Letter to the Editor Mitigation of hydride embrittlement of zirconium by yttrium I.S. Batra a , R.N. Singh b, * , P. Sengupta a , B.C. Maji a , K. Madangopal a , K.V. Manikrishna a , R. Tewari a , G.K. Dey a a Materials Science Division, Bhabha Atomic Research Centre, Modular Laboratories, Trombay, Mumbai 400085, India b Mechanical Metallurgy Section, Bhabha Atomic Research Centre, Modular Laboratories, Trombay, Mumbai, Maharashtra 400085, India article info Article history: Received 31 December 2008 Accepted 24 February 2009 abstract Brittle hydrides of plate-shaped morphology are known to embrittle the host zirconium matrix. The embrittlement effect is a strong function of the aspect ratio of hydride plates and their major dimension, as thinner plates behave akin to cracks resulting in stress-concentration around their edges, especially when tensile load is acting normal to the broad face of the plates. The embrittlement of the host matrix is due to loss in load bearing area as a result of cracking of the hydride plates under load and severe local- ized deformation of the ligaments joining the hydride plates. In this work, mitigation of hydride embrit- tlement was attempted by exploiting the synergistic effect of yttrium addition to zirconium and microstructural modification of the Zr–Y alloy by quenching. This was expected to enable creation of a very high density of nucleation sites much stronger than those available otherwise and, thus, facilitate precipitation of much smaller hydrides that do not embrittle the host matrix. The results obtained in the present work on a dilute Zr–Y alloy do support this idea. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Dilute Zr-alloys are used as pressure boundary for hot coolant in PHWR and RBMK reactors [1,2]. These pressure boundary compo- nents are subjected to aqueous corrosion leading to hydrogen/deu- terium ingress into these components during service. The maximum amount of hydrogen that can be retained in solid solu- tion without precipitation of hydrides is called terminal solid solu- bility (TSS) [3–5]. Hydrogen in excess of solid solubility causes precipitation of plate-shaped brittle hydride phase, which makes the pressure boundary components like pressure tubes of the aforementioned reactors susceptible to hydride induced embrittle- ment [6]. Two forms of hydride embrittlement (HE) have been rec- ognized for hydride forming metals, namely, gross and localized [7]. The former requires certain minimum volume fraction of the hydride phase, and it results in overall reduction in tensile ductil- ity, impact and fracture toughness. Gross embrittlement is strongly influenced by the orientation of hydride plates since the plates that are oriented normal to tensile load can significantly enhance the degree of embrittlement by providing an easy path for the growth of cracks through the hydrides [8]. Hydrogen in solid solution is not reported to cause embrittle- ment of Zr-alloys [9]. Hence, TSS at the operating temperature can be considered as a safe hydrogen concentration limit, even though, for localized form of hydride embrittlement, safe hydrogen concentration limit is just a fraction of TSS [10]. Considering the fact that the rate of hydrogen pick-up in present generation Zr-al- loys is less than 1 ppm per year for these components [2], any ef- fort to increase the TSS even by a few ppm would surely enhance the safe hydrogen concentration limit and, thereby, the life of these components. Therefore, the first objective of this work was to look for an alloying addition, in which, intrinsically, the solid solubility for hydrogen is high. Yttrium is known [11] to have TSS that is about two orders of magnitude higher than that for zirconium at a given temperature. It has earlier been tried as a getter [2] for hydrogen and, in conjunction with other elements, as an alloying addition to mitigate HE [12]. Large hydride plates are likely to enhance the degree of embrit- tlement by causing stress-concentration at the edges of hydride plates and severe localized plasticity of ligaments joining these hy- dride plates. It may be worthwhile to note that RBMK tubes of Zr– 2.5 wt% Nb alloy, which are produced by quenching followed by ageing (Q&A), exhibit smaller hydride plates under optical micros- copy as compared to those observed in cold-worked and stress-re- lieved (CWSR) tubes of the same material for identical hydrogen concentration [13]. Also, for the same concentration of hydrogen, in contrast to CWSR tubes, Q&A tubes exhibit a higher hydride plate number density. In case of CWSR tubes, a-grains are elon- gated. The preferentially aligned grain boundaries in these act as the nucleation sites for hydrides [14]. Manufacturing of Q&A tubes imparts in them a microstructure that comprises of Widmanstät- ten a-grains and very fine b-grains [15]. Apart from grain bound- aries, sub-grain boundaries and inter-lath boundaries also act as the nucleation sites in these, which probably explains the observed smaller size of the hydride plates and their higher number density in the RBMK tubes. This prompted us to make an attempt to some- how increase the density of relatively stronger nucleation sites so 0022-3115/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2009.02.036 * Corresponding author. Tel.: +91 22 25593817; fax: +91 22 25505151. E-mail address: rnsingh@barc.gov.in (R.N. Singh). Journal of Nuclear Materials 389 (2009) 500–503 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat