3FTFBSDI

Intrinsic Martensite Formation in Neutron Irradiated V-1.6%Y
Alloys with Fine-grained Structure of Highly Pure Matrix
It is known that in vanadium gaseous interstitials (H, N,O) cause transformation from the bcc to bct structure byelongating one axis relative to the other two to accommodate
the induced strain. This paper describes the first finding ofintrinsic martensite formation that is not associated with theinterstitials for neutron irradiated V-1.6Y (in wt%) with a finegrained structure of a highly pure matrix [1]Development of refractory transition metals with improved
resistance against mechanical property degradation by highenergy particle irradiation is required for their use underirradiation environments. It has been shown that the mosteffective microstructure to improve the radiation resistanceconsists of fine grains and finely dispersed particles, the finer
the better because grain boundaries and particles can serve
as effective sinks for irradiation induced point defects. For
vanadium such a microstructural refinement requires aprocessing method that enables to make the vanadium matrix
free from gaseous interstitials, particularly nitrogen andoxygen. This is because vanadium is chemically very reactivewith the interstitials and their dissolution into the matrix leads
to significant embrittlement. We have proposed an advanced
powder metallurgical (P/M) method utilizing mechanicalalloying and hot isostatic pressing and successfully developed
ductile V-Y and V-Y-Cr alloys with fine grains of a highly purematrix free from gaseous interstitials and very fine dispersoidsof Y2O3 and YN [2-4]. The Y2O3 and YN dispersoids areformed as a result of consuming solute oxygen and nitrogenimpurities.
V-1.6Y alloys were irradiated at 290 and 600°C to 0.25
and 0.6 displacement per atom (dpa), respectively, in theJapan Materials Testing Reactor to examine the effect ofneutron irradiation. It has been demonstrated that the alloys
exhibit much improved radiation resistance [5] Anothernoticiable finding is that the irradiated V-1.6Y exhibits theintrinsic martensite formation of a vanadium bct structure thatis not caused by gaseous interstitials. The emergence ofintrinsic martensite is the first to be observed in vanadium orits alloys not only in neutron irradiated states but also inunirradiated states. The main results and conclusions are as
follows [1]:1. The martensite emerges heterogeneously in some of
the bcc grains of V-1.6Y and has a high density of {101)bct
microtwins as the lattice invariant shear strain. The bct
structure exhibits a tetragonality (a/c) of 1.06 as measured
with Kikuchi line analyses.
2. The habit planes between the bct martensite and thesurrounding bcc matrix are {101)bct // {110}bcc, which can beregarded as the close-packed plane with coherency even forthe bct structure because of the slight changes in the lattice
constants of both the phases.
3. The features of the P/M V-1.6Y alloys, an interstitialscavenged matrix and extremely fine grains, are considered
to be responsible for the martensite formation caused byirradiation.
4. The transformed vanadium bct lattice provides

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IMR KINKEN Research Highlights 2007

preferential sites as the host phase for hydrogen atoms tooccupy with an ordered arrangement (Fig. 1). The orderedarrangement of hydrogen atoms occurs in a very thinspecimen area that is susceptible to hydrogen pickup
associated with TEM specimen preparation after irradiation,
but does not occur in a thicker specimen area.

Fig. 1 TEM micrograph of dark field image from a very thin
specimen area with a transformed region that provides
preferential sites for hydrogen atoms to occupy with anordered arrangements in V-1.6Y irradiated at 290°C to0.25dpa.
5. The hydrogen content (H/V) in the ordered bct structureis estimated to be approximately H/V = 0.24. This content ismuch lower than H/V~ 0.41, the lowest content of the β1
hydride phase that can exist in the thermoequilibrium state,
supporting the absence of hydride formation in the bct
phase.
References[1] H. Kurishita, S. Kobayashi, K. Nakai, T. Kuwabara and M.
Hasegawa, J. Nucl. Mater. 358, 217 (2006).
[2] T. Kuwabara, H. Kurishita and M. Hasegawa, J. Nucl.
Mater. 283-287, 611 (2000).
[3] T. Kuwabara, H. Kurishita and M. Hasegawa, Mater. Sci.
& Eng. A 417, 16 (2006).
[4] H. Kurishita, T. Kuwabara and M. Hasegawa, Mater. Sci.
& Eng A. 433, 32 (2006).
[5] H. Kurishita, T. Kuwabara, M. Hasegawa, S. Kobayashi
and K. Nakai, J. Nucl. Mater. 343, 318 (2005).
Contact toHiroaki Kurishita (International Research Center for Nuclear
Materials Science)e-mail: kurishi@imr.tohoku.ac.jp

Advanced Nuclear Energy Materials