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Adiabatic shear band formation during dynamic torsional deformation of an HY-100 steel
Acta metall, mater. Vol. 41, No. 3, pp. 923-932, 1993
0956-7151/93$6.00+ 0.00 Copyright © 1993PergamonPress Ltd
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ADIABATIC SHEAR B A N D FORMATION D U R I N G D Y N A M I C TORSIONAL D E F O R M A T I O N OF A N HY-100 STEEL KYUNG-MOX CHO 1, SUNGHAK LEE 2, S. R. NUTT 3 and J. DUFFY3 Department of Metallurgical Engineering, Pusan National University, Pusan, 609-735 Korea, 2Department of Materials Science and Engineering, Pohang Institute of Science and Technology, Pohang, 790-600 Korea and 3Division of Engineering, Brown University, Providence, RI 02912, U.S.A. (Received 20 April 1992; in revised form 10 June 1992)
A~tract--Dynamic torsion experiments have been conducted on thin-walled tubular specimens of a tempered martensitic HY-100 steel, causing adiabatic shear bands to form. The strain rates imposed were 103s-t and local temperature increases up to 600°C within the shear bands were measured. The shear band microstructure was examined by transmission electron microscopy (TEM), revealing two distinct microstructures. In some regions, highly elongated narrow subgrains extended in the shear direction, while in other regions, fine equiaxed cells were characteristic. The proportions of the two microstructures varied for different specimens,and the observations were interpreted to indicate that a process of dynamic recovery accompanying large deformation and a high temperature rise occurred within the shear band. Although thermal effects were apparent, there was no evidence to support a phase transformation to austenite followed by martensite formation. On the basis of present findings, it appears that the thermodynamic stability of the original microstructure can influence the tendency toward shear localization under dynamic loading conditions.
1. INTRODUCTION Highly localized deformation, generally referred to as adiabatic shear bands, can occur in a variety of metals when deformed at high strain rates. This can occur during machining, ballistic impact and highvelocity shaping and forming operations. Materials susceptible to shear localization under dynamic loading include steels, titanium alloys, aluminum alloys and copper alloys [1,2]. Dynamic shear bands are called "adiabatic" because there is generally insufficient time for any significant portion of the heat associated with the deformation to dissipate before thermal softening occurs within the band. Strain localization is often followed by fracture, although even without fracture, localization generally implies failure of a component through loss of load-carrying capacity within the shear band [1-5]. Adiabatic shear banding is thus an important phenomenon in dynamic deformation, and both mechanical and metallurgical approaches have been employed to understand the process [8-12]. A characteristic feature of adiabatic shear bands in steels is that they appear white when etched with a nital solution. This observation dates back to Zener and Hollomon [13], and subsequent investigators have attributed the etching behavior to a phase transformation from the parent phase to untempered martensite via reverse transformation to austenite. Indeed, microhardness measurements show that the shear band material is considerably hardened with respect to material outside the band
[14]. However, there is a question as to whether the local conditions associated with the shear band are sufficient to allow the necessary transformations to occur. In previous studies of shear banding in martensitic HY- 100 steel, ultra-high speed photography of strain localization and simultaneous measurement of local temperature at the shear band were employed [4, 6]. The measured temperature rise at the shear band was ~600°C, leading the investigators to conclude that local heating alone was unlikely to cause the phase transformations in question. They reasoned that for a martensitic steel, two successive transformations were required, first to austenite, and second, upon quenching, to untempered martensite. However, this reasoning does not allow for the possibility of a transformation assisted by the high local strain (1500 ~ 2000%) and stress, so the possibility of such transitions occurring within shear bands remains an open question. In addition to phase transformations, other thermally induced processes, including carbide dissolution, dynamic recovery, and dynamic recrystallization have been reported to occur during shear localization [16-21]. The extent to which these processes are involved in dynamic shear localization can be revealed by direct observations using transmission electron microscopy (TEM). The objective of the present study is to investigate the microstructural development of adiabatic shear bands in martensitic HY-100 steel, and to determine the metallurgical processes involved in dynamic shear localization.
923
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CHO et al.: ADIABATIC SHEAR BAND FORMATION Table 1. Chemical composition of HY-100 steel (wt.%)
C 0.19
Mn 0.29
P 0.006
S 0.014
Si 0.23
Ni 2.56
Cr 1.68
Mo 0.45
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2. EXPERIMENTAL The composition of the steel used in this study is given in Table 1. HY-100 is a low-carbon highstrength steel used in structural applications. The material was obtained from Lukens Steel Company, where it was heat-treated by austenitizing at 900°C for 1 h, followed by a water quench, then tempering at 638°C for 1 h, followed by a water quench. This treatment produced a tempered martensitic structure with a hardness of 26 on the Rockwell C scale. For dynamic shear experiments, steel specimens were machined into short thin-walled tubes and mounted in a torsional Kolsky bar (split-Hopkinson bar). Fine grid-lines were applied to the outer surface of the tube by a photoresist technique to facilitate measurement of local strains by ultra-high-speed photography. Passage of a torsional pulse through the specimen produces nominal strain rates of ~ 103/s. Measurement of local temperature was achieved by use of an optical system designed to focus infrared radiation emitted by the specimen onto an array of InSb detectors [22]. Details of the test configuration and measurement techniques appear in Refs [3-7]. A simple dilatometry experiment was performed in order to investigate the possibility of a ferrite-toaustenite transformation during shear band formation. Cylindrical tube specimens (10 × 3 x 1 mm thick) were abruptly heated to 1000°C at a rate of 500°C/s, generating a dilatation-temperature profile. Adiabatic shear bands invariably formed near the center of the tubular specimens during dynamic deformation, and post-test characterization was accomplished by TEM. After an experiment, an electrodischarge machine (EDM) was used to trepan a 3 mm disk from the tube wall such that the shear band bisected the disk. Disks were mechanically polished to a thickness of 0.1 mm, then lightly etched with nital to reveal the shear band. The location of the band was marked on the disk edge for reference during electropolishing. Disks were further thinned by dimpling to a minimum thickness of 40 ~ 50/~m centered on the shear band. Final electropolishing was carried out in a solution of 10% perchloric acid and 90% acetic acid. Thin foils were examined using a Philips CM30 TEM operated at 300 kV. 3. RESULTS Dynamic shear experiments
A typical stress-strain curve from a dynamic shear experiment on HY-100 steel is shown in Fig. 1, along with corresponding high-speed photographs of the grid pattern on the specimen surface [4]. The response is characterized by a sharp initial rise in stress, followed by a long plateau, and a steep drop in stress.
