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D. M. Harwani et al.
fabricate airframes (from aluminum and titanium alloys) and engine components of
gas turbines (from nickel alloys) [2]. It also finds applications in aerospace, rail and
road transport, medical implants, communication and architecture sectors. Common
techniques of SPF are blow forming, hollow cavity forming, diffusion bonding, quick
plastic forming and high cycle blow forming [3].
Despite the ability of producing near-to-net-shape unitized parts without use of
any joints, SPF has not been commercially popular for many alloys. One reason is
its slow forming rates [4]. The other limiting factor is that this metal forming method
can be applied only in special materials which have undergone pre-processing for
achieving microstructure, consisting of fine grains or dual phases [1].
This paper briefly discusses the concept and pre-requirements for developing
superplasticity in metals. An overview of various severe plastic deformation methods
to attain ultrafine grains (average grain size of about 1 μm) and nano-sized grains
(average grain size of about 10 nm) and subsequent superplastic properties in nonferrous metals has been given. It concludes with the present challenges in the field
of SPF and scope for future research studies.
2 Superplasticity
The foundation of SPF lies in developing superplastic behavior in materials at
elevated temperatures. The capability of any metallic material to get enhanced tensile
elongations more than 200% through plastic deformation is called superplasticity [5].
The essence of superplasticity is large permanent deformations before tensile failure.
Extremely large elongations of about more than 5000% have been reported in various
materials [6]. Superplastically deformed parts exhibit favorable isotropic mechanical
properties and extremely good surface finish [7].
The major pre-requisites to induce superplastic features in most of the metals are:
• Fine grain size (<10 μm)
• Elevated temperatures (0.5–0.8 T m , where T m is melting point of metal matrix in
degree Kelvin)
• Equiaxed grains
• High angular grain boundaries (HAGB)
• Thermal stability of microstructure at high temperatures
• Strain rates in the range of 10
−2 /s to 10
−6 /s [8].
Apart from these aforementioned conditions, the presence of second-phase
particles or liquid phase in the microstructure facilitates easy development of
superplasticity in metals [9, 10].
Unusually long and neck-free elongations are obtained during superplasticity as
the most commonly associated deformation mechanism of grain boundary sliding
(GBS) occurs, accompanied by diffusion, dislocation or grain rotation [11]. Another
important parameter related to superplastic deformation is strain rate sensitivity, m. It
is the measurement of change in flow stress corresponding to the increasing change
D. M. Harwani et al.
fabricate airframes (from aluminum and titanium alloys) and engine components of
gas turbines (from nickel alloys) [2]. It also finds applications in aerospace, rail and
road transport, medical implants, communication and architecture sectors. Common
techniques of SPF are blow forming, hollow cavity forming, diffusion bonding, quick
plastic forming and high cycle blow forming [3].
Despite the ability of producing near-to-net-shape unitized parts without use of
any joints, SPF has not been commercially popular for many alloys. One reason is
its slow forming rates [4]. The other limiting factor is that this metal forming method
can be applied only in special materials which have undergone pre-processing for
achieving microstructure, consisting of fine grains or dual phases [1].
This paper briefly discusses the concept and pre-requirements for developing
superplasticity in metals. An overview of various severe plastic deformation methods
to attain ultrafine grains (average grain size of about 1 μm) and nano-sized grains
(average grain size of about 10 nm) and subsequent superplastic properties in nonferrous metals has been given. It concludes with the present challenges in the field
of SPF and scope for future research studies.
2 Superplasticity
The foundation of SPF lies in developing superplastic behavior in materials at
elevated temperatures. The capability of any metallic material to get enhanced tensile
elongations more than 200% through plastic deformation is called superplasticity [5].
The essence of superplasticity is large permanent deformations before tensile failure.
Extremely large elongations of about more than 5000% have been reported in various
materials [6]. Superplastically deformed parts exhibit favorable isotropic mechanical
properties and extremely good surface finish [7].
The major pre-requisites to induce superplastic features in most of the metals are:
• Fine grain size (<10 μm)
• Elevated temperatures (0.5–0.8 T m , where T m is melting point of metal matrix in
degree Kelvin)
• Equiaxed grains
• High angular grain boundaries (HAGB)
• Thermal stability of microstructure at high temperatures
• Strain rates in the range of 10
−2 /s to 10
−6 /s [8].
Apart from these aforementioned conditions, the presence of second-phase
particles or liquid phase in the microstructure facilitates easy development of
superplasticity in metals [9, 10].
Unusually long and neck-free elongations are obtained during superplasticity as
the most commonly associated deformation mechanism of grain boundary sliding
(GBS) occurs, accompanied by diffusion, dislocation or grain rotation [11]. Another
important parameter related to superplastic deformation is strain rate sensitivity, m. It
is the measurement of change in flow stress corresponding to the increasing change