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The transgranular brittle fracture of strong aluminium alloys under anodise cracks and other acute stress raisers
July 1997
ELSEVIER
Materials Letters 32 (1997) 17-24
The transgranular brittle fracture of strong aluminium alloys unde,r anodise cracks and other acute stress raisers P.J.E. Forsyth Farnham, Surrey Gill9 8QY, UK
Received 13 September 1996; accepted 5 December 1996
Abstract Under conditions of high plastic constraint such as exist beneath anodise coatings and also at the roots of sharp V notches, a strong commercial aluminium alloy exhibited limited brittle transgranular tensile fracture. The transgranular cracks that formed under a 40 p,m thick oxide were observed to extend to a maximum depth of about three times the coating thickness, whereas those cracks that formed beneath notches, in large grain size material, could, in isolated instances, traverse several grains to a depth of 0.5 mm before the onset of ductile failure. These brittle fracture facets exhibited both features of cleavage and a more isotropic mode of cracking which might be described as conchoidal. Keywords: Transgranular brittle fracture; Aluminium; Anodise cracks; Stress raisers
1. Introduction The strongest commercial aluminium alloys, as used for aircraft construction, are those based on the Al-Zn-Mg-Cu system with the addition of essential recrystallisation inhibitors such as zirconium. Without these additions the qu,atemary compositions would recrystallise to form large equiaxed grains. For experimental simplicity, much of the basic research on fracture behaviour has been carried out on such recrystallised microstructures, whereas those of service components are far more complex. Over the years, there have been a number of recorded cases of the occurrence of cleavage-like facets, usually sporadically distributed, appearing on tension loaded fractures of these alloys. From theoretical considerations cleavage should not occur in the face-centred cubic crystal structure of aluminium with its low elastic anisotropy, 1.2, and high value of ~~.,,,/r,,, and because of this, emphasis has always been placed on the need for some bond weakening agency for cleavage to occur, such as a liquid metal [II, corrosion [21, or the presence of some detrimental trace element in the alloy [3]. However, cleavage-like facets have been observed under conditions that suggest that this brittle behaviour may be an intrinsic form of failure in these strong alloys [4]. During other work on anodised aluminium alloys, principally AA7010, it was noticed hat tensile fractures emanating from cracks in thick anodise coatings sometimes exhibited small brittle origins that had cleavage-like features. This prompted a further investigation into fracture from acute stress raisers such as anodise cracks and sharp rooted notches. 00167-577X/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-577X(96)00301-1
P.J.E. Forsyth/Materials
18
Letters 32 f19971 17-24
2. Experimental details The AA 7010 composition
range is as follows:
Zn
Mg
Cu
Mn
Fe
Si
Ti
Zr
6.7 5.7
2.6 2.1
2.0 1.5
0.1 -
0.15 -
0.12 -
0.06 -
0.16 0.1
max.% min. %
All 7010 specimens used in this work were extracted from forged pieces within this compositional range and heat treated to a yield strength of approximately 500 MPa. Specimens were also cut from squeeze castings of a similar compositional range to the 7010, but with some experimental variations that will be mentioned in the text. These castings were heat treated to yield strengths within the range 470-500 MPa. The test piece forms were either rectangular in section and edge notched, or sharply grooved round bars. Anodising was carried out in a 10% H, 504 electrolyte at a current density of 3 A/dm’ producing coatings of 40 pm thickness. All fractures were produced in plane bending, and other experimental details are given in the text.
