Fundamentals and Theory

Cambridge University Press 978-1-107-41004-6 - Materials Research Society Symposium Proceedings: Volume 196: Superplasticity in Metals, Ceramics, and Intermetallics Editors: Merrilea J. Mayo, Masaru Kobayashi and Jeffrey Wadsworth Excerpt More information

PARTI

© in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-1-107-41004-6 - Materials Research Society Symposium Proceedings: Volume 196: Superplasticity in Metals, Ceramics, and Intermetallics Editors: Merrilea J. Mayo, Masaru Kobayashi and Jeffrey Wadsworth Excerpt More information

OBSERVATIONS ON HISTORICAL AND CONTEMPORARY DEVELOPMENTS IN SUPERPLASTICITY

Oleg D. Sherby* and Jeffrey Wadsworth+ * Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 + Lockheed Missiles & Space Company, Inc., Research and Development Division, 0/93-10, B204, 3251 Hanover St., Palo Alto CA 94304

ABSTRACT The history of superplasticity is reviewed. The first scientific observation of superplasticity dates back to 1912. Sporadic reports of subsequent observations appear in the literature up to the review by Underwood in 1962. Since that time there have been a number of milestone achievements that have propelled the technology of superplasticity forward. These include commercial developments based on nickel, titanium, iron, and aluminum alloys. In more recent times the advent of superplasticity in ceramics and intermetallics has stimulated yet further interest in the field. At the present time there are major research efforts underway in the USA, USSR, Japan, China, and Europe to apply superplastic forming technology to the manufacture of complex components. HISTORICAL EVENTS It is often thought that superplasticity is a relatively recently-discovered phenomenon. It is intriguing to speculate, however, that the phenomenon may have had its first application in ancient times. For example, Geckinli [1] has raised the possibility that the ancient arsenic bronzes, containing up to 10 wt% As, used in Turkey in the early Bronze Period could have been superplastic. This is because the materials are two-phase alloys that may have developed the required, stable, fine grained structure during hand forging of intricate shapes. Furthermore, the ancient steels of Damascus, in use from 300 BC to the late 19th Century, are very similar in composition to modern ultrahigh carbon steels that have recently been developed, in large part, for their superplastic characteristics [2,3]. A perspective of the historical context of superplastic studies with respect to the modern time frame is shown in Fig. 1.

Modern Study of Superplasticity

\ Ancient Bronzes Damascus Steel Processing (Cu-As) (1.6-1.9%C Steels) J I I 3000BC 2000BC 1000BC BC/AD 1000 AD 2000AD Figure 1 Historical perspective of the development of superplasticity from possible ancient origins to the present time. We now believe that 1990 is the seventy-eighth anniversary of the first scientific report on superplasticity. During the course of our research into the history of Damascus steels we came across a paper published in 1912, by Bengough [4], that we believe contains the first description of superplasticity in a metallic material. Bengough describes how "a certain special brass which he had examined pulled out to a fine point, just like glass would do, having an enormous elongation." The quote (which even today represents a useful definition of superplasticity) was made by Bengough in a written discussion following a paper by Rosenhain and Ewen [5] on the "amorphous cement" theory. Examination of the original work by Bengough [4] shows that the special brass was an ct+p brass and exhibited a maximum elongation of 163% at 700°C; the original data from Bengough's paper is shown on the left hand side of Fig. 2. Considering the crude equipment (shown on the right hand side of Fig. 2), poor temperature control, large gage

length, and probable relatively high strain rate of Bengough's experiment, his results are really rather remarkable. It is of interest to note that similar materials to those studied by Bengough have been processed to be superplastic in recent times. For example, Cu-40%Zn brasses have been developed for superplasticity in the temperature range 600-800°C [6].

800

700 / 600

i>00 o y 4oo N j / cot1PLEX BRA55 f 200

100 1 0 I 20 40 6o so IOO

Figure 2 Data from the original paper by Bengough [4] in 1912 in which superplasticity was first scientifically documented are shown on the left hand side of the figure. The elongation to failure of an

Other observations can occasionally be found in the early literature to viscous-like behavior in fine- grained metals (see, e.g., Refs. [7,8]). Jenkins achieved elongations of 300 to 400% in Cd-Zn and Pb-Sn eutectics after thermomechanical processing and provided the first photographic evidence of superplastic behavior in a tested sample in 1928 [8] as shown in Fig. 3. Interestingly, Jenkins did not find the observation to be important enough to include in either the synopsis or the conclusions of his paper. In 1934 Pearson [9] dramatically demonstrated, using a Bi-Sn sample that had been deformed to nearly 2000% and then coiled as shown in Fig. 4, that unusually large elongations could be achieved in certain fine-structured, two-phase materials. As a result he is often erroneously credited with having first demonstrated superplasticity. Although the results of Pearson were viewed in the West only as a scientific curiosity, work in the USSR specifically addressed the phenomenon and Bochvar and Sviderskaya [10] coined the term "sverhplastichnost" (ultrahigh plasticity) in their 1945 paper on superplastic alloys. Apparently the term "superplasticity" appeared for the first time in the English language in a 1959 paper by Lozinsky and Simeonova [11] on the subject of "Superhigh Plasticity of Commercial Iron under Cyclic Fluctuations of Temperature." A summary of the key observations and discoveries through this time period in superplasticity is shown in Fig. 5.

