Negative Strain‐rate Sensitivity in a Nanostructured Aluminum Alloy

Abstract

Since grain boundaries and triple junctions occupy a considerable volume fraction in nanostructured materials, grainboundary mediated deformation mechanisms, for instance, Coble diffusion creep or grain boundary sliding, are expected to play a significant role in the deformation of nanostructured materials, even at room temperature. The grain-boundary mediated deformation mechanisms, may lead to a large value of strain rate sensitivity in nanostructured materials. Recently, the strain rate sensitivity of several nanostructured/ ultrafine-grained (UFG) materials has been investigated at room temperature to understand the deformation mechanisms. [1][2][3][4][5][6][7][8] Indeed, nanostructured Cu or Ni are found to have much higher strain rate sensitivity than coarse-grain Cu or Ni at room temperature. [1,3,4,[6][7][8] However, although nanostructured/UFG fcc materials have elevated strain rate sensitivity compared to their coarse-grained counterparts, [5] nanostructured/UFG bcc Fe and Ta were found to have lower strainrate sensitivity than coarse-grained Fe and Ta. [5] The reasons for these observations are not fully understood. These observations, however, indicate that the nanostructures have an effect on the strain rate sensitivity, and thus on their mechanical properties, especially their ductility. Therefore, it is of interest to study the strain rate sensitivity of nanostructured materials.The mechanical properties of nanostructured or UFG face centered cubic (fcc) Al alloys have been investigated recently. [9][10][11][12] While a positive strain rate sensitivity of flow stress at room temperature was observed in the compression deformation of nanocomposite Al-Ti-Cu alloy, [9] a small negative strain rate sensitivity was noticed in the tensile deformation of UFG Al-Mg alloy [12] in a strain rate range of 10 -4 to 10 -2 s -1 . Since the negative strain rate sensitivity observed in coarse-grained Al-Mg alloys is a consequence of dynamic strain aging, [13][14][15][16] it would be of interest to know whether the dynamic strain aging also plays a role in the plastic deformation of nanostructured or UFG Al alloys. The purpose of the present study is twofold: (a) to investigate the influence of strain rate on the deformation behavior of a nanostructured 5083 Al alloy processed by cryomilling and hot extrusion, and (b) to provide insight into the underlying deformation mechanisms of nanostructured or UFG Al alloys.The microstructure, taken from a section parallel to the extrusion direction, of the as-extruded cryomilled 5083 Al alloy, is shown in Figure 1(a). Although the ultrafine grains of 100-200 nm are slightly elongated along the extrusion direction, the selected-area electron diffraction pattern does not reveal preferred crystallographic orientation. In a recent investiga- COMMUNICATIONSADVANCED ENGINEERING MATERIALS 2006, 8, No. 10Fig. 1. Microstructure of the as-extruded cryomilled 5083 Al alloy in a section (a) parallel to and (b) normal to the extrusion direction.

Temperature-Dependent Mechanical Response of an UFG Aluminum Alloy at High Rates

“…The QS results show a maximum flow stress at low strains followed by nearly perfectly plastic behavior, and perhaps even weak strain softening. Similar behavior has been observed for a similar cryomilled Al-5083 material [33]. This behavior is perhaps associated with deformation instabilities arising from dynamic strain aging effects [33,34].…”

“…Similar behavior has been observed for a similar cryomilled Al-5083 material [33]. This behavior is perhaps associated with deformation instabilities arising from dynamic strain aging effects [33,34]. In summary, the CS tests suggest that microstructure evolution is limited in this cryomilled material at these temperatures.…”

Micro-mechanical and tribological properties of aluminum-magnesium nanocomposites processed by high-pressure torsion

“…There are several reports showing negative strain rate sensitivities in nanostructured metals of Ti after ECAP followed by cold rolling [55] and Al alloys produced through cryomilling [56,57]. The consistent behavior of these metals involves a strain-rate dependent instability of plasticity attributed to the occurrence of dynamic strain aging (DSA) leading to the local formation of solute clusters on forest dislocations resulting in a strengthening of dislocation junctions [58].…”

Micro‐Mechanical Behavior of an Exceptionally Strong Metal Matrix Nanocomposite Processed by High‐Pressure Torsion

“…[41] Further investigation of a cryomilled Al-5083 alloy demonstrated that the negative strain rate sensitivities in both nanostructured and coarsegrained Al alloys were caused by DSA. [42] Specifically, DSA occurs due to the formation of solute clusters on forest dislocations resulting in a strengthening of dislocation junctions where the presence of a cluster on a forest dislocation modifies the strength of the junction formed by the interaction of dislocations with mobile unclustered dislocations. Thus, faster deformation allows for less clustering by diffusion of solute atoms to the mobile dislocations and this produces junctions having lower strength.…”

“…Thus, faster deformation allows for less clustering by diffusion of solute atoms to the mobile dislocations and this produces junctions having lower strength. [39,42] It should be noted that the PLC effect is generally observed over a limited range of temperature and strain rate and thus the testing conditions of room temperature to 50 C and strain rates of <10 À1 s À1 were suggested for commercial Al alloys demonstrating a negative strain rate sensitivity. [36] An earlier experiment on a discontinuously reinforced metal matrix composite of a powder-consolidated aluminum 6092/B 4 C demonstrated a negative strain rate sensitivity at strain rates <1.0 s À1 .…”