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THE EFFECT OF QUENCHING RATE ON THE TEMPERATURE AND STRAIN-RATE SENSITIVITY
OF URANIUM 2w/o MOLYBDENUM ALLOY
G. Boyd, J. Harding, P. Bleasdale, K. Dunn, G. Turner
To cite this version:
G. Boyd, J. Harding, P. Bleasdale, K. Dunn, G. Turner. THE EFFECT OF QUENCH- ING RATE ON THE TEMPERATURE AND STRAIN-RATE SENSITIVITY OF URANIUM 2w/o MOLYBDENUM ALLOY. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-487-C5-494.
�10.1051/jphyscol:1985561�. �jpa-00224793�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au n08, Tome 46, aoOt 1985 page C5-487
THE EFFECT OF QUENCHING RATE ON THE TEMPERATURE AND STRAIN-RATE S E N S I T I V I T Y OF URANIUM 2W/o MOLYBDENUM ALLOY
G.A.C. Boyd, J. Harding, P.A. leasd dale', K. ~ u n n ' and G. urn er'
Department o f Engineering Science, University o f Oxford, Parks Road, Oxford OX1 3PJ, U. K .
'AWIIE, AZdermaston, Reading RG7 4PR, U.K.
Rdsumd - Les p r o p r i d t Q s mLcaniques d e l ' a l l i a g e LM02 dans l e s d t a t s trempds d e p u i s y p u i s r e v e n u s , r e f r o i d i s 2 l l a i r p u l s d e t b r u t s d ' e x t r u s i o n s o n t c o y a r b e s e n t r a c t i o n pour des v i t e s s e s de s o l l i c i t a t i o n comprises e n t r e
10- e t 2 . 1 0 ~ ~ - l . Des n i v e a u x de c o n t r a i n t e s i g n i f i c a t i v e m e n t d l e v s s o n t Q t b obs'ervds s u r l e s d c h a n t i l l o n s trempbs d e p u i s y pour t o u t e s l e s v i t e s s e s de d d f o r m a t i o n e t Le mgcanisme d e r u p t u r e a dtL d t u d i d e n m i c r o s c o p i e o p t i q u e e t G l e c t r o n i q u e e n b a l a y a g e .
Abstract - The mechanical properties of a U2w/oMo alloy in the Y-quenched, fast- gas-cooled and as-extruded states a r e compared at tensile strain r a t e s from 10-4 to
~ 2 0 0 0 / s . Significantly higher s t r e s s levels a r e observed in the Y-quenched speci- mens a t all strain r a t e s and failure processes a r e studied by optical and scanning electron microscopy.
I - INTRODUCTION
Apart from an early paper by Maiden (1) Little i s known of the effect of strain r a t e on the mech- anical properties of uranium and its alloys. In recent work (2), however, it has been shown that a t tensile strain r a t e s from 1 0-4 to ~ 3 0 0 0 / s on several grades of depleted uranium small differences in impurity content and fabrication history can Lead to significant differences in the strain-rate sensitivity of both the flow s t r e s s and the failure strain with, in some cases, a marked reduction in the fracture strain a s the strain r a t e i s raised above about l/s. No simi- l a r studies of the response of UMo and UTi alloys have a s yet been published. A particular problem with these alloy systems is the occurrence of quench-cracking when large sections a r e cooled too rapidly from the y-phase field. This has led to an interest in the effect of slower cooling r a t e s on the mechanical properties. In particular there i s a need to determine whether s i m i l a r strength Levels can be developed in the slower cooled, non age- hardenable, alloys a s in the more commonly encountered y-quenched and aged condition.
The present investigation seeks to obtain mechanical data over a wide range of s t r a i n r a t e s f o r specimens of U 2 w / o ~ o which have been cooled from the y-phase field a t three rates, i.e.
quenched (and aged), fast-gas-cooled at %40°C/minute and at a slower r a t e in a static argon atmosphere corresponding to the natural cooling r a t e during processing.
