The strain rate and temperature dependence of the dynamic impact properties of 7075 aluminum alloy
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Journal of Materials Processing Technology 100 (2000) 116±122 The strain rate and temperature dependence of the dynamic impact properties of 7075 aluminum alloy Woei-Shyan Lee*, Wu-Chung Sue, Chi-Feng Lin, Chin-Jyi Wu Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC Received 24 August 1998 Abstract The dynamic impact properties of 7075 aluminum alloy are studied experimentally using a split Hopkinson bar. Cylindrical specimens of 10 mm height and 10 mm diameter are compressed dynamically at temperatures ranging from 25 to 3008C and at constant strain rates of from 103 to 5103 sÿ1. The in¯uence of strain rate and temperature on the mircrostructural evolution, the fracture mechanisms and the occurrence of shear localization is investigated. It is found that the compressive stress±strain response depends sensitively on the applied strain rate and test temperature. Considering the effects of strain rate, temperature, strain hardening, rate sensitivity and thermal softening of the material, a constitutive equation is used successfully to describe the dynamic impact deformation behavior of 7075 Al alloy. Microstructural observations reveal that the size of the initial coarse equi-axial grains is reduced as the strain rate and temperature increase due to dynamic recrystallization. In contrast, the second phase increases in size in response to increasing strain rate and temperature. SEM observation of the fracture surfaces makes evident an adiabatic shearing mechanism along the fracture planes accompanying crack formation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: 7075 Al alloy; Split Hopkinson bar; Strain rate and temperature effects; Adiabatic shearing mechanism; Grain re®nement 1. Introduction the impact response of metals, alloys and, more recently, many non-metallic and composite materials, has been stu- 7075 aluminum alloy is one of the most important engi- died, and several reviews of this ®eld have been published neering alloys and has been utilized extensively in aircraft [4±7]. It is clear that, to a greater or lesser extent, most structures because of its high strength-to-density ratio. A materials show a signi®cant change in mechanical response considerable amount of work has been carried out on the under increased strain rates or temperatures. Several plastic ¯ow of this material under low strain rates and mechanisms such as dislocation damping have been pro- various temperatures [1,2]. However, up to now, there has posed to understand high velocity deformation. It is gen- been little work concerning the systematic effects of strain erally recognized that thermally-activated strain-rate rate and temperature on the plastic ¯ow response, as well as analysis can provide fundamental insight into temperature the evolution of the microstructure, during dynamic impact and strain rate effects on stress [8,9]. deformation. From the deformability viewpoint and for It is also essential to clarify the structural evolution during structural design purposes, it is necessary to characterize deformation over a wide range of temperatures and strain the mechanical properties of 7075 Al alloy over a wide range rates. Several authors have found that the signi®cant strain- of temperatures and strain rates up to impact loading. rate and temperature dependence of the ¯ow stress of The compression split Hopkinson bar (SHPB) is used materials such as copper and titanium alloy in the high widely to determine the mechanical properties of structural strain-rate ranges can be understood in connection with the materials under high loading rates . Using this technique, evolution of the structure during deformation [10,11]. Actu- ally, the mechanical behavior of a material depends not only on the strain rate and temperature but also on its current * Corresponding author. Tel.: 886-6-2757575 (ext. 62174); fax: 886- microstructure, and changes in microstructure result in 6-2352973. changes of the plastic ¯ow behavior. Hence, the establish- E-mail address: email@example.com (W.-S. Lee). ment of more physically-based constitutive models to 0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 4 6 5 - 3
W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 117 describe the complex loading processes of 7075 alloy Ti±6A1±4V rods of 12.7 mm diameter. A good straightness requires a knowledge of the coincident in¯uences of tem- was achieved and tests have shown the ability of such perature, strain rate and microstructure on the high-strain- materials to support high impact stresses without plastic rate mechanical responses of the alloy. deformation or rupture. A short rise time was used for the The objective of this study is to characterize the behavior incident wave in order to secure a rapid material response. of 7075 aluminum alloy during dynamic compression using The stress waves in the input and output bars were sensed by a split Hopkinson bar over a temperature range of from 25 to strain gages cemented at the positions indicated in Fig. 1 and 3008C, at constant strain rates of from 103 to 5103 sÿ1. The