The alloy 690 is a nickel-based alloy (60% Ni, 30% Cr, 10% Fe) used in nuclear Pressurized Water Reactors for different components (steam generator tubes, reactor vessel clevis etc.). These components are subjected to long thermal ageing for 40 to 60 years at 325 °C.
For these kind of alloys, thermal ageing can produce an order-disorder transformation due to the formation of the Ni₂Cr ordered phase by a nucleation growth mechanism. This transformation strongly modifies mechanical properties (strength increasing and ductility decreasing) as well as electrical properties when the orthorhombic ordered domains grow in the cubic disordered matrix.
In order to correlate the ordering state of these alloys to the evolution of their macroscopic properties, TEM studies are conducted. Ordered domains are generally very small (a few nanometers) so that imaging is not always feasible. Hence, ordering state is investigated using electron diffraction where we want to measure the intensity ratio between superlattice reflexions, related to the ordered domains, to the fundamental ones, related to both ordered domains and the disordered matrix (see figure 1). Indeed, in the kinematical approximation, the square of the order parameter s is proportional to Is/If, where Is and If are the intensities of the superlattice and fundamental reflections respectively. We want to appreciate the validity of this approach using Precession Electron Diffraction (PED) coupled to dynamical calculations of the intensities¹. For this kind of quantitative study, PED is advantageous since it strongly reduces sensitivity to experimental parameters such as sample thickness and exact orientation, which renders comparison with intensity calculations much more reliable. Reflections of the <011> cubic matrix zone axis pattern are targeted. In this orientation, superlattice reflections come from only one of the six possible orientation variants of the ordered phase related to the disordered one (see figure 2). This particular type of zone axis thus gives the richest information about the ordered phase.
In the present study, model alloys with iron content of 0 to 3 wt. % Fe are aged between 24 and 20 000 h in the range of 325 to 500 °C. The precipitation of Ni₂Cr ordered phase is detected by an increasing Vickers hardness and ThermoElectric Power (TEP) at the macroscopic scale.
We first need to validate our dynamical calculations on the fully ordered orthorhombic domains. In this attempt, specifically aged alloys are also studied, where ordered domains are well extended (a few tens of nanometers). Microdiffraction patterns are then collected on single domains and a systematic study of the intensity ratio Is/If as a function of the sample thickness is conducted in order to compare experimental and calculated data. Generalization of the procedure to the mixed alloys containing both ordered and disordered domains in various proportions will then be proposed.

Acknowledgements: The authors gratefully acknowledge L. Legras², J. Stodolna² and D. Loisnard² of for useful advice for thin foil preparation and observation; Y. Fontaine and his team² for all the machining work; C. Vincent² for the thermal treatments and P. Stadelmann³ for helpful discussions about JEMS software.

Figures:

Figure 1 : SADP of the [011] cubic zone axis showing fundamental (bright spots) and superlattice spots (1 variant only) on a 66Ni-33Cr alloy aged 500 h at 500 °C. The black and white indices represent the cubic and orthorhombic bases respectively with respect of the following orientation relations between the ordered (orthorhombic) and disordered phase (cubic): [011]c // [100]o; [0-11]c // [010]o; [100]c // [001]o (see figure 2).

Figure 2 : Direct space representation of the orientation domain in the disoriented matrix diffracted in figure 1. Light and dark grey atoms are Ni and Cr respectively. Each site of the disoriented matrix has 33% chance to be filled with a Cr atom. The orientation relations between the domain and his matrix are [011]c // [100]o, [0-11]c // [010]o, [100]c // [001]o.

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EMC Abstracts - https://emc-proceedings.com/abstract/a-precession-electron-diffraction-study-of-ordered-disordered-phases-in-ni-cr-based-alloys/