I1 - EXPERIMENTAL DETAILS
T o cover the desired range of strain r a t e three types of testing machine w e r e required, a
standard Instron screw-driven Loading machine at the lowest rates, in the range l o e 4 to 10-'/s,
a hydraulically-operated loading machine for tests at intermediate rates, between 0.2 and 50/s,
and a tensile version of the split Hopkinson's p r e s s u r e bar apparatus (3) a t the highest r a t e s of
strain, from about 500 to 2000/s. Full details of the experimental procedure have been given
elsewhere (2). All specimens w e r e cut from 36mm diameter extruded bar. The same origi-
nal billet of U2W/oMo alloy was used for both the fast-gas-cooled and the as-extruded speci-
mens. The fabrication process entailed holding the billet at 9000C for 2 hours, extruding in a
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985561
C5-488 JOURNAL DE PHYSIQUE
copper sheath, during which time the temperature fell to about 8000C. i. e. the y structure was retained throughout, and then cooling slowly to room temperature in an argon atmosphere. The copper sheath was then removed and the bar divided into two parts. The f i r s t was reheated in vacuo to BOOOC, held for $ hour and then fast-gas-cooled to room temperature a t a r a t e of @ O W / minute in dry, cooled and recirculated argon. It was then reheated to 500oC and aged f o r 2 hours in vacuo before again fast-gas-cooling to room temperature. The second received only the ageing treatment at 500°C and subsequent fast-gas-cool, the y-anneal and fast cool from the y region being omitted. The s a m e fabrication process, on a different batch of U2w/oMo alloy, was used f o r the y-quenched specimens. After removing the copper sheath specimen blanks w e r e cut from the bar. These blanks w e r e then solution treated in vacuo for $ hour a t 8000C,
oil-quenched to robm temperature and aged in vacuo f o r 6 hours a t 4500C, to give an overaged structure, followed by furnace cooling to room temperature in a static argon atmosphere.
Tensile specimens having a nominal gauge length of O.35in and gauge diameter of 0.125in w e r e cut with their axes parallel to that of the extruded bar. Apart from a few specimens of Y-quen- ched material, only the peripheral region of the bar was used for specimen preparation, thereby avoiding problems associated with any variation in properties a c r o s s the section. A full chemi- cal analysis of each of the three specimen materials i s given in Table I. No significant dif- ferences between the three materials a r e apparent.
Table I Chemical Analysis of Specimen Materials
Material Mo C Si A 1 Fe Mn Mg C r N i T i Cu
(Z) (ppm) (ppm) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ( P P ~ ( P P ~ )
y-quenched 2.21 140 15 130 90 10 50 20 80 10 30
Fast-gas-cooled 2.07 120 15 35 55 20 45 10 15 10 20
As-extruded 2.10 160 25 30 55 15 35 10 15 10 20
Using X-Ray diffractometry all samples showed peaks corresponding to the presence of both a and Y phases. The relative quantities of a , the main phase, and Y remained s i m i l a r in all samples. The a peaks w e r e sharp, indicating well-formed, large and strain-free crystallites whereas they peaks were much broader suggesting smaller crystallites and, possibly, the presence of some residual strain. The relative magnitudes of the a peaks corresponding to the (002), (021) and (110) orientations in all three materials did not differ significantly from those characteristic of a randomly oriented structure. However, for t h e y -quenched material the a and Y peaks w e r e both smaller in magnitude and l e s s sharp than the corresponding peaks for the other two materials, indicating a markedly finer microstructure following the Y-quench and ageing treatment. When examined under the optical microscope at low magnification (x150) both the fast-gas-cooled and as-extruded materials showed large equiaxed a grains with regions of (a + Y ) phase mixture a t grain boundaries. At higher magnification ( ~ 6 7 0 0 ) in the scanning electron microscope the structure in this region w a s found to be of a fine lamellar (or pearlitic) type for the fast-gas-cooled material, s e e fig. l a , compared with a rather c o a r s e r structure, closer in type to the 'basket-weave' morphology associated with fast-gas-cooling treatments in the range 1 2 to 300C/minute, in the as-extruded material, s e e fig.1b. These contrast with the y -quenched material, fig. l c , which shows a much finer overall structure of precipitated Y in a
matrix of a .
111 - RESULTS Stress-Strain Curves
Average tensile stress-strain curves w e r e obtained a t room temperature and s i x strain r a t e s
(seven for as-extruded material) in the range to 2000/s. F o r the fast-gas-cooled and the
as-extruded specimens additional tests were performed at an intermediate strain r a t e of 2/s
and temperatures of -150, -50 and +100oC. Resulting stress-strain curves f o r the fast-gas-
a ) Fast-gas-cooled m a t e r i a l b) As-extruded m a t e r i a l c) y-quenched m a t e r i a l Fig.1 MICROGRAPHS OF SPECIMEN MATERIALS ( ~ 5 0 0 0 )
L
5 I
StrainFig.2 STRESS-STRAIN CURVES FOR FAST-GAS-COOLED SPECIMENS
cooled specimens, showing both the effect of temperature and strain rate, a r e presented in fig. 2. A very s i m i l m stress-strain response w a s found in tests on the as-extruded specimens.
