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On the creep behaviour of grain boundary engineered nickel 1
Materials Science and Engineering A237 (1997) 168- 172
On the creep behaviour of grain boundary engineered nickel’ E.M. Lehockey *, G. Palumbo Ontario
Hydra
Technologies,
Btdding
K.R.,
800 Kipling
Aremte,
Toronto,
Ontario
df8Z
5S4, Cnnndu
Received 29 April 1996
Abstract
Grain boundariesdescribedby low-C CSL relationships(i.e. Z I 29) have previously been shown to be resistant to grain boundary sliding,cavitation and fracture. The presentwork reportson efforts to reducecreepratesin conventional polycrystalline nickel by increasingthe frequency with which these‘special’interfacesoccur in the microstructure. Suitable thermomechanical processingwasemployedto enhancethe frequency of ‘special’grain boundaries(Z 2 29) in 99.99%Ni from 13to 66%, resulting mostly from the formation of twins (C3) and crystallographically-relatedX9 and 227 boundaries.This 53%increasein the fraction of low-Z boundariesproduced reductions of 16-fold in the steady-statecreep rate and six-fold in the primary creep strain. Microstructures having ‘special’boundary frequenciesof lessthan 50% exhibited significantcavitation almostexclusively along ‘random’ boundaries(i.e. Z > 29) at or near triple points. No grosscavitation wasevident in microstructurescontaining ‘special boundary fractions of 66%. Such improvementsin creep propertiesprovide considerablepromisefor the application of a ‘grain boundary engineering’approach to developinginterfacial materialsfor structural applications.0 1997Elsevier ScienceS.A. Ke~worcls: Creep behavior; Grain boundary;Engineered nickel
1. Introduction Use of interfacial materials for structural applications is often limited by susceptibility to creep at relatively low temperatures. One means of alleviating this effect may be to control the structure of grain boundaries. Considerable evidence exists to indicate that interfaces at or near low-C misorientations (C I 29) described by the Coincident Site Lattice (CSL) model display improved physical and chemical properties, as compared with ‘random’ boundaries characterized by higher-order C relationships [l-3]. These include enhanced resistance to: corrosion (SCC), fracture and grain boundary sliding [l-4]. Further, it has been demonstrated that the frequency of low-z CSLs can be manipulated by processing through means such as solute additions, and twinning generated by thermomechanical treatments [5,6]. In materials where the ratio of grain boundary energy to twin energy is large, twinning events are * Corresponding author. Tel.: -I- 1 416 2076745; fax: + 1 416 2360979. ’ Presented at the Engineering Foundation Conference on Mechanical Properties of Interfacial Materials, Kona, HI, 14-19 January 1996. 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(97)00126-3
known to play an important role in generating low-Z CSL boundaries [S]. This offers the possibility of altering grain boundary structure to enhance material performance. This contribution evaluates, ( 1) the magnitude of improvement in the bulk creep resistance of pure nickel possible by processing the material to achieve a high frequency of ‘special’ boundaries, and (3) the dependence of cavitation and grain boundary sliding on grain boundary structure.
2. Experimental
procedure
Coupons of as-cast 99.99% Ni (80 x 130 x 3 mm) having an average grain size of 50 pm were used. Selected samples were given a cold reduction-in-thickness (by rolling) of 60% followed by an anneal in a Nz atmosphere at temperatures between 950 and 1150°C for between 1 and 15 min. Annealing conditions were varied within the above limits to generate a range of microstructures at grain sizes consistent with the original cast material. Grain boundary character distributions (GI3CD) of all samples were evaluated using a JEOL 840 SEM
EM.
Lehockey,
G. Palumbo
Table 1 Comparison of the grain boundary character distribution Boundary type C
/ !Ilarerials
Science
and Engineering
A237
(1997)
in cast versus two wrought @recessed) 99.99% nickel samples
Boundary frequency (%) Cast (GS = 50 urn)
Wrought
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 > 29 (Random)
4.0 1.6 0.9 0.7 1.3 0.3 1.0 0.2 0.3 0.3 0.7 0.6 0.4 0.8 0 86.9
6.6 27.9 0.6 0.6 3.9 0.4 0.7 0.6 0.2 0.5 0.3 03 ~~ 0.4 1.s 0.3 54.9
8.0 46.1 0.2 0.5 5.5 0.7 0.5 0.5 0.1 0.7 0.2 0.1 0.1 2.2 0.1 35.5
Total 2129
13.1
45.1
66.2
‘special’ fraction (Y/o)
169
168-172
# 1 (GS = 35 urn)
Wrought
# 2 (GS = 25 urn)
GS, grain size.
