Titanium Influence on the Microstructure of FeCrAl Alloys Used for 4R Generation Nuclear Power Plants

VICTOR GEANTA1, IONELIA VOICULESCU1, RADU STEFANOIU1*, ADRIAN JIANU2, IOAN MILOSAN3, ELENA MANUELA STANCIU3, ALEXANDRU PASCU3, ION MIHAI VASILE1 1Politehnica University of Bucharest, 313 Splaiul Independentei, 060042, Bucharest, Romania 2Karlsruhe Insititute of Technology Institute for Pulsed Power and Microwave Technology, Campus Nord, Bldg. 421, HermannvonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 3Transilvania University of Brasov, 29 Eroilor Blvd., 500036, Brasov, Romania

Molten lead or lead-bismuth eutectic (LBE) is considered an attractive coolant for various nuclear applications, especially generation fourth (Gen-IV) and small-and medium-sized reactor (SMR) ones. Its low melting point, low reactivity, low vapour pressure, low viscosity, good gamma shielding, high boiling point, excellent chemical stability, lack of fire or explosion risk, as well as neutron transparencyand high neutron yield make LBE-cooled reactors and spallation sources good candidates for advanced energy systems. The main difficulty in using LBE as a coolant lies actually in understanding and controlling the corrosion of candidate structural materials in LBE. It is believed that protective oxide layers can be formed on LBEexposed steel in order to prevent further corrosion, while ensuring that oxide layers formed on steels are able to resist to the system operating conditions (high flow rate, high temperature etc.) for a long period of time (25 -30 years in the case of reactors).
As a coolant for fast reactor systems, liquid lead or leadbismuth eutectic (LBE) provides two advantages over liquid sodium. First, lead alloys do not react exothermically with water or air as sodium does. Second, the boiling temperature of lead alloys is much higher than that of sodium, providing greater safety margins and the ability to operate at higher temperature.
The development of lead alloy coolant technologies will allow successful application with maximized efficiency and enhanced safety.The LBE-cooled nuclear energy systems, however, have been exposed to several issues * email: radu.stefanoiu@upb.ro; Phone: 0744606588 arising from the limitation of the system life due to the relatively low corrosion resistance of structural materials in LBE environments. The corrosion behaviours on several types of austenitic and ferritic/martensitic (FM) steels have been extensively studied to investigate the corrosion performance of structural materials and to improve the materials life performance worldwide [1][2][3][4][5][6][7][8][9].
The resistance of structural alloys to rapid degradation in aggressive environments at elevated temperatures usually depends on the ability of the alloys to form and maintain a protective reaction product scale.
In recent years, substantial efforts have been made to investigate and improve the resistance of candidate materials to typical corrosion mechanisms observed in liquid lead/lead alloys, which are dissolution attacks (loss of alloying elements) and/or extensive oxide scale growths. These research activities showed that thin, slow-growing oxide scale can represent a suitable corrosion barrier for long term protection and adequate heat transfer (important in core components) [10,11].
FeCrAl alloys, along with the ZrCrAl alloys class, can be used to achieve the metallic structures of the reactor in nuclear power plants, generation 4R. The components that can be made from these modern alloys are: outer jacket of the reactor, recirculation pipes for the liquid metal, pumps for the distribution of the cooling medium etc. Modern type LFR reactors working in closed cycle with optional fuel flexibility and fast neutron spectrum are cooled with metallic media like Pb or Pb-Bi alloys. This particularity ensures a much higher level of security at a lower overall volume compared to the previous generation reactors. Such materials must provide excellent mechanical and technological characteristics, among which: plasticity and ductility, mechanical workability, weldability, thermal shock resistance and mechanical strength, creep resistance at high temperature, high chemical stability in specific environmental conditions (temperatures above 400 -800 0 C), corrosion and erosion resistance, high thermal conductivity to ensure optimum heat transfer.
Iron-chromium-aluminium alloys containing 15-20 wt.% Cr and 4-6 wt.% Al have shown excellent corrosion resistance in the temperature range up to 600°C or higher in liquid lead and lead-bismuth eutectic environments by forming protective Al 2 O 3 layers. However, higher Cr and Al concentrations in ferritic alloys could be problematic because of severe embrittlement in the manufacturing process as well as in use, caused by the formation of brittle phases. The resistance of FeCrAl alloys to high-temperature oxidation depends on the properties of the built-up aluminium oxide [12,13].
The design of new alloys was focused on the optimization of Cr and Al levels for the formation of an external Al 2 O 3 layer which can provide excellent oxidation and corrosion resistance in liquid lead alloys in the temperature range 400-600 0 C without severe embrittlement caused by the formation of brittle phases such as Cr-rich α' and σ phase precipitates.
Minor amounts of reactive elements (RE´s), e.g. Y, Hf, Zr, Ti and Ce are usually mixed into FeCrAl alloys to improve oxidation properties at elevated temperatures. All elements may oxidise in an FeCrAl alloy depending on the temperature and the exposure environment. Initially (during heating), all elements present at the alloy surface will oxidise and form a mixed oxide scale called transient oxide.An addition of reactive elements, particularly Zr and Hf, results in better adhesion of the scale to the base.
The effectiveness of Zr stems from the fact that the latter quickly penetrates into the scale, changing its morphology (the column structure is replaced by small equiaxial grains), producing pores and ZrO 2 inclusions in it and preventing chromium carbides from forming at grain boundaries [14 -17].
Two main positive effects of the RE addition on the oxidation resistance of alumina forming alloys are accepted. The first one is the suppression of the outward diffusion of Al-cations in the alumina scale. This results in a decrease in the oxidation rate and changes in the scale structure and morphology. The second important RE-effect is attributed to the prevention of the deleterious sulphur segregation on the scale/metal interface.
Yttrium was added to enhance the oxidation resistance at elevated temperatures. The addition of such reactive elements is known to reduce the growth rate of the alumina scales and to improve the adherence resulting in improved oxidation resistance compared to that of base alloys which do not contain yttrium.
Hafnium is used for nuclear reactor control rods because of its ability to absorb neutrons and its good mechanical and corrosion resistance qualities. Its neutron-capture cross-section is about 600 times larger than that of zirconium. Hafnium resists to corrosion due to the formation of an oxide film on exposed surfaces. In FeCrAl alloys microalloyed with hafnium, a complex aluminium oxide that contains both aluminium and hafnium forms on the metallic surface. The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.
Titanium is an alpha phase stabiliser, having limited solubility in the ferrite due to the formation of compound Fe 2 Ti. Titanium reacts with oxygen at temperatures of 600-800 °C according to the reaction: (1) Titanium has high affinity for oxygen, which means that it takes part in the deoxidation processes of the molten metal. The affinity of titanium for oxygen can be evaluated using the equilibrium constant (K Ti = [Ti][O] 2 ), considering that the total amount formed varies with the temperature according to: (2) Since the K TI value is 6.638x10 -8 at 1600 0 C, it follows that the deoxidation power of titanium is greater comparatively with silicon and aluminium. Titanium is also used as a deoxidizer and as an alloying element in ferrous alloys like FeCrAl. In addition, titanium has a strong effect on the precipitation of sulphides in metallic alloys [18,19].
The paper presents several results related to how the presence of titanium into the experimental alloys, FeCrAl type, influence the formation and stability of superficial oxide layers. Some chemical composition analyses are performed both in the oxide crust and in the material volume, compared with the values of microhardness measurements. It was found that the experimental alloys obtained show a good microstructural homogeneity, balanced distribution of alloying elements and microhardness values within the usual limits for this class.

