鋁合金等通道擠壓晶粒細化機制

黃 巖(布魯奈爾大學材料與制造學院，英國布魯奈爾UB89 3PH)

鋁合金等通道擠壓晶粒細化機制

黃 巖
(布魯奈爾大學材料與制造學院，英國布魯奈爾UB89 3PH)

本文討論鋁合金在等通道擠壓過程中的晶粒細化機制.發(fā)生的晶粒細化主要通過三種機制完成：1)取向分裂誘發(fā)形變帶；2)應(yīng)變集中產(chǎn)生的宏觀或微觀剪切帶；3)高角度晶界隨應(yīng)變增加.形變條件和路徑、模具幾何及材料參數(shù)決定形變組織的演化.亞結(jié)構(gòu)和顯微剪切帶的取向與模具剪切面一致但在原則上與材料的晶體位錯滑移系統(tǒng)無關(guān).形變帶的晶體取向傾向接近在路徑A下穩(wěn)定織構(gòu)的取向.在高應(yīng)變，由于顯微組織的壓縮和拉長造成的晶界面積增加成為主要晶粒細化機制.變形至一定應(yīng)變后，形變進入穩(wěn)態(tài)，晶粒細化不再發(fā)生.

劇烈塑性形變；等通道擠壓；晶粒分解；取向分裂；剪切帶

The last two decades haveseena dramatic rise in interest in the use of severe deformation techniques to produce ultra-fine grained materials. The potential benefits of submicron-grained aluminium alloys, such as enhanced strength and superplasticity, are well-documented[1-2], but are offset bysome negative properties, such asa tendency for low stable elongation and shear localization[3]. Several processing methods have been shown to be effective for the production of ultra-fine grained metals, which include Equal Channel Angular Extrusion (ECAE) and Accumulative Roll Bonding (ARB)[4].More recently, continuous variants of the ECAE process have been developed, which in the future will allow more commercially viable production[5]. All of these techniques involve deformation to ultra-high plastic strains (typically a true strain of 6-10) without a net billet shape change, which would otherwise result in a foil or filament. The deformation history in such processes is frequently complex, sensitive to the tooling and friction conditions and does not generally involve a constant strain path. This is especially true for the ECAE process (e.g[6-8]).

Submicron grained material, produced by a severe plastic deformation process,is defined as requiring the average spacing of high angle boundaries (HABs), misorientated by >15(°), to be less than 1 μm in all directions andthe fraction of HAB area to be greater than 70%[9]. This definition was originally proposed as in many cases materials deformed by severe deformation techniques are simply called submicron grained, based on measurement of their cell size in the TEM, without discriminating between high and low angle boundaries[10]. Understanding how deformation structures evolve to ultra-high strains and the grain refinement mechanisms involved is thus of considerable interest not least because,as will be seenwith certain material and deformation route combinations, ultra-fine grained materials, according to the above definition, are not obtainedand the materials produced still predominantly contain subgrains[11].

The general behaviour of the deformation structure evolution using the ECAE process has been previously reported, particularly using route A (no billet rotation)[9-12]. The increase in HAB area with strain originates from grain subdivision processes, such as deformation banding via orientation splitting and micro- shearbanding (which predominantly occurs at low stains <3), and the extension of grain boundary area with strain.The importance of orientation splitting and shear banding has been noted by other researchers[7,13-14]. With deformation processes involving continuous material element distortion, (e.g. ECAE -Route A) theHABs rotate and compress in spacing with strain to form a lamellar structure.The spacing of the lamellar boundaries reduces until it converges with the subgrain size forming thin ribbon grains, by a strain of 5～7. At higher strains the microstructure refines by the break-up of the ribbon grains into lower aspect ratio submicron grains.This occurs inhomogeneously and starts to develop locally in bands at strains as low asεvm～4[9,12]. The high-strain breakup and spheroidisation of lamellar HAB structures has been attributed to heterogeneity in plastic flow, diffuse shear banding, and dynamic recovery/recrystallisation[9,12]. Where severe plastic deformation involves no net element shape change, due to a redundant strain, the development of the deformed state is less well understood, but grain refinement predominately occurs by deformation banding and shear banding[9, 12, 14-16].