The effect of quenching r a t e on the stress-strain curves i s shown in f g. 3 where results for room temperature tests on the three specimen materials a r e compared at the highest and the Lowest r a t e s of straining. Apart from the reduced elongation to f'racture in tests on t h e y - quenched material at the highest impact rate, the general shape of the stress-strain curves i s similar for each material. A11 show a significant effect of strain rate, the average flow s t r e s s being raised by between 200 and 300 MPa (i.e. by about 20%) a s the strain r a t e i s increased by some 7 o r d e r s of magnitude. However, a t both ends of the s t r a i n r a t e range the fast-gas- cooled specimens a r e only marginally stronger than those in the as-extruded state whereas the Y -quenched material i s very significantly stronger than both, by about 400MPa (i. e. about 30%).
Effect of Strain Rate on the Mechanical P r o p e r t i e s
The effect of s t r a i n r a t e on the relative strengths of the three materials may also be seen in fig.4 where the variation of the flow s t r e s s at 1% plastic strain, a t 2% plastic strain and a t the ultimate Load with the logarithm of the mean plastic strain r a t e i s shown. Overall, the strain- r a t e sensitivity of the three materials, a s defined by the slope of the flow s t r e s s v. log strain r a t e relationship, i s very similar. The average value of the rate-sensitivity parameter n, where rl
= & U / &(log C), increases slightly, from about 25MPa/decade f o r the fast-gas-cooled specimens to about 3 0 ~ P a / d e c a d e for the as-extruded specimens and about 4OMPa/decade f o r the y -quenched specimens. However, both the as-extruded and the y-quenched materials show an increasing r a t e sensitivity with increasing strain rate, the effect being most marked for the quenched material where, at impact rates, rl for the flow s t r e s s at 1% plastic strain appears to increase by around an order of magnitude.
The effect of s t r a i n r a t e on the f r a c t u r e elongation for the three specimen materials i s shown
JOURNAL DE PHYSIQUE
Fig. 3
EFFECT OF QUENCHING RATE ON STRESS-STRAIR CURVES
-20001s
--- y-quenched Fast-aas ., cooled As extruded
0 Strain ('l0)
I I l I I
0 2.5 5 7.5 10 12.5 15 in fig. 5. In each c a s e a ductility minimum, corresponding to a f r a c t u r e elongation of between 6 and g%, i s apparent a t a n intermediate strain rate, the effect being most marked for t h e y - quenched material and least marked f o r the as-extruded material. The s t r a i n r a t e at which the ductility minimum occurs is greater the higher the r a t e at which the specimen material had been cooled from the y region, increasing from %0.2/s for as-extruded material to %50/s for Y -quenched material. The Y-quenched material, however, differs f r o m the other two in that it shows at the highest r a t e of s t r a i n &:second decrease in fracture elongation, from 16.7
+2% i n t h r e e tests at %1400/s to 6.5 + 0.8% in three tests at %l 700/s. In view of the number of specimens tested at each strain r a t e and the relatively small scatter in the measured elonga- t i o n ~ , s e e fig. 5, it would seem that these variations of fracture elongation with s t r a i n r a t e a r e reasonably well established.
Effect of Temperature on the Mechanical Properties
The effect of temperature, over the range -150 to +1000C, on the strength and ductility of the fast-gas-cooled material, i s shown in fig. 6 f o r tests a t a strain r a t e of 2/s, close to that f o r the ductility minimum. Very s i m i l a r behaviour was shown by the as-extruded material. A schematic representation of the stress-strain curve i s given a t each test temperature. A completely brittle mechanical response was observed in tests at -1 500C with negligible elonga- tion to f r a c t u r e s o that only the f r a c t u r e strength could be measured. In contrast a t +100oC the specimen showed signs of necking and the load at fracture followed, and w a s l e s s than, the ultimate load. At the two intermediate temperatures, -50oC and ambient, fracture o c c u r r e d : at the ultimate load a t a plastic strain g r e a t e r than 2%. F r o m the variation of s t r e s s with , temperature a ductility transition would appear to occur a t about -5OoC. The apparent lack of any sudden reduction in fracture strain at o r around this temperature may be due to the absence of experimental data at temperatures between -500C and - 150°C.
