The strain rate and temperature dependence of the dynamic impact properties of 7075 aluminum alloy

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


   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 [3]. 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: (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 [12].
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 [3]. 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 [19]. Kobayashi and Dodd
                                                                               [20] 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 ˆ 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
s0ˆ510.58 MPa, nˆ6.810ÿ2, mˆ0.144 and bˆ1.35                             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

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4. Conclusions                                                                 Metals and Alloys (ICSMA7), Pergamon Press, Oxford, 1985, p.
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   Dynamic impact experiments on 7075 Al alloy have been                       Aluminum Alloys Ð Contemporary Research and Applications,
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ment and the National Science Council of the Republic of
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