Here results are presented that use a selection of model Al-alloys, processing routes, and die angles, to clarify in more detail some important aspects of the deformation structure development during ECAE processing. In particular, the role of the die’s shear plane orientation in the formation of cell bands (or cell blocks) and shear bands, along with orientation splitting, at low strainsis investigated, as well asif a steady state is ultimately achieved at ultra high strains. It is worth noting that in terms of developing a more complete understanding of the general development of deformation structures in metals ECAE offers certain advantages compared to plane strain deformation, astheoretically under ideal conditions there is one principal shear plane with an orientation defined by the die geometry, and this makes it easier to determine the crystallographic, or non-crystallographic, alignment of features such as cell bands and microshear bands.

1 Experimental conditions

All the results presented were obtained by ECAE using a split die held in a hydraulic clamp, so that extrusion could be stopped partway through a cycle and samples extracted. Deformation was carried out at room temperature, on well lubricated 15 mm diameter by 100 mm billets, using a ram speed of 50 mm/min and either a 90 or 120(°) die (=2φ), with only a small blend radius between the die channels. This restricted the deformation zone to a narrow fan of less than 2 mm in width about the theoretical shear plane[5]. Data from three materials is shown; a single phase Al-0.1%Mg, a fine 20 nm Al3Sc dispersiod containing (Vf=0.004)Al-Sc alloy and AA8079, which contained coarse 2 μm Al13Fe4, particles (Vf=0.025), deformed by route A and BC. Most data shown below is for the Al-0.1%Mg alloy, except where comparisons are made to example data from the other materials. Full details of the materials and their pre-processing can be found in[12,17-18]. EBSD orientation maps were acquired from samples at different strain levels, using a high resolution FEGSEM.Data analysis was carried out using VMAP, an in-house software package.HABs were defined as having misorientations ≥ 15(°) and low-angle boundaries (LABs) misorientations<15(°). A minimum misorientation cut-off of 1.5(°) was normally used to eliminate excessive misorientation noise.

2 Mechanisms of refinement

2.1 Cell bands

After the first pass of ECAE deformation the most characteristic feature of the fine scale deformation microstructure in dilute Al-alloys with a reasonable initial grain size (>10 μm) are aligned dense dislocation walls (DDWs) 0.7～1.5 μm in spacing (Fig.1). Similar features have been widely reported in rolling and ECAE[9-12,19-20]. These structures have been termed ‘cell blocks’, where they have been suggested to represent volume elements operating on a similar set of slip systems, or are differentiated from their neighbouring blocks in terms of different local strains,and have been claimed to show preferential alignment with the active {111} slip plane[20]. However, it has been shown recently that they are probably not crystallographically related and tend to align with the principle shear direction (e.g.36(°) to the rolling plane)[19]. In idealized ECAP deformation with a narrow deformation zone, the principal shear plane is theoretically the plane of intersection of the two die channels, and this allows a simple test of these two ideas,although pure shear does not generally occur in ECAE due to die friction and spreading of the deformation zone through an arc, which introduces other minor strain components[6-7]. Nevertheless, after 1 ECAE pass in all materials examined the DDWsin every grain were always found to be strongly aligned with the die shear plane across the centre of the billet, for both 90(°) and 120(°) dies, at 45(°) and 60(°) to ED respectively (Fig.1).Another set of cell bands was also seen within some grains aligned with the extrusion direction, which may reflect the presence of other strain components in imperfect ECAE shear. Slip plane trace analysis clearly showed that there was no preferential {111} alignment, as can be seen from the example pole figure in Fig.1c. The DDWs are typically only one cell wide, and contain very low angle transverse boundaries. Such structures are thus more accurately described as cell bands, as proposed by Humphreys and Hurley[19], and are formed by the coalescence of dislocation networks into sheets by climbontoplanes where there is no out of plane shear. The average misorientation across the cell bands was 3～4(°) for the 120(°) die and 4～5(°) for the 90(°) die, which reflects the higher strain intensity of the lower die angle.No new HABs were formed at cell band boundaries, except where there was observed to be an underlying orientation gradient, either due to grain interactions, or deformation banding (see Fig.4b).The cell band structure thus initially develops independently of orientations splitting, which is driven by crystal instability.

