- Email: [email protected]
Influence of machining conditions on tensile stress and ductility of a mechanically alloyed Fe–40Al intermetallic
Scripta Materialia 46 (2002) 843–850 www.actamat-journals.com
Influence of machining conditions on tensile stress and ductility of a mechanically alloyed Fe–40Al intermetallic D.G. Morris *, C. Garcia Oca, J. Chao, M.A. Mu~ noz-Morris Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain Received 24 January 2002; received in revised form 11 February 2002
Abstract The influence of a variety of machining conditions on the tensile behaviour of a mechanically alloyed FeAl intermetallic has been examined. Yield behaviour is hardly affected by machining conditions, and ductility is only weakly sensitive to machining speed or cutting depth. This behaviour seems to be related more with the extent of subsurface damage or work hardening than with the roughness of the machined samples. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Machining; Intermetallic compound; Mechanical properties; Ductility
1. Introduction Iron aluminide materials of composition near Fe–40Al (at.%) are of interest as possible structural materials for several industrial applications because of their reasonable strength combined with good ductility and excellent oxidation and corrosion resistance in certain industrial environments [1–3]. A current European programme aims to develop commercial materials by mechanical alloying and leading to a high strength and high ductility version of the Fe–40Al base [4–9]. As such materials graduate from being interesting laboratory alloys to real industrial materials, it is essential to evaluate the role of surface roughness, and surface preparation conditions in general, on
*
Corresponding author. E-mail address: [email protected] (D.G. Morris).
the mechanical behaviour of sample pieces. This has been the objective of the present study, to prepare tensile test samples using a variety of machining, grinding and polishing methods, and to evaluate the influence on surface parameters and the tensile behaviour.
2. Experimental procedure Materials for study were supplied either by CEREM, CEA (Centre de Recherche en Materiaux, Atomic Research Centre) of Grenoble, France, or by PLANSEE AG of Germany, as part of a European research programme (FIAC). Materials were prepared by milling pre-alloyed powders of nominal composition (at.%) Fe–40Al– 0.05Zr–50 ppmB with 1 vol% of Y2 O3 powders, and subsequently consolidated by extrusion at 1100 °C. Further details have been reported elsewhere
1359-6462/02/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 0 6 3 - 5
844
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Table 1 Effect of machining/finishing parameters on tensile behaviour of MA Fe–40Al Sample code
Machining speed (m/min)
Machining depth (mm)
Upper yield stress (MPa)
Lower yield stress (MPa)
Yield stress drop (MPa)
Ductility (%)
A B C D E F G H X I J K L M
9 50 25 9 50 25 polishedþ ground þ ground 3 h 1100 °C polished þ 3 h 1100 °C machined machined
0.01 0.01 0.01 0.1 0.1 0.2 1 h 1100 °C 1 h 1100 °C
1175 1205 1160 1192 1150 1150 950 930 1160
1090 1120 1080 1160 1100 1117 875 890 1085 795 800 760 910 925
85 85 80 32 50 33 75 40 75
5 3.7 5.3 3.35 3.3 2.0 9.7 9.0 5.05 10 10.7 8.2 6.3 10.2
þpolished 3 h 1100 °C þ600gr SiC þpolished
[6,7,10]. Tensile samples with a gauge section of diameter 3 mm and length 20 mm were prepared by machining, grinding or polishing using the range of conditions detailed in Table 1. Machining was carried out using a standard KX type brazed carbide tool at a fixed feed rate of 0.2 mm/turn and a variety of cutting speeds and depths of cut. In addition some samples were prepared using standard commercial grinding conditions. A few samples were also prepared by diamond polishing using 6 lm diamond paste on a lathe and/or by annealing after surface preparation. Tensile testing was carried out using a standard universal testing machine at a strain rate of 3 104 /s, and both fracture surfaces and machined/ ground surfaces examined by scanning electron microscopy (SEM) using a JEOL 840 instrument. Surface roughness was measured using a Mitutoyo Surftest instrument.