3. Observations Fractures of specimens extracted from various forged pieces did not consistently exhibit cleavage like facets, and the causes of this variability will be considered later. Figs. 1 and 2 illustrate some of the best examples seen, and such features appeared frequently with specimens cut from this source material. The shallow fracture facets extended along the oxide/metal interface and were co-planar with the oxide fracture itself, there being a maintained bond between oxide and metal. They were generally oriented at right angles to the interface, although some curvature was present. They had curved fronts reminiscent of fatigue cracks, and their growth had clearly been terminated by the onset of ductile fracture. No cracks deeper than about three oxide coating thicknesses have been observed in the present work. The nature of these fractures will be discussed in more detail at a later stage, but they are strikingly like those produced by fatigue of these high strength alloys [5]. Following these observations the oxide coatings were then removed in warm 10% H,SO, which also etched the grain boundaries as exposed on the free surface of the specimen, and on the fracture facets themselves. Fig. 3 shows traces of the roots of the oxide cracks on the metal interface, and Fig. 4,indicates some that have initiated brittle fracture, causing gaping of the crack walls. Overlapping cracks frequently joined by deformation
Fig. 1. Brittle fracture facet generated by oxide coating crack showing curved growth arrest.
P.J.E. Forsyth/Materials
LRtters 32 (1997) 17-24
19
Fig. 2. Brittle crack showing growth patterns.
and tearing of connecting ligaments, as can be seen in Fig. 5, and at this stage brittle crack extension would have been suppressed by local ductile failure. As can be seen in Fig. 3-5, these brittle cracks generating from straight or smoothly curved oxide cracks, crossed the boundary network without deviation, and even on the brittle fracture facets no surface tilts could be detected that might have indicated that the crack path had been influenced by crystal structure. However, by plastically deforming uncoated specimens it could be seen that, in relation to the boundary network, the traversing slip bands showed virtually no deviations, thus revealing that there was very little disorientation between neighbouring grains, or what would be more correctly described as sub-grains. A region of such slip banding is shown in Fig. 6. Thus even if the crack had been closely following a cleavage plane, the fracture face would not have tilted enough to be detected under the microscope. It has been mentioned that this particular piece of material showed a greater tendency to crack in the manner described than any of the other forging samples tested, and to investigate this point the various microstuctures have been metallographically examined. It was found that the most susceptible material was a very lightly worked forging, the cast grain size being of the order of 250 km, and virtually equiaxed in the three dimensions. It had recrystallised around ihese original grains with some degree of grain growth leaving the grain interiors, which constituted the main bulk of the material, in a lightly polygonised state. On the other hand, forged material that seemed to be immune from this form of brittle fracture, although having much the same cast grain size as the susceptible forging, had been heavily worked, at least a three to one reduction. This had, after solution heat
Fig. 3. Specimen surface after removal of oxide showing initial crack traces.
20
P.J.E. Forsyth/Materials
Letters 32 (1997) 17-24
Fig. 4. The presence of brittle cracks denoted by opening displacement.
Fig. 5. Neighbouring
Fig. 6. Surface of susceptible
brittle cracks with connecting
alloy after plastic deformation,
ligament.
showing long slip bands across sub-grains.
P.J.E. Forsyth / Materials Letters 32 (1997) 17-24
21
Fig. 7. Dendritic fracture pattern with spines oriented at 450 to one another.
treatment, resulted in a very fine uniform grain or sub-grain size of approximately 20 Frn with no evidence of residual “ingotism”, nor any grain growth. The nature of slip de formation in the susceptible material has already been illustrated in Fig. 6 where the maximum free slip path was only limited by the cast grain boundary regions, i.e. about 200 pm. In contrast, the non-susceptible material deformed in a more homogeneous manner by fine slip which was concentrated at the coarser particles present, and at the boundaries of the larger grains, initiating cracks at both features. Looking now in more detail at these facets that developed from oxide cracks, Fig. 7 shows that the spines of the dendritic pattern Iof crack growth in the metal form two sets that are consistently oriented at 45” to one another, and, rather curiously, seem to weave over and under at crossing points. This is in contrast to the features exhibited by a neighbouring area of fracture, shown in Fig. 8, which was far from flat, and showed no evidence of any preferred crack growth directions, being what might be best described as conchoidal. Using squeeze cast alloys of a somewhat similar composition to 7010 but tested as round V notched bar specimens, revealed an incidence of brittle fracture of the type already described, both in the anodised and unanodised condition. These materials had, after homogenisation and solution heat treatment, a grain size of approximately 200 p_rn, similar to the susceptible forging material. The grains were equiaxed, and contained no detectable substructure, only a few isolated interdendrite residues. The brittle fracture facets, examples of which are shown in Fig. 9, appeared singly or in groups extending over two or three neighbouring grains, resulting in
Fig. 8. Structure insensitive brittle fracture.