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Fig. 1. The stress-strain behavior in shear of HY-100 steel. The numbered arrows indicate the nominal strain at which each of the high speed photographs was taken. In frame 1, the inclination of the grid lines is constant indicating a homogeneous strain (stage 1). By frame 4 the strain distribution is inhomogeneous (stage 2). A shear band has developed by frame 5 (stage 3) and a crack has appeared within the shear band in frame 6. Note that the plotted strain is nominal shear strain, as determined by the relative displacement of the hex flanges connected to the thin-walled tube. This type of response can be analysed as three stages of dynamic shear deformation. The first stage starts immediately after yielding and is characterized by homogeneous deformation, as shown in frames 1 and 2. Stage 1 is followed by a second stage in which deformation is inhomogeneous, as shown beginning in frame 3 and more strongly in 4. During Stage 2, strain begins to concentrate in a broad band that encircles the specimen. As deformation continues, the strain becomes more localized, and a narrow shear band is formed, as shown in frame 5. This marks the beginning of Stage 3, in which large local strains accumulate, causing a sharp increase in local temperature. Local softening accompanies the temperature increase, and a precipitous drop in flow stress is observed. Note that in this stage, the local strain can vary with the circumferential coordinate, so the shear band does not necessarily circumscribe the specimen [4]. As deformation proceeds, more and more strain is concentrated in the shear band, and the local plastic strain can exceed 1000% with local strain rates >105s -~. Fracture often initiates and propagates within the band, as shown in frame 6. In numerous
CHO et al.: ADIABATIC SHEAR BAND FORMATION
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(A¢~) of the HY-100 steel was measured to be 771°C from the dilatometry experiment. This transformation temperature is well above the measured temperature of ~ 600°C, although the effect of deformation on the transformation behavior has been left uncounted. Thus a phase transformation seems quite unlikely because the temperature rise in the shear band is not great enough. Based on present and previous measurements, thermally-induced metallurgical processes can be expected to occur within the shear band, as discussed in subsequent sections. Shear band characterization
Fig. 2. Optical micrograph of a shear band formed in HY-100 steel during dynamic torsion experiment. Arrows indicate direction of applied shear stress. specimens of this steel, however, fracture did not occur at all. The temperature distribution during shear band formation was measured using an array of solid state detectors and an optical system focused on the outer wall of the tube. The spatial resolution of the system, as described in [6], was 17/~m, slightly less than the typical width of the shear bands (~20/~m). The maximum temperature measured in the tests was ~ 600°C, although previous studies have shown that the local temperature in the shear band varies with different local strains in the samples [4, 6]. The temperature distribution provides a conservative measure of the peak temperature in the band because of the experimental technique employed. Each detector sees a "spot" of diameter 17#m or 35 #m [4, 6]. If the shear band falls between two adjacent spots, the detectors measure an average temperature of the shear band and the (cooler) adjacent material. Allowing for this possibility, Duffy and Chi estimated that the maximum temperature could reach 700°C [6]. The ferrite-austenite transformation start temperature "~ 450
~"-'7"--~~ SHEAR BAND
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Metallographic inspection of shear bands revealed several salient macroscopic features, as shown in Fig. 2, a micrograph of a polished and etched surface. The average width of shear bands formed in HY-100 steel was ~ 2 0 # m , athough the boundaries of the band were never well-defined. Concentrated plastic flow was evident at the edges of the shear band as flow striations, observed previously by other workers [21]. Near the shear band, these flow striation lines were closely spaced and well-aligned with the shear band direction. Within the shear band, the flow lines were too dense to be resolved. Here, the local strain was 1000%, although the value of the local maximum strain varied both within individual shear bands and between specimens [4, 6]. Microhardness measurements were made at different positions across the shear band, generating the hardness profile shown in Fig. 3. The hardness within the shear band was substantially greater than in the adjoining matrix, as reported previously for other steels [23]. This can be attributed to work hardening and other microstructural changes within the band, as discussed below. Note that the "flanks" of the shear band exhibit pronounced softening relative to the surrounding matrix, effectively indicating a heat-affected zone. The affected zone extends 100/~ m to either side of the band center.
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DISTANCE ( p r o ) Fig. 3. Microhardness profile across shear band formed in HY-100 steel during dynamic torsion test.
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Fig. 4. Bright field TEM image of undeformed microstructure of HY-100 steel showing tempered martensite laths and carbide particles.
T E M observations
To establish a reference for comparison purposes, undeformed specimens of heat-treated HY-100 steel were examined by TEM, and a typical region is shown in Fig. 4. The undeformed microstructure was a tempered martensite consisting of packets of fine parallel laths. Tempering at 638°C greatly reduced the density of dislocations within the laths and resulted in fine carbide precipitation along lath boundaries and within the laths. Coarser spherical carbide particles were also occasionally present within the laths. Examination of dynamically deformed material showed that the microstructure within the shear band had been completely changed and bore little resemblance to the parent (martensitic) microstructure. Figure 5 shows a montage of bright-field TEM
MATRIX •
I'
micrographs that includes the entire width of the shear band and some of both flanks. The approximate boundaries of the shear band are marked, indicating a width of ~ 2 0 #m. The packets of parallel martensite, still visible on the flanks, have been replaced by a more equiaxed structure within the shear band. Along the flanks of the shear band, the martensite laths are bent and tend to align along the direction of material flow in shear. They are also narrower than before as shown particularly on the fight hand side of the shear band. These regions correspond to the flow striations that appeared along the shear band flanks shown in the optical micrograph in Fig. 2. An enlargement of these regions is presented later. Within the band, microstructural features are extremely fine and require closer examination. Two microstructures were observd in the center of the shear bands: (1) highly elongated narrow subgrains (laths) that extended in the shear direction, and (2) fine equiaxed cells with high dislocation densities. In general, a mixture of these two features was observed, although one of the microstructures would often be dominant in a specimen, and there was variation between specimens. Another montage of TEM micrographs obtained from a different specimen is associated with different thermomechanical histories as shown in Fig. 6. On the flanks, martensite laths were deformed and elongated along the shear direction. The elongated substructure within the shear band consisted of narrow subgrains 0.1 ~ 0.3 #m wide, with typical aspect ratios of > 10 and as high as 30 ~40. The subgrain diameters varied slightly along the length and small-amplitude undulations were clearly seen. Here again, the center of the shear band contained a number of fine equiaxed structures.
SHEAR BAND
'1
- MATRIX
Fig. 5. Montage of bright field TEM images spanning the entire width of the adiabatic shear band.
CHO et aL: ADIABATIC SHEAR BAND FORMATION
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Fig. 6. Montage of bright field TEM images, showing highly elongated subgrains within the shear band.
Regions of relatively equiaxed cells were also observed in shear bands, as shown in Fig. 7(a, b). The micrographs show cell sizes of 0.2 ~ 0.5/tm and cell walls composed of dense tangles of dislocations. In contrast to the microstructures in the previous figures (Figs 5 and 6), subgrain elongation is much less pronounced. It appears that extensive grain partitioning has occurred as a result of the development of broad, well-defined cell walls consisting of densely tangled dislocations. Coarse, spherical carbide particles are frequently observed along the cell walls [see Fig. 7(b)]. Figure 8 (a, b) show an example of the microdiffraction analyses of this equiaxed cell region, where equiaxed cells share a common (111) ferrite zone axis. The microdiffraction patterns 1-5 were taken from cells 1-5, respectively. The misorientations between cells were small as shown in Fig. 8(b). By the same method, many individual pairs of cells were analyzed for the shear band microstructure. In almost 80% of random cell pairs examined, misorientations were generally small, e.g. less than about 5°, although it was not uncommon to detect much larger misorientations. Thus the equiaxed cells were identified mainly as subgrains, and bore little resemblance to the parent structure of martensite laths, shown in Fig. 4. As shown previously in Fig. 2, shear bands formed in HY-100 exhibited a transition region on the flanks that was characterized by ductile flow striations which appeared to emanate from the interior of the band and extend into the matrix. The microstructure of one such flow striation is shown in Fig. 9, extending diagonally across the frame. The boundary between the flow line and the (less heavily deformed)
Fig. 7. Microstructures in adiabatic shear bands, showing equiaxed cell structures and inter-cellular carbide particles in (b).
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CHO et al.: ADIABATIC SHEAR BAND FORMATION 4. DISCUSSION
b
i
Fig. 8. Five grains with a common (I 11)~ zone axis: (a) bright field image, and (b) corresponding microdiffraction patterns. matrix is remarkably distinct and abrupt, separating elongated subgrains within the flow line from distorted laths of martensite in the matrix. The similarity between these elongated subgrains and the shear band microstructure shown in Fig. 6 is unmistakable. However, the local strain, and consequently the peak temperature, were undoubtedly much less than in the shear band.
Fig. 9. Bright field TEM image of a flow striation formed on the shear band flank, showing elongated subgrains and cells extending in the direction of the striation.
