Research and Development of Superplastic Materials -Recent Progresses and Future Prospects -* By Masaru KOBAYASHI** and Matsuo MIYAGAWA** Key words: superplasticity; grain refinement; superplastic forming; that are based on eutectic or eutectoid compositions. internal cavity. These materials are characterized by the microduplex equiaxed grain structure with dispersion of a second I. Introduction phase sufficiently to inhibit grain coarsening during Superplasticity, as a generally accepted metallurgi- deformation. Findings of these microstructural char- cal concept, is the phenomenon of enhanced ductility acteristics lead to the developments of a variety of under a low resistance to deformation, associated with nonferrous alloys with the base of Ag, Al, Bi, Cd, Go, the microstructural behavior such as slip, twinning, Cr, Cu, Mg, Pb, Sn, and Zn. grain boundary migration, phase transformation, pre- Progresses in grain refinement techniques have pro- cipitation, and recrystallization. vided the various methods of synthesizing materials There are now a large number of investigations on for attaining superplastic properties. Rapidly solidi- the mechanisms and contributing factors of unusually fled powders and films have been found to prepare high tensile elongation. Industrial applications of superplastic alloys with fine but non-equiaxed grain superplasticity have recently been developed for plas- structures. A variety of alloys have been shown to be tic forming. Other applications as the functional rendered superplastic : These materials include the material are also in progress, because some of super- alloys of normal grain size with a dispersion of fine plastic materials have the ability of diffusion bonding second phase particles, those of controlled grain and the high damping capacity. boundary structure and morphology, those exhibiting With the progress of fabrication technology, the ex- " temporary superplasticity " under deformation at a tended applications of superplasticity are considered relatively high strain-rate to synchronize with the in terms of both intrinsic material factors and external grain growth rate, various types of duplex alloys, age- factors such as stress state. Both factors are combined hardening alloys, dispersion hardening alloys, alloys to optimize the ductility and strength for the object of containing intermetallic compounds as well as single- fabrication with respect to materials and processes. phase alloys and pure metals. Thus a wide appli- Superplasticity can be developed as a novel and im- cations of commercial materials are expected as portant means for microstructural control by thermo- superplastic materials. mechanical processing as a combination of mechanical Superplasticity has been the subject of recent in- working and heat treatment. ternational conferences : " Superplastic Forming of The absence of a satisfactory mechanistic explana- Structural Alloys " at San Diego, USA, in 1982; tion for superplasticity and the related phenomena " Superplasticity in Aerospace -Aluminium " at Cran- invalidates the definition of superplasticity. How- field, U.K., in July 1985; " Superplasticity " at ever, grain boundary sliding appears to be the dom- Grenoble, France, in September 1985. Bilateral inant deformation mechanism during superplastic symposia on superplasticity between China and Japan flow on the basis of microstructural observations on have been held in Beijing in 19851 and in Yokohama abnormal elongation. In the present review on the in 1986.2) recent developments and applications of superplas- The trends of research and development of super- tic materials, emphasis will be focused on the fine plasticity, as seen in these international conferences, grain superplasticity that is associated with equiaxed are the rapid progress toward industrial superplastic fine grain structure having high angle boundaries. forming; applications and practical use of Al-Li This review can not be extended to dynamic super- alloys, Ti-6A1-4V alloys and superalloys as frame- plasticity accompanied by phase transformation and work components and engine parts for air- and space- transformation-induced plasticity as well as creep be- craft. To ensure the reliability for structural use, havior proceeding at the state of solid solubility limit extensive studies have been made on the cavitation, or progressing concurrently with recrystallization and strength, toughness, and fatigue of superplastically aging. formed materials. The fundamental physical metal- lurgy of superplasticity has been making progress for II. Development of Superplastic Materials the identification of deformation mechanisms. Fur- In the early stage of development, superplastic ma- thermore, the superplastic forming is now extended