P.J.E. Forsyth/Materials
22
Letters 32 (1997) 17-24
Fig. 9. Cleavage facets on squeeze cast aluminium
alloy following
twin orientation.
cracks of over 0.5 mm in depth. The example shown in Fig. 9 reveals that the crack had been diverted by twin boundaries. Such casting twins are sometimes found in these materials, and persist through the homogenising and solution heat treatments. Although these facets were more frequently found in the squeeze cast material, they were also present on the fracture of the lightly forged 7010 material, in the vicinity of an unanodised notch.
4. Discussion The geometric alignment of the spines shown in Fig. 7 and the planar tilting of the fracture facets in Fig. 9 indicate a degree of structure sensitivity that is not shown by the fracture illustrated in Fig. 8, where the crack had been growing as through a continuum, following only the dictate of the local stress. This dual behaviour might be expected in a material with weak elastic anisotropy such as aluminium. Most of the topographic features that appear on cleavage fractures, or for that matter on conchoidal fractures in elastically isotropic materials, arise from the joining of cracks advancing through the crystal at different levels. Where the elastic anisotropy is pronounced, for example in zinc single crystals, break-through may occur by secondary cleavage, in other instances it has been achieved by shear. Fig. 10 is a schematic illustration of the way that such joining seems to happen in the present case. It is based on the observation that the ends of
Break-through of parallel cleavage planes (a) Smgle fold - One preferred growth dlrectlon (b) Two preferred growth dtrections. Fig. 10. Schematic
illustration
of overlap break-through
and the effect of two preferred crack growth directions.
P.J.E. Forsyth /Materials Letters 32 (1997) 17-24
23
approaching offset c:racks pass one another and then turn to form what might be described as a “sigmoidal” junction [6]. The condition for this to occur rather than joining by shear is that the two advancing crack tip plastic zones should be too small to interact with one another, and the rotation of the cracks that results is as would be predicted from linear elastic fracture mechanics principles. Fig. 1Oa shows the way that a ledge would develop, and it is of interest that this form of cracking, as well as occurring with metals, has been observed on fatigue fractures of polymethylmethacrylate where the sigmoidal enclosure curled away freely from the fracture surface in the form of a filament, the overlap being oriented in the crack growth direction [6]. Fig. lob shows how, when there are two preferred crack growth directions, the dendrite pattern develops by periodic diversions from the original spine, and because the fracture does not continually follow the preferred plane, overlaps occur. There would appear to be two principal requirements for brittle fracture to occur in these high strength aluminium alloys: (1) they must have a very high intrinsic flow stress, and (2) there must be a sufficient degree of plastic constraint.Furthermore, for this brittle fracture to manifest itself with a semblance of cleavage, the elastic anisotropy of the aluminium lattice, small as it is, must have the freedom to become effective, and this requires large grains. It should be noted that these coarse, lightly worked microstructures are inevitable in large forgings manufactured from cast stock in a conventional manner, but what has, perhaps, not been fully appreciated, is that the cores of thesa large grains have deformation and fracture characteristics that are similar to those of the simpler fully recrystallised materials. Referring now to the way that these brittle cracks are thought to occur, Fig. 11 shows, in schematic form, the disposition of the constrained metal under the oxide coating in relation to the more freely plastically deforming interior, and A and I3 relate crack formation to the stress/strain curves for the composite arrangement. It has been shown that large grain size is essential for the development of recognisable cleavage, which is in accord with experience with other materials where it is a more common mode of fracture. It now seems clear that these transgranular brittle fractures that have been loosely described as cleavage, are, in fact, essentially conchoidal, where the crack path is influenced only by the local stress direction, and the perturbations caused by microstructural discontinuities, such as particles and defects, deflect the crack front to produce the irregularity that is observed. Over and above this, the weak elastic anisotropy of the aluminium lattice is still powerful enough to draw the crack path towards and into a low bond strength plane, but not strong enough to maintain it there permanently against the fluctuations described. It is envisaged that the crack tip advances through highly disturbed material which distances it to some extent, depending on the plastic zone radius, from the influence of the elastic anisotropy of the undisturbed surroundings. Because the material within this zone has lost its crystalline identity, fracture within it tends to be conchoidal, although it cannot be described, within the strict meaning of the word, as brittle. Nevertheless, in engineering terms this is academic, the fact remaining that these are low energy fractures by virtue of the extremely small plastic volumes involved.
