Hardening by crystallization during superplastic flow in a powder-metallurgy-processed Zr 65 Al 10 Ni 10 Cu 15 glass met
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W.J. Kima) Department of Materials Science and Engineering, Hong-Ik University, Seoul 121-791, Korea (Received 26 October 2004; accepted 24 February 2005)
The superplastic behavior of the Zr65Al10Ni10Cu15 glass metallic alloy produced by the powder metallurgy method was examined in the supercooled liquid region. A tensile elongation as large as 750% was obtained at 6.3 × 10−3 s−1 at 697 K. Large strain hardening took place during the course of deformation and systematic trend was observed in the hardening behavior. Plots of stress versus strain and strain rate versus stress at 697 K showed that Newtonian viscous flow governed the plastic flow until the onset of strain hardening. Microstructure and differential scanning calorimetry analyses as well as flow stress versus testing time curves provided consistent evidence that the strain hardening was induced by crystallization. Crystallization was enhanced in the gauge region (deformed region) as compared to the grip region (undeformed region). Crystallization is expected to decrease tensile ductility by decreasing the strain-rate-sensitivity value and increasing the degree of brittleness. Hardening by crystallization, however, can contribute to neck stability if crystallization is enhanced in the neck region. The strain hardening and plastic stability parameters were measured as a function of strain for different strain rates at 696 K. The strain hardening parameter remained highly positive until failure. Because of this, the neck stability parameter (I) could be I < 0 in the entire hardening region. The contribution of hardening by crystallization to neck stability was found to be much more significant than that by grain growth in the superplastic metallic alloys. Reducing the specimen heating-and-holding time was suggested to promote superplasticity deformation without delaying initiation of crystallization. The largest tensile strain in the hardening region where crystallization may be obtained at the strain rates and temperatures where crystallization rate is controlled to be the lowest while maintaining I 艋 0 throughout deformation.
I. INTRODUCTION
The deformation behavior of amorphous metallic alloys has been widely investigated. It is typically divided into either an inhomogeneous deformation or homogeneous deformation mode. While inhomogeneous deformation usually occurs at low temperatures by the formation and propagation of localized shear bands with exhibiting very limited ductility, homogeneous deformation occurs at high temperatures with neck formation and is associated with large plasticity. Several amorphous alloy systems showing superplasticity in the supercooled liquid region defined by the difference between the glass a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0185 J. Mater. Res., Vol. 20, No. 6, Jun 2005
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transition temperature (Tg) and the crystalline temperature (Tx) have been studied.1–4 The homogenous deformation behavior of those a
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