Y.T.Zhu et al.Scripta Materialia 51 (2004)825-830 829 SPD-processed metals may obviate the need for sub- perature superplasticity [45]in bulk nanostructured sequent surface finishing steps.This will reduce the materials,where high strain rates refer to the tensile manufacturing cost by eliminating or simplifying the testing of samples at rates at and above 10-2s-!and low processing steps.Forging is used to create product temperatures refer to tensile testing at homologous shapes in the aerospace and automotive industry and temperatures below 0.5Tm.For example,there are re- there is evidence that the forging temperatures can be ports of tensile elongations of up to >2000%at a strain significantly reduced when forging SPD-processed alu- rate of 1 s-in a Zn-Al alloy processed by ECAP [46] minum alloys for aerospace applications.In addition, and the occurrence of superplasticity at homologous the times for subsequent heat treatments may be re- temperatures as low as ~0.36Tm for an electrodeposited duced by as much as 50%.Thus,for alloys with heat nickel [45]. treatments in excess of 12 h,the energy and time savings The production of bulk nanostructured alloys in are substantial. sheet form,with ultrafine grains that are fairly stable at elevated temperatures,has the potential to expand the 3.2.2.Formability through superplasticity superplastic forming niche into a processing regime that The NS alloys processed by SPD can be formed su- will be effective in producing components for a very wide perplastically at lower temperatures and faster rates range of commercial applications.The recent demon- than is possible in conventional superplastic alloys. stration of the ECAP processing of plate samples [47] Superplasticity is a flow process in which polycrystalline suggests that it may be a fairly easy task to produce materials exhibit high elongations prior to ultimate superplastic NS materials that can be readily utilized in failure.This type of flow is the characteristic feature of forming operations.However,even in the absence of the superplastic forming industry in which complex sheet production,there are several potential applications components,often having multiple curved surfaces,are for these materials in bulk form:an example of current formed from superplastic sheet metals.The essential interest is the production of superplastic seismic damp- requirements for achieving a superplastic forming ing devices [48]. capability are small grain sizes,typically less than ~10 um,and high forming temperatures,typically above 0.5 Tm,where Tm is the absolute melting point of the material.At the present time,the superplastic forming 4.Summary and conclusions industry occupies a small but viable cost-effective niche through the production of high-cost low-volume com- 1.Processing through the application of SPD is attrac- ponents associated primarily with the aerospace,archi- tive for the production of bulk NS materials.These tectural and sports industries [41].Expansion beyond materials can be tailored to exhibit both superior per- this niche,into automotive and other high-volume formance and superior properties. applications,is currently restricted by the slow strain 2.A primary advantage of SPD is the development of rates involved in the forming process(typically ~10-3 materials having good machinability,forgability, s-)and the consequent long forming times (~20-30 and formability at potentially low processing cost. min)associated with the production of each separate This makes these NS materials especially attractive component. for use in specialized structural applications such The introduction of bulk NS materials provides a as medical implants,biomedical devices,and high- potential for overcoming the inherent limitations asso- performance bicycles.In the longer term,when ciated with conventional coarse-grained superplastic continuous-processing methods are developed,it is materials.Thus,it is now well established,both theo- reasonable to anticipate large-scale applications in retically and experimentally [42,43],that the rate of flow the automotive and other fields. within the superplastic regime varies inversely with the 3.Amorphous and NS materials also have unique phys- grain size raised to a power that is close to ~2.It is ical properties that are attractive for optical and elec- anticipated,therefore,that a decrease in the grain size trical applications.The high strength of NS materials by one order of magnitude will lead to an increase in the makes them ideal for micro-devices optimal superplastic forming rate by approximately two orders of magnitude and thus,in effect,the total forming time will be reduced to ~20-30 s.It can be shown also that this reduction in grain size will lead to the advent of Acknowledgements a superplastic forming capability which occurs at lower temperatures than those generally associated with con- This work was supported by the US Department of ventional superplastic flow.Early experimental results Energy IPP program(YTZ TCL)and by the National provided very clear demonstrations of the occurrence of Science Foundation under Grant No.DMR-0243331 both high strain rate superplasticity [44]and low tem- (TGL).SPD-processed metals may obviate the need for sub￾sequent surface finishing steps. This will reduce the manufacturing cost by eliminating or simplifying the processing steps. Forging is used to create product shapes in the aerospace and automotive industry and there is evidence that the forging temperatures can be significantly reduced when forging SPD-processed alu￾minum alloys for aerospace applications. In addition, the times for subsequent heat treatments may be re￾duced by as much as 50%. Thus, for alloys with heat treatments in excess of 12 h, the energy and time savings are substantial. 