Fig. 11. !Schematic illustration
of the conditions
that lead to brittle cracking under a thick oxide coating.
24
P.J.E. Forsyth/Materials
Letters 32 (1997) 17-24
Although agencies have been identified that have had a specific association with this mode of fracture, it may well be that they have only exacerbated what is an intrinsic weakness in these alloys. From the practical viewpoint, the most important thing is that such fractures can occur in material, that is not just an experimental composition, but one that in all respects would be considered typical of its type, and fit for service. However, the true engineering significance of this should be judged against the fact that other material with a microstructure that indicated that it had received more hot working, which would be considered a preferable condition, exhibited no cleavage but failed with a considerable degree of intergranular separation. Where transgranular cracking susceptibility might be of more concern would be with hard (thick) anodised material used in a fatigue environment with the probability of an overload that could produce the type of gaping crack that has been illustrated. Even if no fatigue cracking ensued, these crevices would provide easy entry places for a corrodent. There is one further specific point that should be made that will be of particular concern to failures investigators. These tensile fracture facets, being relatively smooth, are highly reflective compared with the rougher textured area of ductile fracture. They are generally semi-elliptical in shape and extremely like fatigue cracks as they might appear in a similar situation. Even the absence of fatigue fracture striations in such small origins might not entirely disqualify them as being fatigue related, where it could reasonably be argued that the cracks had grown too slowly to produce resolvable striations. The abrupt change from bright to dull fracture texture would then be accepted as evidence of exploitation by a critical load much greater than any hitherto experienced. Where broken structural elements become available for a detailed fracture examination, either those that result from laboratory tests, or even crash damaged pieces, it is usual practice to look for fatigue origins that might highlight unexpected design weaknesses. It is important that in any failure analysis this possibility of mistaking tensile cleavage for fatigue should be recognised.
5. Conclusions (1) 40 p,rn thickness anodise coatings provided sufficient plastic constraint to the underlying metal to cause brittle transgranular fracture origins in plain edge-notched specimens extracted from a commercial AA7010 forging. (2) Other acute stress raisers such as sharp V notches, even in the absence of anodise coatings, promoted similar cracks in the above material, and also in squeeze cast material of a similar composition. (3) These brittle fractures, in some instances, showed attributes of cleavage, the crack path being influenced by crystal structure. However, in some respects they behaved as in a continuum. This duality of crack path preference is attributed to the weak elastic anisotropy of the aluminium lattice. (4) Only large (200-500 km) grain size microstructures fractured in this cleavage-like mode, but where forgings can only be lightly worked such microstructures are not uncommon. (5) Such crack origins as have been described, if they appeared on tensile fractures, such as those brought about by crash damage, might easily be mistaken for fatigue cracks.
References [l] A.P. Reynolds and GE. Stoner, Metall. Trans. A 22 (1991) 1849. [2] J.C. Scully, The theory of stress corrosion cracking in alloys, Conference Proceedings 1971). [3] W.S. Miller, M.P. Thomas and .I. White, Scripta Metall. 21 (1987) 663. [4] T.M. Yue, Ph.D. Thesis, University of Southampton (1987). [5] P.J.E. Forsyth, C.A. Stubbington and D.J.I. Clark, Metals 90 (1961-Z) 238. [6] P.J.E. Forsyth, Intern. J. Fatigue, January 1983, pp. 3-14.
