Effect of
Ti
addition in refinement of oxide dispersoids in
Fe‐Y2O3‐Ti
model
ODS
alloys during milling and subsequent annealing
Authors: Parida Pradyumna Kumar , Dasgupta Arup , Raghavendra K. G. , Chakravarty Sujay , Jayasankar K , Saibaba Saroja 
Published: December 2016

The oxide dispersion strengthened (ODS) steels have very high thermal stability and creep strength due to reinforcement of hard and stable nano‐crystalline ceramic oxides in a metallic matrix which act as barriers to dislocation motion. These steels are candidate core structural materials for advanced fission and fusion reactors. The dispersoid size and their distribution in metal matrix play an important role in deciding physical and mechanical property of the steel. However understanding the formation mechanism and crystallographic structure of the nano‐dispersoids is an ongoing activity of various research groups. The yttria based ODS steel, which contains 0.35wt% of yttria and 0.2 wt% of Ti amounting to ~ 0.56 vol % is very difficult to characterize by conventional XRD; TEM is possibly the only method to characterize the nano‐sized (~2‐5nm) dispersoids. But to understand the mechanism of formation of nano‐dispersoid, crystallinity and structural changes one needs to employ both XRD and TEM. For this purpose, a Fe‐15wt% Y
2
O
3
–xwt%Ti (0, 5 and 15) type of concentrated alloy has been used. However, characterisation of samples prepared by conventional methods for TEM studies continues to be difficult owing to magnetic nature of ferritic steel. Hence FIB was employed to extract electron transparent samples which are of micrometer dimensions. The model ODS alloys were synthesized by ball milling for various time durations in inert atmosphere and with subsequent annealing at 1273K for 1h. The structural studies for the powders prior to milling as well as after milling and subsequent annealing were carried out using electron microscopy and XRD techniques. It was earlier reported by us [2] that yttria dispersoids gets refined and even amorphised during mechanical alloying (MA) and upon annealing yttria re‐precipitates to micron size [2]. It is believed that Ti can inhibit the growth of nano‐dispersoid during annealing by formation of Y‐Ti‐O complex oxides such as Y
2
Ti
2
O
7
or Y
2
TiO
5
or YTiO
3
[3‐5]. In this study XRD result revealed amorphisation of Y
2
O
3
/ Ti during milling for 60h in Ar atmosphere and evolution of Y
2
Ti
2
O
7
andYTiO
3
complex oxide upon annealing for 1h at 1273K, which is shown in figure‐1. The scanning electron microscopy studies reveal the refinement of alloy powders from ~50 microns to sub micron level during milling, which is shown in figure 2
.
Figure 3(a) and (b) represent the Selected Area Diffraction pattern (SADP) of 60h milled powders of Fe‐15wt% Y
2
O
3
and Fe‐15wt% Y
2
O
3
‐5wt%Ti respectively. Analysis of these patterns showed the 1
st
ring correspond to Y
2
O
3
(411) was broader in case of the alloy without Ti as compared to alloy with Ti. Figure 3(c) and (d) represents the bright field (BF) TEM micrographs of Fe‐15wt% Y
2
O
3
‐5wt%Ti model ODS after 60h of milling and subsequent annealing respectively, the corresponding SAD patterns are shown as inset. The analysis of BF‐TEM micrograph showed the recrystallization of 100‐200nm size crystallites upon annealing of milled powder, where as analysis of SADP reveal formation of Y‐Ti‐O complex oxides such as Y
2
Ti
2
O
7
, Y
2
TiO
5
and YTiO
3
in annealed alloy powder. EDS analysis from the 60h milled and annealed powder of Fe‐15wt%Y
2
O
3
‐5wt%Ti showed evolution of Y, Ti ,O rich complex oxide which is shown in figure 4. The results point towards a direct involvement of Ti in restricting amorphisation of yttria during milling, and formation of Y‐Ti‐O complex nano‐dispersoid upon annealing. Details of these studies will be presented in the paper.