One of the most important (and least understood) aspects of adiabatic shear bands in steels is the metallurgical processes that are involved in their formation, and the microstructural factors influencing the process. In previous work, Wittman et ai. studied the microstructure in AISI 4340 steel after ballistic impact [16]. They reported that shear bands, which formed at estimated local strain rates > 106 s -1, were characterized by martensitic microstructures with ~ carbides that resulted from the heat generated by the shear localization. No phase transformation to austenite was observed, despite the classical whiteetching behavior. Derep, on the other hand, reported a possible phase transformation from a-ferrite to -ferrite during ballistic impact of an armor steel [15]. Glenn and Leslie also claimed a phase transformation during ballistic impact of a quenched and tempered carbon steel (0.6%C, 1.0%Ni, 0.5%Mo and 0.5% Cr) to untempered martensite [24]. Their conclusions were based on analysis of diffraction patterns and the white etching behavior of the shear band. Wingrove reported high dislocation densities and a cell structure in white-etched shear bands formed in quenched and tempered 1.0% C-1.0% Cr steel, and concluded that a phase transformation was the probable process involved [25]. Finally, Yeung and Duggan reported that the equiaxed crystallites found in shear bands in a-brass deformed by hot-rolling were in fact a recovered structure derived from extremely large and localized deformation during shearing [26]. These works provide a useful backdrop for the following discussion of our results. The process of shear band formation in HY-100 steel can be understood by first considering the less-heavily deformed flanks of the shear band. Here, the microstructure is representative of initial stages of shear localization, although in fact, much of the deformation probably occurred subsequent to the formation of the shear band. The flank regions show evidence of plastic flow and alignment of martensite laths in the shear direction, as well as initiation of a thermally affected microstructure within the flow striations. The microstructure within the striation resembles the shear band microstructure, although the local strain is undoubtedly much lower. Two features are particularly remarkable about these striations. First, the elongated subgrains in the flow striations are similar in size to those present within the shear band, indicating that the shear band can undergo enormous additional strain while the microstructure in the shear band remains essentially unchanged. The microstructure that develops on the shear band flank is thus representative of an early stage of microstructural development in the shear band. Secondly, the boundary between the flow striation and the adjoining matrix is discrete and resembles the boundaries separating the elongated grains within the striation, suggesting a similar
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CHO et al.: ADIABATIC SHEAR BAND FORMATION a
b MARTENSITE LATHS
CENNCrI'I~S
INTR CEMENT1TES
ELONGA LATHS (SUBGRAINS)
UNDEFORMED STRUCTURE (TEMPERED MARTENSITE)
c
REORIENTATION AND ELONGATION OF MARTENSITE LATHS IN SHEAR DIRECHON
d
~
ED CELLS
CARBIDES
CE BY DISLOCATION
MOVEMENT PARTITIONING OF ELONGATED SUBGRAINS INTO RECTANGULAR CELLS
FORMATION OF EQUIAXED CELLULAR STRUC'IZIRE
Fig. 10. Schematic illustration of the microstructural development occurring during formation of adiabatic shear bands in HY-100 steel. mechanism of formation. These boundaries are unidirectional with the shear direction and exhibit fine-scale undulations. Transverse cell walls are another characteristic feature, and the overall microstructure within the flow striation resembles the shear band microstructure shown in Fig. 6, only in an earlier stage of development. These observations can be understood by considering the probable mechanisms by which the microstructure develops. Based on the present observations and results, it is suggested that dynamic recovery is the dominant metallurgical process involved in adiabatic shear banding in HY-100 steel (under the present condition of dynamic shear). A schematic diagram of the process of microstructural development is shown in Fig. 10(a-d). When tempered martensite [Fig. 10(a)] is deformed at dynamic strain rates, the severe plastic flow induces reorientation and elongation of the martensite laths in the shear direction, as shown in Fig. 10(b). A dynamic recovery process begins, in which dislocations begin to arrange in cell boundaries. The tendency for dislocations to form a cell structure is strong and exists even to low temperatures [27-29]. This process is enhanced by AM
41/3~S
a simultaneous increase in local temperature as the deformation localizes. Aided by the local increase in temperature, dislocations condense into tangles, producing regions of high and low dislocation density and forming clearly defined subgrain boundaries, as shown previously in Figs 7-9. The laths elongate, forming long subgrain boundaries with fine-scale undulations, as shown in Figs 6 and 9. These elongated grains are often partitioned by transverse cell walls of variable widths, as shown in Fig. 10(c). As the local strain increases, one of two microstructures can evolve. In one case, a dynamically recovered microstructure develops which consists of highly elongated narrow subgrains. With increasing strain, the subgrains elongate parallel to the stress axis, providing soft channels for glide of dislocations along the shear direction. Undulations of the subgrain boundaries can impinge on one another, and when this happens, some grains can be "pinched off" or perforated, as shown in Fig. 10(c). The regions protruding through such perforations can give the impression of dynamically recrystallized grains, even without the nucleation and growth of new grains, as discussed by MeQueen et al. [30, 31]. In this process,
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termed "geometric dynamic recrystallization", deforming grains lengthen progressively until the thickness reaches subgrain dimensions and, due to serrations, come into contact [31, 32]. With largestrain deformation, the grains can acquire large misorientations, and in this sense the "recrystallization" term is considered appropriate, although there is no nucleation-and-growth as in static recrystallization. The process can lead to reduction in length of some subgrains and increase in local diameter of others, leading to a "steady-state" microstructure and grain size. The second microstructure that can evolve in the shear band consists of equiaxed cells, as shown in Fig. 10(d) and also in Fig. 7(a, b). This microstructure is also a consequence of dynamic recovery processes, in which dislocation cell walls have formed but are not elongated or sharpened to the same extent as in other regions. Large spherical carbide particles are frequently observed in these regions, suggesting the possible inhibition of grain boundary migration. However, the most plausible explanation for the variability in shear band microstructures is that the local strain and stress state can vary within the shear band because of "flow turbulence". Elongated subgrains that are partitioned by transverse cell walls, as shown in Fig. 10(c), can apparently break down into a more equiaxed cellular structure during this process, whereupon flow patterns at the microscopic level become complex. Similar equiaxed substructures observed in quasi-statically deformed metals have been attributed to repolygonization and/or subgrain boundary migration [30]. Thermal effects are undoubtedly critical to the development of the microstructure within the shear band and the associated dynamic and quasi-static properties. During the dynamic experiment, substantial heat is generated within the shear band that undoubtedly causes thermal softening. Because of the large strains imposed at high rates, the mobility of dislocations is expected to momentarily increase and enhance the process of dynamic recovery. We believe this to be the dominant metallurgical process operative in this steel under these conditions. While it is not impossible that dynamic recrystallization occurs concurrently with recovery and deformation, these is no clear evidence to support this. The equiaxed cellular microstructure shown in Fig. 7(a, b) is not interpreted as dynamic recrystallization because the misorientations across cell walls and boundaries are primarily small, and recrystallized regions (in the conventional sense) are not apparent. However, the possibility of dynamic recrystallization occurring at higher strain rates cannot be ruled out, and there is some evidence to support this [18]. The process of dynamic recovery appeared to be highly localized and confined to the shear band and the flow striations that emanated from it. However, evidence of a heat-affected zone outside the