equipped with orientation imaging microscopy (OIM), after electro-polishing in a 10% solution of perchloric acid in methanol maintained at - 50°C. Misorientations across grain boundaries were determined based upon changes in the backscattered electron diffraction patterns as the incident beam scans sequentially across the raster area. The CSL character of grain boundaries in the material was classified according to Brandon’s criterion given by A0 = 15” C - l/2 [7]. A 1 x 1 mm area in each sample was analysed encompassing a total of approximately 400 grain boundaries. ASTM Type V dogbones [8] were electro-discharge machined from the cast and wrought (processed) coupons. Samples were subsequently creep tested in air using a constant load configuration in accordance with ASTM El39 [9]. Tests were conducted at temperatures of 450°C under nominal stress of 84 MPa in the grain boundary sliding regime [IO].
stantiates the direct relationship between the frequency of low-C CSL boundaries and twinning proposed by Palumbo et al. [5]. Creep curves for each material considered are presented in Fig. 1. Both the primary creep strain and steady-state creep rate among the three materials are summarized as a function of ‘special’ boundary content in Fig. 2. Steady-state creep rates decreased monotonically with increasing ‘special’ grain boundary fraction. Increases in the frequency of ‘special’ boundaries from 13% in the cast nickel to 66%, in the optimally processed (wrought # 2) material resulted in a factor of 16 reduction in creep rate from 314 to 20%/year, respectively. It should be emphasized that this result is partic-
3. Results and discussion The grain boundary character distribution and corresponding grain size obtained from the as-cast and wrought (processed) Ni samples are compared in Table 1. Thermomechanical processing increased the frequency of ‘special’ boundaries (SJ from 13% in the original as-cast condition to a maximum of 66% in the wrought # 2 microstructure. Twins (X3) and crystallographically-related boundaries (i.e. C9 and X27) collectively accounted for 70-80% of the total population of ‘special’ boundaries in the wrought materials. This sub-
100 150 200 Time (Hours)
250
300
Fig. 1. Creep curves for cast and qrought (processed) polycrystalline nickel.
E.M.
Lehockey,
G. Palumbo
s’Materials
Science
und Engineering
A237
(1997)
168-172
6 52 lb .E 4g
r
3i 2: 1 .E 2 0 IO
20
30
40
Special Boundary
50 Fraction
60
70
(%)
Fig. 2. Effect of ‘special’ boundary frequency on steady-state creep rate and total primary creep strain.
ularly significant given the somewhat larger grain size of the cast material (GS = 50 pm) which would be expected to minimize the steady-state creep rate. However, the rate of reduction in steady-state creep diminishes once ‘special’ boundary fractions of 50-60% are achieved. According to Fig. 2, increases in the frequency of low-x CSL boundaries are also accompanied by a decline in the total accumulated primary creep strain. Unfortunately, separating contributions due to primary creep by glissile dislocations in the matrix from those in the grain boundaries is tenuous. However, this trend is consistent with an enhanced ‘slide-hardening’ of ‘special’ boundaries compared with ‘random’ boundaries (i.e. C > 29) reported by Watanabe [I ,3]. Collectively, these results demonstrate the potential for improving creep performance by altering the grain boundary structure in conventional polycrystalline nickel. A micrograph and corresponding OIM map showing grain boundary structure surrounding cavitation in the wrought # 1 material (,S,= 45%) are shown in Fig. 3(a) and (b), respectively. Voids appeared almost exclusively along ‘random’ boundaries (delineated in black), while twins (in red) and higher order ‘special’ grain boundaries for 3 < C I 29 (in yellow) remained unaffected. Resistance of ‘special’ boundaries to cavitation is further emphasized in Fig. 4 showing the proportion of ‘special’ and ‘random’ boundaries cavitated in wrought # 1 material compiled from a total of 93 boundaries. Cavitation was apparent along 70% of ‘random’ boundaries examined, with only 8% of ‘special’ boundaries being similarly affected. SEM micrographs showing the extent of cavitation in the wrought nickel are presented in Fig. 5. Gross cavitation is clearly evident in the wrought # 1 microstructure (S, = 45%) after strains of 1.80,/o,as indicated by the arrows in Fig. 5(a) and (b). In contrast, no voids were detected at similar strain levels in the wrought # 2 microstructure
i-l
P
:> 1 ,.n - ‘0 ,:c,, bekald.ril.;i*.s.ot
1h.v
Fig. 3. Micrograph (a) and corresponding 01M map (b) of the grain boundary character distribution surrounding cavitation in the wrought # 1 sample
containing 66”/0 ‘special’ boundaries, as depicted in Fig. 5(c). In general, the large magnitude of change in creep resistance of nickel can be rationalized on the basis that 70
0
Cl - 2;29
2>29
Grain Boundary Type Fig. 4. Relative proportion of ‘random’ and ‘special’ low-C CSL boundaries in the wrought # 1 material showing cavitation after a total creep strain of 1.8%.