F r a c t u r e Appearance
Scanning electron micrographs a t a magnification of XI 000 a r e presented in fig. 7 showing the f r a c t u r e surfaces of specimens of as-extruded material tested at 2/s a t three temperatures.
Very s i m i l a r micrographs w e r e obtained for the fast-gas-cooled material. At 1 OOOC, fig. 7a,
both materials failed in a ductile manner. Inclusions a r e apparent in the fracture surface and
there a r e large regions showing a dimpled structure. At -1500C. fig. 7c, cleavage facets pre-
dominate on the f r a c t u r e surface, corresponding to a brittle fracture mode, and many cracks
a r e seen. An intermediate behaviour was observed at room temperature, fig. 7b, both cleavage
facets and dimpled regions being apparent on the fracture surface. This mixed failure mode
was present in t e s t s at room temperature on both materials at all s t r a i n r a t e s from 1 0 - ~ to
50/s. At %1000/s in t e s t s on the as-extruded material there i s some tendency for the cleavage
--
As exlruded-X- Ultimale lensile stress
-
1.2-
-+- Flow slress at 2% plastic strain-L -3 -2 -1 0 1 2 3 L
Stra~n rste -log
l i ' l F i g . 4
EFFECT OF STRAIN RATE ON FLOW STRESS
*
Flow stress at 1% plastic strain o Flow slress at 2% plastic strainUltimate tensile stress A Fracture stress
Temperature - l o c i
C
F i g . 6 F i g . 7
FAST-GAS-COOLED MATERIAL FRACTOGRAPHS FOR AS-EXTRUDED MATERIAL TEMPERATURE DEPENDENCE OF MECHANICAL TESTED AT 2 1 s ( x 7 5 0 )
RESPONSE AT 2 1 s a) 100°C b ) 2 0 ° c
C )- 1 5 0 ~ ~
JOURNAL DE PHYSIQUE
mode to predominate. This relative insensitivity of the fracture mode to strain r a t e was also observed, despite the wide fluctuations in the elongation to fracture, in tests on the y-quenched material. SEM fractographs showed a ductile fine dimpled fracture surface after testing at all r a t e s and, in particular, after tests at %25/s and %1700/s, both corresponding to minimum duc- tility levels. It may be noted that in previous work on unalloyed uranium (2), despite a marked decrease in f r a c t u r e strain at strain r a t e s above about 2/s SEM fractographs again revealed no corresponding change in the fracture mode.
On the macroscopic scale, however, marked differences w e r e apparent in the fracture appear- ance of the various y-quenched specimens. Those tested a t % 2 / s and %1400/s, where f r a c t u r e followed the ultimate Load, showed a 'cup-and-cone' type fracture with significant porosity o r void formation i n the necked region while those tested at %25/s and %1700/s, where fracture was close to the ultimate load and at a Low elongation, showed a macroscopically flat fracture surface with no necking or void formation. At the lowest s t r a i n rates, where f r a c t u r e was again a t the ultimate load but following a considerable elongation a t this Load, s e e fig. 3, a flat fracture without necking or significant void formation was observed. Instead of necking the specimen gauge section showed a tapered profile corresponding to a continuously decreasing cross-section from the shoulders to the f r a c t u r e surface. A similar complex combination of features was also apparent on macroscopic examination of the fast-gas-cooled and as-extruded specimens with the difference that, although no specimen showed a cup-and-cone o r necking type failure,
some void formation was apparent in a l l specimens. At higher magnification a fast-gas-cooled specimen tested a t the lowest r a t e and examined before and after etching, figs. 8a and 8b r e s - pectively, showed c r a c k s in both cuboid carbide inclusions and at interfaces between prior Y grains. Above the ductility minimum void formation is more marked, s e e fig. 9a for an as- extruded specimen tested a t %50/s, and the propagation of f r a c t u r e would appear to be by void coalescence, s e e fig. 9b f o r the s a m e specimen.