shear band was manifest in the microhardness data, as shown in Fig. 3. Two competing mechanisms appeared to operate in the shear band: hardening associated with the development of a fine dislocated cell structure, and softening associated with recovery processes and a temperature rise up to 600°C. In the center of the shear band, the temperature increase is greatest, and the effects of thermal softening are probably greatest during the dynamic experiment. However, upon cooling, the resultant cell structure and elongated subgrains are responsible for the observed hardening shown in Fig. 3. The hardening outweighs the softening effects, which disappear rapidly as the heat is dissipated. On the other hand, in the transition region between the shear band and the matrix, the hardness values are slightly lower than in the matrix. This is a heat-affected zone in which the microstructure is affected by the heat generated in the band, causing carbide coarsening and dislocation recovery. Here, the local strain is smaller and the softening effect is dominant, causing lower hardness. Despite the evidence of thermal effects on microstructure and properties, there was no evidence to support a phase transformation to untempered martensite via austenite. Were such a transformation to occur, the resultant microstructure would consist of fresh martensite and retained austenite. This was not observed in our study. Carbide dissolution has also been reported to occur in adiabatic shear bands formed during high-strain-rate experiments [16]. However, the size of the carbide particles were considerably consistent within the shear band and in the parent martensitic structure, and it is thus unlikely that significant dissolution (and subsequent re-precipitation) of these phases occurred. Stability argtonents
It is generally believed that strain localization in metals is initiated by a thermomechanical instability, i.e. when thermal softening overcomes the effects of strain hardening [8]. In our experiments; localization begins at some point near the center of the thinwalled tube between the two flanges. There are two probable causes for this. First, the wall-thickness inevitably varies with location because of uneven polishing of the machined surface, thus causing a slight increase in the applied shear stress at that point [8]. Secondly, heat generated by localized plastic deformation is conducted away by the flanges, leaving the center of the tube somewhat warmer than the ends. Once localization begins, the process is accentuated because any increase in temperature causes further softening. Eventually, the influence of thermal softening outweighs the effect of strain hardening, resulting in a loss in load-carrying capacity [1, 33]. Thus, thermo-plastie instability contributes to localization of plastic flow. It should be noted that HY-100 steel exhibits two properties conducive to adiabatic shear banding: high
CHO et al.: ADIABATIC SHEAR BAND FORMATION ductility and an unstable microstructure. It has been noted that martensitic steels, as opposed to pearlitic steels, have a relatively strong tendency to form shear bands [23]. From a thermodynamic perspective, martensitic microstructures are metastable and have a high free energy. Given adequate time and temperature, these microstructures would evolve into tempered martensite and eventually into spheroidized steel. Preliminary tests indicate that spheroidized steel does not form shear bands readily, at least in the present experimental apparatus [34], although deforming to large strains (140%) at a strain rate of 1100 s-1. The influence of microstructural stability on shear band formation can be further illustrated by comparising AISI 1018 cold-rolled steel with AISI 1020 hot-rolled steel, in which the microstructure is presumably more stable. In the former steel, shear bands form at about 25% nominal shear strain, while in the latter steel, 90% strain is achieved prior to shear localization. Similarly, AISI 4340 steels with different isothermal tempers show the same trend, i.e. lower tempering temperatures translate to shear band formation at smaller strains. Thus, while adiabatic shear band formation is known to be related to the work-hardening rate, thermal capacity, and strain rate sensitivity of a steel, it may also depend on the thermodynamic stability of the microstructure. This consideration is potentially useful for predicting material performance. A metal with a lower hardness and an unstable microstructure is relatively more likely to form shear bands before fracture. A sufficiently stable microstructure should be more resistant to shear localization. Further investigation of this phenomenon would be useful, for example, by selecting a steel with a relatively unstable microstructure for which the shear banding behavior was well understood, then heat treating the same material to form a more stable microstructure, and conducting dynamic shear experiments. The formation of adiabatic shear bands in metals is often attributed to a variety of microstructural factors, such as inclusions or precipitate particles, grain boundary configurations, geometrical defects and phase distributions. It is particularly useful to understand how these factors affect shear band formation in order to control the phenomenon. For example, in AISI 1018 cold-rolled steel where pearlite grains are highly elongated and aligned with the rolling direction, hard pearlite grains appear to inhibit initiation and propagation of the shear bands [3]. Similarly, ballistic impact experiments with whisker-reinforced aluminum composites showed that whiskers oriented transverse to propagating shear bands effectively restrained matrix flow in adjacent regions and in some instances, deflected the shear band toward the whisker orientation [35]. Conversely, in the HY-100 steel studied here, there are no hard microstructural features to block shear band propagation. Martensite lath boundaries
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are similar to low-angle grain boundaries, and the eementite particles are much too small to serve as a deterrent. 5. CONCLUSIONS TEM observations have been used to study the microstructural development of adiabatic shear bands formed in HY-100 steel during torsional loading at strain rates of ~ 1500 s- i. The primary metallurgical process occurring within the shear bands is dynamic recovery, which resulted in highly elongated subgrains, as well as more equiaxed cellular structures. Thermal effects are critical to the development of the shear band microstructure and to the associated mechanical properties both during and after the dynamic experiment. The temperature rise in the shear band, which can be as high as 600°C, enhances the dynamic recovery process and causes local softening in the shear band. The final structure is achieved by a series of steps: (i) alignment and elongation of martensite laths parallel to the direction of applied shear stress, and formation of elongated subgrains in the shear direction; (ii) partitioning of elongated subgrains by transverse cell walls; (iii) breakdown of elongated subgrains and formation of equiaxed cellular structures. No evidence was found to support a phase transformation to anstenite and subsequently to martensite during this process. Furthermore, dynamic recrystallization was not observed in our investigation, although it is possible that under a different mode of loading, e.g. a higher strain rate and/or bi-axial stress state, dynamic recovery might proceed into dynamic recrystallization. An important motivation for research on shear bands is to identify factors controlling the initiation of shear bands, such as microstructure, composition, geometric defects or physical characteristics. In this regard, it appears that thermodynamic stability of the microstructure may be an important factor, metastable or unstable microstructures being inherently more susceptible to shear localization under dynamic loading. The microstructural features which influence shear band formation remains an unanswered question. Heterogeneities such as inclusions, defects, interfaces, etc., would appear to be likely suspects, although certain types of heterogeneities, such as short fiber reinforcements (in the proper orientation) have been shown to inhibit the propagation of shear bands. Our work has shown that dynamic recovery is an important metallurgical process that transpires during shear localization at dynamic strain rates. A logical approach to avoid shear band formation might then be to consider microstructural modifications that also inhibit the process of dynamic recovery.
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CHO et al.: ADIABATIC SHEAR BAND FORMATION
Acknowledgements--This work was supported by the Army Research Office under contract DAAL03-91-G-0025, Materials Research Group on Micro-mechanics of FailureResistant Materials at Brown University founded by the National Science Foundation, grant DMR-9002994, and by the Korea Science and Engineering Foundation (KOSEF) through the Center for Advanced Aerospace Materials. The authors are also thankful to Professor Nack J. Kim (POSTECH) and Dr Chang Sun Lee (RIST) for their helpful discussion of this work, and to Mr Yoon-I1 Chung (POSTECH) for his help with TEM experiments. REFERENCES
1. H. C. Rogers, A. Rev. Mater. Sci. 9, 283 (1979). 2. H. Kobayashi, Ph.D. thesis, Univ. of Reading, U.K. (1987). 3. K. Cho, Y. C. Chi and J. Duffy, Metall. Trans. 21A, 1162 (1990). 4. A. Marchand and J. Duffy, J. Mech. Phys. Solids 36, 251 (1988). 5. K. A Hartley, J. Duffy and R. H. Hawley, J. Mechn. Phys. Solids 35, 283 (1986). 6. J. Duffy and Y. C. Chi, Mater. Sci. Engng, in press. 7. A. Marchand, K. Cho and J. Duffy, Brown University Technical Report, ONR, September (1988). 8. A. Molinari and R. J. Clifton, J. appL Mech. 55, 806 (1987). 9. T. W. Wright and J. W. Walter, J. Mech. Phys. Solids 35, 701 (1987). 10. T. G. Shawki and R. J. Clifton, Mech. Mater. 8, 13 (1989). ! 1. L. Anand, K. H. Kim and T. G. Shawki, J. Mech. Phys. Solids 38, 407 (1987). 12. J. W. Hutchinson (editor), Scripta metall. 18, 421 (1984). 13. C. Zener and J. H. Holloman, J. appl. Mech. 15, 22 (1944). 14. Y. Me-Bar and D. S. Sheahtman, Mater. Sci. Engng 58, 181 (1983). 15. J. D. Derep, Acta metall. 35, 1245 (1987). 16. C. L. Wittman, M. A. Meyer and H.-r. Pak, Metall. Trans. 21A, 707 (1990).
