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Irreversible effect of hydrostatic pressurization on yielding in niobium
Scripta
METALLURGICA
Vol. 9, pp. 1351-1353, 1975 Printed in the United States
Pergamon Press, Inc,
IRREVERSIBLE EFFECT OF HYDROSTATIC PRESSURIZATION ON YIELDING IN NIOBIUM D. Aichbhaumik and W. L. Haworth Department of Chemical Engineering and Material Sciences College of Engineering _ Wayne State University, Detroit, Michigan (Received October 4, 1975)
Irreversible effects of hydrostatic pressure on the mechanical properties of impure B.C.C. metals have been reported for iron (1,2,3,4), steel (5), silicon-iron (6), chromium (7,8,9) and molybdenum (10). These studies show that prior hydrostatic pressure may suppress the yield drop, reduce the yield stress, and decrease the ductile brittle transition temperature on subsequent mechanical testing at atmospheric pressure. These phenomena are generally attributed to the generation of mobile dislocations at internal elastic discontinuities such as particle-matrix interfaces due to stresses developed by differential compression of the particle and the matrix during hydrostatic pressurization. Conclusive evidence of dislocation generation at particle-matrix interfaces due to pressurization has been obtained by electron microscopy in the case of iron (11) and chromium (8,12,13). In this communication we present some recent results on the irreversible effect of hydrostatic pressurization on the mechanical behavior of polycrystalline niobium, and compare these results with those previously obtained for iron, steel, and chromium (which have elastic properties comparable to niobium). Test specimens in the form of right circular cylinders approximately 10 mm long and 5 mm in diameter were machined from cold rolled polycrystalline niobium rod (Materials Research Corporation VP grade), and annealed in dry argon at Interstitial impurities in 1000°C to produce a mean grain intercept of 20um. wt. ppm after annealing were: C 90 ppm, N 40 ppm, H 44-51 ppm and 0 350-660 ppm. The specimens were then electropolished, and pressurized hydrostatically for 10 minutes at pressures up to 1000 MPa before testing at atmospheric pressure in uniaxial compression at a strain rate of 2 x low3 set-' at temperatures between 80°K and 298°K. Load-displacement curves for the compression tests were recorded autographically. For prior pressures up to and including 3.45 MPa, there is no detectable change in the yield stress (Figs. 1 and 3). Prior pressurization at 6.9 MPa eliminates the yield drop and sharp yield point at room temperature (Fig. 1) and reduces the yield stress at all temperatures (Figs. 2 and 3). Pressurization at higher pressures causes a correspondingly greater decrease in yield stress (Figs. 2,3, and 4). The fractional change in yield stress for a given prior pressure is greater at lower temperatures (Fig. 3). The mechanical behavior of pre-pressurized niobium differs from that of other B.C.C. metals like iron and chromium mainly in that the critical pressure to induce an irreversible effect on mechanical properties is about two orders of magnitude lower than those previously observed (1,5,8,9,13). The effects of prior pressurization in niobium (i.e. reduction of yield strength and elimination of yield drop) can be understood qualitatively in terms 1351
1352
YIELDING IN NIOBIUM
Vol. 9, No. 12
FIG.l. LOAD-DISPLACEMENT CURVES FOR NIOBIUM IN COMPRESSION AT ROOM TEMPERATURE. Prior hydrostatic pressure is indicated.
3. LOWER YIELD STRESS OF NIOBIUM VERSUS TEMPERATURE FOR INDICATED PRIOR PRESSURES. Data for parts (a), (b) and (c) is for groups of samples annealed in different batches.
FIG.2. LOAD-DISPLACEMENT CURVES FOR NIOBIUM IN COMPRESSION AT INDICATED TEMPERATURES. Prior hydrostatic pressure is indicated in MPa.
I
I"
sTe=.YI-*, n?.
‘00
FIG. 4. VARIATION OF LOWER YIELD STRESS OF NIOBIUM WITH PRIOR HYDROSTATIC PRESSURE. Stresses are normalized to the unpressurized lower yield stress as unity. Data is for room temperature.