At high strains, when the minimum grain fragment HAB spacing reduces to <5 μm no regular cell bands are seen[9]and irregular subgrains are found, because when the crystal fragment size reduces below a certain level slip becomes less well defined with respect to the macroscopic shear stress, due to grain to grain interactions. A similar result has been reported for deformation structures in fine grained materials by Jazaeri and Humphreys[21].

2.2 Shear bands

In the dilute alloys studied, microshear bands werenot seen in the first pass. In the second pass with ECAE processing there is always a strain path change which depends on the die angle and processing route[8]. Except for deformation in route C, it represents a Bauschinger type reversal and involves a rotation of the idealized shear plane and the activation of latent slip systems.For route A, the theoretical shear planealternates byπ-2φevery cycle and should thus intercept the cell bands by 60(°) or 90(°) , with the 120(°) and 90(°) dies respectively. Such strain path changes are known to promote shear banding as the collapse of the lamellar DDWs barriers forming the cell bands, when subjected to an orthogonal shear, can lead to transient flow softening[22]. In Fig.2 after the 2ndECAE pass microshear bands, only one cell in width, can now be seen cutting through the cell band structure aligned closely with the die’s theoretical shear plane in both dies. Their average misorientation thickness and spacing are given in table 1. The cell band substructure has rotated towards the extrusion direction 25(°) and 15(°) to ED for the 120(°) and 90(°) die respectively, from the expected 60 and 45 degrees, due to the highly localized shear within the microshear bands, but the alignment is spread and depends on the local shear band intensity. The cell band misorientations have remained at only 4～6(°) . Some microshear bands were seen in packets of two, to five, bands and extended across grain boundaries, although they did not generally propagate across the whole sample. It can thus be considered that there is a mixture of inter-granular microshear bands and microshear packets that are developing into trans-granular macroshear bands. In comparison in the same alloy.

Single microshear bands are more commonly observed in rolling at similar strains, due to the lower intensity of the shear, and lack of an abrupt strain path change[19]. Substantial lattice rotationsoccur within the shear bands of 6～30(°) (table 1) around TD towards the extrusion direction,which readily generates new HAB segments across their boundaries. The microshear bands were more regularly distributed and slightly higher in misorientation with the 90(°) than the 120(°) die, which reflects the higher strain intensity each pass. More intense and regular microshear bands were found within larger stable crystal volumes, that often coincided with when a {111} crystallographic plane aligned with the dies shear plane, compared to areas subdivided by more closely spaced deformation bands, which reduces the intensity of intergranualmicroshear bands by restricting the slip line length.

The microshear bands have mediummisorientations and are permanent features of the deformation structure.Once formed, with route A, they undergo a rigid-body rotation towards the extrusion direction on subsequent cycles and their spacing is compressed[9,12]. This process, along with orientation splitting, is eventually responsible for the formation of lamellar boundary structures and the elongated “ribbon grains” observed at high strains[9,12]. At intermediate strains it causes a complex microstructure to develop by subsequent micro/shear bands kinking the rotated cell bands and earlier microshear bands. Micro/shear bands are also observed to form by route Bc, but their comparative intensity has not yet been quantified. However, they are less likely to form with processes where there is a direct Bauschinger strain path reversal[22], as typified by route C. The importance of micro/shear bands to grain refinement in ECAE,at low strains, can be seen from the much higher rate of HAB area generation in a single phase alloy(Fig.3a), relative to that in an Al-0.2%Sc alloy, where theirformation is suppressed because of slip homogenisaiton due to the presence of a high density of non-shearble fine particles (Fig.2d)[16].

Coarse macroscopictrans-billet shear banding can also be observed to occur in ECAE under unstable deformation conditions[6]. This particularly occurs in aluminium alloys when the material has been heavily work hardened and contains high levels of Mg. Ultimately, this results in fracture of the billet, and can not thus be exploited to produce a homogeneous ultra-fine grain structure.Diffuse trans-granular shear bands at high strains havealso been observed to contribute to the breakup of ribbon grains,produced by route A, into ultrafine grains[9,12].