17. S. P. Timothy and I. M. Hutchings, Acta metall. 33, 667 (1985). 18. J. H. Beatty, L. W. Meyer, M. A. Meyers and S. Nemat-Nasser, U.S. Army Materials Technical Laboratory Report, MTL TR 90-54, September (1991). 19. M. A. Meyers and H.-r. Pak, Acta metall. 34, 2493 (1986). 20. M. Hartherly and A. S. Malin, Scripta metall. 18, 449 (1984). 21. M. E. Backman and S. A. Finnegan, in Metallurgical Effects at High Strain Rates (edited by R. W. Rohde, B. M. Butcher, J. R. Holland and C. H. Karnes), pp. 531-543. Plenum Press, New York (1973). 22. E. E. Crisman, J. Duffy and Y. C. Chi, in Micromechanics: Experimental Techniques, AMD Vol. 102 (edited by W. N. Sharpe Jr), pp. 164-176 (1989). 23. H. C. Rogers and C. V. Shastry, in Shock Waves and High-Strain-Rate Phenomena in Metals (edited by M. A. Meyers and L. E. Murr), pp. 285-298. Plenum Press, New York (1981). 24. R. C. Glenn and W. C. Leslie, Metall. Trans. 2, 2945 (1971). 25. A. L. Wingrove, J. Aust. Inst. Metals 16, 67 (1971). 26. W.Y. Yeung and B. J. Duggan, Mater. Sci. Tech. 2, 552 (1986). 27. R. E. Reed-Hill, Physical Metallurgy Principles. Van Nostrand, Princetown, NJ (1973). 28. W. C. Leslie, The Physical Metallurgy of Steels. McGraw-Hill, New York (1982). 29. T. H. Alden, Metall. Trans. 7A, 1057 (1976). 30. H.J. McQueen, in Hot Deformation of Aluminum Alloys (edited by T. G. Langdon, H. D. Merchant, J. G. Morris and M. A. Zaidi). The Minerals, Metals and Materials Society (1991). 31. J. K. Solberg, H. J. McQueen, N. Ryum and E. Nes, Phil. Mag. A. 60, 441 (1989). 32. M. E. Kassner, M. M. Myshlyaev and H. J. MeQueen, Mater. Sci. Engng A 108, 45 (1989). 33. S. P. Timothy, Acta metall. 35, 301 (1987). 34. C. Mgbokwere, Brown Univ., unpublished results. 35. S. Lee, K. Cho, K. C. Kim and W. B. Choi, Metall. Trans. in press.
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ADIABATIC SHEAR B A N D FORMATION D U R I N G D Y N A M I C TORSIONAL D E F O R M A T I O N OF A N HY-100 STEEL KYUNG-MOX CHO 1, SUNGHAK LEE 2, S. R. NUTT 3 and J. DUFFY3 Department of Metallurgical Engineering, Pusan National University, Pusan, 609-735 Korea, 2Department of Materials Science and Engineering, Pohang Institute of Science and Technology, Pohang, 790-600 Korea and 3Division of Engineering, Brown University, Providence, RI 02912, U.S.A. (Received 20 April 1992; in revised form 10 June 1992)
A~tract--Dynamic torsion experiments have been conducted on thin-walled tubular specimens of a tempered martensitic HY-100 steel, causing adiabatic shear bands to form. The strain rates imposed were 103s-t and local temperature increases up to 600°C within the shear bands were measured. The shear band microstructure was examined by transmission electron microscopy (TEM), revealing two distinct microstructures. In some regions, highly elongated narrow subgrains extended in the shear direction, while in other regions, fine equiaxed cells were characteristic. The proportions of the two microstructures varied for different specimens,and the observations were interpreted to indicate that a process of dynamic recovery accompanying large deformation and a high temperature rise occurred within the shear band. Although thermal effects were apparent, there was no evidence to support a phase transformation to austenite followed by martensite formation. On the basis of present findings, it appears that the thermodynamic stability of the original microstructure can influence the tendency toward shear localization under dynamic loading conditions.
1. INTRODUCTION Highly localized deformation, generally referred to as adiabatic shear bands, can occur in a variety of metals when deformed at high strain rates. This can occur during machining, ballistic impact and highvelocity shaping and forming operations. Materials susceptible to shear localization under dynamic loading include steels, titanium alloys, aluminum alloys and copper alloys [1,2]. Dynamic shear bands are called "adiabatic" because there is generally insufficient time for any significant portion of the heat associated with the deformation to dissipate before thermal softening occurs within the band. Strain localization is often followed by fracture, although even without fracture, localization generally implies failure of a component through loss of load-carrying capacity within the shear band [1-5]. Adiabatic shear banding is thus an important phenomenon in dynamic deformation, and both mechanical and metallurgical approaches have been employed to understand the process [8-12]. A characteristic feature of adiabatic shear bands in steels is that they appear white when etched with a nital solution. This observation dates back to Zener and Hollomon [13], and subsequent investigators have attributed the etching behavior to a phase transformation from the parent phase to untempered martensite via reverse transformation to austenite. Indeed, microhardness measurements show that the shear band material is considerably hardened with respect to material outside the band
[14]. However, there is a question as to whether the local conditions associated with the shear band are sufficient to allow the necessary transformations to occur. In previous studies of shear banding in martensitic HY- 100 steel, ultra-high speed photography of strain localization and simultaneous measurement of local temperature at the shear band were employed [4, 6]. The measured temperature rise at the shear band was ~600°C, leading the investigators to conclude that local heating alone was unlikely to cause the phase transformations in question. They reasoned that for a martensitic steel, two successive transformations were required, first to austenite, and second, upon quenching, to untempered martensite. However, this reasoning does not allow for the possibility of a transformation assisted by the high local strain (1500 ~ 2000%) and stress, so the possibility of such transitions occurring within shear bands remains an open question. In addition to phase transformations, other thermally induced processes, including carbide dissolution, dynamic recovery, and dynamic recrystallization have been reported to occur during shear localization [16-21]. The extent to which these processes are involved in dynamic shear localization can be revealed by direct observations using transmission electron microscopy (TEM). The objective of the present study is to investigate the microstructural development of adiabatic shear bands in martensitic HY-100 steel, and to determine the metallurgical processes involved in dynamic shear localization.
923
924
CHO et al.: ADIABATIC SHEAR BAND FORMATION Table 1. Chemical composition of HY-100 steel (wt.%)
C 0.19
Mn 0.29
P 0.006
S 0.014
Si 0.23
Ni 2.56
Cr 1.68
Mo 0.45
800-
HYIO0
Pb 0.013
I
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R.T. 2
3
I i
I
4
5
6
I
I.
I ,
~ =
1200/$
600t
2. EXPERIMENTAL The composition of the steel used in this study is given in Table 1. HY-100 is a low-carbon highstrength steel used in structural applications. The material was obtained from Lukens Steel Company, where it was heat-treated by austenitizing at 900°C for 1 h, followed by a water quench, then tempering at 638°C for 1 h, followed by a water quench. This treatment produced a tempered martensitic structure with a hardness of 26 on the Rockwell C scale. For dynamic shear experiments, steel specimens were machined into short thin-walled tubes and mounted in a torsional Kolsky bar (split-Hopkinson bar). Fine grid-lines were applied to the outer surface of the tube by a photoresist technique to facilitate measurement of local strains by ultra-high-speed photography. Passage of a torsional pulse through the specimen produces nominal strain rates of ~ 103/s. Measurement of local temperature was achieved by use of an optical system designed to focus infrared radiation emitted by the specimen onto an array of InSb detectors [22]. Details of the test configuration and measurement techniques appear in Refs [3-7]. A simple dilatometry experiment was performed in order to investigate the possibility of a ferrite-toaustenite transformation during shear band formation. Cylindrical tube specimens (10 × 3 x 1 mm thick) were abruptly heated to 1000°C at a rate of 500°C/s, generating a dilatation-temperature profile. Adiabatic shear bands invariably formed near the center of the tubular specimens during dynamic deformation, and post-test characterization was accomplished by TEM. After an experiment, an electrodischarge machine (EDM) was used to trepan a 3 mm disk from the tube wall such that the shear band bisected the disk. Disks were mechanically polished to a thickness of 0.1 mm, then lightly etched with nital to reveal the shear band. The location of the band was marked on the disk edge for reference during electropolishing. Disks were further thinned by dimpling to a minimum thickness of 40 ~ 50/~m centered on the shear band. Final electropolishing was carried out in a solution of 10% perchloric acid and 90% acetic acid. Thin foils were examined using a Philips CM30 TEM operated at 300 kV. 3. RESULTS Dynamic shear experiments
A typical stress-strain curve from a dynamic shear experiment on HY-100 steel is shown in Fig. 1, along with corresponding high-speed photographs of the grid pattern on the specimen surface [4]. The response is characterized by a sharp initial rise in stress, followed by a long plateau, and a steep drop in stress.