3. Results Table 1 lists the sample preparation methods used as well as the results of mechanical testing. Typical tensile test stress–strain curves of samples A–F are shown in Fig. 1. These curves show that the initiation of plastic strain was marked by a well-defined yield drop, followed by a yield plateau before work hardening set in. Samples A–F cor-
Fig. 1. Tensile test curves for samples A–F, illustrating the influence of machining conditions on the yield behaviour and the extent of work hardening before final failure.
respond to the as-extruded (Plansee) material in the as-machined state, G and H are the same Plansee material finished by grinding or diamond polishing and given a short stress relaxing anneal. Sample X is the Plansee material in the as-ground state. Samples I–M correspond to CEREM material prepared by a variety of methods and heat treatments. Data from these samples have been reported previously [9]. Note that for all materials the grain size was close to 1 lm and the yttria particle size was about 20 nm. The tensile data illustrated in Fig. 1 and summarised in Table 1 show that the state of the as-
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
prepared sample surface has no effect on the lower yield (or flow) stress, but does modify the extent of the yield drop and the amount of work hardening achieved by affecting the plastic strain before failure. Fig. 2 shows the influence of machining depth and machining speed on the tensile ductility (with the ground sample considered to correspond to conditions of very high cutting speed and very small depth). It may be seen that ductility is little affected by the machining speed but appears to be linearly degraded by increases in machining depth. The behaviour of the ground sample is also well covered by this correlation. In a similar way to the changes of ductility, the extent of yield drop is sensitive to the machining depth but is insensitive to the machining speed, as clearly seen in Fig. 3. The influence of sample preparation conditions on the surface roughness is shown in Fig. 4. The roughness parameter shown is the average roughness Ra , measured after 3 scans over a 4 mm distance along the tensile sample length, with other roughness parameters (Rz , Rp , etc.) showing similar variations between samples as Ra . It may be noted that the scanning length is much greater than the material grain size (of the order of 1 lm) and the machining feed rate (0.2 mm). Contrary to the
845
trends of tensile parameters presented in Figs. 2 and 3, the roughness shows no dependence on machining depth but increases approximately linearly with machining speed. In this figure the ground sample is again represented as a high speed, low cutting depth preparation condition, but shows a much lower roughness than would machining at an equivalent speed, reflecting the different material removal conditions of grinding and machining. The surface quality has also been examined by SEM, and some examples are shown in Fig. 5. The general surface appearance changes greatly from the condition of deep machining (Fig. 5a) to the condition of shallow machining (Fig. 5b) and to the grinding condition (Fig. 5c). More important than apparent roughness, however, is the damage seen as occasional fine transverse (to tensile axis) cracks and deeper longitudal tears after deep machining cuts (Fig. 5a), as occasional fine transverse cracks after shallow machining cuts (Fig. 5b), and no visible cracking after grinding. Fig. 5c shows a ground sample after failure in tension and the transverse crack visible was produced during the tensile testing with no cracks being visible in the as-ground state before testing.
Fig. 2. Influence of machining conditions on tensile ductility. Increasing machining depth (left) leads to a decrease of ductility, while machining speed (right) has no influence. The as-ground sample (X) is characterised approximately as a high speed, low cutting depth preparation condition.
846
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 3. Influence of machining conditions on yield drop at the initiation of plasticity. Increasing machining depth (left) leads to a reduction of yield drop, while machining speed (right) has no influence. The as-ground sample (X) is characterised approximately as a high speed, low cutting depth preparation condition.
Fig. 4. Influence of machining conditions on surface roughness ðRa Þ. Increasing machining depth (left) has no effect on roughness, while increasing machining speed (right) leads to a steady increase in roughness. The as-ground sample (X) is represented as a high speed, low cutting depth preparation condition.
Fracture surfaces of a selection of samples after tensile testing are shown in Fig. 6. Fig. 6a and b show low magnification photographs of the entire fractured sample illustrating how failure occurs for
samples machined roughly (Fig. 6a––samples D, E, F) and machined/ground carefully (Fig. 6b–– sample X, with A, B, C similar). In Fig. 6a failure appears to have started readily around the entire
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 5. External surface of samples of (a) material F––deep machining cut; (b) material C––small machining cut; and (c) material X––ground state. In all cases the tensile test axis is approximately vertical.