Although occasional papers appeared on the subject after this date, the major increase in interest came with Underwood's [12] review article in 1962 on work in the USSR. In this review a Soviet graph, reproduced in Fig.6, was included which illustrated the ductility of Zn-Al alloys after quenching from 375°C. A ductility maximum of 650% was achieved at 250°C for a 20% Al and 80% Zn alloy (the monotectoid composition). This remarkable result, achieved on a sample that required only a quenching treatment, attracted the attention of Professor Backofen at M.I.T. With his students[13], Backofen studied details of the Zn-Al monotectoid alloy, as well as a Pb-Sn eutectic composition material. Of great significance is that Backofen and his colleagues showed

Figure 3 The first published photograph of a superplastically deformed sample was by Jenkins[8] in 1928. The material is either a cadmium-zinc alloy or a lead - tin alloy (Jenkins does not specify which it is) and has undergone about 300% elongation.

Figure 4 Famous photograph by Pearson [9] in 1934 of a Bi-Sn alloy that has undergone 1950% elongation. Note the undeformed specimen on the right hand side.

Pearson publishes Sauveur discovers dramatic picture of internal stress 2000% deformed superplasticity sample of Bi-Sn alloy Jenkins Bochvar and colleagues carry out publishes Bengough research on superplasticity in first picture observes the USSR and coin the term of superplasticity superplasticity superplastic in a brass in Sverhplastichnost sample 1912 i 1910 1920 1930 1940 1950 1960

Figure 5 Key discoveries in the early and middle part of the 20th century prior to the major development of interest in the Western World in the 1960s. that the superplastic Zn-Al alloy could be formed into a practical shape, a hemisphere, by a simple air pressure forming operation, as in glass blowing. An example of such a hemisphere is shown in Fig. 7. This practical example, showing the spectacular formability of a superplastic metallic alloy, was probably the single biggest initiator of the rapid growth in the field of superplasticity that took place after the publication of the paper in 1964. By 1968, just four years after the Backofen demonstration of superplastic forming, the first review paper on the subject appeared by Chaudhari [14] and in the next year the first book was published by Presnyakov [15]. Since then, a number of monographs, by Western [6, 16-18], Soviet [19-24], and Chinese [25] researchers, have been published, as have numerous review articles [12, 26-41]. Two extensive reviews on the subject have also been presented by the present authors [16,42]. A conference entitled "Superplastic Forming of Structural Alloys," held in 1982 [43] showed how great was the interest in the subject of fine-structure superplasticity, from both academic and commercial viewpoints.

Figure 6 The ductility of Zn-Al alloys taken Figure 7 A superplastic hemispherical shape from the Underwood review from Zn-Al alloy from Backofen article [12] of 1962. et al [13] in 1964.

The increase in commercial interest reflects the fact that superplastic materials exhibit low resistance to plastic flow (in specific temperature and strain-rate regimes) as well as high plasticity. This combination of properties is ideal since it is possible to form a complex shape with a minimum expenditure of energy, and the fine structure can give better service properties in the finished product. The feasibility of the commercial application of superplasticity has recently been reviewed for alloys based on titanium [44], nickel [45], aluminum [46,47], and iron [48].

Since the 1982 International Conference on Superplastic Forming of Structural Alloys in San Diego [43], a number of other international symposia have been held. Superplastic forming was the topic of a symposium held in Los Angeles in 1984 [49]. A conference on superplastic aerospace aluminum alloys was held in Cranfield, United Kingdom, in July 1985 [50], and a general conference on superplasticity was held in Grenoble, France, in September 1985 [51]. NATO/AGARD selected superplasticity as one of their lecture series in 1987 and again in 1989 [52]. Bilateral symposia on superplasticity between China and Japan have been held in Beijing in 1985 [53] and in Yokohama in 1986 [54]. In the first Cino-Japan symposium, 32 separate papers were presented over a 4-day period with considerable emphasis placed on superplastic ferrous- and aluminum-base alloys. Both the Chinese and Japanese governments selected superplasticity, in 1980, for intense national research and development studies. They envision superplasticity "as a future technology into the next century." An international conference on superplasticity was held in Blaine, Washington, in August 1988 [55], and attracted a large number of delegates from the Iron Curtain countries including five from the Soviet Union. The Soviet delegation was headed by Professors O. Kabaiyshev and O. Smirnov. Kabaiyshev is author of a book on commercial superplastic alloys [56] and directs an Institute on Superplasticity involving over 400 personnel. Smirnov is Program Director of a 30-person effort on superplasticity at the Moscow Institute for Steel and Alloys. Interest in the field continues unabated as seen by the data in Fig. 8 which shows the number of publications in fine structure superplasticity from 1960 to the present time.