~
4OO-
.~ 200-
.
0 0.0
, I0.0
I,
20.0
NOMINAL
30.0
40.0
,
, 50.0
,~.
, 60.0
70.0
SHEAR S T R A I N -
Fig. 1. The stress-strain behavior in shear of HY-100 steel. The numbered arrows indicate the nominal strain at which each of the high speed photographs was taken. In frame 1, the inclination of the grid lines is constant indicating a homogeneous strain (stage 1). By frame 4 the strain distribution is inhomogeneous (stage 2). A shear band has developed by frame 5 (stage 3) and a crack has appeared within the shear band in frame 6. Note that the plotted strain is nominal shear strain, as determined by the relative displacement of the hex flanges connected to the thin-walled tube. This type of response can be analysed as three stages of dynamic shear deformation. The first stage starts immediately after yielding and is characterized by homogeneous deformation, as shown in frames 1 and 2. Stage 1 is followed by a second stage in which deformation is inhomogeneous, as shown beginning in frame 3 and more strongly in 4. During Stage 2, strain begins to concentrate in a broad band that encircles the specimen. As deformation continues, the strain becomes more localized, and a narrow shear band is formed, as shown in frame 5. This marks the beginning of Stage 3, in which large local strains accumulate, causing a sharp increase in local temperature. Local softening accompanies the temperature increase, and a precipitous drop in flow stress is observed. Note that in this stage, the local strain can vary with the circumferential coordinate, so the shear band does not necessarily circumscribe the specimen [4]. As deformation proceeds, more and more strain is concentrated in the shear band, and the local plastic strain can exceed 1000% with local strain rates >105s -~. Fracture often initiates and propagates within the band, as shown in frame 6. In numerous
CHO et al.: ADIABATIC SHEAR BAND FORMATION
925
(A¢~) of the HY-100 steel was measured to be 771°C from the dilatometry experiment. This transformation temperature is well above the measured temperature of ~ 600°C, although the effect of deformation on the transformation behavior has been left uncounted. Thus a phase transformation seems quite unlikely because the temperature rise in the shear band is not great enough. Based on present and previous measurements, thermally-induced metallurgical processes can be expected to occur within the shear band, as discussed in subsequent sections. Shear band characterization
Fig. 2. Optical micrograph of a shear band formed in HY-100 steel during dynamic torsion experiment. Arrows indicate direction of applied shear stress. specimens of this steel, however, fracture did not occur at all. The temperature distribution during shear band formation was measured using an array of solid state detectors and an optical system focused on the outer wall of the tube. The spatial resolution of the system, as described in [6], was 17/~m, slightly less than the typical width of the shear bands (~20/~m). The maximum temperature measured in the tests was ~ 600°C, although previous studies have shown that the local temperature in the shear band varies with different local strains in the samples [4, 6]. The temperature distribution provides a conservative measure of the peak temperature in the band because of the experimental technique employed. Each detector sees a "spot" of diameter 17#m or 35 #m [4, 6]. If the shear band falls between two adjacent spots, the detectors measure an average temperature of the shear band and the (cooler) adjacent material. Allowing for this possibility, Duffy and Chi estimated that the maximum temperature could reach 700°C [6]. The ferrite-austenite transformation start temperature "~ 450
~"-'7"--~~ SHEAR BAND
o'3 Z
Metallographic inspection of shear bands revealed several salient macroscopic features, as shown in Fig. 2, a micrograph of a polished and etched surface. The average width of shear bands formed in HY-100 steel was ~ 2 0 # m , athough the boundaries of the band were never well-defined. Concentrated plastic flow was evident at the edges of the shear band as flow striations, observed previously by other workers [21]. Near the shear band, these flow striation lines were closely spaced and well-aligned with the shear band direction. Within the shear band, the flow lines were too dense to be resolved. Here, the local strain was 1000%, although the value of the local maximum strain varied both within individual shear bands and between specimens [4, 6]. Microhardness measurements were made at different positions across the shear band, generating the hardness profile shown in Fig. 3. The hardness within the shear band was substantially greater than in the adjoining matrix, as reported previously for other steels [23]. This can be attributed to work hardening and other microstructural changes within the band, as discussed below. Note that the "flanks" of the shear band exhibit pronounced softening relative to the surrounding matrix, effectively indicating a heat-affected zone. The affected zone extends 100/~ m to either side of the band center.
LOAD : 25 g
4O0
< o~ 350 ~D >
0 300 c_) 25o -200-160-120-80-40
0
40
80
120
160
200
DISTANCE ( p r o ) Fig. 3. Microhardness profile across shear band formed in HY-100 steel during dynamic torsion test.
926
CHO et al.: ADIABATIC SHEAR BAND FORMATION
Fig. 4. Bright field TEM image of undeformed microstructure of HY-100 steel showing tempered martensite laths and carbide particles.
T E M observations
To establish a reference for comparison purposes, undeformed specimens of heat-treated HY-100 steel were examined by TEM, and a typical region is shown in Fig. 4. The undeformed microstructure was a tempered martensite consisting of packets of fine parallel laths. Tempering at 638°C greatly reduced the density of dislocations within the laths and resulted in fine carbide precipitation along lath boundaries and within the laths. Coarser spherical carbide particles were also occasionally present within the laths. Examination of dynamically deformed material showed that the microstructure within the shear band had been completely changed and bore little resemblance to the parent (martensitic) microstructure. Figure 5 shows a montage of bright-field TEM
MATRIX •
I'
micrographs that includes the entire width of the shear band and some of both flanks. The approximate boundaries of the shear band are marked, indicating a width of ~ 2 0 #m. The packets of parallel martensite, still visible on the flanks, have been replaced by a more equiaxed structure within the shear band. Along the flanks of the shear band, the martensite laths are bent and tend to align along the direction of material flow in shear. They are also narrower than before as shown particularly on the fight hand side of the shear band. These regions correspond to the flow striations that appeared along the shear band flanks shown in the optical micrograph in Fig. 2. An enlargement of these regions is presented later. Within the band, microstructural features are extremely fine and require closer examination. Two microstructures were observd in the center of the shear bands: (1) highly elongated narrow subgrains (laths) that extended in the shear direction, and (2) fine equiaxed cells with high dislocation densities. In general, a mixture of these two features was observed, although one of the microstructures would often be dominant in a specimen, and there was variation between specimens. Another montage of TEM micrographs obtained from a different specimen is associated with different thermomechanical histories as shown in Fig. 6. On the flanks, martensite laths were deformed and elongated along the shear direction. The elongated substructure within the shear band consisted of narrow subgrains 0.1 ~ 0.3 #m wide, with typical aspect ratios of > 10 and as high as 30 ~40. The subgrain diameters varied slightly along the length and small-amplitude undulations were clearly seen. Here again, the center of the shear band contained a number of fine equiaxed structures.
SHEAR BAND
'1
- MATRIX
Fig. 5. Montage of bright field TEM images spanning the entire width of the adiabatic shear band.
CHO et aL: ADIABATIC SHEAR BAND FORMATION
927
Fig. 6. Montage of bright field TEM images, showing highly elongated subgrains within the shear band.