847
sample circumference and final failure has led to a chisel-shaped sample. In Fig. 6b, failure clearly began at one surface location and propagated, leaving river lines, from one side to the other leaving a flat overall surface. Detailed examination at high magnification near fracture initiation failed to find any specific site where crack nucleation occurred. For samples annealed after preparation (such as G, H, J) a thin surface layer of recrystallised material was detected, both by optical microscopy and from the large cleavage facets propagating in from the surface, Fig. 6c. Such recrystallisation has been discussed before and can be understood by the subsurface deformation due to machining/grinding inducing recrystallisation during annealing [9,11]. For the strain imposed by grinding and 1100 °C annealing, Fig. 6c shows that about 15 lm recrystallised below the surface, while diamond polishing after grinding has reduced the depth of deformed layer such that only about 5 lm recrystallised, Fig. 6e. Despite this recrystallised layer, the fracture ductility remains unaffected and is the same for samples with shallow recrystallised surface layers as for samples polished sufficiently to remove any such surface layers. Fig. 6d shows the transition of fracture morphology as failure propagates from the recrystallised surface layer to the underlying region, where failure remains at least partially intergranular for some tenths of a millimetre. Fig. 6e shows a similar evolution for the ground, polished and annealed sample G. For all samples the final stage of failure, occupying much of the fracture surface, is ductile transgranular failure, as shown in Fig. 6f, and as discussed elsewhere [9]. The influence of surface state and yield stress on tensile ductility is summarised in Fig. 7 for machined, ground and polished samples. This is an extension of an earlier work relating ductility to the yield stress [9]. Fig. 7 shows that ductility has a weak inverse dependence on yield stress and depends on the method used to prepare the surface of the samples. The sensitivity to surface state is perhaps greater for samples of higher strength. It should be noted that the scatter of ductility data for machined samples in Fig. 7 is due to the wide range of machining parameters considered and not due to data scatter for a given set of preparation conditions.
848
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 6. Tensile fracture surfaces: (a) chisel fracture morphology after rough machining (F); (b) flat fracture morphology after fine machining (C); (c) fracture initiation in surface recrystallised layer (H); (d) transition of fracture from recrystallised to unrecrystallised region (H); (e) thin recrystallised surface layer (G), and transition of fracture from recrystallised to unrecrystallised region (G); (f) typical final trangranular failure (H).
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
849
strength and good ductility for reasonable machining conditions. Both yield drop and tensile ductility are shown to be sensitive to machining depth but not to machining speed. The fact that surface roughness is independent of machining speed implies that it is not roughness as such but subsurface damage––both as cracking and as work hardening––that is important: surface cracks have been shown in Fig. 3, and work hardening demonstrated in the tendency to recrystallise during subsequent annealing (has also been shown by nano/microhardness testing on sectioned samples). Such surface microcracking and subsurface work hardening leads both to easier initiation of plastic deformation (hence yield drops diminish or are lost) and to easier strain localisation leading to crack initiation. Fig. 7. Variation of tensile ductility with yield stress and sample preparation method––machining using a wide range of conditions, abrasion/grinding, and polishing.
4. Discussion Tensile fracture of the mechanically alloyed Fe– 40Al material begins by a process of crack nucleation (a poorly understood process occurring after significant plastic deformation), is followed by a period of slow stable crack growth affected by atmospheric attack with a significant amount of cleavage cracking, and is followed by final rapid transgranular failure [9]. Polished samples spend most of their ductilities in the crack nucleation stage, most of the remaining strain in the slow crack growth stage, with final failure occupying only a very short time/strain [12]. It is the first stage of crack nucleation that is considerably accelerated by surface roughness: for example for samples of yield stress about 800 MPa and 10% ductility as polished (Fig. 7), about 8–9% strain is spent in producing the first detectable cracks [12]. Rough sample surfaces reduce this nucleation stage to about 6–7% strain (for ground samples) or to perhaps 4% strain (for machined samples). The most important result of the present study is the demonstration of limited sensitivity of tensile ductility to preparation state, polishing, grinding or machining, and that it is possible to retain high
Acknowledgements We should like to thank especially our colleagues at CEREM, CEA, Grenoble and at PLANSEE for the supply of materials. This work was funded by a European GROWTH project, FIAC, contract number G5RD-CT-99-00070. We also thank the Spanish Ministry of Science and Technology for complementary support under project MAT2000-1885-CE. We want also to thank B.J. Fernandez Glez for surface roughness measurements.