were recorded in a Nicolet Pro 60 oscilloscope. The stress, stress±strain relation, the evolution of the microstructure and strain, and strain-rate were analyzed in digital form using the fracture characteristics are discussed in terms of the test one-dimensional stress-wave theory. A full description of conditions. A constitutive equation expressing the plastic the utilization of this analytic technique for obtaining ¯ow behavior is developed by analysis and regression of the dynamic properties data is included in . test results. Temperature variations of between 25 and 3008C on the split Hopkinson bar were achieved utilizing a clam shell radiant-heating furnace with an internal diameter of 25 mm 2. Experimental procedure and a heating element of 300 mm length. The sample temperature was monitored using a Chromel±Alumel ther- The material tested was a commercially produced 7075 mocouple attached directly to the sample. The microstruc- aluminum alloy with the following chemical composition: ture was examined using optical and scanning electron Zn 5.65, Mg 2.7, Cu 1.58, Cr 0.2, Fe 0.07, Ti 0.02, Mn 0.05, microscopy (SEM). Specimens for optical metallography balance Al (all in mass pct). This material was supplied as were sectioned perpendicular to the loading axis from the hot-rolled plates of 12.7 mm thickness, which were formed deformed samples, polished conventionally and etched in the annealed (O temper) condition and subsequently heat using an acidic solution (10 ml H2O, 10 ml HNO3, 10 ml treated to the T6 temper (1208C for 24 h). Cylindrical HCl and 5 ml HF). An MeF3 optical microscope was used compression specimens of height 10 mm and diameter for microstructural observations. Fracture specimens were 10 mm were prepared from the plates with the compression examined using a Jeol JXA-840 scanning electron micro- axis parallel to the rolling direction. All of the specimens scope operating at an accelerating potential of 20 kV. presented a maximum ¯atness defect of 0.01 mm and a maximum parallelism defect of 0.05 mm between their faces. 3. Results and discussion Dynamic compression tests were conducted in the strain- rate range of from 103 to 5103 sÿ1 and at temperatures of 3.1. Stress±strain behavior 25 to 3008C utilizing a split Hopkinson method . Fig. 1 illustrates schematically the apparatus and the measuring The true compressive stress±strain curves of 7075 Al circuits. The striker, the input and the output bars are alloy deformed at four temperatures under strain rates of Fig. 1. Schematic presentation of the compression split Hopkinson bar apparatus.
118 W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 Fig. 2. True stress±strain curves of 7075 Al alloy deformed at 1.3103 sÿ1 Fig. 4. As for Fig. 2, but for a strain rate of 3.1103 sÿ1. for all of the temperature tested. 1300, 2400 and 3100 sÿ1 are shown in Figs. 2±4, respec- rate on ¯ow stress. Flow softening appears more clearly as tively. The ¯ow stress as well as the shape of the ¯ow curves the temperature increases, even when the specimen is loaded are sensitively dependent on temperature and strain rate. For at a high strain rate. all of the specimens, after initial yielding, the ¯ow stress increases monotonically with different strain-hardening 3.2. Strain-rate effect rates. Comparing these curves with one another, it is found that, for a speci®c strain rate, the ¯ow stress decreases As can be seen from the stress±strain curves shown in markedly with temperature. Further, changes in temperature Figs. 2±4, the strain rate has obvious effects on the ¯ow have a signi®cant effect on the work-hardening rate: the stress for all temperatures. Fig. 5 shows a plot of ¯ow stress degree of work hardening is considerably smaller during versus log strain-rate at various temperatures for a ®xed true deformation at the higher temperature of 3008C, at this strain of 0.1. It is clear that the relationship between the true temperature a nearly horizontal line being obtained, sug- stress and the strain rate at any temperature is represented by gesting that the rate of work-hardening is being balanced by a single straight line, the slope of which gives information the rate of thermal softening. In contrast, for a ®xed tem- about the rate sensitivity of the ¯ow stress. This type of perature, the ¯ow stress generally increases as the strain rate stress±strain rate relationship has been reported for many increases due to an increase of dislocation density and the metals [13±18]. It is shown that, over a wide range of strain dislocation multiplication rate. When the ¯ow stress relative rates, two regions of strain-rate sensitivity exist. When the to the temperature is compared to the ¯ow stress relative to strain-rate is in the range e_ < 102 ÿ103 sÿ1 , the strain rate the strain rate, there is no doubt that the effect of temperature has only a slight in¯uence on the ¯ow stress. In this range, on the ¯ow stress is more pronounced than that of the strain the stress is proportional to the logarithm of the strain rate. Fig. 5. In¯uence of the strain rate on the ¯ow stress at a constant true Fig. 3. As for Fig. 2, but for a strain rate of 2.4103 sÿl. strain of 0.1 as a function of temperature.