Experimental part Materilas and methods
The aim of the research were to design and manufacture FeCrAl alloys as mini-ingot, using a VAR (Vacuum Arc Remelting) furnace, documenting the procedure and analysing the microstructure in order to estimate the effect of the addition of titanium on the alloys' characteristics. The samples were designed and manufactured in the ERAMET laboratory, at the Politehnica University of Bucharest, Material Science and Engineering Faculty (www.eramet.wix.com/eramet), using a MRF ABJ 900 furnace [21]. Three experimental FeCrAl alloys were obtained, with different contents of titanium: 0.5%wt Ti, 1.0%wt Ti and 1.5%wt Ti, added into the same metallic matrix of the experimental alloy Fe-14Cr-5Al.
For obtaining the high alloyed material, high purity alloying elements were introduced into the base material: metallic chromium 99.5 % Cr; electrolytic aluminium 99.4 % Al; titanium 99.5 % purity. The raw material on the plate of the VAR equipment is presented in fig. 1. After obtaining the stable vacuum of 1x10 -4 mBar, the furnace chamber was filled with argon (Ar 5.3) to insure the electric arc stability. Each mini-ingot was remelted and solidified five times, in order to obtain the microstructural homogeneity ( fig. 2).
The assimilation coefficient for this particular alloy class was over 99.5 %, due to the low vapour losses of the components in the electric arc remelting process. Mini ingots had quasi-constant weight (39.63g -39.96 g) (table  1).