References [1] McKamey CG, DeVan JH, Tortorelli PF, Sikka VK. J Mater Res 1991;6:1779. [2] Gaydosh DJ, Draper SL, Noebe RD, Nathal MV. Mater Sci Eng A 1992;150:7. [3] Stoloff NS, Liu CT. Intermetallics 1994;2:75. [4] Tassa O, Testani C, Bocher P, Lefort A. In: Proc Materials Development in Rail, Tire, Wing, Hull Transportation, September 22–24, Genoa. London: Elsevier Applied Science; 1992. p. 2.101. [5] Lefort A, Webb G, Wolski K, Lecoze J, Thevenot F, Franzoni U, Tassa O. In: Niku-Lari A, editor. Proc MATTEC-92, 14–16 October, Grenoble. Paris: IITTInternational; 1992. p. 327. [6] Baccino R, San Filippo D, Moret F, Lefort A, Webb G. In: Proc PM’94, Powder Metallurgy World Congress,
850
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
June, Paris, vol. II. Les Ulis: Editions de Physique; 1994. p. 1239. [7] Morris DG, Gunther S. Mater Sci Eng A 1996;208:7. [8] Briguet C, Morris DG. Acta Mater 1997;45:4939. [9] Chao J, Morris DG, Mu~ noz-Morris MA, Gonzalez-Carrasco JL. Intermetallics 2001;9:299.
[10] Morris-Mu~ noz MA, Garcia Oca C, Morris DG. Acta Mater, in press. [11] Morris DG, Gunther S, Briguet C. Scripta Mater 1997;37:71. [12] Morris DG, Chao J, Garcia Oca C, Morris-Mu~ noz MA. Mater Sci Eng, in press.
Influence of machining conditions on tensile stress and ductility of a mechanically alloyed Fe–40Al intermetallic D.G. Morris *, C. Garcia Oca, J. Chao, M.A. Mu~ noz-Morris Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain Received 24 January 2002; received in revised form 11 February 2002
Abstract The influence of a variety of machining conditions on the tensile behaviour of a mechanically alloyed FeAl intermetallic has been examined. Yield behaviour is hardly affected by machining conditions, and ductility is only weakly sensitive to machining speed or cutting depth. This behaviour seems to be related more with the extent of subsurface damage or work hardening than with the roughness of the machined samples. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Machining; Intermetallic compound; Mechanical properties; Ductility
1. Introduction Iron aluminide materials of composition near Fe–40Al (at.%) are of interest as possible structural materials for several industrial applications because of their reasonable strength combined with good ductility and excellent oxidation and corrosion resistance in certain industrial environments [1–3]. A current European programme aims to develop commercial materials by mechanical alloying and leading to a high strength and high ductility version of the Fe–40Al base [4–9]. As such materials graduate from being interesting laboratory alloys to real industrial materials, it is essential to evaluate the role of surface roughness, and surface preparation conditions in general, on
*
Corresponding author. E-mail address: [email protected] (D.G. Morris).
the mechanical behaviour of sample pieces. This has been the objective of the present study, to prepare tensile test samples using a variety of machining, grinding and polishing methods, and to evaluate the influence on surface parameters and the tensile behaviour.
