- Email: [email protected]
The effect of pressurization and quenching on the yield behaviour in low carbon steel
THE
EFFECT
OF PRESSURIZATION AND QUENCHING BEHAVIOUR IN LOW CARBON STEEL* D. J. CAPP,$
P. G. Mc~ORMICK~
and
HUGH
UN THE
YIELD
MUIR?
The effeot of pressurization on tlw subsequent yield behaviour of a low cmrbon steel has been investigated. Measurements of preyield properties, the anelastic limit and the preyield microstrain, were found to be sensitive to the level of pressurization at relatively low pressures, while macroyield measurements were found to be inoreasingly dependent on pressure at higher levels of pressurization. The results indioate that the preyield micro&rain parameters are sensitive to the presence of small numbers of grams containing activated slip soumes, whereas a relatively large fraction of active grains must be present to modify the macroyield behaviour significantly. The effect of quenching on yield behaviour was also investigated. Comparison of the yield properties of quenched and pressurrzed specimens indicates that the quenching process introduces a more homogeneous distribution of slip sources than does pressurization. INFLUENCE DE LA PRESSION ET DE LA TREMPE SUR LE COMPORTEMENT ELASTIQUE DES ACIERS A F-4IBLE TENEUR EN CARBONE L’influence de la pression sur le comportament Qlastique ulterieur d’un aoier 8, faible teneur en oarbone a et6 btudi6e. Les auteurs trouvent que les proprietes mesurees avant la limite elastique, tels que la limite anelastique et la mierod~fo~ation avant la limite Blastique, sont sensibles B la pression pour ies pressions relativement faibles, alors que ies mesures m~ro~l~tiques d~~ndant d’autant plus de la pression que celle-ci est plus &e&e. Les r&ultats montrent que les parametres de microdeformation avant la limite tslastiqnesont aensibles B la presence d’un petit nombre de grains contenant des sources de glissement active, alors qu’une proportion relativement importante de grams actifs doit etre p&se&e pour modifier de faeon signifieative le eompor~ment maczoelastique. L’influence de la trempe sur b compo~eme~t Blastique a 6th Btudiee egalement. La comparaison des proprietes &stiques d’echantillons trempes et pressuris& montre que la trempe introduit tme distribution des souroes de glissement plus homogene que la pressurisation. DER
EINFLUD EINER DRUCK- UND ABSCHRECKBEHANDLUNG AUF FL~E~~R~LTEN VON KO~LENSTOFFARMEM STAHL
DAS
Der Ein%B einer Druok~handlu~ auf das naehfolgende Flie~verhal~n eines kohlensto~-en St&s wurde untersuoht. Messungen der Eigenschaften vor Beginn des Flieaens, der aneiaetisohen Grenze und der Mikrodehnung h&ngen bei relstiv kleinen Rrucken sehr empflndlich vom Druck ab; dagegen zeigt das Makroflief3verhalten bei hiiheren Druoken eine zunehmende Druokabhiingigkeit. Die Ergebnisss deuten daranf hin, faB die ~~i~odehnungsparameter sehr empflndlich auf das Vorhandensein einer kleinen Zahl von Kiirnern mit aktiven Gle~tquellen reagieren; dagegen bedarf es eines ml&iv groDen Anteils aktiver Kiirner, urn das Makroflieljverhalten betraohtlich zu beeinflussen. Der EinfluD des Abschreokens auf das FlieBverhalten wurde ebenfalls untersucht. Ein Vergleich der FlieBeigenschaften abgeschreckter und druckbehandelter Proben zeight, da3 beim Absohreckvorgang eine homogenere Verteilung der Gieitquellen entsteht als bei der Druckbehandlnng. 1. INTRODUCTION
on these effects has been reported. Trester,(s) using a 0.18 %C steel, found that the introduction of mobile dislocations by pressurization resulted in a significantly lower anelastic limit than that measured on prestrained specimens. Worthington,@) made etch pit studies on a 3 % silicon-iron alloy in the preyield region and found that for a given stress, pressurization caused an increase in the number of yielded grains and an increase in the number of slip lines per grain over that observed in unpressurized specimens. In addition slip break-through was found to occur at lower stresses in pressurized samples. This paper describes the effects, on the micro and macroyield behaviour of a low carbon steel, of increasing numbers of mobile dislocations introduced by pressurization. Experimental me~urement~s in which the yield behaviour of pressurized specimens is compared with that of specimens in which mobile dislocations are produced by quen~h~g are also reported.
is well known that the formation of mobile dislocations in iron and other b.c.c. metals by methods such as hy~os~tic presau~zation,(l) quenahi~g(2) or cyclic prestressing@) may significantly modify the subsequent yield behaviour. In iron the injection of mobile dislocations into the matrix at elastic discontinuities, such as second phase particles, by hydrostatic p~ssurization causes both the upper and lower yield stresses to decrease, and at a sufficiently high level of pressurization the discontinuous yield behaviour is eliminated.(@) Direct evidence for the pressure-induced formation of dislocations at second phase particles in iron has been obtained by Radcliffe and Warlimont.(‘) In spite of the fact that measurement of the effects of pressurization on the preyield behaviour should be a sensitive guide to changes in the mobile dislocation density caused by pressurization, lit,tIe work It
* Received May 25, 1972.