terials were limited to the classical group of materials to the area of ceramics. Partly published in Tetsu-to-Hagane, 72 (1986), 2001, in Japanese. Manuscript received on May 12, 1987 accepted in the final form on May 15, 1987. © 1987 ISIJ * * The Technological University of Nagaoka, Kamitomioka-cho, Nagaoka 940-21. Review (685) (686) Transactions ISIJ, Vol. 27, 1987 In Japan, " the Research Group of Superplasticity sign of superplastic materials and processing are di- in Japan " contributes to the effective interchange of rected toward superplastic forming of the materials of information relevant to the research and development poor workability including intermetallic compounds of superplasticity. " The Research and Development and ceramics. Institute of Metals and Composites for Future In- One of the authors has reviewed7) on the various dustry " promotes the research activities of super- techniques of grain refinement for rendering super- plasticity, in particular, the development of Ni-base plasticity. These methods include (1) formation of superalloys as heat-resistant materials and Ti-alloys duplex structures by eutectoid reaction, (2) eutectic as high toughness materials with high strength to reaction, (3) static recrystallization and precipitation density ratio. The activities have reported and com- after heavy deformation, (4) thermomechanical treat- piled annually in the proceedings.3-6) ments,8) (5) optimum thermal cycling, (6) spray at- omization, compaction and sintering through cold III, Grain Refinement for Superplasticity isostatic pressing (CIP), hot isostatic pressing (HIP), The present concerns on commercial application of or combined CIP and HIP, (7) spray atomization superplastic alloys are focused on the Al, Cu, Fe, Ni, and collection process (Osprey process), (8) liquid and Ti based systems rather than the classical Zn-Al dynamic compaction (LDC) process,9) (9) hot extru- eutectoid alloy. The current trends of alloy and proc- sion of splat cooled ribbons, and (10) development ess design are mainly to attain the reduction of de- of modulated structure by spinodal decomposition. formation resistance, the strengthening of grain bound- In the following, grain refinement techniques will ary together with grain refinement and the moderate be described for the typical alloy systems and the su- ductility in place of the traditional elongation as large perplastic phenomena of the products. Table 1 is a as several thousand percent. The recent trends of de- summary of the methods for grain refinement of re- Table 1. Grain refinement methods for recently developed superplastic alloys. Review Transactions ISIJ, Vol. 27, 1987 (687) cently developed alloys. Extensive reviews10-13) are ever, the resultant grain size is difficult to refine below also available for Al, Cu, and Ti based alloys and Ni 12 µm. Therefore, the products are not satisfactory powder alloys. in reliability for the strength, since cavities are formed at a strain 232 % during superplastic deformation. 1. High Strength Aluminum Alloys Flow stress of A7475 alloy is known to decrease with Commercially available alloys are Supral 100, Al- decreasing grain size,18~attempts have been made to Li alloys, and NEOPRAL. Five methods, as shown find the effective means for refinement such as the in Fig. 1,14-17) have been proposed for the grain refine- use of spray atomized powders. The superplasticity ment of these alloys. These methods utilize the pin- of these alloys is thought to proceed by the mechanism ning effects of dislocations by fine particles, with a of grain boundary sliding, as suggested in observa- size of about 0.75 µm, which are precipitated from tions on the changes in size and morphology of grains solute atoms such as Cu, Mg, and Zn that are brought and in preferred orientation. Significant contribu- into solution in the process of solution treatment and tions arise from dynamic recovery in the early stage overaging. The dislocations generated in deforma- and dynamic recrystallization in the latter stage.19) tion are inhibited to migrate and rearrange. As a The methods for grain refinement of Supral 100 consequence, the formation of cell structure and the and Al-Li20> alloy are not disclosed in detail. Both nucleation of recrystallization grains on the interface alloys may be refined by recrystallization after heavy of precipitates are promoted to provide the sites of deformation through the processing of solution treat- recrystallization nuclei for grain refinement. How- ment and warm working. In the warm working, fine dispersion of metastable A13Zr and stable CuAl2 in Supral 100 and A15CuLi3 (Al2CuLi for higher con- tents of Cu) in Al-Li alloys will interact with disloca- tions to promote nucleation of recrystallization grains