2. Experimental procedure Materials for study were supplied either by CEREM, CEA (Centre de Recherche en Materiaux, Atomic Research Centre) of Grenoble, France, or by PLANSEE AG of Germany, as part of a European research programme (FIAC). Materials were prepared by milling pre-alloyed powders of nominal composition (at.%) Fe–40Al– 0.05Zr–50 ppmB with 1 vol% of Y2 O3 powders, and subsequently consolidated by extrusion at 1100 °C. Further details have been reported elsewhere
1359-6462/02/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 0 6 3 - 5
844
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Table 1 Effect of machining/finishing parameters on tensile behaviour of MA Fe–40Al Sample code
Machining speed (m/min)
Machining depth (mm)
Upper yield stress (MPa)
Lower yield stress (MPa)
Yield stress drop (MPa)
Ductility (%)
A B C D E F G H X I J K L M
9 50 25 9 50 25 polishedþ ground þ ground 3 h 1100 °C polished þ 3 h 1100 °C machined machined
0.01 0.01 0.01 0.1 0.1 0.2 1 h 1100 °C 1 h 1100 °C
1175 1205 1160 1192 1150 1150 950 930 1160
1090 1120 1080 1160 1100 1117 875 890 1085 795 800 760 910 925
85 85 80 32 50 33 75 40 75
5 3.7 5.3 3.35 3.3 2.0 9.7 9.0 5.05 10 10.7 8.2 6.3 10.2
þpolished 3 h 1100 °C þ600gr SiC þpolished
[6,7,10]. Tensile samples with a gauge section of diameter 3 mm and length 20 mm were prepared by machining, grinding or polishing using the range of conditions detailed in Table 1. Machining was carried out using a standard KX type brazed carbide tool at a fixed feed rate of 0.2 mm/turn and a variety of cutting speeds and depths of cut. In addition some samples were prepared using standard commercial grinding conditions. A few samples were also prepared by diamond polishing using 6 lm diamond paste on a lathe and/or by annealing after surface preparation. Tensile testing was carried out using a standard universal testing machine at a strain rate of 3 104 /s, and both fracture surfaces and machined/ ground surfaces examined by scanning electron microscopy (SEM) using a JEOL 840 instrument. Surface roughness was measured using a Mitutoyo Surftest instrument.
3. Results Table 1 lists the sample preparation methods used as well as the results of mechanical testing. Typical tensile test stress–strain curves of samples A–F are shown in Fig. 1. These curves show that the initiation of plastic strain was marked by a well-defined yield drop, followed by a yield plateau before work hardening set in. Samples A–F cor-
Fig. 1. Tensile test curves for samples A–F, illustrating the influence of machining conditions on the yield behaviour and the extent of work hardening before final failure.
respond to the as-extruded (Plansee) material in the as-machined state, G and H are the same Plansee material finished by grinding or diamond polishing and given a short stress relaxing anneal. Sample X is the Plansee material in the as-ground state. Samples I–M correspond to CEREM material prepared by a variety of methods and heat treatments. Data from these samples have been reported previously [9]. Note that for all materials the grain size was close to 1 lm and the yttria particle size was about 20 nm. The tensile data illustrated in Fig. 1 and summarised in Table 1 show that the state of the as-
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
prepared sample surface has no effect on the lower yield (or flow) stress, but does modify the extent of the yield drop and the amount of work hardening achieved by affecting the plastic strain before failure. Fig. 2 shows the influence of machining depth and machining speed on the tensile ductility (with the ground sample considered to correspond to conditions of very high cutting speed and very small depth). It may be seen that ductility is little affected by the machining speed but appears to be linearly degraded by increases in machining depth. The behaviour of the ground sample is also well covered by this correlation. In a similar way to the changes of ductility, the extent of yield drop is sensitive to the machining depth but is insensitive to the machining speed, as clearly seen in Fig. 3. The influence of sample preparation conditions on the surface roughness is shown in Fig. 4. The roughness parameter shown is the average roughness Ra , measured after 3 scans over a 4 mm distance along the tensile sample length, with other roughness parameters (Rz , Rp , etc.) showing similar variations between samples as Ra . It may be noted that the scanning length is much greater than the material grain size (of the order of 1 lm) and the machining feed rate (0.2 mm). Contrary to the
845
trends of tensile parameters presented in Figs. 2 and 3, the roughness shows no dependence on machining depth but increases approximately linearly with machining speed. In this figure the ground sample is again represented as a high speed, low cutting depth preparation condition, but shows a much lower roughness than would machining at an equivalent speed, reflecting the different material removal conditions of grinding and machining. The surface quality has also been examined by SEM, and some examples are shown in Fig. 5. The general surface appearance changes greatly from the condition of deep machining (Fig. 5a) to the condition of shallow machining (Fig. 5b) and to the grinding condition (Fig. 5c). More important than apparent roughness, however, is the damage seen as occasional fine transverse (to tensile axis) cracks and deeper longitudal tears after deep machining cuts (Fig. 5a), as occasional fine transverse cracks after shallow machining cuts (Fig. 5b), and no visible cracking after grinding. Fig. 5c shows a ground sample after failure in tension and the transverse crack visible was produced during the tensile testing with no cracks being visible in the as-ground state before testing.