2.3 Orientation splitting

The formation of deformation bands in unstable grains, which subdivide into slab morphology volumes and rotate towards diverging end orientations, that define the deformation texture for a given deformation mode, is a well-known phenomenon[23]. The pattern of deformation banding within a given grain appears to become established at relatively low strains, and occurs in ECAE during the first pass as a grain enters the dies deformation zone (Fig.4a). High angle misorientationsare generated between each band and typically alternate for regular bands as shown Fig.4c. In the first pass the DDWs that make up the cell band boundaries remain aligned with the shear plane, but where they coincide with steep orientation gradients narrow transition bands form between the deformation bands with high HAB segments. This can occur irregularly and form closely spaced parallel HAB segments as can be seen in area C in Fig.4a. The misorientation features of these HAG segments can be seen in Fig.4d. Typically, two length scales of deformation banding were found (table 2), coarse primary deformation bands that have high misorientations of 30～55(°)(Fig.4a A and B) and a spacing of 20～30 μm (after the first pass) and much finer secondary deformation bands, that can form within the primary bands in some grains with spacings as fine as 5 μm (Fig.4a C). In subsequent passes with route A, these regions rapidly develop into local bands containing a high density of lamellar HABs. In the current work sufficient statistical data has not yet been obtained to correlate general crystal orientations to grain stability in such regions. However, both regular coarse and irregular fine deformation bands were found to rotate towards a stable orientation defined by the end texture of the material, which is (111)[-211] for both 90(°) and 120(°) dies. Fig.4b shows that deformation bands A, B and C all evolve towards (111)[-211], regardless of their initial orientations. However, theoretical analysis by Barnett and Montheillet[24]has suggested that, due to the material spin in simple shear, which results in no truly stable final deformation texture, orientation splitting is more significant than in other deformation modes, such as plane strain.

Despite reorientation of the shear plane, on subsequent ECAE passes by route A the pattern of deformation banding formed in the first pass does not appear to change, (i.e. no more subdivision occurs), although the band width does reduce due to continued element distortion, and the misrorentation across bands increases resulting in the formation of continuous HABs at secondary bands. Similarly, with route Bc where there is no continuous element distortion, again the pattern of deformation banding developed after the first pass appears to largely remain during subsequent passes, unless the structure is destroyed by intense shear banding, such as that seen in high Mg content alloys. Crystal plasticity simulations have shown similar results, in that the pattern of orientation subdivision is generally determined at relatively low strains[13].

Cell bands can also develop HABs in regions near grain boundary surfaces and triple points, in the original grain structure, due to high local orientation gradients resulting from grain-grain interactions[9]. New HABs can also be promoted by orientation gradients associated with coarse second phase particles, within their deformation zones and through particle stimulated deformation bands. This greatly increases the rate of grain refinement and reduces the strain required to produce an ultra-fine grain structure[17], as can be seen from Fig.3a.

2.4 Importance of geometric boundary compression

In most conventional deformation processes continuous distortion of a material element occurs. This leads to an increase in grain boundary area simply due to the geometric shape change imposed on a grain[23]. In ECAE deformation continuous element distortion does take place by route A and BA, but not by Route Bc or C. The grain subdivision mechanisms of orientation splitting,and repeated micro/shear banding at low strains and transgranularshear banding at high strains,on alternate shear planes, are therefore primarily responsible for grain refinement by Route BC. With route A, as well as orientation splitting and shear banding resulting in grain subdivision there is also an increase in HAB area due to the geometric requirements of shear associated with the development of a fibrous deformation structure, which results in a lamellar HAB structure at strains grater than 3～4. At higher strains of 5～7 the lamellar boundary spacing approaches the subgrain size and ribbon grains are formed which subsequently breakup and reduce in aspect ratio to form submicron grains at ultra-high strains[8,12]. In Fig.3b the measured increase in HAB area with strain,for different original grain sizes, is compared to that calculated from the geometric increase in HAB area, based on the theoretically distortion of grains that deform homogenously. From Fig.3b it can be seen that at low strains the geometric increase in HAB area becomes extremely important when the initial grain size reduces below 5 μm, and at high strains all the data converges with the predicted behaviour for an initial grain size of 10 μm. Jazaeri and Humphreys[25]have shown that in rolling all new HAB area can be attributed to the geometric increase in boundary area for grain sizes below 10～15 μm. However, for smaller initial grain sizes the reduction in HAB spacing falls below that expected from the geometrical requirements of strain, which suggests a minimum HAB spacing is approached that is controlled by dynamic recovery. An effective limit to grain refinement is also observed in ECAE processing, when the grain size converges with the subgrain size, as will be shown below.