2. EXPERIMENTAL
t School of Metallurgy, University of New South Wales, Kensington, N.S.W., Australia. $ Now at: Department of Metallurgy, University of Oxford, Oxford, England, ACTA
METALLURG~CA,
VOL.
21. JANUARY
1973
METHOD
The material used in this study was a commercial low carbon steel containing 0.08% C, 0.44% Mn, 43
ACTA
44
METALLURGICA,
0.04 % P and 0.04% S. Tensile specimens with a gauge length of 1.4 in. and of diameter 0.14 in. were annealed for I hr at 620°C and either slow cooled in vermiculite or quenched into brine. The grain size was 0.018 mm. The specimens were polished chemically subsequent to heat treatment to remove any surface defects. ~essu~zation was accomp~sh~ by suspending the specimens in the bore of a high pressure vessel. Pressures up to 12.0 Kbar were obtained using a Harwood high pressure generator. The pressure was measured by means of a calibrated manganin gauge. Each specimen was held at a pressure within 0.05 Kbar of the value quoted, for a period of approximately 5 min. After pressurization or quenching the specimens were stored for short times at -40°C until the commencement of testing. Tensile testing was conducted using an Instron testing machine. A strain gauge extensometer with a gauge length of 1 in. was used to measure strain. The strain sensitivity was 2 x 10-6. The tensile cycling technique,(ls*li~ was used to determine the anelastic limit oA (co~s~n~ng to the stress required for a plastic strain of 5 x 10”) and the preyield microstrain, E,. The measurements of a, were accurate to -&1.O kg/mm2. Measurements of the upper and lower yield stresses were also made for each level of pressurization. Considerable care was taken to ensure uniaxial loading conditions. 3. RESULTS
The effect of the level of pressurization on the anelastic limit and total preyield microstrain is shown in Figs. 1 and 2 respectively. The preyield microstrains for the unpressurized specimens and specimens pressurized to 4.1 kbar were less than the 2 x 1O-5 sensitivity of the extensometer and the anelastic limits were taken as equal to the upper yield stress
0
2
4
6 P,
8 k
IO
bar
Fro. 1. Effect of pressurization on the anelastic limit.
12
d
VOL.
21,
1973
0 -
3
0
2
4
6
8
IO
12
p, k bar FIG. 2. Et%ct of pressurization on the preyield microstrain
values. For levels of pressurization greater than 4.1 kbar the preyield microstrain increased and a, decreased with increasing pressure. At levels of pressurization approaching that required to secure continuous yielding U~ appeared to approach a minimum value, whereas E, continued to increase rapidly with increasing pressure. Each preyield mi~rostrain value reported is the tot81 preyield microstrain measured when the Liiders band initiated within the gauge length of the extensometer. The total preyield microstrain may be separated into two components : a “homogeneous” component not associated with the Liiders band initiation (Ebb,,,)and a heterogeneous component (E& which is directly associated with the formation of the embryonic Liidem band prior to the yield drop. In some tests, since the extensometer gauge length was less than the specimen gauge length, Liiders band initiation oocurred outside the gauge length of the extensomekr, thus allowing measurement of E,,,_,. In these tests E,,,,,,,was found to be approximately 75 per cent of the total preyield microstrain, irrespective of pressure. The effect of pressurization on the upper and lower yield stress values is shown in Fig. 3. Above 4.1 Kbar buys decreased with increasing pressure. A similar decrease in uLYs occurred after a pressure of 8 Kbar had been exaeeded. Extrapolation of the curves in Fig. 3 indicates that a pressure of 11.8 kbar is required to eliminate the yield drop. Comparison of Figs. 1 and 3 shows that pressurization has a more marked effect on CT,than it has on buys and oLYs. The effect of repeated pressurizations at a given level on the subsequent yield behaviour was also examined. The repeated pressurizations (15 oyoles at 6.2 kbar and 3 cycles at 11 kbar) were not found to produce any effect on the yield parameters different from that measured after one pressure cycle.
CAPP, MCCORMICK
t IO
l
%YS
l
ULYS
MUIR:
AND
t
I1 0
I
2
11
I
4
I
I
I,,
6 P, k bar
8
OF PRESSURE
EFFECT
,
IO
,I 12
Fra. 3. Effect of pressurization on the upper and lower yield stresses.
