Influence of Mechanical Alloying Time on Morphology and Properties of 15Cr-ODS Steel Powders

Introduction

Generation IV nuclear fission reactors such as supercritical pressurized water reactor (SCPWR) and lead bismuth-cooled fast reactor (LFR) are aimed at making revolutionary improvements in our lives to cope with climate change [1, 2]. Developing Cladding material is very challenging due to high level of neutron displacement damage [3]. Nanostructured oxide dispersion strengthened (ODS) steels with chromium (Cr) concentration 9–18 mass% are the leading structural material candidates for advanced fission and fusion reactor application due to their excellent irradiation resistance as well as high-temperature creep strength [4–8]. High-density nanoscale Y–Ti–O-rich clusters/precipitates play a key role in improving the radiation tolerance and high-temperature properties [9]. For SCPWR and LFR with highly corrosive coolants, an Al-alloyed high-Cr-ODS steel with Zr addition was successfully developed to meet the urgent requirements of fuel cladding materials [10, 11]. Due to the low solubility limit of the oxides in metals, it is not easy to process ODS steels through conventional methods such as melting, working and casting. Sakasegawa et al. found that oxide particles aggregate together and coarsen during conventional casting processes [12]. Benjamin et al. firstly used mechanical alloy (MA) for elaboration of the Ni-based ODS alloys [13]. It is beneficial to utilize this technique to form uniform dispersion of nanosized oxides in the metal matrix [14, 15]. After MA, during the consolidation and heat treatment processes, nanoscaled oxide particles formed with a very high density [16]. Nanosized dispersoids act as obstacles against the movement of dislocations and growth of grains through dispersion interaction offers excellent high-temperature creep resistance [17]. However, MA is a complex process and a series of process variables should be considered in order to achieve the desired performance-related properties. It has been reported that the morphology, grain size distribution of the milled powders strongly depend on the process variables in MA such as milling atmosphere, etc. [18]. However, the effects of milling time on the microstructure and properties of precursor powders have not been understood comprehensively yet.

In this research, Fe–15Cr–4.5Al–0.3Ti–0.3Zr–0.3Y₂O₃ was selected. The objective of this work was conducted to study the effects of milling time on the morphology, grain size and Vickers hardness of milled powders systematically.

Experimental

The raw materials used in this study were elemental powders of Fe, Cr, Al, W, Zr and Ti (a purity of 99.9% and particle sizes between 1 and 50 μm) and Y₂O₃ particles (about 50 nm in size). MA was carried out in FRITSCH Pulverisette 5 high-energy planetary mill with the chemical composition of Fe–15Cr–4.5Al–0.3Ti–0.3Zr –0.3Y₂O₃ (in weight percent). The high chromium content was added to enhance strength and improve corrosion resistance. The addition of aluminum led to the formation of a stable, adherent α-Al₂O₃ scale providing excellent high temperature oxidation resistance. A small amount of Ti can improve the adherence of the alumina surface scales. The creep strength of the Al-added ODS ferritic steels was drastically improved by Zr addition. The vial loaded with powders was sealed in a glove box under an argon (99.9999%) atmosphere in order to avoid oxidation of powders during ball milling. The ratio of ball-to-powder mass was 10:1. The rotation speed was 260 r/min using stainless steel balls, and the milling time of 5 min, 30 min, 1 h, 2 h, 4 h, 8 h, 12 h, 20 h, 30 h, 40 h, 50 h, 60 h and 70 h were adopted, respectively. The morphologies and the distribution of element of MA powders were observed on JSM-6510A scanning electron microscope (SEM). The element distribution was characterized by using the energy dispersive spectroscopy (EDS) equipmenting on Shimadzu SSX-550 SEM instrument. LA-920 laser scattering particle size analyzer was applied to measure the mean powder size of milled powders. The phase constitutions of milled powders were determined by using X’Pert Pro X-ray diffraction (XRD), with a Cu Kα target, scanning at a voltage of 40 kV, a current of 40 mA and a speed of 2°/min. Grain size (D) and lattice strain (ε) of MA powders were estimated from XRD patterns according to Scherrer formula (1) and (2), respectively:

where θ is the Bragg angle, λ is the X-ray wavelength and β is the full width at half maximum (FWHM). The Vickers hardness of MA powders was measured on 401MVDTM Vickers hardness tester. The load value and loading time were 0.01 N and 10 spots, respectively.

