures [13].
Even though the DSA-generated serrations in Figure 1, at 10−3 s−1 for example, are small compared to the wave dispersion-based irregularities in the SHPB curves, the slowest strain rate curve is raised by DSA to near-coincidence with the SHPB result at a strain rate of 2500 s−1. The DSA-influenced strain hardening is also comparable to or greater than that of any of the SHPB curves. Thus, the DSA influence interferes with ‘normal’ material behavior that is characterized by an increase in flow stress with increasing strain rate. An example of the latter concern occurred for the prediction of alloy 5086 explosive impact behavior where thermally-activated solute motion does not occur.
Physically-based constitutive equations, for example, of Zerilli-Armstrong type [14], that are derived for thermally-activated dislocation-lattice or dislocation-dislocation interactions, are susceptible to interference from DSA. The recent case in point, relating to Figure 1, concerned the use of commercial aluminum alloy 5086 when employed as a confinement vessel for monitoring the explosive behavior of nonideal energetic materials [15]. For this reason, clarification of the importance of strain aging in accounting for laboratory tests of the stress-strain behavior of the alloy has been investigated. Of special interest was the solute influence on the combined yield and strain hardening behavior.
Section snippets
Experimental
Alloy 5086 is a commercial Al-Mg alloy whose major constituents are 4.0 wt% Mg, 0.4 wt% Mn and 0.15 wt% Cr [16]. The material was received in seamless pipe form subjected to an H32 tempering treatment that consists of 20% cold work followed by a proprietary recovery anneal. The end result is a material with a specified yield strength of approximately 205 MPa, tensile strength of 290 MPa and 12 percent elongation. Preliminary tensile tests of the as-received material for this study showed, at a
Results
Load versus crosshead displacement data from each of the tests were reduced to true stress versus true strain results [19]. Comparison of the stress-strain curves at 10−3 s−1 and 0.36 s−1 rates in Figure 2 shows that a reverse strain rate effect is measured in each case because of the greater influence of DSA at the lower strain rate. Accordingly, lower flow stresses at maximum load, and lower strain hardening, are shown to have occurred in each case at the higher strain rate. The behavior is
Discussion
The yield point behavior for the annealed small grain size results of Figure 2 is similar to the results reported by Thomas [6]. This is reasonable since the grain sizes are comparable. However, both Lüders type plateaus and band markings were absent in the current study where the bulk specimens showed a more conventional strain hardening behavior.
Normally the H-P relationship is tested at yield or at a constant strain value as in, equation 1 where σ0ϵ is a friction stress
Summary
The stress-strain behavior of Al-Mg alloy 5086 in an H32 condition is shown in conventional laboratory tests to be sensitively dependent on the combined influences of strain rate, grain size and the occurrence of dynamic strain aging (DSA). A reverse strain rate effect occurs at room temperature in conventional testing where serrated flow occurs. The alloy behavior was compared informatively with that for fully annealed material or material subjected only to the 20% cold work part of the H32
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