A comparison of the effects of pressurizing and quenching on the yield behaviour is shown in Table 1. The quenched specimens exhibited a lower anelastic limit, larger preyield microstrain and lower upper yield stress values than the unquenched and unpressurized specimens. However, the lower yield stress remained approximately the s&me. No yield. drop was observed in the quenched specimens. Specimens pressurized at a level which would give the same value of auYS as the quenched specimen, exhibited significantly lower values of ua and 5Lps and higher values of E,. TABLEI Treatment Slow cooled Quenched Slow cooled and pressurized to 11 kbar
(kg$m’)
E,( x 10’)
39.6 22.6 9.5
3.: 15.0
C&l-S
(kTjgrna) (kg/mmz) 39.5 24.0 24.0
24.5 24.0 20.8
4. DISCUSSION 4.1
Effect of pressurization
The marked dependence of the yield behaviour of iron on pressurization is due to the formation of mobile dislocations and dislocation sources at second phase particles in a matrix in which essentially all other dislocations are strongly locked by solute atoms. Models of dislocation nucleation at particles(i2*13)predict that a critical pressure is required to activate inaoherent particles of a given size and compressibility and indicate that the critical pressure increases with deoreasing particle size. With an increase in pressure it is apparent that the dislocation density about active particles of given size will
AND QUENCHING
ON YIELD
45
increase. More importantly, provided there is a distribution of particle sizes, an increase in pressure will give rise to an increase in the number of active particles and, hence, the sources of slip. In polycrystalline specimens the yield behaviour should be dependent on the number of grains which contain aotive particles at a given level of pressurization and on the density of slip sources in the active grains.cq’ Recent studies’**id) have suggested that activated particles located at grain boundaries may have the largest effect on yield behaviour and that pressurization may modify the slip resistance of grain boundary regions. The present results indicate that the preyield ~crostrain parameters 5, and E,, are more sensitive to initial changes in the mobile dislocation density caused by pressurization than the upper and lower yield stress values. Id fine grained, polycrystalline material, such as that used in this study, extensive slip break-through is not expected to occur until stresses near the upper yield stress are reached.(i5) Therefore in the preyield region slip is confined to in~~dual grains aon~i~ng activate particles. On this basis the preyield microstrain for a given level of pressurization should be directly dependent on the number of grains containing active dislocation sources and its variation with the level of pressurization should reflect the particle density and size distribution present. The anelastic limit, at least initially, would also be expected to be a sensitive function of the number of active grains. At high levels of pressurization, near that required to secure continuous yielding, the anelastic limit appears to approach a minimum value. In this region it appears that most grains contain active particlea and that there is no longer any significant shortage of mobile dislocations and dislocation sources. Further increases in the number of dislocations and sources would therefore be expected to have less effect on Us and it is likely that above a critical level aA may increase with further increases in pressure. It may be noted that in continuously yielding materials a, has been found to increase with small amounts of prestrainc17*18)and recent work(l*) on the effects of pressu~zation in a Cu-Sn alloy, conniving a dispersion of Pb particles has shown that the flow stress increases with increasing levels of pressurization. The pressure at which the first detectable changes in 5A and E, occur would be expected to decrease with increasing strain sensitivity and it is possible that, with an increase in strain sensitivity, the change in 5A would occur over a broader pressure range.
ACTA
46
METALLURCICA,
The lower initial sensitivity of cuys and aLys to the level of pressurization appears to be due to the fact that these parameters depend on the propagation of slip through grain boundaries, rather than slip within individual activated grains. In polycrystalline material the upper yield stress is usually identified with the stress required to cause a stress concentration a% the head of an embryonic Liiders band which is su~~iently high to propagate slip into an unfavourably oriented grain. The embryonic Liiders band may exist in just one grain or a cluster of yielding grains, depending on experimental conditions. The local stress concentration increases as the square root of the embryo diameter.(18) It appears that the primary effect of pressu~zation on cuys is a~oeia~ with changes in the size of the Liiders embryo. In unpressurized specimens the critical embryo consists of essentially one grain. However, the increase in the number of active grains with pressure, particularly at higher levels of pressure where it is probable that some adjacent grains are activated, will cause s,n increase in the size of the Uiders embryo and the local stress concentration thus decreasing the stress required to propagate slip. The lower yield stress also becomes increasingly dependent on pressure at high levels of pressurization, where the number of active grains encountered by the Liiders band front becomes significant. Measurements of the heterogeneous preyield microstrain also indicate that the size of the emb~onic Liiders band at cruys increases with pressure. The heterogeneous preyield microstrain may be expressed in terms of the number of grains comprising the critical cluster size, N,, as: N.