Regions of relatively equiaxed cells were also observed in shear bands, as shown in Fig. 7(a, b). The micrographs show cell sizes of 0.2 ~ 0.5/tm and cell walls composed of dense tangles of dislocations. In contrast to the microstructures in the previous figures (Figs 5 and 6), subgrain elongation is much less pronounced. It appears that extensive grain partitioning has occurred as a result of the development of broad, well-defined cell walls consisting of densely tangled dislocations. Coarse, spherical carbide particles are frequently observed along the cell walls [see Fig. 7(b)]. Figure 8 (a, b) show an example of the microdiffraction analyses of this equiaxed cell region, where equiaxed cells share a common (111) ferrite zone axis. The microdiffraction patterns 1-5 were taken from cells 1-5, respectively. The misorientations between cells were small as shown in Fig. 8(b). By the same method, many individual pairs of cells were analyzed for the shear band microstructure. In almost 80% of random cell pairs examined, misorientations were generally small, e.g. less than about 5°, although it was not uncommon to detect much larger misorientations. Thus the equiaxed cells were identified mainly as subgrains, and bore little resemblance to the parent structure of martensite laths, shown in Fig. 4. As shown previously in Fig. 2, shear bands formed in HY-100 exhibited a transition region on the flanks that was characterized by ductile flow striations which appeared to emanate from the interior of the band and extend into the matrix. The microstructure of one such flow striation is shown in Fig. 9, extending diagonally across the frame. The boundary between the flow line and the (less heavily deformed)
Fig. 7. Microstructures in adiabatic shear bands, showing equiaxed cell structures and inter-cellular carbide particles in (b).
928
CHO et al.: ADIABATIC SHEAR BAND FORMATION 4. DISCUSSION
b
i
Fig. 8. Five grains with a common (I 11)~ zone axis: (a) bright field image, and (b) corresponding microdiffraction patterns. matrix is remarkably distinct and abrupt, separating elongated subgrains within the flow line from distorted laths of martensite in the matrix. The similarity between these elongated subgrains and the shear band microstructure shown in Fig. 6 is unmistakable. However, the local strain, and consequently the peak temperature, were undoubtedly much less than in the shear band.
Fig. 9. Bright field TEM image of a flow striation formed on the shear band flank, showing elongated subgrains and cells extending in the direction of the striation.
One of the most important (and least understood) aspects of adiabatic shear bands in steels is the metallurgical processes that are involved in their formation, and the microstructural factors influencing the process. In previous work, Wittman et ai. studied the microstructure in AISI 4340 steel after ballistic impact [16]. They reported that shear bands, which formed at estimated local strain rates > 106 s -1, were characterized by martensitic microstructures with ~ carbides that resulted from the heat generated by the shear localization. No phase transformation to austenite was observed, despite the classical whiteetching behavior. Derep, on the other hand, reported a possible phase transformation from a-ferrite to -ferrite during ballistic impact of an armor steel [15]. Glenn and Leslie also claimed a phase transformation during ballistic impact of a quenched and tempered carbon steel (0.6%C, 1.0%Ni, 0.5%Mo and 0.5% Cr) to untempered martensite [24]. Their conclusions were based on analysis of diffraction patterns and the white etching behavior of the shear band. Wingrove reported high dislocation densities and a cell structure in white-etched shear bands formed in quenched and tempered 1.0% C-1.0% Cr steel, and concluded that a phase transformation was the probable process involved [25]. Finally, Yeung and Duggan reported that the equiaxed crystallites found in shear bands in a-brass deformed by hot-rolling were in fact a recovered structure derived from extremely large and localized deformation during shearing [26]. These works provide a useful backdrop for the following discussion of our results. The process of shear band formation in HY-100 steel can be understood by first considering the less-heavily deformed flanks of the shear band. Here, the microstructure is representative of initial stages of shear localization, although in fact, much of the deformation probably occurred subsequent to the formation of the shear band. The flank regions show evidence of plastic flow and alignment of martensite laths in the shear direction, as well as initiation of a thermally affected microstructure within the flow striations. The microstructure within the striation resembles the shear band microstructure, although the local strain is undoubtedly much lower. Two features are particularly remarkable about these striations. First, the elongated subgrains in the flow striations are similar in size to those present within the shear band, indicating that the shear band can undergo enormous additional strain while the microstructure in the shear band remains essentially unchanged. The microstructure that develops on the shear band flank is thus representative of an early stage of microstructural development in the shear band. Secondly, the boundary between the flow striation and the adjoining matrix is discrete and resembles the boundaries separating the elongated grains within the striation, suggesting a similar
929
CHO et al.: ADIABATIC SHEAR BAND FORMATION a
b MARTENSITE LATHS
CENNCrI'I~S
INTR CEMENT1TES
ELONGA LATHS (SUBGRAINS)
UNDEFORMED STRUCTURE (TEMPERED MARTENSITE)
c
REORIENTATION AND ELONGATION OF MARTENSITE LATHS IN SHEAR DIRECHON
d
~
ED CELLS
CARBIDES
CE BY DISLOCATION
MOVEMENT PARTITIONING OF ELONGATED SUBGRAINS INTO RECTANGULAR CELLS
FORMATION OF EQUIAXED CELLULAR STRUC'IZIRE
Fig. 10. Schematic illustration of the microstructural development occurring during formation of adiabatic shear bands in HY-100 steel. mechanism of formation. These boundaries are unidirectional with the shear direction and exhibit fine-scale undulations. Transverse cell walls are another characteristic feature, and the overall microstructure within the flow striation resembles the shear band microstructure shown in Fig. 6, only in an earlier stage of development. These observations can be understood by considering the probable mechanisms by which the microstructure develops. Based on the present observations and results, it is suggested that dynamic recovery is the dominant metallurgical process involved in adiabatic shear banding in HY-100 steel (under the present condition of dynamic shear). A schematic diagram of the process of microstructural development is shown in Fig. 10(a-d). When tempered martensite [Fig. 10(a)] is deformed at dynamic strain rates, the severe plastic flow induces reorientation and elongation of the martensite laths in the shear direction, as shown in Fig. 10(b). A dynamic recovery process begins, in which dislocations begin to arrange in cell boundaries. The tendency for dislocations to form a cell structure is strong and exists even to low temperatures [27-29]. This process is enhanced by AM
41/3~S
a simultaneous increase in local temperature as the deformation localizes. Aided by the local increase in temperature, dislocations condense into tangles, producing regions of high and low dislocation density and forming clearly defined subgrain boundaries, as shown previously in Figs 7-9. The laths elongate, forming long subgrain boundaries with fine-scale undulations, as shown in Figs 6 and 9. These elongated grains are often partitioned by transverse cell walls of variable widths, as shown in Fig. 10(c). As the local strain increases, one of two microstructures can evolve. In one case, a dynamically recovered microstructure develops which consists of highly elongated narrow subgrains. With increasing strain, the subgrains elongate parallel to the stress axis, providing soft channels for glide of dislocations along the shear direction. Undulations of the subgrain boundaries can impinge on one another, and when this happens, some grains can be "pinched off" or perforated, as shown in Fig. 10(c). The regions protruding through such perforations can give the impression of dynamically recrystallized grains, even without the nucleation and growth of new grains, as discussed by MeQueen et al. [30, 31]. In this process,
930
CHO et aL: ADIABATIC SHEAR BAND FORMATION
termed "geometric dynamic recrystallization", deforming grains lengthen progressively until the thickness reaches subgrain dimensions and, due to serrations, come into contact [31, 32]. With largestrain deformation, the grains can acquire large misorientations, and in this sense the "recrystallization" term is considered appropriate, although there is no nucleation-and-growth as in static recrystallization. The process can lead to reduction in length of some subgrains and increase in local diameter of others, leading to a "steady-state" microstructure and grain size. The second microstructure that can evolve in the shear band consists of equiaxed cells, as shown in Fig. 10(d) and also in Fig. 7(a, b). This microstructure is also a consequence of dynamic recovery processes, in which dislocation cell walls have formed but are not elongated or sharpened to the same extent as in other regions. Large spherical carbide particles are frequently observed in these regions, suggesting the possible inhibition of grain boundary migration. However, the most plausible explanation for the variability in shear band microstructures is that the local strain and stress state can vary within the shear band because of "flow turbulence". Elongated subgrains that are partitioned by transverse cell walls, as shown in Fig. 10(c), can apparently break down into a more equiaxed cellular structure during this process, whereupon flow patterns at the microscopic level become complex. Similar equiaxed substructures observed in quasi-statically deformed metals have been attributed to repolygonization and/or subgrain boundary migration [30]. Thermal effects are undoubtedly critical to the development of the microstructure within the shear band and the associated dynamic and quasi-static properties. During the dynamic experiment, substantial heat is generated within the shear band that undoubtedly causes thermal softening. Because of the large strains imposed at high rates, the mobility of dislocations is expected to momentarily increase and enhance the process of dynamic recovery. We believe this to be the dominant metallurgical process operative in this steel under these conditions. While it is not impossible that dynamic recrystallization occurs concurrently with recovery and deformation, these is no clear evidence to support this. The equiaxed cellular microstructure shown in Fig. 7(a, b) is not interpreted as dynamic recrystallization because the misorientations across cell walls and boundaries are primarily small, and recrystallized regions (in the conventional sense) are not apparent. However, the possibility of dynamic recrystallization occurring at higher strain rates cannot be ruled out, and there is some evidence to support this [18]. The process of dynamic recovery appeared to be highly localized and confined to the shear band and the flow striations that emanated from it. However, evidence of a heat-affected zone outside the
