On this seventy-fifth anniversary of the first scientific report of true superplastic flow, it is appropriate both to look back and examine the major developments that established the present understanding of superplasticity and to look to the future to the new opportunities that are made possible by new processing techniques, based on the application of severe plastic deformation, that permit the production of fully dense bulk materials with submicrometer or nanometer grain sizes. This review proposes a minor modification to the present definition of superplasticity, it provides an overview of the current understanding of the flow of superplastic metals and ceramics and then it examines, and gives examples of, the new possibilities that are now available for achieving exceptional superplastic behavior.

Seventy-five years of superplasticity: historic developments and new opportunities

The evolution of technology for materials processing over the last 50 years: The automotive example

Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Friction stir processing: An effective technique to refine grain structure and enhance ductility

General Motors has developed Quick Plastic Forming (QPF) as a hot blow forming process capable of producing aluminum closure panels at high volumes. This technology has been successfully implemented for automotive liftgates and decklids with complex shapes. This talk will review key elements of the QPF process, describe some of the technical achievements realized in this process, and identify areas for future research in process, material, and lubricant development.

Overview of Quick Plastic Forming Technology

The plastic deformation of seven 5083 commercial aluminum materials, produced from five different alloy heats, are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to govern plastic deformation in AA5083 over the strain rates, strains, and temperatures of interest for these forming technologies: grain-boundary-sliding (GBS) creep and solutedrag (SD) creep. Quantitative analysis of stress transients following rate changes clearly differentiates between GBS and SD creep and offers conclusive proof that SD creep dominates deformation at fast strain rates and low temperature. Furthermore, stress transients following strain-rate changes under SD creep are observed to decay exponentially with strain. A new graphical construction is proposed for the analysis and prediction of creep transients. This construction predicts the relative size of creep transients under SD creep from the relative size of changes in an applied strain rate or stress. This construction reveals the relative size of creep transients under SD creep to be independent of temperature; temperature dependence resides in the “steady-state” creep behavior to which transients are related.

Deformation mechanisms in superplastic AA5083 materials

The mechanisms of tensile failure in four 5083 aluminum sheet materials are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to control failure of the AA5083 materials under uniaxial tension at elevated temperatures: cavitation and flow localization (i.e., necking). Conditions for which failure is controlled by cavitation correspond to those under which deformation is primarily by grain-boundary-sliding creep. Conditions for which failure is controlled by flow localization correspond to those under which deformation is primarily by solutedrag creep. A geometric parameter, Q, is used to determine whether final failure is controlled by cavitation or by flow localization. Differences in elongations to failure between the different AA5083 materials at high temperatures and slow strain rates are the result of differences in cavitation behaviors. The rate of cavitation growth with strain is nearly constant between the AA5083 materials for identical testing conditions, but materials with less tensile ductility evidence initial cavitation development at lower strain levels. The rate of cavitation growth with strain is shown to depend on the governing deformation mechanism; grain-boundary-sliding creep produces a faster cavitation growth rate than does solute-drag creep. A correlation is found between the early development of cavitation and the intermetallic particle-size population densities of the AA5083 materials. Fine filaments, oriented along the tensile axis, are observed on fracture surfaces and within surface cavities of specimens deformed primarily under grain-boundary-sliding creep. As deformation transitions to control by solute-drag creep, the density of these filaments dramatically decreases.

Failure mechanisms in superplastic AA5083 materials

A modified AA5083 aluminum sheet material containing a Cu addition of 0.61 wt pct has been investigated under conditions relevant to commercial hot-forming technologies. This material was produced by continuous casting followed by industrial hot and cold rolling into sheet. Deformation and failure mechanisms at elevated temperatures were investigated through mechanical testing, thermal analysis, and microscopy. The effects of Cu addition are evaluated by comparisons with data from AA5083 sheet materials without Cu addition, produced both by continuous and direct-chill (DC) casting techniques. At low temperatures and fast strain rates, for which solute-drag (SD) creep governs deformation, the Cu addition slightly increases tensile ductility at 450 °C but does not otherwise alter deformation behaviors. At high temperatures and slow strain rates, for which grainboundary-sliding (GBS) creep governs deformation, the Cu addition decreases flow stress and, at 450 °C, improves tensile ductility. A strong temperature dependence for tensile ductility results from the Cu addition; tensile ductility at 500 °C is notably reduced from that at 450 °C. The Cu addition creates platelike particles at grain boundaries, which produce incipient melting and the observed mechanical behavior.

Deformation and Failure of a Superplastic AA5083 Aluminum Material with a Cu Addition

Solute-drag (SD) creep in Class I alloys is characterized by several features. Among these is the presence of “inverse” creep transients, which are unique to these solid-solution alloys and the SD creep mechanism. Creep transients in commercial AA5083 materials under SD creep are analyzed using a model based on a graphical construct previously proposed. It is observed that transient behavior can be represented in a general fashion which predicts the decay in relative transient size as a function of strain. Experimental data for SD creep are presented using the proposed graphical construct to determine the dependence of dislocation glide speed on stress and the dependence of equilibrium mobile dislocation density on stress. It is observed that the high stress exponents of the commercial AA5083 materials under SD creep, relative to low-impurity, binary Al–Mg materials, are primarily the result of an increased dependence of dislocation glide speed on stress.

W. Paul Green

Papers

Deformation mechanisms in superplastic AA5083 materials

Failure mechanisms in superplastic AA5083 materials

Deformation and Failure of a Superplastic AA5083 Aluminum Material with a Cu Addition

Analysis, representation, and prediction of creep transients in Class I alloys