&E &het= NT
where sE is the effective strain in the Liiders embryo and NT is the total number of grains contained in the gauge length of the extensometer. With increasing pressure, chet was found to increase from 1215x 10e6 at 6.5”K bar to ~5 x lo4 at 11.2 kbar. Although the increase in whetwith level of pressurization may be due to an increase in Ni or E,,,_~or both, it would seem that the increase in the number of active grains with pressure would have the greatest effect in increasing whetby increasing Ni. The absence of any effect of repeated pressurization on the yield parameters is in agreement with the work of Bullen et uZ.(Ql) For a single pressurization, dislocation tangles are observed at activated particles.‘7) It is thought that the back stresses associated with these tangles prevent the formation of any new
VOL.
21,
1973
dislocations on re-pressurizing.c4~21) However, it is also possible that the failure of repeated pressurization to modify the yield behaviour is simply due to the fact that no new particles are activated (i.e. acquire free dislocations) on re-pressurization. This interpretation suggests that the yield behaviour is determined by the number of activated slip sources and that the number of dislocations formed about an activated particle is of little consequence. 4.2 Effect of quenching The differences in yield behaviour exhibit.ed by the quenched and the pressurized specimens cannot be reconciled simply in terms of different densities of mobile ~sIo~ations pr~uc~ by the two treatment. Rather, it appears that the differences in the yield behaviour relate to differences in the distributions of mobile dislocations and slip sources. In pressurized specimens the activated slip sources are present in a limited number of grains, the number of active grains increasing with the level of pressurization. However, in quenched specimens the dislocation source distribution should be much more uniform, with mobile or lightly pinned dislocations being present in all grains. Whether mobile or lightly pinned dislocations are secured by quenching is not well understood. The high anelastic limit exhibited by the quenched specimens suggests that yielding initiates by the unpinning of lightly pinned dislocations in favourably oriented grains. Yield propagation then occurs by further unlocking in grains with progressively less favourable orientations, leading to the formation of a diffuse Liiders front, with little or no load drop. When differences in stress concentration are taken into account, it is not surprising that the Liiders bands for quenched and slow cooled specimens propagate under approximately the flame applied stress. ACKNOWLEDGEMENT
The authors would like to express their appreciation of support of the Australian Research Grants Committee. REFERENCES
RADCLIFFE, Mechanical BehwimcT of .kfc&ri& under Presswe, edited by H. LL. D. PUQH, Elsevier (1970). A. H. CO~RELL and R. M. FISHER,in Sympotziumon the .Relat&m bawls Strwtwe md M~~n~u~ Proper&e of Met& H.M.S.O. (1963). A. ABEL and H. MUIR, Acta Met. in peas. F. P. BULLEN, F. HENDERSOK M. M. HUTCRISON and H. L. WAIN, Phil. Mag. 9,285 (1964). S. V. RADCLIFFE,Irreversible Effects of High Preseure and Tempereture 0% MatertiEs. ASTM (1964). M. YAJIMA and M. ISHII, Acta Met. 15, 651 (1967). S. V. RADCLIFFE and. H. WABLIMONT, Phye. Status Sol&%1,64 (1964). P. TRESTEX,M. S. Thesis Case IF&. of Technologg (1967). P.J. W~RTE~~TO~, ~~iZ.~ug.l8,663 (1969}.
1. S. V. 2. 3. 4. 6. 6. 7. 3. 9.
CAPP,
MCCORMICK
AND
MUIR:
EFFECT
OF PRESSURE
10. H. MUIR, B. L. AVERBACH and M. ~OEEN, Tmwm Am. sec. Metals 47, 340 (1956). 11. J. M. ROBERTS and N. BROWX, Tmm. Am. Inst. Min. Engr8 218,454 (1960). 12. M. F. ASEBY, S. H. GELLES and L. E. TANNER, Phil. Mag. 19,757 (1969). 13. G. DAS and S. V. RADCLIFFE, Phil, Mag. 20, 589 (1969). 14. P. J. WORTHINQTON, Scripta Met. S, 895 (1969). 15. P. J. WORTHINGTON, Acta Met. 16, 1795 (1967).
AND
QUENCHING
OS
YIELD
47
16. S. V. RADCLIFFE, Scripta Met. 3, 59 (1969). 17. A. R. ROSENFIELD and B. L. AVERBACH, Acta Net. 10,i'l (1962). 18. W. BONFIELD and C, H. LI, Acta Met. 13,317 (1965). 19. M. M. HUTCHISON and R. T. PASCOE, Metal&i. J. 4, 177 (1970). 20. D. W. Moow and T. VREELAND, Acta Met. 17,989 (1968). 21. F. RI. C. BESAG andF. P. BULLES, Phil. Mag. 12,41 (1965).