2.5 Severely deformed microstructures

Withdeformation paths that do not involve a net element shape change, such as route BC, if deformation occurs homogeneously there is no geometric requirement for a compression of the boundary spacing, or increase in HAB area with strain. However, grain subdivision processes will still result in grain refinement and a geometric increase in boundary area can still occur locally if deformation occurs heterogeneously within intense macroshear bands.Thus, at ultra high strains by ECAE, both routes A and BCcan produce ultra-fine grained materials, but result in distinctively differentdeformation structures, examples of which are given in Fig.5. Materials produced by route A,with continuous element distortion, (Fig.5a, b) have fibrous ultra-fine grain structures, which tend to be quite homogeneous in the transverse direction, but have a lower average boundary misorientation in the grain elongation direction and still contain surviving long ribbon grain fragments. With route BC, shear banding on alternate planes and the lack of an overall compression of the structure with strain, results in a more chaotic microstructure, which contains lower misorientationsubgrain islands within a matrix of ultra-fine grains (Fig.5c). These submicron grains are frequently finer in scale and elongated within shear bands created in the last pass.

If a mechanism of grain subdivision, such as micro/shear banding is suppressed by the inclusion of high densities of dispersoids, which are frequently used in superplastic alloys[16], both route A and route BCresult in more heterogeneous structures. However, route BCis much more significantly affected due to the lack of any general geometric boundary extension. With route A larger unrefined fibrous grains are retained (Fig.5b), which correspond to volumes with more stable orientations, which are no longer subdivided by micro/shearbanding[16]. But, with route BC, due to the suppression of shear banding, the material can no longer be described as submicron grained and contains only 45% HABs (Fig.5d). In contrast, materials produced by any deformation route can be made more homogenous by including coarse second phase particles, which encourages matrix orientation divergence at low strains and breaks up lamellar HAB structuresat high strains[17].

2.6 Steady state and dynamic recovery

If the deformation conditions are maintained constant, ultimately a steady state must be approached in severe deformation where there is no further grain refinement. An example of this behaviour can be seen in Fig.6a, where statistical data obtained by EBSD shows little significant difference in the fraction of HAB area, and a negligible reduction in grain width, in the strain range 8～17, using route A. At this stage the grain width has converged with the cell size. There is, however, still a noticeable reduction in average grain length. This occurs due to the breakup of the final remaining long fibrous ribbon grain fragments, resulting in an increase in homogeneity of the material.Due to the geometric requirements of the imposed shear, the grain width should be reducing with strain at a much higher rate, although this is less significant than in plane strain where a similar limiting grain width has been observed by Jazaeri et al.[25-26]. This is illustrated in Fig.6a where the theoretical grain width is geometrically required by shear to reduce by the order of 0.1-0.02 μm per extrusion cycle in this strain range.

If a constant grain width is maintained dynamically, with a ram speed of 50 mm/min and deformation zone width of 2 mm, this corresponds to a boundary migration velocity of 0.01～0.04 μm·s-1. The conventional thermally activated migration rate of a boundary is given by[18]:

V=Moexp(-Q/RT)P

(1)

If the average driving force for boundary migration P is equivalent to 3γ/d, using typical measured values ofγ=0.3 J·m-2, a grain sized0.7 μm,M0～0.1 m4·J-1·s-1, and an activation energy of 130 kJ·mol-1[26], this gives, assuming a deformation temperature due to plastic work of ～50 ℃ a velocity of 1×10-10μm·s-1, which is too lowby a factor of 108to maintain a constant grain width.However estimated, unrealistic values of P would be required to greatly increase V. In comparison an effective activation energy and a value of 80 kJ·mol-1will result in the required migration rate. Similar values of Q have been reported for static grain growth at low temperatures in severely deformed alloys, using TEM measurements, but these results are generally confusing as they can also be attributed to misinterpretation resulting from the heterogeneous nature of the deformation structures[27]. However, overall the data suggests that a limiting grain size is achieved due to dynamic grain boundary migration occurring at a far higher rate than for conventional static annealing. This could result from the interaction of dislocations with HABs, a higher vacancy concentration, and higher local driving pressures due to non-equilibrium boundary configurations.