Fig. 2. Influence of machining conditions on tensile ductility. Increasing machining depth (left) leads to a decrease of ductility, while machining speed (right) has no influence. The as-ground sample (X) is characterised approximately as a high speed, low cutting depth preparation condition.
846
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 3. Influence of machining conditions on yield drop at the initiation of plasticity. Increasing machining depth (left) leads to a reduction of yield drop, while machining speed (right) has no influence. The as-ground sample (X) is characterised approximately as a high speed, low cutting depth preparation condition.
Fig. 4. Influence of machining conditions on surface roughness ðRa Þ. Increasing machining depth (left) has no effect on roughness, while increasing machining speed (right) leads to a steady increase in roughness. The as-ground sample (X) is represented as a high speed, low cutting depth preparation condition.
Fracture surfaces of a selection of samples after tensile testing are shown in Fig. 6. Fig. 6a and b show low magnification photographs of the entire fractured sample illustrating how failure occurs for
samples machined roughly (Fig. 6a––samples D, E, F) and machined/ground carefully (Fig. 6b–– sample X, with A, B, C similar). In Fig. 6a failure appears to have started readily around the entire
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 5. External surface of samples of (a) material F––deep machining cut; (b) material C––small machining cut; and (c) material X––ground state. In all cases the tensile test axis is approximately vertical.
847
sample circumference and final failure has led to a chisel-shaped sample. In Fig. 6b, failure clearly began at one surface location and propagated, leaving river lines, from one side to the other leaving a flat overall surface. Detailed examination at high magnification near fracture initiation failed to find any specific site where crack nucleation occurred. For samples annealed after preparation (such as G, H, J) a thin surface layer of recrystallised material was detected, both by optical microscopy and from the large cleavage facets propagating in from the surface, Fig. 6c. Such recrystallisation has been discussed before and can be understood by the subsurface deformation due to machining/grinding inducing recrystallisation during annealing [9,11]. For the strain imposed by grinding and 1100 °C annealing, Fig. 6c shows that about 15 lm recrystallised below the surface, while diamond polishing after grinding has reduced the depth of deformed layer such that only about 5 lm recrystallised, Fig. 6e. Despite this recrystallised layer, the fracture ductility remains unaffected and is the same for samples with shallow recrystallised surface layers as for samples polished sufficiently to remove any such surface layers. Fig. 6d shows the transition of fracture morphology as failure propagates from the recrystallised surface layer to the underlying region, where failure remains at least partially intergranular for some tenths of a millimetre. Fig. 6e shows a similar evolution for the ground, polished and annealed sample G. For all samples the final stage of failure, occupying much of the fracture surface, is ductile transgranular failure, as shown in Fig. 6f, and as discussed elsewhere [9]. The influence of surface state and yield stress on tensile ductility is summarised in Fig. 7 for machined, ground and polished samples. This is an extension of an earlier work relating ductility to the yield stress [9]. Fig. 7 shows that ductility has a weak inverse dependence on yield stress and depends on the method used to prepare the surface of the samples. The sensitivity to surface state is perhaps greater for samples of higher strength. It should be noted that the scatter of ductility data for machined samples in Fig. 7 is due to the wide range of machining parameters considered and not due to data scatter for a given set of preparation conditions.
848
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
Fig. 6. Tensile fracture surfaces: (a) chisel fracture morphology after rough machining (F); (b) flat fracture morphology after fine machining (C); (c) fracture initiation in surface recrystallised layer (H); (d) transition of fracture from recrystallised to unrecrystallised region (H); (e) thin recrystallised surface layer (G), and transition of fracture from recrystallised to unrecrystallised region (G); (f) typical final trangranular failure (H).