(NATO Scientific
Affairs
Division,
Brussels,
ELSEVIER
Materials Letters 32 (1997) 17-24
The transgranular brittle fracture of strong aluminium alloys unde,r anodise cracks and other acute stress raisers P.J.E. Forsyth Farnham, Surrey Gill9 8QY, UK
Received 13 September 1996; accepted 5 December 1996
Abstract Under conditions of high plastic constraint such as exist beneath anodise coatings and also at the roots of sharp V notches, a strong commercial aluminium alloy exhibited limited brittle transgranular tensile fracture. The transgranular cracks that formed under a 40 p,m thick oxide were observed to extend to a maximum depth of about three times the coating thickness, whereas those cracks that formed beneath notches, in large grain size material, could, in isolated instances, traverse several grains to a depth of 0.5 mm before the onset of ductile failure. These brittle fracture facets exhibited both features of cleavage and a more isotropic mode of cracking which might be described as conchoidal. Keywords: Transgranular brittle fracture; Aluminium; Anodise cracks; Stress raisers
1. Introduction The strongest commercial aluminium alloys, as used for aircraft construction, are those based on the Al-Zn-Mg-Cu system with the addition of essential recrystallisation inhibitors such as zirconium. Without these additions the qu,atemary compositions would recrystallise to form large equiaxed grains. For experimental simplicity, much of the basic research on fracture behaviour has been carried out on such recrystallised microstructures, whereas those of service components are far more complex. Over the years, there have been a number of recorded cases of the occurrence of cleavage-like facets, usually sporadically distributed, appearing on tension loaded fractures of these alloys. From theoretical considerations cleavage should not occur in the face-centred cubic crystal structure of aluminium with its low elastic anisotropy, 1.2, and high value of ~~.,,,/r,,, and because of this, emphasis has always been placed on the need for some bond weakening agency for cleavage to occur, such as a liquid metal [II, corrosion [21, or the presence of some detrimental trace element in the alloy [3]. However, cleavage-like facets have been observed under conditions that suggest that this brittle behaviour may be an intrinsic form of failure in these strong alloys [4]. During other work on anodised aluminium alloys, principally AA7010, it was noticed hat tensile fractures emanating from cracks in thick anodise coatings sometimes exhibited small brittle origins that had cleavage-like features. This prompted a further investigation into fracture from acute stress raisers such as anodise cracks and sharp rooted notches. 00167-577X/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-577X(96)00301-1
P.J.E. Forsyth/Materials
18
Letters 32 f19971 17-24
2. Experimental details The AA 7010 composition
range is as follows:
Zn
Mg
Cu
Mn
Fe
Si
Ti
Zr
6.7 5.7
2.6 2.1
2.0 1.5
0.1 -
0.15 -
0.12 -
0.06 -
0.16 0.1
max.% min. %
All 7010 specimens used in this work were extracted from forged pieces within this compositional range and heat treated to a yield strength of approximately 500 MPa. Specimens were also cut from squeeze castings of a similar compositional range to the 7010, but with some experimental variations that will be mentioned in the text. These castings were heat treated to yield strengths within the range 470-500 MPa. The test piece forms were either rectangular in section and edge notched, or sharply grooved round bars. Anodising was carried out in a 10% H, 504 electrolyte at a current density of 3 A/dm’ producing coatings of 40 pm thickness. All fractures were produced in plane bending, and other experimental details are given in the text.
