Correlation between the trap state spectra and dielectric behavior of CaCu3Ti 4 O 12
- PDF / 827,693 Bytes
- 12 Pages / 584.957 x 782.986 pts Page_size
- 72 Downloads / 149 Views
The interplay of large strain and large strain rate during high-rate severe plastic deformation (HR-SPD) lead to dynamic temperature rise in situ that engenders a recovered microstructure whose characteristics are not just a function of the strain, but also of the strain rate and the coupled temperature rise during the deformation. In this work, we identify three classes of microstructures characterized by multistage recovery phenomena that take place during the high strain rate SPD of Cu. It is found that the first stage of this recovery is similar to the first stage of static recovery, which is characterized mainly by annihilation of dislocations. The second stage starts around 360 K and was characterized by dislocations getting arranged in tight cell boundaries and eventually into subgrain. Recovery stages were found to be followed by a stage of grain growth and recrystallization when the temperature in the deformation zone approaches 480 K.
I. INTRODUCTION
Severe plastic deformation (SPD) in metallic systems is accompanied not only by grain refinement, which endows it with strength,1–4 but also by generation and organization of high density of crystal defects. The energies stored in various configurations of these defects provide the driving force for most observed recovery and recrystallization phenomena.5–9 Dislocations that contribute to most of this stored energy in severely deformed materials are mostly integrated into grain boundaries, cell boundaries, or complex entanglements that are prone to annihilation in response to heating, resulting in microstructural transformation. Thus far, most of the work on SPD has used low strain rates (;0.1/s to 10/s), except for a few recent works wherein strain rates of the order of 103 were used10–12 and resulted in intriguing property combinations. One of these studies (Ref. 10) used dynamic plastic deformation instead of SPD where they obtained high levels of plastic deformation through repeated compressive deformation at high strain rates. While in the works mentioned in Refs. 11 and 12, we used high shear strain rates for SPD where strains of the order 3–5 and strain rates of the order of ;103 were obtained in one pass of deformation. The interplay of large shear strains and high strain rates and the supplementary effect of temperature rise near the deformation zone that accompanies the high strain rates, as modeled in our earlier work, gives rise to novel microstructures and subsequently, enhanced material a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.28 J. Mater. Res., Vol. 26, No. 3, Feb 14, 2011
http://journals.cambridge.org
Downloaded: 15 Feb 2015
properties.11,12 We have shown that high strain rate SPD can lead to dense dispersion of nanotwins in low stacking fault energy (SFE) material, like brass.11 In the case of copper, we found that high strain rate SPD and the accompanying flash heating lead to bimodal grain size distribution with modes at 500 and 2000 nm.12 Such multimodality was shown to lead to a significan
Data Loading...