Results and discussion

Particle sizes and morphology

Figure 1 shows the SEM morphologies of Fe-Cr-Al-W-Ti-Zr-Y₂O₃ powders with different milling time, which distinctively exhibited the progressive changes in morphology of mechanically alloyed particles. In the initial 5 min, most of powders maintained the original shape, and only a small portion of alloy powders was flattened, as shown in Figure 1(a). After 1 h milling, the ductile FeCrAl particles were soft and flattened. The mean powder size reaches to the maximum of 41 μm due to the repeated flattened and cold welding (Figure 1(c)). After 2 h milling, there was a significant change in the particle size down to about 24 μm because of dominated work hardening and fracture as shown in Figure 1(d). With continual deformation, the particles were work hardened and fractured by fatigue failure or fragmentation. Up to 20 h, mean powder size decreased significantly. From 20 to 50 h, the mean powder size decreased gradually, and the shape of MA powders trended to become spherical again. It can be inferred that the milling process is close to steady-state equilibrium. After 60 h MA, the mean powder size and morphology of alloy powders showed no obvious change compared to 50 h MA powders. Extending milling time to 70 h, the powder agglomerated again and mean powder size increases. The mean size of powders with the different milling time is shown in Figure 2. The results obtained from the laser scattering particle size analyzer were in good agreement with the SEM data.

Figure 1:

SEM morphologies of 15Cr-ODS powders milled for (a) 5 min, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 4 h, (f) 8 h, (g) 12 h, (h) 20 h, (i) 30 h, (j) 40 h, (k) 50 h, (l) 60 h and (m) 70 h.

Figure 2:

Particle size distribution of 15Cr-ODS steel MA powders as a function of milling time.

XRD and EDS analysis

The XRD spectra of the milled powders with different time are shown in Figure 3. These indicated that almost all the alloying elements dissolved into the matrix. It could be seen that it was difficult to distinguish between Fe and Cr in XRD spectra as their positions of reflection peaks were close to each other and partially overlapped. Therefore, the detection of solid solubility of nanostructured 15Cr-ODS alloys could only be done by measuring changes in the reflection peaks of Al, W and Y₂O₃. Comparison of different milling duration showed that the Al peak disappeared after 1 h milling and Y₂O₃ peak disappeared after 0.5 h milling. With the increasing of milling time, the peak of W gradually decreased until disappear after 50 h milling. As the increasing of milling time, it was clear that the diffraction peak intensity decreased and diffraction peak width increased gradually, indicating grain refinement and/or increasing lattice strains. The peak position of α-(Fe, Cr) reflections slightly shifted to lower 2θ angles as the mill time increases, implying an increase in the lattice parameter of Fe, which was consistent with Fe forming a solid solution with Cr W or Al. Particles suffered severe plastic deformation during the long time milling which provided a high density of defects associated with large local strains. Figure 4 presents EDS element mapping of the MA powders after 50 h milling. It demonstrated that Al, Y₂O₃ and W were completely solid solution into the matrix.

Figure 3:

XRD patterns of 15Cr-ODS steel MA powders with different MA time.

Figure 4:

EDS elemental mapping of 15Cr-ODS steel MA powders after 50 h milling.

Crystallite size and lattice distortion analysis

The crystallite size of MA powders estimated from XRD patterns using Scherrer equation as the function of milling time is shown in Figure 5. It is illustrated that the crystallite size progressively decreased with the increasing of milling time (around 16.5 nm for 50 h milling). After that, the crystallite size showed no change obviously. Figure 5 shows an increase in the lattice distortion with the increasing of milling time (about 0.56% for 70 h milling). This implied that the MA not only refined the size of powders but also caused lattice strain and hence increased the number of dislocations in the powder crystals.

Figure 5:

Dependence of grain size and lattice distortion of 15Cr-ODS steel MA powders with different MA time.

Vickers hardness analysis

Figure 6 shows the Vickers hardness change of the MA powders with different milling time. From 5 min to 50 h, the MA powders subjected severe deformation and the dislocation density inside the grain increased rapidly. Hardness of the particles were measured and appeared to increase with increasing milling time. It remained constant after 50 h of milling. Otherwise N. Baluc et al. [19] proved that the defect or dislocation density saturation in the particles will increase Vickers hardness.

Figure 6:

Vickers hardness of 15Cr-ODS steel MA powders as a function of milling time.

Conclusion

During MA, the morphology of powders changed from the initial spherical or flattened to flattened structure, and finally to nearly spherical again. The particle size progressively decreased with increasing milling time. XRD spectra demonstrated that disappearance of the Al, W and Y₂O₃ peak and the formation of α-(Fe, Cr) solid solution is evident with the increasing of milling time. All α-(Fe, Cr) reflections exhibited broadening and intensity reducing with increasing milling time, indicating grain refined and lattice strain increased. Mean powder size, grain size and Vickers hardness of MA powders reached to steady-state equilibrium beyond 50 h of the milling time.