3. Observations Fractures of specimens extracted from various forged pieces did not consistently exhibit cleavage like facets, and the causes of this variability will be considered later. Figs. 1 and 2 illustrate some of the best examples seen, and such features appeared frequently with specimens cut from this source material. The shallow fracture facets extended along the oxide/metal interface and were co-planar with the oxide fracture itself, there being a maintained bond between oxide and metal. They were generally oriented at right angles to the interface, although some curvature was present. They had curved fronts reminiscent of fatigue cracks, and their growth had clearly been terminated by the onset of ductile fracture. No cracks deeper than about three oxide coating thicknesses have been observed in the present work. The nature of these fractures will be discussed in more detail at a later stage, but they are strikingly like those produced by fatigue of these high strength alloys [5]. Following these observations the oxide coatings were then removed in warm 10% H,SO, which also etched the grain boundaries as exposed on the free surface of the specimen, and on the fracture facets themselves. Fig. 3 shows traces of the roots of the oxide cracks on the metal interface, and Fig. 4,indicates some that have initiated brittle fracture, causing gaping of the crack walls. Overlapping cracks frequently joined by deformation
Fig. 1. Brittle fracture facet generated by oxide coating crack showing curved growth arrest.
P.J.E. Forsyth/Materials
LRtters 32 (1997) 17-24
19
Fig. 2. Brittle crack showing growth patterns.
and tearing of connecting ligaments, as can be seen in Fig. 5, and at this stage brittle crack extension would have been suppressed by local ductile failure. As can be seen in Fig. 3-5, these brittle cracks generating from straight or smoothly curved oxide cracks, crossed the boundary network without deviation, and even on the brittle fracture facets no surface tilts could be detected that might have indicated that the crack path had been influenced by crystal structure. However, by plastically deforming uncoated specimens it could be seen that, in relation to the boundary network, the traversing slip bands showed virtually no deviations, thus revealing that there was very little disorientation between neighbouring grains, or what would be more correctly described as sub-grains. A region of such slip banding is shown in Fig. 6. Thus even if the crack had been closely following a cleavage plane, the fracture face would not have tilted enough to be detected under the microscope. It has been mentioned that this particular piece of material showed a greater tendency to crack in the manner described than any of the other forging samples tested, and to investigate this point the various microstuctures have been metallographically examined. It was found that the most susceptible material was a very lightly worked forging, the cast grain size being of the order of 250 km, and virtually equiaxed in the three dimensions. It had recrystallised around ihese original grains with some degree of grain growth leaving the grain interiors, which constituted the main bulk of the material, in a lightly polygonised state. On the other hand, forged material that seemed to be immune from this form of brittle fracture, although having much the same cast grain size as the susceptible forging, had been heavily worked, at least a three to one reduction. This had, after solution heat
Fig. 3. Specimen surface after removal of oxide showing initial crack traces.
20
P.J.E. Forsyth/Materials
Letters 32 (1997) 17-24
Fig. 4. The presence of brittle cracks denoted by opening displacement.
Fig. 5. Neighbouring
Fig. 6. Surface of susceptible
brittle cracks with connecting
alloy after plastic deformation,
ligament.
showing long slip bands across sub-grains.
P.J.E. Forsyth / Materials Letters 32 (1997) 17-24
21
Fig. 7. Dendritic fracture pattern with spines oriented at 450 to one another.
treatment, resulted in a very fine uniform grain or sub-grain size of approximately 20 Frn with no evidence of residual “ingotism”, nor any grain growth. The nature of slip de formation in the susceptible material has already been illustrated in Fig. 6 where the maximum free slip path was only limited by the cast grain boundary regions, i.e. about 200 pm. In contrast, the non-susceptible material deformed in a more homogeneous manner by fine slip which was concentrated at the coarser particles present, and at the boundaries of the larger grains, initiating cracks at both features. Looking now in more detail at these facets that developed from oxide cracks, Fig. 7 shows that the spines of the dendritic pattern Iof crack growth in the metal form two sets that are consistently oriented at 45” to one another, and, rather curiously, seem to weave over and under at crossing points. This is in contrast to the features exhibited by a neighbouring area of fracture, shown in Fig. 8, which was far from flat, and showed no evidence of any preferred crack growth directions, being what might be best described as conchoidal. Using squeeze cast alloys of a somewhat similar composition to 7010 but tested as round V notched bar specimens, revealed an incidence of brittle fracture of the type already described, both in the anodised and unanodised condition. These materials had, after homogenisation and solution heat treatment, a grain size of approximately 200 p_rn, similar to the susceptible forging material. The grains were equiaxed, and contained no detectable substructure, only a few isolated interdendrite residues. The brittle fracture facets, examples of which are shown in Fig. 9, appeared singly or in groups extending over two or three neighbouring grains, resulting in
Fig. 8. Structure insensitive brittle fracture.