W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 119 This is generally acknowledged to be a consequence of the role of thermal activation in the control of deformation mechanics. Above 103 sÿ1, the ¯ow stress increases more rapidly with strain rate, with an approximately linear rela- tionship. This behavior can be interpreted to be an indication that velocity-dependent ``drag'' on dislocation motion is causing an additional limitation to deformation. On the basis of the data in Fig. 5, it is clear that, under the present testing conditions, the deformation behavior of 7075 Al alloy is dominated by a dislocation viscous drag mechanism. In addition, at 3008C, the ¯ow stress is found to be relatively less sensitive to the strain rate, which suggests that thermal softening plays a major role in the strain-rate effect for higher temperature regimes. 3.3. Temperature effect Fig. 7. Variation of the ¯ow stress as a function of the temperature for a strain rate of 3.1103 sÿ1 for different true strains. A substantial decrease in the ¯ow stress with increasing temperature for 7075 Al alloy is observed in Figs. 2±4. ture and an augmentation of the dislocation annihilation rate Indeed, the manner in which the ¯ow stress decreases with with increasing temperature. Since 7075 Al alloy is strength- increasing temperature, at a given strain level, can be seen ened by precipitates that form during the decomposition of a more clearly by plotting the ¯ow stress directly as a function supersaturated solid solution, the strength of this material of the test temperature. Figs. 6 and 7 illustrate such behavior results from lattice strain interactions between precipitates for the two strain rates of 1.3103 and 3.1103 sÿ1. As can and dislocations, and is dependent highly on the type, be seen, for both of the strain rates, the ¯ow stress decreases distribution and size of the precipitates. During high-tem- moderately up to about 2008C and then decreases fairly perature loading, dissolution of the precipitates is involved dramatically. This difference in slopes is suf®ciently large as in the thermally-activated process. Increasing the tempera- to imply that different temperature sensitivity and micro- ture accelerates precipitate dissolution, the rate of disloca- structural evolution appear in the material at theses two tion annihilation and, subsequently, the rate of softening. temperature ranges. With regard to the effect of strain rate on These physical and metallurgical reactions result in a reduc- the temperature sensitivity, it is found that the ¯ow stress tion of ¯ow stress and, correspondingly, the material variations with temperature for both of the strain rates are becomes relatively softer and weaker. Although high rate very similar. This observation demonstrates clearly that the deformation leads to an enhancement of strength due to the temperature sensitivity of the tested material is independent increase of work hardening and dislocation density, it is of strain rate. obvious visually that the deformation resistance of 7075 Al Actually, on the micro-scale, the slope of the ¯ow stress alloy at the high temperature range is dominated by the versus the temperature indicates a variation of microstruc- temperature effect. 3.4. Deformation constitutive equation Computer simulations have been used extensively to predict the responses of complex structures at high strain rates. In order to obtain realistic data from these simulations, it is necessary to formulate an accurate constitutive relation- ship that describes the material behavior over a range of strains, strain rates, and temperatures. Many constitutive equations have been developed to describe the behavior of materials under dynamic loading. Some of these models describe only the behavior of the yield stress with strain-rate changes, whilst others describe strain and strain-rate hard- ening effects without softening effects. A collection of these equations is discussed by Meyer . Kobayashi and Dodd  proposed the following equation with a term for tem- perature softening: Fig. 6. Variation of the ¯ow stress as a function of the temperature for a strain rate of 1.3103 sÿ1 for different true strains. s s0 en e_ m 1 ÿ bDT (1)
120 W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 Fig. 8. Comparison between the predicted and measured stress±strain curves for a strain rate of 3.1103 sÿ1. where s is the ¯ow stress, e the strain, n the work-hardening coef®cient, e_ the strain rate, m the strain-rate sensitivity index, T the temperature and s0 and b are constants. This equation is used to predict the high rate and high temperature deformation behavior of 7075 Al alloy. The data in the plots of Figs. 2±4 are used to determine the coef®- cients and exponents of the constitutive equations by a regression analysis technique that determines the best-®t parameters for all of the data for a given alloy. To ®t the experimental data to Eq. (1), the values of four speci®c material parameters for 7075 Al alloy are determined as Fig. 9. Optical micrographs taken from specimens deformed at: (a) 258C s0510.58 MPa, n6.810ÿ2, m0.144 and b1.35 with a strain rate of 1.3103 sÿ1; and (b) 3008C with a strain rate of 10ÿ3. By putting these four obtained material parameters 3.1103 sÿ1. into the proposed constitutive equation, the high rate and high temperature deformation behavior of 7075 Al alloy under mathematical conditions that are equivalent to the appear within grains and grain boundaries. These lath and present actual experimental test conditions is predicted. Fig. particular precipitates of MgZn2 are insoluble at the solute- 8 shows an example for specimens tested at a strain rate of treated temperature. 