all mechanisms contributing to grain boundary (GB) sliding and Coble creep are suppressed along low-C interfaces. Increased structural order and reduced free volume of low-1 CSL boundaries may inhibit dislocation motion by vacancy diffusion and climb, as well as absorption/dissociation of extrinsic dislocations necessary for GB sliding and void formation [11,12]. As a result, the critical size for void growth may be greater on ‘special’ boundaries compared with ‘random’ boundaries. This may account for the preferential cavitation observed on ‘random’ boundaries in Fig. 4 and substantiates reports by Watanabe and others regarding resistance of ‘special’ boundaries to sliding and cavitation [l-3]. (a)
4. Summary
Significant reductions in bulk primary creep strain and steady-state creep rate can be realized in nickel by increasing the frequency of low-C CSL (C I 29) grain boundaries. Preferential occurrence of cavitation and void nucleation along ‘random’ grain boundaries (for C > 29) is considered responsible for this effect. This is consistent with a reduced structural order along ‘random’ boundaries compared with low-C,C,SL interfaces, facilitating increased: absorption mobility: and dissociation of extrinsic dislocations necessary for GB sliding.
Acknowledgements
The authors wish to acknowledge the technical assistance provided by D. Limoges and Z. Grazdanovski in sample preparation, testing and materials characterization.
References
Fig. 5. (a) Cross section SEM micrograph showing the extent of cavitation present in the wrought # 1 sample (S, = 45%) after a total creep strain of 1.8”/u. (b) Additional examples of cavitation in the wrought # 1 sample of (a) (Sr = 45%). (c) Cross section SEM micrograph showing absence of cavitation (S, = 66%) following a total strain of 1.8%.
[I] G. Palumbo, P.J. King, P.C. Lichtenberger, K.T. Aust, E. Erb, Grain Boundary Structure Control for Intergranular S,$e@ Corrosion Resistance, in: W.A.T. Clark, U. Dahman, C.L..Briant (Eds.), Structure and Properties of Interfaces in Materials, MRS, Pittsburg, 1992, p. 31 I. + . [2] T. Watanabe, Res. Mechanica 11 (1984) 25-l. “+ I [3] H. Hiroyukiz T. Watanabe, S. Karashima, Phil.~Mag. A44-i(1981) 1239. [4] K.T. Aust. U. Erb. G. Palumbo. Interface Structures and’Properties, in; Mechanical Properties and Deformation Behaabur of Materials Having Ultra-Fine Microstructures, Kluwer, Netherlands, 1993. [5] G. Palumbo. K.T. Aust, P.J. King, A.M.1 Brennenstuhl:~ P.G.-Lichtenberger! Physica Status Solidi A131 (1992) 425.
172
EN. Lehockey, G. Pahmbo /Materials
[6] K.T. Aust, J,W. Rutter, Trans. Met. Sot. AIME 215 (1959) 119. pi] D.G. Braadon, Acta Metallurgica 14 (1966) 1479. 181 ASTM Committee D-20, Specification D638-91, ASTM Annual Book of Standards 8.01 (1991) 159. [9] ASTM Committe E-20, Specification E139-83, Annual Book of
Science and Engineering A237 (1997) 166-112 ASTM Standards 3.01 (1995) 257. [IO] M. Ashby, Acta Metallurgica 20 (1972) 887. [ll] G.R. Kegg, C.A.P. Horton, J.M. Silcock, Phil. Mag. A27 (1973) 1041. [12] R.C. Pond, D.A. Smith, Phil. Mag. A36 (1977) 353.