EFFECT
OF PRESSURIZATION AND QUENCHING BEHAVIOUR IN LOW CARBON STEEL* D. J. CAPP,$
P. G. Mc~ORMICK~
and
HUGH
UN THE
YIELD
MUIR?
The effeot of pressurization on tlw subsequent yield behaviour of a low cmrbon steel has been investigated. Measurements of preyield properties, the anelastic limit and the preyield microstrain, were found to be sensitive to the level of pressurization at relatively low pressures, while macroyield measurements were found to be inoreasingly dependent on pressure at higher levels of pressurization. The results indioate that the preyield micro&rain parameters are sensitive to the presence of small numbers of grams containing activated slip soumes, whereas a relatively large fraction of active grains must be present to modify the macroyield behaviour significantly. The effect of quenching on yield behaviour was also investigated. Comparison of the yield properties of quenched and pressurrzed specimens indicates that the quenching process introduces a more homogeneous distribution of slip sources than does pressurization. INFLUENCE DE LA PRESSION ET DE LA TREMPE SUR LE COMPORTEMENT ELASTIQUE DES ACIERS A F-4IBLE TENEUR EN CARBONE L’influence de la pression sur le comportament Qlastique ulterieur d’un aoier 8, faible teneur en oarbone a et6 btudi6e. Les auteurs trouvent que les proprietes mesurees avant la limite elastique, tels que la limite anelastique et la mierod~fo~ation avant la limite Blastique, sont sensibles B la pression pour ies pressions relativement faibles, alors que ies mesures m~ro~l~tiques d~~ndant d’autant plus de la pression que celle-ci est plus &e&e. Les r&ultats montrent que les parametres de microdeformation avant la limite tslastiqnesont aensibles B la presence d’un petit nombre de grains contenant des sources de glissement active, alors qu’une proportion relativement importante de grams actifs doit etre p&se&e pour modifier de faeon signifieative le eompor~ment maczoelastique. L’influence de la trempe sur b compo~eme~t Blastique a 6th Btudiee egalement. La comparaison des proprietes &stiques d’echantillons trempes et pressuris& montre que la trempe introduit tme distribution des souroes de glissement plus homogene que la pressurisation. DER
EINFLUD EINER DRUCK- UND ABSCHRECKBEHANDLUNG AUF FL~E~~R~LTEN VON KO~LENSTOFFARMEM STAHL
DAS
Der Ein%B einer Druok~handlu~ auf das naehfolgende Flie~verhal~n eines kohlensto~-en St&s wurde untersuoht. Messungen der Eigenschaften vor Beginn des Flieaens, der aneiaetisohen Grenze und der Mikrodehnung h&ngen bei relstiv kleinen Rrucken sehr empflndlich vom Druck ab; dagegen zeigt das Makroflief3verhalten bei hiiheren Druoken eine zunehmende Druokabhiingigkeit. Die Ergebnisss deuten daranf hin, faB die ~~i~odehnungsparameter sehr empflndlich auf das Vorhandensein einer kleinen Zahl von Kiirnern mit aktiven Gle~tquellen reagieren; dagegen bedarf es eines ml&iv groDen Anteils aktiver Kiirner, urn das Makroflieljverhalten betraohtlich zu beeinflussen. Der EinfluD des Abschreokens auf das FlieBverhalten wurde ebenfalls untersucht. Ein Vergleich der FlieBeigenschaften abgeschreckter und druckbehandelter Proben zeight, da3 beim Absohreckvorgang eine homogenere Verteilung der Gieitquellen entsteht als bei der Druckbehandlnng. 1. INTRODUCTION
on these effects has been reported. Trester,(s) using a 0.18 %C steel, found that the introduction of mobile dislocations by pressurization resulted in a significantly lower anelastic limit than that measured on prestrained specimens. Worthington,@) made etch pit studies on a 3 % silicon-iron alloy in the preyield region and found that for a given stress, pressurization caused an increase in the number of yielded grains and an increase in the number of slip lines per grain over that observed in unpressurized specimens. In addition slip break-through was found to occur at lower stresses in pressurized samples. This paper describes the effects, on the micro and macroyield behaviour of a low carbon steel, of increasing numbers of mobile dislocations introduced by pressurization. Experimental me~urement~s in which the yield behaviour of pressurized specimens is compared with that of specimens in which mobile dislocations are produced by quen~h~g are also reported.
is well known that the formation of mobile dislocations in iron and other b.c.c. metals by methods such as hy~os~tic presau~zation,(l) quenahi~g(2) or cyclic prestressing@) may significantly modify the subsequent yield behaviour. In iron the injection of mobile dislocations into the matrix at elastic discontinuities, such as second phase particles, by hydrostatic p~ssurization causes both the upper and lower yield stresses to decrease, and at a sufficiently high level of pressurization the discontinuous yield behaviour is eliminated.(@) Direct evidence for the pressure-induced formation of dislocations at second phase particles in iron has been obtained by Radcliffe and Warlimont.(‘) In spite of the fact that measurement of the effects of pressurization on the preyield behaviour should be a sensitive guide to changes in the mobile dislocation density caused by pressurization, lit,tIe work It
* Received May 25, 1972.