1000
Vol. 9, No. 12
YIELDING IN NIOBIUM
1353
of the generation of free dislocations during pressurization. However, when the results of this study are corn ared with those for other B.C.C. metals, it is evident that either the critica P pressure for dislocation generation at internal interfaces in iron and chromium is about two orders of magnitude higher than in niobium, or that dislocation generation is irreversible at relatively low pressures only in the case of niobium - that is, dislocations generated in iron or chromium run back to their sources when the hydrostatic pressure is reduced. There is no evidence to support the second of these hypotheses. Ashby and Johnson (14) have shown that a size effect can be expected for dislocation generation at incoherent or semicoherent particle-matrix interfaces, the critical pressure being lower for large particles. There is no evidence, however, that the particle size distribution in our niobium samples differs radically from that of earlier studies on other materials. However, the presence of faceted particles could reduce the critical pressure for dislocation generation substantially because of stress concentrations along facet edges. We have observed faceted particles (probably oxides) up to 5pm in diameter in our niobium samples; these particles may account for the low critical pressure necessary for the mechanical property changes that we observe in niobium. ACKNOWLEDGEMENT: We are grateful to Wayne State.University for a Research Grant-in-Aid which helped us to initiate this study, and to the Dow Chemical Corporation for donating high pressure equipment. REFERENCES. 1. F. P. Bullen, F. Henderson, M. M. Hutchinson and H. L. Wain, Phil. Mag. 2, 285 (1964). 2.
S. V. Radcliffe, "Effect of High Pressure and Temperature on the Mechanical Properties of Metals and Alloys", International Conference on Materials, Feb. 1964. American Society for Testinz Materials. Philadelphia 1965.
3. M. Yajima and M. Ishii, Trans. Iron. Steel. Inst. Japan 7, 45 (1967). 4.
A. Oguchi, S. Yoshida and M. Nobuki, Trans. Japan. Inst. Met. 13, 63 (1972).
5. D. J. Capp, P. G. McCormick and H. Muir, Acta. Met. z, 43 (1973). 6.
P. J. Worthington, Phil. Mag. 19, 663 (1969).
7.
F. P. Bullen, F. Henderson, H. L. Wain and M. S. Paterson, Phil. Mag. 2, 803 (1964).
8.
A. Ball and F. P. Bullen, Phil. Mag. 11, 301 (1970).
9.
H. G. Mellor and A. S. Wronski, Acta. Met. Is, 765 (1970).
10.
H. G. Mellor and A. S. Wronski, Ocean. Eng . 1, 235 (1969).
11.
S. V. Radcliffe and H. Warlimont, Phys. Stat. Solid 7, K67 (1964).
12.
R. I. Garrod and H. L. Wain, J. Less-Common Metals 2. 81 (1965).
13.
A. Ball, F. P. Bullen and H. L. Wain, Mat. Science Eng . 2, 28'3 (1969).
14.
M. F. Ashby and L. Johnson, Phil. Mag. 20, 1009 (1969).
METALLURGICA
Vol. 9, pp. 1351-1353, 1975 Printed in the United States
Pergamon Press, Inc,
IRREVERSIBLE EFFECT OF HYDROSTATIC PRESSURIZATION ON YIELDING IN NIOBIUM D. Aichbhaumik and W. L. Haworth Department of Chemical Engineering and Material Sciences College of Engineering _ Wayne State University, Detroit, Michigan (Received October 4, 1975)
Irreversible effects of hydrostatic pressure on the mechanical properties of impure B.C.C. metals have been reported for iron (1,2,3,4), steel (5), silicon-iron (6), chromium (7,8,9) and molybdenum (10). These studies show that prior hydrostatic pressure may suppress the yield drop, reduce the yield stress, and decrease the ductile brittle transition temperature on subsequent mechanical testing at atmospheric pressure. These phenomena are generally attributed to the generation of mobile dislocations at internal elastic discontinuities such as particle-matrix interfaces due to stresses developed by differential compression of the particle and the matrix during hydrostatic pressurization. Conclusive evidence of dislocation generation at particle-matrix interfaces due to pressurization has been obtained by electron microscopy in the case of iron (11) and chromium (8,12,13). In this communication we present some recent results on the irreversible effect of hydrostatic pressurization on the mechanical behavior of polycrystalline niobium, and compare these results with those previously obtained for iron, steel, and chromium (which have elastic properties comparable to niobium). Test specimens in the form of right circular cylinders approximately 10 mm long and 5 mm in diameter were machined from cold rolled polycrystalline niobium rod (Materials Research Corporation VP grade), and annealed in dry argon at Interstitial impurities in 1000°C to produce a mean grain intercept of 20um. wt. ppm after annealing were: C 90 ppm, N 40 ppm, H 44-51 ppm and 0 350-660 ppm. The specimens were then electropolished, and pressurized hydrostatically for 10 minutes at pressures up to 1000 MPa before testing at atmospheric pressure in uniaxial compression at a strain rate of 2 x low3 set-' at temperatures between 80°K and 298°K. Load-displacement curves for the compression tests were recorded autographically. For prior pressures up to and including 3.45 MPa, there is no detectable change in the yield stress (Figs. 1 and 3). Prior pressurization at 6.9 MPa eliminates the yield drop and sharp yield point at room temperature (Fig. 1) and reduces the yield stress at all temperatures (Figs. 2 and 3). Pressurization at higher pressures causes a correspondingly greater decrease in yield stress (Figs. 2,3, and 4). The fractional change in yield stress for a given prior pressure is greater at lower temperatures (Fig. 3). The mechanical behavior of pre-pressurized niobium differs from that of other B.C.C. metals like iron and chromium mainly in that the critical pressure to induce an irreversible effect on mechanical properties is about two orders of magnitude lower than those previously observed (1,5,8,9,13). The effects of prior pressurization in niobium (i.e. reduction of yield strength and elimination of yield drop) can be understood qualitatively in terms 1351
1352
YIELDING IN NIOBIUM
Vol. 9, No. 12
FIG.l. LOAD-DISPLACEMENT CURVES FOR NIOBIUM IN COMPRESSION AT ROOM TEMPERATURE. Prior hydrostatic pressure is indicated.
