Grain size and hydrogen concentration effects on the ductility return in a refractory alloy
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I.
INTRODUCTION
HYDROGEN doped group VA metals, if deformed at low strain rates, will often exhibit a pronounced ductile-brittleductile behavior, l-s In other words, after a typical ductilebrittle transition, the ductility returns at low temperatures. Such a ductile-brittle-ductile behavior has also been observed now in some hydrogenated solid solution refractory metal alloys. 9,l~ It has been suggested that the return of ductility may be related to the hydride particle size 12or to the low diffusivity of hydrogen at these temperatures.4-6'13'14The latter explanation was reconciled with the observation that the ductility minimum created by this ductile-brittle-ductile behavior becomes less pronounced as the strain rate is increased. Since the diffusion coefficients for some of the alloys are also well known,15 some judgment on the effects of diffusivity on the ductility return can now be made. In Figure 1, we have depicted ductility (R.A.)-temperature profiles of a hydrogenated 25 at. pct V-75 at. pet Nb alloy taken from two different investigations. 10.16In the top curve it is seen that for a grain size of 25/~m this alloy exhibits a pronounced return of ductility at low temperatures in the presence of 0.3 at. pet of hydrogen) ~ The bottom curve shows no such ductility return for this alloy at a grain size of 175/xm even though the hydrogen concentration is the same. Unfortunately, the start-up materials used in this comparison were not the same. In order to exclude the possibility of an effect of the residual elements on the ductility return, this more detailed study on the grain size effect of H doped 25V-75Nb was initiated.
H.
MATERIALS AND PROCEDURES
The vanadium as well as the niobium for this investigation were obtained from the Wah Chang Corporation. The niobium was further purified by electron beam melting before being alloyed with the as-received vanadium. Table I shows the interstitial content of both materials before alloying. C.V. OWEN, Associate Metallurgist, and O. BUCK, Senior Metallurgist, are with Ames Laboratory--U.S.-D.O.E., Iowa State University, Ames, IA 50011. D. -S. CHEONG, formerly a Graduate Student at Ames Laboratory, is a Graduate Assistant at Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted June 6, 1986. METALLURGICALTRANSACTIONS A
100,'
,
!
,
,
,
25V- 75Nb+0.3 at.% H. 25/zm
G.S.
*: 175 y m G.S. 6O "6 = 40 .s
n--
20 0
1
70
II0
I
150 190 250 Test Temperature, K
!
270
31(
Fig. 1 - - Temperature dependence of ductility (R.A.) for the hydrogenated 25V-75Nb alloy at the grain sizes indicated.
Table I.
Chemical Analysis in Atomic Percent
Material O* N* Vanadium 0.041 0.005 Niobium 0.078 0.013 *Spark sourcemass spectrometricanalysis
C* 0.017 0.009
A 25V-75Nb alloy, in the form of a finger-shaped ingot, was prepared from these materials by arc-melting under an atmosphere of purified argon. This 1.8 cm diameter by 12 cm long as-cast ingot was first encapsulated in a stainless steel jacket and reduced to a diameter of 1.0 cm by swaging at I2
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