2. EXPERIMENTAL
t School of Metallurgy, University of New South Wales, Kensington, N.S.W., Australia. $ Now at: Department of Metallurgy, University of Oxford, Oxford, England, ACTA
METALLURG~CA,
VOL.
21. JANUARY
1973
METHOD
The material used in this study was a commercial low carbon steel containing 0.08% C, 0.44% Mn, 43
ACTA
44
METALLURGICA,
0.04 % P and 0.04% S. Tensile specimens with a gauge length of 1.4 in. and of diameter 0.14 in. were annealed for I hr at 620°C and either slow cooled in vermiculite or quenched into brine. The grain size was 0.018 mm. The specimens were polished chemically subsequent to heat treatment to remove any surface defects. ~essu~zation was accomp~sh~ by suspending the specimens in the bore of a high pressure vessel. Pressures up to 12.0 Kbar were obtained using a Harwood high pressure generator. The pressure was measured by means of a calibrated manganin gauge. Each specimen was held at a pressure within 0.05 Kbar of the value quoted, for a period of approximately 5 min. After pressurization or quenching the specimens were stored for short times at -40°C until the commencement of testing. Tensile testing was conducted using an Instron testing machine. A strain gauge extensometer with a gauge length of 1 in. was used to measure strain. The strain sensitivity was 2 x 10-6. The tensile cycling technique,(ls*li~ was used to determine the anelastic limit oA (co~s~n~ng to the stress required for a plastic strain of 5 x 10”) and the preyield microstrain, E,. The measurements of a, were accurate to -&1.O kg/mm2. Measurements of the upper and lower yield stresses were also made for each level of pressurization. Considerable care was taken to ensure uniaxial loading conditions. 3. RESULTS
The effect of the level of pressurization on the anelastic limit and total preyield microstrain is shown in Figs. 1 and 2 respectively. The preyield microstrains for the unpressurized specimens and specimens pressurized to 4.1 kbar were less than the 2 x 1O-5 sensitivity of the extensometer and the anelastic limits were taken as equal to the upper yield stress
0
2
4
6 P,
8 k
IO
bar
Fro. 1. Effect of pressurization on the anelastic limit.
12
d
VOL.
21,
1973
0 -
3
0
2
4
6
8
IO
12
p, k bar FIG. 2. Et%ct of pressurization on the preyield microstrain
values. For levels of pressurization greater than 4.1 kbar the preyield microstrain increased and a, decreased with increasing pressure. At levels of pressurization approaching that required to secure continuous yielding U~ appeared to approach a minimum value, whereas E, continued to increase rapidly with increasing pressure. Each preyield mi~rostrain value reported is the tot81 preyield microstrain measured when the Liiders band initiated within the gauge length of the extensometer. The total preyield microstrain may be separated into two components : a “homogeneous” component not associated with the Liiders band initiation (Ebb,,,)and a heterogeneous component (E& which is directly associated with the formation of the embryonic Liidem band prior to the yield drop. In some tests, since the extensometer gauge length was less than the specimen gauge length, Liiders band initiation oocurred outside the gauge length of the extensomekr, thus allowing measurement of E,,,_,. In these tests E,,,,,,,was found to be approximately 75 per cent of the total preyield microstrain, irrespective of pressure. The effect of pressurization on the upper and lower yield stress values is shown in Fig. 3. Above 4.1 Kbar buys decreased with increasing pressure. A similar decrease in uLYs occurred after a pressure of 8 Kbar had been exaeeded. Extrapolation of the curves in Fig. 3 indicates that a pressure of 11.8 kbar is required to eliminate the yield drop. Comparison of Figs. 1 and 3 shows that pressurization has a more marked effect on CT,than it has on buys and oLYs. The effect of repeated pressurizations at a given level on the subsequent yield behaviour was also examined. The repeated pressurizations (15 oyoles at 6.2 kbar and 3 cycles at 11 kbar) were not found to produce any effect on the yield parameters different from that measured after one pressure cycle.
CAPP, MCCORMICK
t IO
l
%YS
l
ULYS
MUIR:
AND
t
I1 0
I
2
11
I
4
I
I
I,,
6 P, k bar
8
OF PRESSURE
EFFECT
,
IO
,I 12
Fra. 3. Effect of pressurization on the upper and lower yield stresses.