3. LOWER YIELD STRESS OF NIOBIUM VERSUS TEMPERATURE FOR INDICATED PRIOR PRESSURES. Data for parts (a), (b) and (c) is for groups of samples annealed in different batches.
FIG.2. LOAD-DISPLACEMENT CURVES FOR NIOBIUM IN COMPRESSION AT INDICATED TEMPERATURES. Prior hydrostatic pressure is indicated in MPa.
I
I"
sTe=.YI-*, n?.
‘00
FIG. 4. VARIATION OF LOWER YIELD STRESS OF NIOBIUM WITH PRIOR HYDROSTATIC PRESSURE. Stresses are normalized to the unpressurized lower yield stress as unity. Data is for room temperature.
1000
Vol. 9, No. 12
YIELDING IN NIOBIUM
1353
of the generation of free dislocations during pressurization. However, when the results of this study are corn ared with those for other B.C.C. metals, it is evident that either the critica P pressure for dislocation generation at internal interfaces in iron and chromium is about two orders of magnitude higher than in niobium, or that dislocation generation is irreversible at relatively low pressures only in the case of niobium - that is, dislocations generated in iron or chromium run back to their sources when the hydrostatic pressure is reduced. There is no evidence to support the second of these hypotheses. Ashby and Johnson (14) have shown that a size effect can be expected for dislocation generation at incoherent or semicoherent particle-matrix interfaces, the critical pressure being lower for large particles. There is no evidence, however, that the particle size distribution in our niobium samples differs radically from that of earlier studies on other materials. However, the presence of faceted particles could reduce the critical pressure for dislocation generation substantially because of stress concentrations along facet edges. We have observed faceted particles (probably oxides) up to 5pm in diameter in our niobium samples; these particles may account for the low critical pressure necessary for the mechanical property changes that we observe in niobium. ACKNOWLEDGEMENT: We are grateful to Wayne State.University for a Research Grant-in-Aid which helped us to initiate this study, and to the Dow Chemical Corporation for donating high pressure equipment. REFERENCES. 1. F. P. Bullen, F. Henderson, M. M. Hutchinson and H. L. Wain, Phil. Mag. 2, 285 (1964). 2.
S. V. Radcliffe, "Effect of High Pressure and Temperature on the Mechanical Properties of Metals and Alloys", International Conference on Materials, Feb. 1964. American Society for Testinz Materials. Philadelphia 1965.
3. M. Yajima and M. Ishii, Trans. Iron. Steel. Inst. Japan 7, 45 (1967). 4.
A. Oguchi, S. Yoshida and M. Nobuki, Trans. Japan. Inst. Met. 13, 63 (1972).
5. D. J. Capp, P. G. McCormick and H. Muir, Acta. Met. z, 43 (1973). 6.
P. J. Worthington, Phil. Mag. 19, 663 (1969).
7.
F. P. Bullen, F. Henderson, H. L. Wain and M. S. Paterson, Phil. Mag. 2, 803 (1964).
8.
A. Ball and F. P. Bullen, Phil. Mag. 11, 301 (1970).
9.
H. G. Mellor and A. S. Wronski, Acta. Met. Is, 765 (1970).
10.
H. G. Mellor and A. S. Wronski, Ocean. Eng . 1, 235 (1969).
11.
S. V. Radcliffe and H. Warlimont, Phys. Stat. Solid 7, K67 (1964).
12.
R. I. Garrod and H. L. Wain, J. Less-Common Metals 2. 81 (1965).
13.
A. Ball, F. P. Bullen and H. L. Wain, Mat. Science Eng . 2, 28'3 (1969).
14.
M. F. Ashby and L. Johnson, Phil. Mag. 20, 1009 (1969).