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
849
strength and good ductility for reasonable machining conditions. Both yield drop and tensile ductility are shown to be sensitive to machining depth but not to machining speed. The fact that surface roughness is independent of machining speed implies that it is not roughness as such but subsurface damage––both as cracking and as work hardening––that is important: surface cracks have been shown in Fig. 3, and work hardening demonstrated in the tendency to recrystallise during subsequent annealing (has also been shown by nano/microhardness testing on sectioned samples). Such surface microcracking and subsurface work hardening leads both to easier initiation of plastic deformation (hence yield drops diminish or are lost) and to easier strain localisation leading to crack initiation. Fig. 7. Variation of tensile ductility with yield stress and sample preparation method––machining using a wide range of conditions, abrasion/grinding, and polishing.
4. Discussion Tensile fracture of the mechanically alloyed Fe– 40Al material begins by a process of crack nucleation (a poorly understood process occurring after significant plastic deformation), is followed by a period of slow stable crack growth affected by atmospheric attack with a significant amount of cleavage cracking, and is followed by final rapid transgranular failure [9]. Polished samples spend most of their ductilities in the crack nucleation stage, most of the remaining strain in the slow crack growth stage, with final failure occupying only a very short time/strain [12]. It is the first stage of crack nucleation that is considerably accelerated by surface roughness: for example for samples of yield stress about 800 MPa and 10% ductility as polished (Fig. 7), about 8–9% strain is spent in producing the first detectable cracks [12]. Rough sample surfaces reduce this nucleation stage to about 6–7% strain (for ground samples) or to perhaps 4% strain (for machined samples). The most important result of the present study is the demonstration of limited sensitivity of tensile ductility to preparation state, polishing, grinding or machining, and that it is possible to retain high
Acknowledgements We should like to thank especially our colleagues at CEREM, CEA, Grenoble and at PLANSEE for the supply of materials. This work was funded by a European GROWTH project, FIAC, contract number G5RD-CT-99-00070. We also thank the Spanish Ministry of Science and Technology for complementary support under project MAT2000-1885-CE. We want also to thank B.J. Fernandez Glez for surface roughness measurements.
References [1] McKamey CG, DeVan JH, Tortorelli PF, Sikka VK. J Mater Res 1991;6:1779. [2] Gaydosh DJ, Draper SL, Noebe RD, Nathal MV. Mater Sci Eng A 1992;150:7. [3] Stoloff NS, Liu CT. Intermetallics 1994;2:75. [4] Tassa O, Testani C, Bocher P, Lefort A. In: Proc Materials Development in Rail, Tire, Wing, Hull Transportation, September 22–24, Genoa. London: Elsevier Applied Science; 1992. p. 2.101. [5] Lefort A, Webb G, Wolski K, Lecoze J, Thevenot F, Franzoni U, Tassa O. In: Niku-Lari A, editor. Proc MATTEC-92, 14–16 October, Grenoble. Paris: IITTInternational; 1992. p. 327. [6] Baccino R, San Filippo D, Moret F, Lefort A, Webb G. In: Proc PM’94, Powder Metallurgy World Congress,
850
D.G. Morris et al. / Scripta Materialia 46 (2002) 843–850
June, Paris, vol. II. Les Ulis: Editions de Physique; 1994. p. 1239. [7] Morris DG, Gunther S. Mater Sci Eng A 1996;208:7. [8] Briguet C, Morris DG. Acta Mater 1997;45:4939. [9] Chao J, Morris DG, Mu~ noz-Morris MA, Gonzalez-Carrasco JL. Intermetallics 2001;9:299.
[10] Morris-Mu~ noz MA, Garcia Oca C, Morris DG. Acta Mater, in press. [11] Morris DG, Gunther S, Briguet C. Scripta Mater 1997;37:71. [12] Morris DG, Chao J, Garcia Oca C, Morris-Mu~ noz MA. Mater Sci Eng, in press.