Zirconium phase transformations measured by “in-situ” X-ray diffraction

J. Říha, P. Šutta

University of West Bohemia, Research Centre New Technologies, Univerzitní 8, 306 14 Plzeň
Czech Republic

janriha@ntc.zcu.cz

Zirconium alloys are very important group of materials used in nuclear energetics. With regard to their properties – high melting temperature, high corrosion resistance and very low absorption cross-section for thermal neutrons the zirconium alloys are used in pressurized- and boiling-water reactors for protective layers of nuclear fuel rods. Except of these properties zirconium has a strong affinity for gaseous oxygen, nitrogen and hydrogen with which it can form stable oxides, nitrides and hydrides [1, 2]. Physical and mechanical properties of zirconium are influenced especially by oxygen presence significantly. In form of solid solution oxygen and also hydrogen stabilize the low-temperature a-Zr modification with HCP lattice and also increases the zirconium hardness. The zirconium a ® b phase transformation temperature is 863 °C.

The development of new Zr-alloys is in nowadays focused on their behaviour optimisation during the Loss of Coolant Accident (LOCA). This type of reactor accident results in a rapid moderator escape in time shorter than 10 seconds, followed by a rapid heating of the Zr alloy in steam environment up to approximately 1000°C. These severe conditions lead partly to a fast high-temperature oxidation and also to a phase transformation of zirconium to high-temperature b-modification with body-centred cubic lattice structure until the reactor core is flood with water and the cladding is quenched back to a-phase. The temperature of Zr phase transformation is strongly influenced by free oxygen placed in interstitial positions of crystal lattice and also by a heating rate, Fig. 1 and 2.

Figure 1. Zirconium – oxygen binary phase diagram.	Figure 2. Zirconium – nitrogen binary phase diagram.

Two pure zirconium foils with dimensions 11 ´ 11 ´ 1 mm supplied by Goodfellow Ltd. were used as experimental samples. During previous experiments [3 - 5] was observed, that the phase transformation of zirconium to b-phase did not proceed even at 1000°C. On the basis of that, the experimental samples were measured under the temperature 1100°C. The XRD measurements proceeded in high-temperature chamber Anton Paar HTK 1200N being a part of automatic powder diffractometer Panalytical X’Pert Pro. This instrument uses a copper X-ray tube (l_Ka = 0.15406 nm) and an ultra-fast semiconductor detector PIXcel. The chamber was evacuated with the aid of turbo-molecular pump Edwards EXT75DX. A dry scroll pump Edwards XDS5 created the initial vacuum. The minimum pressure value achieved during the measurements was about 10^-3 Pa. To observe the influence of thermal heating on material structure at high temperatures the experimental samples were subjected to different heating courses, described in Tab. 1. For the lowest pressure achieving, the deaeration step at 250 °C for 60 minutes was applied on the samples.

Table 1. Heating courses of experimental samples.

Sample	Heating courses
Zr_23	25 °C ® 250 °C (20 °C/min) ® 860 °C (20 °C/min) ® 1100 °C (20 °C/min) ® 30 °C (50 °C/min)
Zr_29	25 °C ® 250 °C (20 °C/min) ® 860 °C (20 °C/min) ® 1100 °C (20 °C/min) ® 900 °C (50 °C/min) ® 850 °C (50 °C/min) ® 800 °C (50 °C/min) ® 30 °C (50 °C/min)

From the XRD results of both experimental samples and also from previous experiments it is evident that zirconium foils from Goodfellow contain a significant amount of nitrogen. This element is in all samples in form of solid solution – diffraction patterns of both samples in initial state show only a presence of a-Zr phase. The nitrogen causes an expressive increasing of a ® b phase transformation temperature.

The b-Zr phase could be identified at the temperature 1100°C in sample Zr_23, Fig. 3. In sample Zr_29 is can be observed only a a-Zr phase at that temperature and a high-temperature b-Zr is seen even during the cooling at the temperature 1000°C, Fig. 4. This is influenced by two factors – heating rate and a nitrogen presence in samples. The influence of heating rate is described in publication [6]. The influence of nitrogen presence in structure of Zirconium is seen in Fig. 2. During the heating nitrogen diffuses on sample surface, where subsequently create the ZrN phase. The diffusion rate can be different in both samples and that is why we can not observe the b-Zr at 1100 °C under the same heating conditions as in case of sample Zr_23.

Figure 3. Partial diffraction pattern of Zr_23 during the heating.	Figure 4. Partial diffraction pattern of Zr_29 during the heating.

During the cooling of Zr_29 sample, the amount of residual interstitial nitrogen amount in the structure of zirconium is very small and the phase transformation of b-Zr to low-temperature a-phase proceeds in accord with the binary phase diagram, Fig. 5.

Figure 5. Partial diffraction pattern of Zr_29 during the cooling.

References

[1] M. E. Dric: Svojstva elementov, spravočnik, Metallurgija Moskva 1985

[2] J. Koutský, J. Kočík,: Radiation damage of structural materials. Praha Academia, 1994.

[3] J. Říha, O. Bláhová, P. Šutta: Fázové změny slitiny Zr-1Nb a jejich vliv na lokální mechanické vlastnosti, Chemické listy, 2008, in print

[4] J. Říha, O. Bláhová, P. Šutta: Mechanical properties and structure of Zr-Nb alloy after high-temperature transformations, Chemické listy, Volume 104, Issue 15, p. 364 – 367.

[5] J. Říha, R. Medlín, A. Vincze, P. Šutta: Zirconium phase transformations observed by “in-situ” XRD analysis

[6] A. R. Massih: Transformation kinetics of zirconium alloys under non-isothermal conditions, Journal of Nuclear Materials, Volume 384, Issue 3, 28 February 2009, Pages 330-335