The effect of temperature on the f r a c t u r e appearance of both fast-gas-cooled and as-extruded specimens w a s m o r e straightforward. At lOOOC and 2/s both showed a tapered profile, a rough f r a c t u r e surface and considerable void formation, similar t o fig. 9% some of which had developed into cracks, s e e fig. 10, a t some distance f r o m the f r a c t u r e surface. In contrast, at -150°C a very flat f r a c t u r e surface w a s obtained with very few voids visible and the only cracks close to the f r a c t u r e surface.
N - DISCUSSION
As shown in sectlon I1 the microstructural differences between the fast-gas-cooled and the as- extruded specimens a r e small and not surprisingly, therefore, their mechanical properties a r e also very similar. Although the fast-gas-cooled specimens at all strain r a t e s a r e marginally stronger, by %5%, both materials show a similar stress-strain response, a s i m i l a r temperature and s t r a i n r a t e dependence and a tendency to a ductility minimum at a strain r a t e of %2/s. In contrast, the y-quenched material has a much finer microstructure leading to a significantly higher strength, by between 25 and 30%, a t all s t r a i n r a t e s and, more surprisingly perhaps, to an increased rate-sensitivity of the flow s t r e s s . The ductility~minimum at intermediate strain r a t e s is m o r e marked and i s followed by a second reduction in ductility at the highest impact rate.
Taking each s e t of specimens alone it might be possible to discount the apparent ductility mini-
mum a t intermediate s t r a i n r a t e s of about 2/s. However, its appearance in tests on all three
materials makes this m o r e difficult. No such effect is observed in very recent work in this
laboratory on fast-gas-cooled UsO/o~i and ~ 2 & N b alloys after similar ageing treatments. In
both c a s e s a continuous decrease in ductility with increasing s t r a i n r a t e i s apparent. An ex-
planation of the present results i s difficult to find since there appears to be no effect of strain
rate, a s opposed to temperature, on the fracture mode. A correlation i s possible, however,
between the shape of the s t r e s s - s t r a i n curve and the macroscopic fracture appearance, suggest-
ing that s o m e complex effect of strain r a t e on the work-hardening behaviour may be a t least
partially responsible.
- .-
' . - , .- .. -- .
v.
' -a ) Unetched, showing cracks i n cuboid b ) Etched, showing g r a i n boundary crack c a r b i d e i n c l u s i o n s
Fig. 8 FAST-GAS-COOLED SPECIMEN TESTED AT I o - ~ / s (XI 50)
b
Fig.9 AS-EXTRUDED SPECIMEN TESTED AT 501s a ) Unetched, showing tapered p r o f i l e and
e x t e n s i v e void formation (x24)
b ) Etched, showing void coalescence ( ~ 1 5 0 )
On the microscopic scale, sections through tested fast-gas-cooled and as-extruded specimens
frequently showed the presence of many small voids, fig. 9a, generally forming a t grain boun-
daries, fig. 8b, o r other interfaces, fig. 11, and not Limited to regions close to the f r a c t u r e sur-
face. These voids would often coalesce to form extended, if somewhat blunt, microcracks,
fig. 9b. It s e e m s likely, therefore, that the process of fracture in these materials involves
several stages - the initiation of voids, their coalescence to form c r a c k s and, when the density
of cracks on a given qross-section is sufficient, their propagation to give failure. Voids w e r e
l e s s commonly found in the y-quenched specimens and w e r e notably absent from the more
brittle of them. Also there was very little evidence for void coalescence in this material,
suggesting that voids w e r e more difficult to f o r m in the finer precipitated y-quenched structure
and that when they did form fracture normally followed without the intermediate stage of void
growth and coalescence. In previous work on a y-quenched U2W/oMo alloy in a similarly
overaged condition Eckelmeyer and Zanner (4) found that the fracture path in quasi-static tests
was unrelated to the microstructure. Evidence i s limited in present t e s t s on y -quenched
specimens because of the very few voids visible in this material. However, in one test at the
highest r a t e of strain a long crack away from the f r a c t u r e surface was observed, the path of
which appeared to bear little relation to the microstructure, suggesting that Eckelmeyer and
Zanner's observations may also hold at impact r a t e s of strain. F o r the fast-gas-cooled and
as-extruded specimens, however, present results show clearly that f r a c t u r e initiates a t micro-
structural interfaces. Since the phases present in a l l three materials a r e the s a m e and in
essentially the s a m e overaged state it seems likely that this difference in behaviour i s a direct
C5-494 JOURNAL DE PHYSIQUE
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