On the creep behaviour of grain boundary engineered nickel’ E.M. Lehockey *, G. Palumbo Ontario
Hydra
Technologies,
Btdding
K.R.,
800 Kipling
Aremte,
Toronto,
Ontario
df8Z
5S4, Cnnndu
Received 29 April 1996
Abstract
Grain boundariesdescribedby low-C CSL relationships(i.e. Z I 29) have previously been shown to be resistant to grain boundary sliding,cavitation and fracture. The presentwork reportson efforts to reducecreepratesin conventional polycrystalline nickel by increasingthe frequency with which these‘special’interfacesoccur in the microstructure. Suitable thermomechanical processingwasemployedto enhancethe frequency of ‘special’grain boundaries(Z 2 29) in 99.99%Ni from 13to 66%, resulting mostly from the formation of twins (C3) and crystallographically-relatedX9 and 227 boundaries.This 53%increasein the fraction of low-Z boundariesproduced reductions of 16-fold in the steady-statecreep rate and six-fold in the primary creep strain. Microstructures having ‘special’boundary frequenciesof lessthan 50% exhibited significantcavitation almostexclusively along ‘random’ boundaries(i.e. Z > 29) at or near triple points. No grosscavitation wasevident in microstructurescontaining ‘special boundary fractions of 66%. Such improvementsin creep propertiesprovide considerablepromisefor the application of a ‘grain boundary engineering’approach to developinginterfacial materialsfor structural applications.0 1997Elsevier ScienceS.A. Ke~worcls: Creep behavior; Grain boundary;Engineered nickel
1. Introduction Use of interfacial materials for structural applications is often limited by susceptibility to creep at relatively low temperatures. One means of alleviating this effect may be to control the structure of grain boundaries. Considerable evidence exists to indicate that interfaces at or near low-C misorientations (C I 29) described by the Coincident Site Lattice (CSL) model display improved physical and chemical properties, as compared with ‘random’ boundaries characterized by higher-order C relationships [l-3]. These include enhanced resistance to: corrosion (SCC), fracture and grain boundary sliding [l-4]. Further, it has been demonstrated that the frequency of low-z CSLs can be manipulated by processing through means such as solute additions, and twinning generated by thermomechanical treatments [5,6]. In materials where the ratio of grain boundary energy to twin energy is large, twinning events are * Corresponding author. Tel.: -I- 1 416 2076745; fax: + 1 416 2360979. ’ Presented at the Engineering Foundation Conference on Mechanical Properties of Interfacial Materials, Kona, HI, 14-19 January 1996. 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PIISO921-5093(97)00126-3
known to play an important role in generating low-Z CSL boundaries [S]. This offers the possibility of altering grain boundary structure to enhance material performance. This contribution evaluates, ( 1) the magnitude of improvement in the bulk creep resistance of pure nickel possible by processing the material to achieve a high frequency of ‘special’ boundaries, and (3) the dependence of cavitation and grain boundary sliding on grain boundary structure.
2. Experimental
procedure
Coupons of as-cast 99.99% Ni (80 x 130 x 3 mm) having an average grain size of 50 pm were used. Selected samples were given a cold reduction-in-thickness (by rolling) of 60% followed by an anneal in a Nz atmosphere at temperatures between 950 and 1150°C for between 1 and 15 min. Annealing conditions were varied within the above limits to generate a range of microstructures at grain sizes consistent with the original cast material. Grain boundary character distributions (GI3CD) of all samples were evaluated using a JEOL 840 SEM
EM.
Lehockey,
G. Palumbo
Table 1 Comparison of the grain boundary character distribution Boundary type C
/ !Ilarerials
Science
and Engineering
A237
(1997)
in cast versus two wrought @recessed) 99.99% nickel samples
Boundary frequency (%) Cast (GS = 50 urn)
Wrought
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 > 29 (Random)
4.0 1.6 0.9 0.7 1.3 0.3 1.0 0.2 0.3 0.3 0.7 0.6 0.4 0.8 0 86.9
6.6 27.9 0.6 0.6 3.9 0.4 0.7 0.6 0.2 0.5 0.3 03 ~~ 0.4 1.s 0.3 54.9
8.0 46.1 0.2 0.5 5.5 0.7 0.5 0.5 0.1 0.7 0.2 0.1 0.1 2.2 0.1 35.5
Total 2129
13.1
45.1
66.2
‘special’ fraction (Y/o)
169
168-172
# 1 (GS = 35 urn)
Wrought
# 2 (GS = 25 urn)
GS, grain size.