3.1103 sÿl, for which a good agreement with the experi- With deformation under higher strain rate and tempera- mental data using Eq. (1) is noted. ture conditions, a distinct difference in the microstructural morphology is observed, as shown in Fig. 9(b), which 3.5. Evolution of the microstructure during impact corresponds to a specimen deformed at a strain rate of deformation 3.1103 sÿl at 3008C. Clearly, high-rate and high-tempera- ture loading leads to a growth of second phase in order to The specimens after impact loading are examined by reduce the total boundary energy. The large difference in the optical microscopy in order to determine the effects of strain amount of second phase may be attributed to the augmenta- rate and temperature on the evolution of the microstructure. tion of the test temperature and impact heating of the Fig. 9(a) shows the microstructure of the specimens specimen during deformation. If the grain morphology is deformed at room temperature under a strain rate of examined, it is found that the Al matrix grains remain 1.3103 sÿ1. Under these conditions, the microstructure roughly equal-axed, but the initial coarse grains have been consists of coarse stable equi-axed grains of Al matrix with uniformly re®ned. The average grain size decreases drama- an average diameter of 40 mm. Each grain is composed of tically from 110 mm at a strain rate of 1.3103 sÿ1 and a many ®ne precipitates of the major alloying elements (Zn, temperature of 258C, to 40 mm at a strain rate of 3.1103 sÿ1 Mg, Al). A nearly continuous network of second phase and a temperature of 3008C. This result suggests the pos- surrounding the equi-axed grains can also be seen. Along sibility of dynamic recrystallization taking place during the grain boundaries, many lath-like precipitates are found, deformation. Fig. 10 shows the changes of average grain and a few precipitate particles larger than about 15 mm size in terms of strain rate and temperature by using a linear
W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 121 Fig. 12. Optical micrograph of the 7075 Al alloy deformed at 1008C and 3.1103 sÿ1, showing arc-type adiabatic shear bands. Fig. 10. Changes in grain size with strain rate at a true strain of 0.15 for different temperature conditions. faster one along the center of inversion into the depth of the specimen to the opposite side, the other as a slow, concurrent line intercept method against the obtained optical micro- lateral growth. The faster propagates at an angle of approxi- graphs. mately 458 to the perpendicular plane. Under dynamic impact loading, an adiabatic shear band 3.6. Fracture feature observations can be de®ned as a stress localization phenomenon generally caused by a plastic instability, in turn caused by thermal Scanning electron micrographs show that fractures softening during adiabatic or quasi-adiabatic deformation. always occurs at an angle of 458 with respect to the At strain rates above 103 sÿl, plastic deformation in the compression axis, and that they are accompanied by adia- impacted material does not have enough time to occur batic shear bands. An example of the fracture appearance is thoroughly. Therefore, the impact energy has to be dissi- shown in Fig. 11, which corresponds to a specimen pated rapidly by an elevation of temperature within the deformed at 2008C and 3.1103 sÿ1. In this case, the material. This adiabatic temperature rise in the material adiabatic shear bands observed in the specimens are narrow can be very high and, furthermore, because of the short and consist of sharp cracks that lead to the separation of the duration of the event, is concentrated within narrow bands at specimen. Fig. 12 shows an optical micrograph of the locations of maximum stress. As a consequence of this sheared area for a specimen deformed at 1008C and temperature rise, the material within the shear band can 3.1103 sÿ1. Examination of the cross-sections of the tested possibly melt or, more usually, its microstructure will be specimens indicates that adiabatic shear bands form near one changed dramatically with the formation of the transformed periphery and run diagonally through the depth of the white phase. Dynamic impact material failure can thus differ specimen to the opposite periphery. The adiabatic shear from the other dynamic fracture modes of void formation bands as observed on any cross-section appear as arcs with and coalescence. Fig. 13 shows fracture surface features their radii towards the specimen center. Hence, adiabatic shear bands after initiation propagate in two directions, the Fig. 11. Fracture appearance of the 7075 Al alloy deformed at 2008C with Fig. 13. Fracture features of specimen tested at 3008C and 3.1103 sÿ1 as a strain rate of 3.1103 sÿ1. observed by SEM.
