Phase transformations of E110G Zr-alloy observed by “in-situ” XRD

 

J. Říha, P. Šutta

 

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

janriha@ntc.zcu.cz

 

The most important area of zirconium alloys usage is today the nuclear energetics. In this sphere the zirconium alloys are mainly used as protective layers of nuclear fuel rods where they create a first barrier against the reactor core atmosphere. For this application the Zr-alloys must ensure a very low absorption cross section for thermal neutrons, high corrosion resistance in water steam at high pressure and temperature a good mechanical properties. In this form these alloys are used in pressurized- and boiling-water reactors. Except of those properties zirconium has a strong affinity for gaseous oxygen, nitrogen and hydrogen with which they 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 nitrogen stabilize the low-temperature a-Zr modification with HCP lattice and also increase the zirconium hardness. The phase transformation temperature of pure zirconium a ® b is 863 °C, Fig. 1 and 2.

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 at the temperature above 1000°C. These severe conditions lead partly to a fast high-temperature oxidation and also to a phase transformation of Zr-alloy 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-alloy phase transformation is strongly influenced by free oxygen and nitrogen placed in interstitial positions of crystal lattice and also by a heating rate [3], Fig. 1 and 2.

 

Figure 1. Zirconium – oxygen binary phase diagram.

 

Figure 2. Zirconium – nitrogen binary phase diagram.

 

 

The Zr-Nb alloy E110G was used as an experimental material, Tab. 1. This material is today most often used for nuclear fuel rods protective layers. With regard to interstitial oxygen and nitrogen influence           on phase transformations the samples of pure Zr supplied by Goodfellow Ltd. were used for the comparison. During previous experiments [4, 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 heated at three temperatures, 1100 °C, 1150 °C and 1200 °C. After that, the samples were cooled down at 1000 °C, 900 °C 800 °C and 30 °C, Fig 3. The diffraction patters were recorded at all these temperatures.

 

Figure 3. Heat treatment of experimental samples

 

 

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          (lKa = 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. For the lowest pressure achieving, the deaeration step at 250 °C for 60 minutes was applied on the samples.

Table 1. Chemical composition of E110G Zr-Nb alloy.

E110G Alloy

 

Element

Nb [%]

Fe [ppm/%]

H [ppm]

N [ppm]

C [ppm]

O [ppm]

Ni [ppm]

Hf [ppm]

1,0 - 1,1

0,055

3

20

100

840

-

~500

 

From the XRD results of both types of experimental materials is evident that they 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, Fig. 2.

A trace amount of high-temperature b-Zr phase can be identified in E110G alloy after heating at 1150 °C, sample Zr-1Nb_12, Fig. 3. This is mainly caused by interstitial nitrogen but also by interstitial oxygen which is contained in the material structure already in initial state, Tab. 1. That is why the b-Zr phase          is  unstable under 1000 °C and transforms back to low temperature a-Zr. Due to high amount of interstitial nitrogen in the structure also a zirconium nitride ZrN have created on the sample surface during the cooling from 1150 °C to 1000 °C. In the case of pure zirconium, sample Zr_29, the small amount b-Zr phase created already after the heating at 1100°C, but during the cooling down at 1000 °C. This is caused by a-Zr surface layer depletion of nitrogen which is used for creation of surface ZrN layer, Fig. 4. During the subsequent cooling down, the residual interstitial nitrogen amount in the structure of zirconium decreases and the phase transformation of b-Zr to low-temperature a-phase proceeds in accord with the binary phase diagram Zr - N, Fig. 5.

Figure 4. Partial diffraction pattern of Zr-1Nb_12

 

Figure 5. Partial diffraction patterns of Zr_29

 

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

 

 

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.       A. R. Massih, J. Nucl. Mat., 384, (2009), pp. 330–335

4.       J. Říha, O. Bláhová, P. Šutta, Chemické listy, 105, (2011), pp. 210-213

5.       J. Říha, R. Medlín, A. Vincze, P. Šutta, Vacuum, 86, (2012), pp. 785-788