Virtually all the discussion in the preceding sections has dealt with the history of fine structure superplasticity. The other type of superplasticity is known as internal stress superplasticity. In these latter materials, in which internal stresses can be developed, considerable tensile plasticity can take place under the application of a low, externally-applied, stress. This is because internal-stress superplastic materials can have a strain-rate sensitivity exponent of as high as unity, i.e. they can exhibit ideal Newtonian-viscous behavior. Such superplastic materials deform by a slip-controlled process. A fine grain size is not a necessary condition for superplasticity. In most cases, internal stress is generated by thermal cycling or pressure cycling conditions. However, it is also possible to achieve Newtonian-viscous flow under isothermal and constant pressure conditions from the presence of internal stress (as in Harper-Dorn creep).

Internal stress superplasticity seems to have been first noted by Sauveur in 1924 [57]. He indicated that concurrent phase transformation, during thermal cycling, lead to exceptional weakness in iron when under a small applied stress. Early researchers who made an attempt to study this effect were Koref [59] in 1926 and Wasserman [60] in 1932. These investigators coined the term "amorphous plasticity" to explain this type of weakening. Koref studied enhanced plasticity of tungsten coils during recrystallization. Wasserman studied enhanced plasticity during phase transformation (austenite to ferrite) in a nickel steel under stress during low temperature cooling. The latter work is the predecessor to the modern TRIP (Transformation Induced Plasticity) steel studies of Zackay, Parker, and their colleagues [61]. The enhanced plasticity in steels at low temperatures is now more clearly understood to be related to a strain hardening effect than to a strain rate sensitivity effectThe modern study of phase transformation plasticity can be considered to have started with the work of de Jong and Rathenau [62], who in 1959 established quantitative relations between stress and elongation during thermal cycling plasticity of iron. Early Soviet work in this field was lead by Lozinsky and his colleagues [11], where they considered that the weakening effect was a result of the " the transient breaking of atomic bonds." An elongation of 700% was achieved in a 52100 steel, by Oelschlagel and Weiss [63], when thermally cycled under an engineering stress of 17MPa. (On a personal note, one of the present authors (O.D.S.) was introduced to phase transformation plasticity in 1958 on a visit to Professor Rathenau's laboratory in Amsterdam, which led to initiation of similar studies at Stanford University with Frank Clinard [64]; through this initial work, the author became interested in fine structure superplasticity after attending, with Charles Packer, an M.I.T. superplasticity conference held in Santa Monica, CA on January 27,1966, which lead to a firstpublicatio n by Packer and Sherby on the subject [65]).

PUBLISHED PAPERS ON FINE-GRAIN SUPERLASTICITY

300 290 METADEX: SUPERLASTIC? AND (METAL? OR ALLOY?) SUPERPLASTIC 280 CERAMICS TOTAL PUBLICATIONS - 2736 270 260

250 TOTAL r PUBUCATIONS 240 |} USSR 230 220 210 200 r 190 SAN DIEGO CONFERENCE 180 170 160 150 140 130 PATENT ISSUED ON UHC STEELS 120 110 FIRST PAPERS ON SUPERPLASTIC 100 COMMERCIAL Al ALLOYS (SUPRAL) 90 80

70 PATENT ISSUED 60 ON SUPERPLASTIC Nl ALLOYS 50 40 30 20 -UNDERWOOD REVIEW 10

1960 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 871988

Figure 8 The above figure shows the increase in the number of published papers per year since 1960 in the area of superplasticity as well as some of the highlights since that time.

Another common type of internal stress superplasticity is that developed in polycrystalline materials with thermal expansion anisotropy characteristics. Lobb, Sykes and Johnson [66] were among the first to show examples of the high ductilities achievable in polycrystalline metals with large anisotropic thermal expansion properties. In their 1972 paper they showed a sample of alpha uranium elongated to about 300% under thermal cycling conditions in contrast to only 50% under optimal isothermal conditions. Their results were recently analyzed by an internal stress superplasticity model by Wu, Wadsworth, and Sherby [67]. Internal stress superplasticity has also been utilized to show that whisker and particle reinforced metal matrix composites can be made to flow superplastically, exhibiting Newtonian-viscous behavior, under thermal cycling conditions. This was specifically demonstrated in SiC whisker reinforced aluminum [68] alloys as well as in an alumina particle reinforced zinc alloy [69]. A record elongation of 1400% has been found in a SiC whisker reinforced606 1 aluminum alloy [70].