It is also apparent that limited dynamic boundary migration could contribute to the grain refinement process at high strains once the HAB spacing in local bands reduces to the subgrain sizeand, for example, facilitate some spheroidisationandthe break up of thin ribbon grains in a manner similar to that observed during static annealing of lamellar HAB structures at higher temperatures[27]. Simulations have shown this occurs becauseeven low angle transverse boundaries exert a large resolved surface tension at nodal points in lamellar boundary structures. A strongly developing texture can also reduce the HAB density during deformation, but the texture is generally relatively weak in ECAE[12]. Further grain refinement can thus only be achieved by increasing the Zener-Hollomon parameter. The remarkable effect of reducing the deformation temperature is illustrated in Fig.6b, where the Al-0.1Mg alloy after processing by ECAE at 20 ℃ temperature toεeff=10 (Fig.5a) has been subsequently deformed in plane strain compression in liquid nitrogen to a further strain ofεtrue=2, resulting in thin ribbon grains of 100 nm in width.

3 Summary

Grain refinement in high stacking fault energy aluminum alloys during severe deformation processes,such as ECAE, occurs predominantly by orientation splitting, micro and macroshear banding, and the geometric requirements for high angle boundary area to increase with strain. The deformation structure evolution is strongly affectedby the processing route, die geometry, and material parameters. Both cell bands and microshear bands show alignment with the dies shear plane, corresponding to the maximum resolved stress but irrelevant of the crystallographic slip systems of the material. Micro/shear bands are promoted by changes in the shear plane orientation in each extrusion cycle. The geometrically required increase in boundary area and compression in boundary spacing with strain is an important aspect of the formation of fibrous ultra-fine grain structures with route A, and increases in significance with smaller initial grain sizes. The introduction of fine non-shearable second phase particles results in a reduction in the rate of grain refinement by the homogenization of slip which results, when combined with processing routes that involve redundant strains (e.g. Route BC), in materials that predominantly contain subgrains. In contrast, coarse second phase particles accelerate the rate of grain refinement and increase homogeneity of the final grain structure. At high strains a steady state is achieved, due to dynamic boundary migration occurring at far higher rate than under static conditions, resulting in a constant minimum grain size. Dynamic boundary migration also contributes to the grain refinement process at high strains when the HAB spacing approaches the subgrain size. Further grain refinement can thus only be achieved by increasing the Zener-Hollomon parameter.

Acknowledgements

Thanks to EPSRC (UK) funding to this research and to Prof J. Humphreys and Prof. P. B. Prangnell for their long term support..

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Mechanisms of grain refinement in aluminium alloys by ECAE

Yan Huang
(BCAST, Institute of Materials and Manufacturing Brunel University London, Kingston Lane, Uxbridge, UB89 3PH UK)

The grain refinement mechanisms operating during severe deformation processes, like ECAE, are discussed using data obtained from model Al-alloys. Refinement occurs predominantly by regular and irregular deformation banding via orientation splitting, micro and macroshear banding due to strain localization, and the geometric requirement for high angle boundary area to increase with strain. The deformation structure evolution is affected strongly by the processing route, die geometry, and material parameters. Both cell bands and microshear bands show strong alignment with the dies shear plane but in principle irrelevant of the crystallographic slip systems of the material.However, deformation bands tend to take an orientation close to that defined by the stable end texture in route A. At high strains, geometrical compression of the existing grain structure becomes more important and eventually a steady state is approached, where the grain size converges with the subgrain size,controlled by dynamic boundary migration.

severe plastic deformation; ECAE; grain subdivision; orientation splitting; shear bands

10.14186/j.cnki.1671-6620.2015.04.002

TG 146.2

A

1671-6620(2015)04-0245-10