A comparison of the effects of pressurizing and quenching on the yield behaviour is shown in Table 1. The quenched specimens exhibited a lower anelastic limit, larger preyield microstrain and lower upper yield stress values than the unquenched and unpressurized specimens. However, the lower yield stress remained approximately the s&me. No yield. drop was observed in the quenched specimens. Specimens pressurized at a level which would give the same value of auYS as the quenched specimen, exhibited significantly lower values of ua and 5Lps and higher values of E,. TABLEI Treatment Slow cooled Quenched Slow cooled and pressurized to 11 kbar
(kg$m’)
E,( x 10’)
39.6 22.6 9.5
3.: 15.0
C&l-S
(kTjgrna) (kg/mmz) 39.5 24.0 24.0
24.5 24.0 20.8
4. DISCUSSION 4.1
Effect of pressurization
The marked dependence of the yield behaviour of iron on pressurization is due to the formation of mobile dislocations and dislocation sources at second phase particles in a matrix in which essentially all other dislocations are strongly locked by solute atoms. Models of dislocation nucleation at particles(i2*13)predict that a critical pressure is required to activate inaoherent particles of a given size and compressibility and indicate that the critical pressure increases with deoreasing particle size. With an increase in pressure it is apparent that the dislocation density about active particles of given size will
AND QUENCHING
ON YIELD
45
increase. More importantly, provided there is a distribution of particle sizes, an increase in pressure will give rise to an increase in the number of active particles and, hence, the sources of slip. In polycrystalline specimens the yield behaviour should be dependent on the number of grains which contain aotive particles at a given level of pressurization and on the density of slip sources in the active grains.cq’ Recent studies’**id) have suggested that activated particles located at grain boundaries may have the largest effect on yield behaviour and that pressurization may modify the slip resistance of grain boundary regions. The present results indicate that the preyield ~crostrain parameters 5, and E,, are more sensitive to initial changes in the mobile dislocation density caused by pressurization than the upper and lower yield stress values. Id fine grained, polycrystalline material, such as that used in this study, extensive slip break-through is not expected to occur until stresses near the upper yield stress are reached.(i5) Therefore in the preyield region slip is confined to in~~dual grains aon~i~ng activate particles. On this basis the preyield microstrain for a given level of pressurization should be directly dependent on the number of grains containing active dislocation sources and its variation with the level of pressurization should reflect the particle density and size distribution present. The anelastic limit, at least initially, would also be expected to be a sensitive function of the number of active grains. At high levels of pressurization, near that required to secure continuous yielding, the anelastic limit appears to approach a minimum value. In this region it appears that most grains contain active particlea and that there is no longer any significant shortage of mobile dislocations and dislocation sources. Further increases in the number of dislocations and sources would therefore be expected to have less effect on Us and it is likely that above a critical level aA may increase with further increases in pressure. It may be noted that in continuously yielding materials a, has been found to increase with small amounts of prestrainc17*18)and recent work(l*) on the effects of pressu~zation in a Cu-Sn alloy, conniving a dispersion of Pb particles has shown that the flow stress increases with increasing levels of pressurization. The pressure at which the first detectable changes in 5A and E, occur would be expected to decrease with increasing strain sensitivity and it is possible that, with an increase in strain sensitivity, the change in 5A would occur over a broader pressure range.
ACTA
46
METALLURCICA,
The lower initial sensitivity of cuys and aLys to the level of pressurization appears to be due to the fact that these parameters depend on the propagation of slip through grain boundaries, rather than slip within individual activated grains. In polycrystalline material the upper yield stress is usually identified with the stress required to cause a stress concentration a% the head of an embryonic Liiders band which is su~~iently high to propagate slip into an unfavourably oriented grain. The embryonic Liiders band may exist in just one grain or a cluster of yielding grains, depending on experimental conditions. The local stress concentration increases as the square root of the embryo diameter.(18) It appears that the primary effect of pressu~zation on cuys is a~oeia~ with changes in the size of the Liiders embryo. In unpressurized specimens the critical embryo consists of essentially one grain. However, the increase in the number of active grains with pressure, particularly at higher levels of pressure where it is probable that some adjacent grains are activated, will cause s,n increase in the size of the Uiders embryo and the local stress concentration thus decreasing the stress required to propagate slip. The lower yield stress also becomes increasingly dependent on pressure at high levels of pressurization, where the number of active grains encountered by the Liiders band front becomes significant. Measurements of the heterogeneous preyield microstrain also indicate that the size of the emb~onic Liiders band at cruys increases with pressure. The heterogeneous preyield microstrain may be expressed in terms of the number of grains comprising the critical cluster size, N,, as: N.