equipped with orientation imaging microscopy (OIM), after electro-polishing in a 10% solution of perchloric acid in methanol maintained at - 50°C. Misorientations across grain boundaries were determined based upon changes in the backscattered electron diffraction patterns as the incident beam scans sequentially across the raster area. The CSL character of grain boundaries in the material was classified according to Brandon’s criterion given by A0 = 15” C - l/2 [7]. A 1 x 1 mm area in each sample was analysed encompassing a total of approximately 400 grain boundaries. ASTM Type V dogbones [8] were electro-discharge machined from the cast and wrought (processed) coupons. Samples were subsequently creep tested in air using a constant load configuration in accordance with ASTM El39 [9]. Tests were conducted at temperatures of 450°C under nominal stress of 84 MPa in the grain boundary sliding regime [IO].
stantiates the direct relationship between the frequency of low-C CSL boundaries and twinning proposed by Palumbo et al. [5]. Creep curves for each material considered are presented in Fig. 1. Both the primary creep strain and steady-state creep rate among the three materials are summarized as a function of ‘special’ boundary content in Fig. 2. Steady-state creep rates decreased monotonically with increasing ‘special’ grain boundary fraction. Increases in the frequency of ‘special’ boundaries from 13% in the cast nickel to 66%, in the optimally processed (wrought # 2) material resulted in a factor of 16 reduction in creep rate from 314 to 20%/year, respectively. It should be emphasized that this result is partic-
3. Results and discussion The grain boundary character distribution and corresponding grain size obtained from the as-cast and wrought (processed) Ni samples are compared in Table 1. Thermomechanical processing increased the frequency of ‘special’ boundaries (SJ from 13% in the original as-cast condition to a maximum of 66% in the wrought # 2 microstructure. Twins (X3) and crystallographically-related boundaries (i.e. C9 and X27) collectively accounted for 70-80% of the total population of ‘special’ boundaries in the wrought materials. This sub-
100 150 200 Time (Hours)
250
300
Fig. 1. Creep curves for cast and qrought (processed) polycrystalline nickel.
E.M.
Lehockey,
G. Palumbo
s’Materials
Science
und Engineering
A237
(1997)
168-172
6 52 lb .E 4g
r
3i 2: 1 .E 2 0 IO
20
30
40
Special Boundary
50 Fraction
60
70
(%)
Fig. 2. Effect of ‘special’ boundary frequency on steady-state creep rate and total primary creep strain.
ularly significant given the somewhat larger grain size of the cast material (GS = 50 pm) which would be expected to minimize the steady-state creep rate. However, the rate of reduction in steady-state creep diminishes once ‘special’ boundary fractions of 50-60% are achieved. According to Fig. 2, increases in the frequency of low-x CSL boundaries are also accompanied by a decline in the total accumulated primary creep strain. Unfortunately, separating contributions due to primary creep by glissile dislocations in the matrix from those in the grain boundaries is tenuous. However, this trend is consistent with an enhanced ‘slide-hardening’ of ‘special’ boundaries compared with ‘random’ boundaries (i.e. C > 29) reported by Watanabe [I ,3]. Collectively, these results demonstrate the potential for improving creep performance by altering the grain boundary structure in conventional polycrystalline nickel. A micrograph and corresponding OIM map showing grain boundary structure surrounding cavitation in the wrought # 1 material (,S,= 45%) are shown in Fig. 3(a) and (b), respectively. Voids appeared almost exclusively along ‘random’ boundaries (delineated in black), while twins (in red) and higher order ‘special’ grain boundaries for 3 < C I 29 (in yellow) remained unaffected. Resistance of ‘special’ boundaries to cavitation is further emphasized in Fig. 4 showing the proportion of ‘special’ and ‘random’ boundaries cavitated in wrought # 1 material compiled from a total of 93 boundaries. Cavitation was apparent along 70% of ‘random’ boundaries examined, with only 8% of ‘special’ boundaries being similarly affected. SEM micrographs showing the extent of cavitation in the wrought nickel are presented in Fig. 5. Gross cavitation is clearly evident in the wrought # 1 microstructure (S, = 45%) after strains of 1.80,/o,as indicated by the arrows in Fig. 5(a) and (b). In contrast, no voids were detected at similar strain levels in the wrought # 2 microstructure
i-l
P
:> 1 ,.n - ‘0 ,:c,, bekald.ril.;i*.s.ot
1h.v
Fig. 3. Micrograph (a) and corresponding 01M map (b) of the grain boundary character distribution surrounding cavitation in the wrought # 1 sample
containing 66”/0 ‘special’ boundaries, as depicted in Fig. 5(c). In general, the large magnitude of change in creep resistance of nickel can be rationalized on the basis that 70
0
Cl - 2;29
2>29
Grain Boundary Type Fig. 4. Relative proportion of ‘random’ and ‘special’ low-C CSL boundaries in the wrought # 1 material showing cavitation after a total creep strain of 1.8%.