P.J.E. Forsyth/Materials
22
Letters 32 (1997) 17-24
Fig. 9. Cleavage facets on squeeze cast aluminium
alloy following
twin orientation.
cracks of over 0.5 mm in depth. The example shown in Fig. 9 reveals that the crack had been diverted by twin boundaries. Such casting twins are sometimes found in these materials, and persist through the homogenising and solution heat treatments. Although these facets were more frequently found in the squeeze cast material, they were also present on the fracture of the lightly forged 7010 material, in the vicinity of an unanodised notch.
4. Discussion The geometric alignment of the spines shown in Fig. 7 and the planar tilting of the fracture facets in Fig. 9 indicate a degree of structure sensitivity that is not shown by the fracture illustrated in Fig. 8, where the crack had been growing as through a continuum, following only the dictate of the local stress. This dual behaviour might be expected in a material with weak elastic anisotropy such as aluminium. Most of the topographic features that appear on cleavage fractures, or for that matter on conchoidal fractures in elastically isotropic materials, arise from the joining of cracks advancing through the crystal at different levels. Where the elastic anisotropy is pronounced, for example in zinc single crystals, break-through may occur by secondary cleavage, in other instances it has been achieved by shear. Fig. 10 is a schematic illustration of the way that such joining seems to happen in the present case. It is based on the observation that the ends of
Break-through of parallel cleavage planes (a) Smgle fold - One preferred growth dlrectlon (b) Two preferred growth dtrections. Fig. 10. Schematic
illustration
of overlap break-through
and the effect of two preferred crack growth directions.
P.J.E. Forsyth /Materials Letters 32 (1997) 17-24
23
approaching offset c:racks pass one another and then turn to form what might be described as a “sigmoidal” junction [6]. The condition for this to occur rather than joining by shear is that the two advancing crack tip plastic zones should be too small to interact with one another, and the rotation of the cracks that results is as would be predicted from linear elastic fracture mechanics principles. Fig. 1Oa shows the way that a ledge would develop, and it is of interest that this form of cracking, as well as occurring with metals, has been observed on fatigue fractures of polymethylmethacrylate where the sigmoidal enclosure curled away freely from the fracture surface in the form of a filament, the overlap being oriented in the crack growth direction [6]. Fig. lob shows how, when there are two preferred crack growth directions, the dendrite pattern develops by periodic diversions from the original spine, and because the fracture does not continually follow the preferred plane, overlaps occur. There would appear to be two principal requirements for brittle fracture to occur in these high strength aluminium alloys: (1) they must have a very high intrinsic flow stress, and (2) there must be a sufficient degree of plastic constraint.Furthermore, for this brittle fracture to manifest itself with a semblance of cleavage, the elastic anisotropy of the aluminium lattice, small as it is, must have the freedom to become effective, and this requires large grains. It should be noted that these coarse, lightly worked microstructures are inevitable in large forgings manufactured from cast stock in a conventional manner, but what has, perhaps, not been fully appreciated, is that the cores of thesa large grains have deformation and fracture characteristics that are similar to those of the simpler fully recrystallised materials. Referring now to the way that these brittle cracks are thought to occur, Fig. 11 shows, in schematic form, the disposition of the constrained metal under the oxide coating in relation to the more freely plastically deforming interior, and A and I3 relate crack formation to the stress/strain curves for the composite arrangement. It has been shown that large grain size is essential for the development of recognisable cleavage, which is in accord with experience with other materials where it is a more common mode of fracture. It now seems clear that these transgranular brittle fractures that have been loosely described as cleavage, are, in fact, essentially conchoidal, where the crack path is influenced only by the local stress direction, and the perturbations caused by microstructural discontinuities, such as particles and defects, deflect the crack front to produce the irregularity that is observed. Over and above this, the weak elastic anisotropy of the aluminium lattice is still powerful enough to draw the crack path towards and into a low bond strength plane, but not strong enough to maintain it there permanently against the fluctuations described. It is envisaged that the crack tip advances through highly disturbed material which distances it to some extent, depending on the plastic zone radius, from the influence of the elastic anisotropy of the undisturbed surroundings. Because the material within this zone has lost its crystalline identity, fracture within it tends to be conchoidal, although it cannot be described, within the strict meaning of the word, as brittle. Nevertheless, in engineering terms this is academic, the fact remaining that these are low energy fractures by virtue of the extremely small plastic volumes involved.