&E &het= NT
where sE is the effective strain in the Liiders embryo and NT is the total number of grains contained in the gauge length of the extensometer. With increasing pressure, chet was found to increase from 1215x 10e6 at 6.5”K bar to ~5 x lo4 at 11.2 kbar. Although the increase in whetwith level of pressurization may be due to an increase in Ni or E,,,_~or both, it would seem that the increase in the number of active grains with pressure would have the greatest effect in increasing whetby increasing Ni. The absence of any effect of repeated pressurization on the yield parameters is in agreement with the work of Bullen et uZ.(Ql) For a single pressurization, dislocation tangles are observed at activated particles.‘7) It is thought that the back stresses associated with these tangles prevent the formation of any new
VOL.
21,
1973
dislocations on re-pressurizing.c4~21) However, it is also possible that the failure of repeated pressurization to modify the yield behaviour is simply due to the fact that no new particles are activated (i.e. acquire free dislocations) on re-pressurization. This interpretation suggests that the yield behaviour is determined by the number of activated slip sources and that the number of dislocations formed about an activated particle is of little consequence. 4.2 Effect of quenching The differences in yield behaviour exhibit.ed by the quenched and the pressurized specimens cannot be reconciled simply in terms of different densities of mobile ~sIo~ations pr~uc~ by the two treatment. Rather, it appears that the differences in the yield behaviour relate to differences in the distributions of mobile dislocations and slip sources. In pressurized specimens the activated slip sources are present in a limited number of grains, the number of active grains increasing with the level of pressurization. However, in quenched specimens the dislocation source distribution should be much more uniform, with mobile or lightly pinned dislocations being present in all grains. Whether mobile or lightly pinned dislocations are secured by quenching is not well understood. The high anelastic limit exhibited by the quenched specimens suggests that yielding initiates by the unpinning of lightly pinned dislocations in favourably oriented grains. Yield propagation then occurs by further unlocking in grains with progressively less favourable orientations, leading to the formation of a diffuse Liiders front, with little or no load drop. When differences in stress concentration are taken into account, it is not surprising that the Liiders bands for quenched and slow cooled specimens propagate under approximately the flame applied stress. ACKNOWLEDGEMENT
The authors would like to express their appreciation of support of the Australian Research Grants Committee. REFERENCES
RADCLIFFE, Mechanical BehwimcT of .kfc&ri& under Presswe, edited by H. LL. D. PUQH, Elsevier (1970). A. H. CO~RELL and R. M. FISHER,in Sympotziumon the .Relat&m bawls Strwtwe md M~~n~u~ Proper&e of Met& H.M.S.O. (1963). A. ABEL and H. MUIR, Acta Met. in peas. F. P. BULLEN, F. HENDERSOK M. M. HUTCRISON and H. L. WAIN, Phil. Mag. 9,285 (1964). S. V. RADCLIFFE,Irreversible Effects of High Preseure and Tempereture 0% MatertiEs. ASTM (1964). M. YAJIMA and M. ISHII, Acta Met. 15, 651 (1967). S. V. RADCLIFFE and. H. WABLIMONT, Phye. Status Sol&%1,64 (1964). P. TRESTEX,M. S. Thesis Case IF&. of Technologg (1967). P.J. W~RTE~~TO~, ~~iZ.~ug.l8,663 (1969}.
1. S. V. 2. 3. 4. 6. 6. 7. 3. 9.
CAPP,
MCCORMICK
AND
MUIR:
EFFECT
OF PRESSURE
10. H. MUIR, B. L. AVERBACH and M. ~OEEN, Tmwm Am. sec. Metals 47, 340 (1956). 11. J. M. ROBERTS and N. BROWX, Tmm. Am. Inst. Min. Engr8 218,454 (1960). 12. M. F. ASEBY, S. H. GELLES and L. E. TANNER, Phil. Mag. 19,757 (1969). 13. G. DAS and S. V. RADCLIFFE, Phil, Mag. 20, 589 (1969). 14. P. J. WORTHINQTON, Scripta Met. S, 895 (1969). 15. P. J. WORTHINGTON, Acta Met. 16, 1795 (1967).
AND
QUENCHING
OS
YIELD
47
16. S. V. RADCLIFFE, Scripta Met. 3, 59 (1969). 17. A. R. ROSENFIELD and B. L. AVERBACH, Acta Net. 10,i'l (1962). 18. W. BONFIELD and C, H. LI, Acta Met. 13,317 (1965). 19. M. M. HUTCHISON and R. T. PASCOE, Metal&i. J. 4, 177 (1970). 20. D. W. Moow and T. VREELAND, Acta Met. 17,989 (1968). 21. F. RI. C. BESAG andF. P. BULLES, Phil. Mag. 12,41 (1965).