all mechanisms contributing to grain boundary (GB) sliding and Coble creep are suppressed along low-C interfaces. Increased structural order and reduced free volume of low-1 CSL boundaries may inhibit dislocation motion by vacancy diffusion and climb, as well as absorption/dissociation of extrinsic dislocations necessary for GB sliding and void formation [11,12]. As a result, the critical size for void growth may be greater on ‘special’ boundaries compared with ‘random’ boundaries. This may account for the preferential cavitation observed on ‘random’ boundaries in Fig. 4 and substantiates reports by Watanabe and others regarding resistance of ‘special’ boundaries to sliding and cavitation [l-3]. (a)
4. Summary
Significant reductions in bulk primary creep strain and steady-state creep rate can be realized in nickel by increasing the frequency of low-C CSL (C I 29) grain boundaries. Preferential occurrence of cavitation and void nucleation along ‘random’ grain boundaries (for C > 29) is considered responsible for this effect. This is consistent with a reduced structural order along ‘random’ boundaries compared with low-C,C,SL interfaces, facilitating increased: absorption mobility: and dissociation of extrinsic dislocations necessary for GB sliding.
Acknowledgements
The authors wish to acknowledge the technical assistance provided by D. Limoges and Z. Grazdanovski in sample preparation, testing and materials characterization.
References
Fig. 5. (a) Cross section SEM micrograph showing the extent of cavitation present in the wrought # 1 sample (S, = 45%) after a total creep strain of 1.8”/u. (b) Additional examples of cavitation in the wrought # 1 sample of (a) (Sr = 45%). (c) Cross section SEM micrograph showing absence of cavitation (S, = 66%) following a total strain of 1.8%.
[I] G. Palumbo, P.J. King, P.C. Lichtenberger, K.T. Aust, E. Erb, Grain Boundary Structure Control for Intergranular S,$e@ Corrosion Resistance, in: W.A.T. Clark, U. Dahman, C.L..Briant (Eds.), Structure and Properties of Interfaces in Materials, MRS, Pittsburg, 1992, p. 31 I. + . [2] T. Watanabe, Res. Mechanica 11 (1984) 25-l. “+ I [3] H. Hiroyukiz T. Watanabe, S. Karashima, Phil.~Mag. A44-i(1981) 1239. [4] K.T. Aust. U. Erb. G. Palumbo. Interface Structures and’Properties, in; Mechanical Properties and Deformation Behaabur of Materials Having Ultra-Fine Microstructures, Kluwer, Netherlands, 1993. [5] G. Palumbo. K.T. Aust, P.J. King, A.M.1 Brennenstuhl:~ P.G.-Lichtenberger! Physica Status Solidi A131 (1992) 425.
172
EN. Lehockey, G. Pahmbo /Materials
[6] K.T. Aust, J,W. Rutter, Trans. Met. Sot. AIME 215 (1959) 119. pi] D.G. Braadon, Acta Metallurgica 14 (1966) 1479. 181 ASTM Committee D-20, Specification D638-91, ASTM Annual Book of Standards 8.01 (1991) 159. [9] ASTM Committe E-20, Specification E139-83, Annual Book of
Science and Engineering A237 (1997) 166-112 ASTM Standards 3.01 (1995) 257. [IO] M. Ashby, Acta Metallurgica 20 (1972) 887. [ll] G.R. Kegg, C.A.P. Horton, J.M. Silcock, Phil. Mag. A27 (1973) 1041. [12] R.C. Pond, D.A. Smith, Phil. Mag. A36 (1977) 353.