Fig. 11. !Schematic illustration
of the conditions
that lead to brittle cracking under a thick oxide coating.
24
P.J.E. Forsyth/Materials
Letters 32 (1997) 17-24
Although agencies have been identified that have had a specific association with this mode of fracture, it may well be that they have only exacerbated what is an intrinsic weakness in these alloys. From the practical viewpoint, the most important thing is that such fractures can occur in material, that is not just an experimental composition, but one that in all respects would be considered typical of its type, and fit for service. However, the true engineering significance of this should be judged against the fact that other material with a microstructure that indicated that it had received more hot working, which would be considered a preferable condition, exhibited no cleavage but failed with a considerable degree of intergranular separation. Where transgranular cracking susceptibility might be of more concern would be with hard (thick) anodised material used in a fatigue environment with the probability of an overload that could produce the type of gaping crack that has been illustrated. Even if no fatigue cracking ensued, these crevices would provide easy entry places for a corrodent. There is one further specific point that should be made that will be of particular concern to failures investigators. These tensile fracture facets, being relatively smooth, are highly reflective compared with the rougher textured area of ductile fracture. They are generally semi-elliptical in shape and extremely like fatigue cracks as they might appear in a similar situation. Even the absence of fatigue fracture striations in such small origins might not entirely disqualify them as being fatigue related, where it could reasonably be argued that the cracks had grown too slowly to produce resolvable striations. The abrupt change from bright to dull fracture texture would then be accepted as evidence of exploitation by a critical load much greater than any hitherto experienced. Where broken structural elements become available for a detailed fracture examination, either those that result from laboratory tests, or even crash damaged pieces, it is usual practice to look for fatigue origins that might highlight unexpected design weaknesses. It is important that in any failure analysis this possibility of mistaking tensile cleavage for fatigue should be recognised.
5. Conclusions (1) 40 p,rn thickness anodise coatings provided sufficient plastic constraint to the underlying metal to cause brittle transgranular fracture origins in plain edge-notched specimens extracted from a commercial AA7010 forging. (2) Other acute stress raisers such as sharp V notches, even in the absence of anodise coatings, promoted similar cracks in the above material, and also in squeeze cast material of a similar composition. (3) These brittle fractures, in some instances, showed attributes of cleavage, the crack path being influenced by crystal structure. However, in some respects they behaved as in a continuum. This duality of crack path preference is attributed to the weak elastic anisotropy of the aluminium lattice. (4) Only large (200-500 km) grain size microstructures fractured in this cleavage-like mode, but where forgings can only be lightly worked such microstructures are not uncommon. (5) Such crack origins as have been described, if they appeared on tensile fractures, such as those brought about by crash damage, might easily be mistaken for fatigue cracks.
References [l] A.P. Reynolds and GE. Stoner, Metall. Trans. A 22 (1991) 1849. [2] J.C. Scully, The theory of stress corrosion cracking in alloys, Conference Proceedings 1971). [3] W.S. Miller, M.P. Thomas and .I. White, Scripta Metall. 21 (1987) 663. [4] T.M. Yue, Ph.D. Thesis, University of Southampton (1987). [5] P.J.E. Forsyth, C.A. Stubbington and D.J.I. Clark, Metals 90 (1961-Z) 238. [6] P.J.E. Forsyth, Intern. J. Fatigue, January 1983, pp. 3-14.
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