shear band was manifest in the microhardness data, as shown in Fig. 3. Two competing mechanisms appeared to operate in the shear band: hardening associated with the development of a fine dislocated cell structure, and softening associated with recovery processes and a temperature rise up to 600°C. In the center of the shear band, the temperature increase is greatest, and the effects of thermal softening are probably greatest during the dynamic experiment. However, upon cooling, the resultant cell structure and elongated subgrains are responsible for the observed hardening shown in Fig. 3. The hardening outweighs the softening effects, which disappear rapidly as the heat is dissipated. On the other hand, in the transition region between the shear band and the matrix, the hardness values are slightly lower than in the matrix. This is a heat-affected zone in which the microstructure is affected by the heat generated in the band, causing carbide coarsening and dislocation recovery. Here, the local strain is smaller and the softening effect is dominant, causing lower hardness. Despite the evidence of thermal effects on microstructure and properties, there was no evidence to support a phase transformation to untempered martensite via austenite. Were such a transformation to occur, the resultant microstructure would consist of fresh martensite and retained austenite. This was not observed in our study. Carbide dissolution has also been reported to occur in adiabatic shear bands formed during high-strain-rate experiments [16]. However, the size of the carbide particles were considerably consistent within the shear band and in the parent martensitic structure, and it is thus unlikely that significant dissolution (and subsequent re-precipitation) of these phases occurred. Stability argtonents
It is generally believed that strain localization in metals is initiated by a thermomechanical instability, i.e. when thermal softening overcomes the effects of strain hardening [8]. In our experiments; localization begins at some point near the center of the thinwalled tube between the two flanges. There are two probable causes for this. First, the wall-thickness inevitably varies with location because of uneven polishing of the machined surface, thus causing a slight increase in the applied shear stress at that point [8]. Secondly, heat generated by localized plastic deformation is conducted away by the flanges, leaving the center of the tube somewhat warmer than the ends. Once localization begins, the process is accentuated because any increase in temperature causes further softening. Eventually, the influence of thermal softening outweighs the effect of strain hardening, resulting in a loss in load-carrying capacity [1, 33]. Thus, thermo-plastie instability contributes to localization of plastic flow. It should be noted that HY-100 steel exhibits two properties conducive to adiabatic shear banding: high
CHO et al.: ADIABATIC SHEAR BAND FORMATION ductility and an unstable microstructure. It has been noted that martensitic steels, as opposed to pearlitic steels, have a relatively strong tendency to form shear bands [23]. From a thermodynamic perspective, martensitic microstructures are metastable and have a high free energy. Given adequate time and temperature, these microstructures would evolve into tempered martensite and eventually into spheroidized steel. Preliminary tests indicate that spheroidized steel does not form shear bands readily, at least in the present experimental apparatus [34], although deforming to large strains (140%) at a strain rate of 1100 s-1. The influence of microstructural stability on shear band formation can be further illustrated by comparising AISI 1018 cold-rolled steel with AISI 1020 hot-rolled steel, in which the microstructure is presumably more stable. In the former steel, shear bands form at about 25% nominal shear strain, while in the latter steel, 90% strain is achieved prior to shear localization. Similarly, AISI 4340 steels with different isothermal tempers show the same trend, i.e. lower tempering temperatures translate to shear band formation at smaller strains. Thus, while adiabatic shear band formation is known to be related to the work-hardening rate, thermal capacity, and strain rate sensitivity of a steel, it may also depend on the thermodynamic stability of the microstructure. This consideration is potentially useful for predicting material performance. A metal with a lower hardness and an unstable microstructure is relatively more likely to form shear bands before fracture. A sufficiently stable microstructure should be more resistant to shear localization. Further investigation of this phenomenon would be useful, for example, by selecting a steel with a relatively unstable microstructure for which the shear banding behavior was well understood, then heat treating the same material to form a more stable microstructure, and conducting dynamic shear experiments. The formation of adiabatic shear bands in metals is often attributed to a variety of microstructural factors, such as inclusions or precipitate particles, grain boundary configurations, geometrical defects and phase distributions. It is particularly useful to understand how these factors affect shear band formation in order to control the phenomenon. For example, in AISI 1018 cold-rolled steel where pearlite grains are highly elongated and aligned with the rolling direction, hard pearlite grains appear to inhibit initiation and propagation of the shear bands [3]. Similarly, ballistic impact experiments with whisker-reinforced aluminum composites showed that whiskers oriented transverse to propagating shear bands effectively restrained matrix flow in adjacent regions and in some instances, deflected the shear band toward the whisker orientation [35]. Conversely, in the HY-100 steel studied here, there are no hard microstructural features to block shear band propagation. Martensite lath boundaries
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are similar to low-angle grain boundaries, and the eementite particles are much too small to serve as a deterrent. 5. CONCLUSIONS TEM observations have been used to study the microstructural development of adiabatic shear bands formed in HY-100 steel during torsional loading at strain rates of ~ 1500 s- i. The primary metallurgical process occurring within the shear bands is dynamic recovery, which resulted in highly elongated subgrains, as well as more equiaxed cellular structures. Thermal effects are critical to the development of the shear band microstructure and to the associated mechanical properties both during and after the dynamic experiment. The temperature rise in the shear band, which can be as high as 600°C, enhances the dynamic recovery process and causes local softening in the shear band. The final structure is achieved by a series of steps: (i) alignment and elongation of martensite laths parallel to the direction of applied shear stress, and formation of elongated subgrains in the shear direction; (ii) partitioning of elongated subgrains by transverse cell walls; (iii) breakdown of elongated subgrains and formation of equiaxed cellular structures. No evidence was found to support a phase transformation to anstenite and subsequently to martensite during this process. Furthermore, dynamic recrystallization was not observed in our investigation, although it is possible that under a different mode of loading, e.g. a higher strain rate and/or bi-axial stress state, dynamic recovery might proceed into dynamic recrystallization. An important motivation for research on shear bands is to identify factors controlling the initiation of shear bands, such as microstructure, composition, geometric defects or physical characteristics. In this regard, it appears that thermodynamic stability of the microstructure may be an important factor, metastable or unstable microstructures being inherently more susceptible to shear localization under dynamic loading. The microstructural features which influence shear band formation remains an unanswered question. Heterogeneities such as inclusions, defects, interfaces, etc., would appear to be likely suspects, although certain types of heterogeneities, such as short fiber reinforcements (in the proper orientation) have been shown to inhibit the propagation of shear bands. Our work has shown that dynamic recovery is an important metallurgical process that transpires during shear localization at dynamic strain rates. A logical approach to avoid shear band formation might then be to consider microstructural modifications that also inhibit the process of dynamic recovery.
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Acknowledgements--This work was supported by the Army Research Office under contract DAAL03-91-G-0025, Materials Research Group on Micro-mechanics of FailureResistant Materials at Brown University founded by the National Science Foundation, grant DMR-9002994, and by the Korea Science and Engineering Foundation (KOSEF) through the Center for Advanced Aerospace Materials. The authors are also thankful to Professor Nack J. Kim (POSTECH) and Dr Chang Sun Lee (RIST) for their helpful discussion of this work, and to Mr Yoon-I1 Chung (POSTECH) for his help with TEM experiments. REFERENCES
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