122 W.-S. Lee et al. / Journal of Materials Processing Technology 100 (2000) 116±122 produced by separation along the adiabatic shear bands. It is China for their ®nancial support. The grant from the NSC is clear that the fracture surface is relatively smooth, with ®ne numbered 86-2212-E006-015. striations running parallel to the shear direction. Also, cracks and secondary micro-cracks are seen in the fracture surface, indicating the brittleness of the material. References  A.K. Ghosh, G. Gandhi, in: H.J. McQueen et al. (Eds.), Strength of 4. Conclusions Metals and Alloys (ICSMA7), Pergamon Press, Oxford, 1985, p. 2065.  M.A. Zaidi, J.A. Wert, in: A.K. Vasudevan, R.D. Doherty (Eds.), Dynamic impact experiments on 7075 Al alloy have been Aluminum Alloys Ð Contemporary Research and Applications, conducted to investigate the in¯uence of the loading rate and Academic Press, New York, 1989, p. 137. the temperature on the mechanical properties and micro-  U.S. Lindholm, J. Mech. Phys. Solids 12 (1964) 317. structure variations. The compressive stress±strain response  P.S. Follansbee, G.T. Gray, Metall. Trans. 20A (1989) 863.  M. Meyers, Y.J. Chen, F.D.S. Marquis, D.S. Kim, Metall. Mater. of this material is found to depend on both the strain rate and Trans. 26A (1995) 2493. the temperature. By means of the experimentally-deter-  A.M. Bragov, A.K. Lomunov, Int. J. Impact Eng. 16(2) (1995) 321. mined material parameters, a proposed deformation consti-  W.S. Lee, S.T. Chiou, Composites: part B 27B (1996) 193. tutive equation is used successfully to describe the behavior  K. Tanaka, K. Ogawa, T. Nojima, in: J. Shioiri, K. Kawata (Eds.), of the material under dynamic loading. Fracture analysis High Velocity Deformation of Solids, Springer, Berlin, 1979, p. 98.  K. Ogawa, T. Nojima, J. Mater. Sci. Jpn. 37 (1988) 41. show that adiabatic shear bands play a key role in the  W.S. Lee, C.F. Lin, J. Mater. Process. Technol. 75 (1998) 127. dynamic failure process of 7075 Al alloy. The fractures  C.Y. Chiem, J. Duffy, Mater. Sci. Eng. A 57 (1983) 233. exhibit relatively smooth surfaces, which contains many  W.S. Lee, C.F. Lin, Mater. Sci. Eng. A 241 (1998) 48. cracks that are characteristic of brittle fracture. Microstruc-  R.C. Dorward, K.R. Hause, J. Mater. Eng. Perform. 4(2) (1995) 216. tural observations reveal that grain re®nement and second-  E. Cerri, E. Evangelista, A. Forellese, H.J. McQueen, Mater. Sci. phase growth are induced during high-rate and high-tem- Eng. A 197 (1995) 181.  D.E. Albert, G.T. Gray III, Acta Mater. 45(1) (1997) 343. perature deformation.  G.R. Johnson, J.M. Hoegfeldt, U.S. Lindholm, A. Nagy, Trans. ASME, J. Eng. Mater. Technol. 105 (1983) 48.  A.J. Holzer, Trans. ASME, J. Eng. Mater. Technol. 101 (1979) 231. Acknowledgements  B.C. Muddle, Metall. Trans. A 15A (1984) 1089.  L.W. Meyer, in: M. Meyer et al. (Eds.), Shock Wave and High Strain Rate Phenomena in Materials, Marcel Dekker, New York, 1992. The authors would like to acknowledge both their depart-  H. Kobayashi, B. Dodd, Int. J. Impact Eng. 8 (1989) 1. ment and the National Science Council of the Republic of
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