Results and discussions
Each cross section sectioned from the mini-ingots was processed according to the metallographic procedure using abrasive grit paper, followed by a final polishing using alumina alpha powder. The metallographic analysis was performed with the aim of highlighting the microstructural characteristics of the experimental alloys, in order to     Table 3 shows the estimate the distribution of the chemical elements in the superficial oxide layer [20][21][22][23]. The microstructural analysis was conducted using a scanning electron microscope FEI QUANTA INSPECT F provided with electron gun with field emission -EGF with a resolution of 1.2 nm and a X-ray spectrometer energy dispersive (EDS) with resolution of 133 eV at MnK [25].

Chemical composition and microstructural analysis
The chemical composition of the samples was determined by EDAX analysis in three successive zones (point 1 located on the surface layer, point 2 located at around 15 ìm below the surface and point 3 located in the middle of the sample), in accordance with figure 3. The The chemical composition of the edge zone of the samples microalloyed with titanium indicates that the value of the Ti content is increasing with the increase of the titanium amount in the alloy. This value decreases in the direction of points 1 to 3 due the migration of this element toward the marginal zone, where it combines with the oxygen dissolved in the sample. Because of the simultaneous presence of the two elements with very high affinity for oxygen (aluminium and titanium), complex oxides of Al and Ti are formed in the marginal crust and their share is given by the amounts of these elements in the alloy.
In the case of the sample NUC11, due to the small amount of Ti in the alloy, the superficial oxide layer is discontinuous (like isolated islands) and does not contain large amounts of titanium oxide, but there are predominantly aluminium oxides ( fig. 4). This is evidenced The primary analysis of the experimental data shows that there is a quasi-constancy of the microhardness values, which reflects an increased homogeneity of all samples obtained in the VAR furnace. For all the samples, one can observe that there is a narrow variation of the microhardness values, in the range of 163-183 HV0.2, solely due to the influence of the alloying elements in the metal alloy composition and to the uniform layout of the constituents in the metal matrix. The Vickers microhardness values for these alloys are higher than of other similar alloys microalloyed with hafnium [26]. Furthermore, there is a downward trend of microhardness with the increase of the Ti content in the chemical composition of the alloy.

Conclusions
This research is aimed at obtaining and characterizing the FeCrAl alloy, microalloyed with titanium, potentially usable in nuclear power plants (generation 4R, type LRF). To highlight the effect of alloying with titanium on the mechanical and microstructural characteristics of the alloy, especially on the superficial oxide layers, three batches of FeCrAl alloy with variable content of titanium (0.5 % wt Ti, 1.0 % wt Ti and 1.5 % wt Ti) were obtained. The EDAX analysis performed in the central zone of the samples shows a similitude between the designed and the obtained chemical composition. The Ti distribution reflects the homogeneity of the alloy, the content of this element in the surface oxide layer increases with the increase of the Ti content in the alloy.
The microstructural aspect of different areas, located in the cross section, shows that the surface layer contains complex oxides of elements like aluminium and titanium, whose proportions are given by the concentration of these elements in the alloy composition. The chemical composition of the marginal crust of the samples microalloyed with titanium indicates that the value of the Ti content is increasing with the increase of the amount of titanium in the alloy. The simultaneous presence of the two elements with very high affinity for oxygen (aluminium and titanium), contribute to the formation of complex oxides of Al and Ti, whose share is given by the amounts of these elements in the alloy. The oxide thickness is variable, ranging from 5.247µm (NUC 12) and 20.04 ìm (NUC 13).
The hardness values for FeCrAl alloys microalloyed with titanium are within the range of 163-183 HV0.2, the normal limits for these materials. This is mainly due to the influence of the alloying elements in the metal alloy composition and to the uniform distribution of the metallographic constituents in the metal matrix.