Superplasticity, then, is the capability of certain polycrystalline materials to undergo extensive tensile plastic deformation, often without the formation of a neck, prior to failure. In certain fine- grained metal alloy systems, tensile elongations of thousands of percent have been documented. In fact, the subject is of sufficient general scientific interest that a world record has been recognized for the phenomenon in metal alloys [71]. Recently, the world record of 4,850% in a Pb-62wt% Sn alloy [72] was exceeded in a commercial aluminum bronze (Cu-10wt% Al based alloy) by a value reported as 5,500% [73]. It has recently been brought to our attention [74,75] that the record has now changed hands twice with a value of elongation-to failure of about 8000% now claimed in the commercial aluminum bronze [75] as shown in Fig.9.

Figure 9 The above sample shows the extraordinary elongation (8000%) that has been achieved in the commercial superplastic aluminum bronze by Higashi and his colleagues [75].

Despite these large elongation values, of several thousand percent or more, most superplastic alloys exhibit optimum tensile elongations of about 300 to 1000%. This range of values is more than sufficient to make extremely complex shapes using superplastic forming technology. It should be recognized that high values of the strain rate sensitivity exponent, m, are also required in order for uniform thinning to accompany high tensile elongations. Large cost and weight savings (through redesign) have provided the driving force in commercial manufacturing for the change from conventional to superplastic forming. A major factor in cost savings arises from the elimination of expensive machining and joining operations. The principal alloy systems which have been exploited commercially for superplastic forming are those based on nickel, titanium, and aluminum. Up to now, iron-based alloys have been pursued to a lesser extent, mostly because iron and its alloys are inexpensive, and machining and joining techniques have been developed so optimally that costs are already minimized. As designers become better acquainted with superplastic materials this pattern is likely to change. Figure 10 summarizes the key events that have been part of the scientific history of superplasticity and poses the question of what exciting discoveries will be made in the next decade.

This question, in part, is already being answered and the progress to date has been addressed in a recent review by the present authors [42]. For example, over the last several years, interest in superplasticity continues to be high and new insights are becoming apparent in the area of modelling superplastic behavior by the integration, into existing models, of concepts based on grain boundary sliding accommodated by slip and the influence of threshold stresses associated with grain boundary sliding. Newtonian-viscous flow can be approached in fine-grained Class I solid solution alloys. Table I includes a summary of the various models since 1969 that have been proposed to describe superplasticity. Superplasticity has been developed in fine microstructures in microduplex stainless steels, aluminum-lithium alloys, aluminum-magnesium alloys, silicon carbide whisker-reinforced aluminum alloys, and mechanically alloyed aluminum, and nickel alloys. In the latter three cases, the phenomenon is observed at high strain rates by comparison with most superplastic alloys. In a major breakthrough, a ceramic (yttria stabilized zirconia) and a ceramic composite (yttria stabilized zirconia containing alumina) have been shown to be superplastic in tension tests. Wakai et al. [86,87] showed that this material, when L = 0.3 ^m ,

The Sixties 1962 Underwood Reviews Soviet Work 1964 Backofen et al Publish at MIT, Demonstrate Superplastic Forming 1966 Research Initiated at Stanford 1969 Superplastic-like Behavior Found in Class I Alloys 1969 Ball-Hutchison Theory on Grain Boundary Sliding 1969 First Book on Superplasticity by Presnyakov

The Seventies 1970 First Patent Issued on Ni-based Superplastic Alloys 1973 SPF/DB Demonstrated in Ti Alloys 1973 Ashby-Verrall Model Published 1975 First Papers Published on SUPRAL Alloys 1976 First Patent Issued on UHC Steels 1976 Edington, Melton, Cutler Review Published 1978 Ahmed and Langdon Claim World Record of 4950%

The Eighties 1982 San Diego Conference on Superplasticity 1984 High Rate Superplasticity Discovered in Al MMC 1984 Ferrous Laminated Composites Found to be Superplastic 1986 Superplasticity in Tension Discovered in a Ceramic by Wakai 1987 Superplasticity Selected as Lecture Series by NATO/AGARD 1988 World Record Improved to 8000% by Higashi 1989 Superplasticity Found in Nickel Silicide Intermetallics

Figure 10 Development of key steps in the history of superplasticity.