Microstructure and mechanical properties at different length scales and strain rates of nanocrystalline tantalum produced by high-pressure torsion

Abstract

Wei, Q.; Pan, Z. L.; Wu, X. L.; Schuster, B. E.; Kecdkes, L. J.; and Valiev, R. Z., "Microstructure and mechanical properties at different length scales and strain rates of nanocrystalline tantalum produced by high-pressure torsion" (2011 AbstractFully dense, nanocrystalline tantalum (average grain size as small as $40 nm) has been processed for the first time by high-pressure torsion. High-resolution transmission electron microscopy reveals non-equilibrium grain boundaries and grains decorated with high-density dislocations. Microhardness measurements and instrumented nanoindentation experiments indicate that the mechanical property is quite uniform except for the central area of the disks. Nanoindentation experiments at different strain rates suggest that the strain rate sensitivity of nanocrystalline tantalum is increased compared to the coarse-and ultrafine-grained counterparts and is accompanied by an activation energy of the order of a few $b 3 (b is the magnitude of the dislocation Burgers vector), implying a shift in the plastic deformation mechanism from the screw dislocation dominated regime. We thus infer the plastic deformation mechanisms of nanocrystalline body-centered cubic (bcc) and face-centered cubic metals converge. To examine the stress-strain behavior, we have used microcompression to measure the compressive stress-strain curves on microscale pillars fabricated by focused ion beam technique. Yield strength as high as 1.6 GPa has been observed. High-strain rate behavior has been investigated using a miniature Kolsky bar system. We have found that at high-strain rates the nanocrystalline tantalum specimens exhibit adiabatic shear banding, a dynamic plastic deformation mode common to many ultrafine-grained and all nanocrystalline bcc metals.

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

“…The Vickers hardness at the saturated level (HV) of a wide range of pure metals and semi-metals with various crystal structures processed through severe plastic deformation (SPD) have been reported [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]28,66]. In this work, we study the increment of hardness (∆HV) of 17 high purity (better than 99.9%) elements due to HPT reported in [4,5,9,10,23,28,66] (see Table 1).…”

“…Following extensive work in the field by a range of researchers, extensive data on the hardness and microstructure of HPT processed metals and alloys is now available (e.g. [4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]). Data on hardness increase due to HPT have been published for over 20 pure metals, and several of the factors influencing the hardness of HPT processed pure metals have been investigated in some detail by Edelati and Horita [4,5].…”

Mechanism of grain refinement and its effect on Adiabatic Shear Bands in 4340 steel and pure copper during impact

“…It has been reported that BCC metals exhibit an increased propensity to ASB when their grain sizes are reduced into the ultrafine-grain and nano-crystalline regime [2]. This is attributed to the associated changes in the strength, strain hardening behaviour and strain rate sensitivity when the grain size changes [2]. Also, it has been observed that the ASBs associated with FCC metals, such as aluminium and copper, are diffused and exhibit a plastic flow phenomenon [3].…”

“…Mechanical and material factors have been reported to influence the appearance and susceptibility of materials to the formation of ASBs [1]. It has been reported that BCC metals exhibit an increased propensity to ASB when their grain sizes are reduced into the ultrafine-grain and nano-crystalline regime [2]. This is attributed to the associated changes in the strength, strain hardening behaviour and strain rate sensitivity when the grain size changes [2].…”

Flow Stress in Submicron BCC Iron Single Crystals: Sample-size-dependent Strain-rate Sensitivity and Rate-dependent Size Strengthening

“…However, the strain-rate sensitivity of BCC metals was found to decrease with the reduction in grain size, especially in the ultrafine-grained and nanocrystalline regimes. [1,[11][12][13] To understand this trend, Wei et al [11,12] used the m ∼ 1/ √ τ scaling relation to explain the grain size effect on m based on the HallPetch-type elevation of the flow stress (τ ) at small grain sizes. More recently, Cheng et al [2] proposed an alternative explanation.…”