Work softening and annealing hardening of deformed nanocrystalline nickel

Abstract

We reported that work softening takes place during room-temperature rolling of nanocrystalline Ni at an equivalent strain of around 0.30. The work softening corresponds to a strain-induced phase transformation from a face-centered cubic ͑fcc͒ to a body-centered cubic ͑bcc͒ lattice. The hardness decreases with increasing volume fraction of the bcc phase. When the deformed samples are annealed at 423 K, a hardening of the samples takes place. This hardening by annealing can be attributed to a variety of factors including the recovery transformation from the bcc to the fcc phase, grain boundary relaxation, and retardation of dislocation gliding by microtwins. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3062849͔The deformation mechanisms of nanocrystalline ͑nc͒ materials are very different from those of coarse grain materials. [1][2][3][4][5][6][7] It has been found that in face-centered cubic ͑fcc͒ nc Ni, mechanisms such as deformation twinning, formation of extended and full dislocations from the grain boundaries ͑GBs͒, GB sliding, and grain rotation can contribute to plasticity. 6,[8][9][10][11][12][13][14][15][16] Recently, we have reported that for nc Ni, a phase transformation from fcc ͑␥͒ to body-centered cubic ͑bcc, ␣͒ can provide another deformation mechanism to realize plastic deformation to relatively large plastic strain during room-temperature rolling, 17 i.e., a far-fromequilibrium microstructure such as nc Ni can accommodate plastic strain via a change in lattice structure upon mechanical loading. In this letter we report that such a strain-induced phase transformation from ␥ to ␣ leads to work softening during cold-rolling and that subsequent annealing at 423 K results in hardening.Fully dense, electrodeposited nc Ni sheets ͑99.8% purity͒ were procured from Goodfellow, Inc. The as-received sheets were 200 m thick with an average grain size of about 20 nm. Samples of the nc Ni of 10ϫ 10 mm 2 in size were rolled at room temperature to various von Mises equivalent strains VM calculated as VM = ͉2 / ͱ 3ln͑1+␦͉͒, where ␦ is the rolling reduction. The microstructure of the samples was examined using x-ray diffraction ͑XRD͒ ͑Riguta D/max-RC͒ and high-resolution transmission electron microscopy ͑JEM 2010F operated at 200 kV͒. Microhardness measurements were taken using a load of 0.196 N ͑HVS-1000͒. The reported hardness values are each averaged from three indents. The hardness provides a measure of the resistance to deformation by surface indentation or by abrasion and is a combined property of elasticity, plasticity, strength, and toughness. However, the hardness value is determined by the resistance both to initial and continuous plastic deformation. In general, a high hardness corresponds to a high tensile strength, although there is not an exact relationship between the two values. Considering the small dimensions of the deformed samples, in the experiment we use the sample hardness to investigate changes in the mechanical properties.The experimental result shows that once a deformation strain ...

Microstructure and micro-texture evolution during large strain deformation of an aluminium–copper–lithium alloy AA 2195

“…Several reasons viz. strain induced phase transformation [19], softening due to texture [20], plastic instability and localization in shear bands [21], localized temperature rise due to adiabatic shear banding [22] have been suggested as the reason for flow softening. Here in case of Al-Li alloys, there is no phase transformation.…”

Effect of iron content on mechanical behavior of nanocrystalline nickel and alloys

“…Similar phenomenon has also been found in Cu and Cu-Ge alloys with low to medium SFE [12]. Another mechanical behavior concerning the softening of nc metals is generally observed when strain hardening reaches the maximum level [13][14][15][16][17]. For simplicity, we refer to this cross-over between strain hardening and strain softening as "mechanical transition".…”

“…Additional experimental details can be found in Ref. [16]. of the nc samples was investigated by XRD (Rigaku D/MAX 2500 PC, 18 kW) operating in a step scan mode with Cu K␣ radiation.…”

Microstructure evolution and mechanical property of nanocrystalline Ni–20Fe alloy during cold rolling

“…Compared with the pure Ni, the hardness of the current nc Ni-20Fe alloy is greatly improved with the Fe alloying effect, which is known as solid solution hardening [20]. In our previous investigation, the pure nc nickel enters into strain hardening slowly and reaches the peak hardness at an equivalent strain of about 0.30 [21]. Noted that the nc Ni-20Fe alloy reaches a peak hardness just at the equivalent strain of 0.10, it can be concluded that the presence of Fe plays a significant role in the strain hardening.…”

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