3.2.2. Formability through superplasticity The NS alloys processed by SPD can be formed su￾perplastically at lower temperatures and faster rates than is possible in conventional superplastic alloys. Superplasticity is a flow process in which polycrystalline materials exhibit high elongations prior to ultimate failure. This type of flow is the characteristic feature of the superplastic forming industry in which complex components, often having multiple curved surfaces, are formed from superplastic sheet metals. The essential requirements for achieving a superplastic forming capability are small grain sizes, typically less than
10 lm, and high forming temperatures, typically above
0.5 Tm, where Tm is the absolute melting point of the material. At the present time, the superplastic forming industry occupies a small but viable cost-effective niche through the production of high-cost low-volume com￾ponents associated primarily with the aerospace, archi￾tectural and sports industries [41]. Expansion beyond this niche, into automotive and other high-volume applications, is currently restricted by the slow strain rates involved in the forming process (typically
103 s1) and the consequent long forming times (
20–30 min) associated with the production of each separate component. The introduction of bulk NS materials provides a potential for overcoming the inherent limitations asso￾ciated with conventional coarse-grained superplastic materials. Thus, it is now well established, both theo￾retically and experimentally [42,43], that the rate of flow within the superplastic regime varies inversely with the grain size raised to a power that is close to
2. It is anticipated, therefore, that a decrease in the grain size by one order of magnitude will lead to an increase in the optimal superplastic forming rate by approximately two orders of magnitude and thus, in effect, the total forming time will be reduced to
20–30 s. It can be shown also that this reduction in grain size will lead to the advent of a superplastic forming capability which occurs at lower temperatures than those generally associated with con￾ventional superplastic flow. Early experimental results provided very clear demonstrations of the occurrence of both high strain rate superplasticity [44] and low tem￾perature superplasticity [45] in bulk nanostructured materials, where high strain rates refer to the tensile testing of samples at rates at and above 102 s1 and low temperatures refer to tensile testing at homologous temperatures below 0.5Tm. For example, there are re￾ports of tensile elongations of up to >2000% at a strain rate of 1 s1 in a Zn–Al alloy processed by ECAP [46] and the occurrence of superplasticity at homologous temperatures as low as
0.36Tm for an electrodeposited nickel [45]. The production of bulk nanostructured alloys in sheet form, with ultrafine grains that are fairly stable at elevated temperatures, has the potential to expand the superplastic forming niche into a processing regime that will be effective in producing components for a very wide range of commercial applications. The recent demon￾stration of the ECAP processing of plate samples [47] suggests that it may be a fairly easy task to produce superplastic NS materials that can be readily utilized in forming operations. However, even in the absence of sheet production, there are several potential applications for these materials in bulk form: an example of current interest is the production of superplastic seismic damp￾ing devices [48]. 4. Summary and conclusions 1. Processing through the application of SPD is attrac￾tive for the production of bulk NS materials. These materials can be tailored to exhibit both superior per￾formance and superior properties. 2. Aprimary advantage of SPD is the development of materials having good machinability, forgability, and formability at potentially low processing cost. This makes these NS materials especially attractive for use in specialized structural applications such as medical implants, biomedical devices, and high￾performance bicycles. In the longer term, when continuous-processing methods are developed, it is reasonable to anticipate large-scale applications in the automotive and other fields. 3. Amorphous and NS materials also have unique phys￾ical properties that are attractive for optical and elec￾trical applications. The high strength of NS materials makes them ideal for micro-devices. Acknowledgements This work was supported by the US Department of Energy IPP program (YTZ & TCL) and by the National Science Foundation under Grant No. DMR-0243331 (TGL). Y.T. Zhu et al. / Scripta Materialia 51 (2004) 825–830 829