Oxide reinforced Ni base composite prepared by spark plasma sintering

Y2O3 is a favourable reinforcement for the oxide dispersion strengthened superalloy due to its good high temperature stability, excellent strength and large elastic modulus. In this work, Y2O3 is introduced into the K4169 superalloy aiming to improve its high temperature properties. The high quality Y2O3 and K4169 powder mixture can be obtained with the addition of the process control agent of stearic acid after high energy ball milling, and the Y2O3/K4169 composite with uniformly dispersed reinforcement of nanoscale Y2O3 particles are finally synthesized with the ball-milled powders by using spark plasma sintering.


Introduction
Different from the technical route of the optimization of composition and microstructure, the oxide dispersion strengthening (ODS) is an effective method to improve the high temperature performances of materials [1].Thus, the synthesis of ODS superalloys has attracted wide attention.Yttria (Y2O3) is a common reinforcement for ODS superalloy due to its high thermal stability, favourable strength and elastic moudulus [2].Benjamin [3] has prepared the Y2O3 reinforced superalloy by using mechanical alloying in 1970s.Later, a few ODS superalloys have been invented, including MA754 and PM3000, in which the concentration of Y2O3 is 0.6wt.%and 0.9wt.%Y2O3 respectively.The superalloys strengthened by Y2O3 have been widely applied in aero engines.Moreover, Y2O3 also can react with the elements in matrix or other added elements to form new reinforcing phases, resulting in better strengthening effect.The Y3Al5O12, YAlO3 and Y4Al2O9 phases are formed in the MA6000 alloy due to the reactions between Y2O3 and Al during the mechanical alloying [4].the addition of Hf in PM1000 ODS superalloy leads to the formation of Y2Hf2O7 which has better interface characteristics with the matrix compared to Y2O3 [5].
K4169 alloy is a commercial superalloy which has similar composition and microstructure with IN718C alloy.The high temperature strength of K4169 is not satisfactory though it has excellent comprehensive properties when the service temperature is lower than 650 o C [6][7][8].
In this work, Y2O3/K4169 composite is prepared by using high energy ball milling (HEBM) and spark plasma sintering (SPS) to improve the high temperature strength of matrix K4169 superalloy.The microstructures of the ball-milled Y2O3 and K4169 powders as well as the microstructure of Y2O3/K4169 composite after SPS are studied.

Materials and method
High-purity Y2O3 powders and K4169 powders are used as raw materials.Spherical K4169 powders (45 μm) and nanoscale Y2O3 powders (50 nm) are added to a ball milling jar.The weight ratio of Y2O3 in the powder mixture is 10wt.%,and 0.5wt.% of stearic acid is used as process control agent for HEBM.The weight ratio of steel balls to the powder mixture is 10:1.The HEBM is conducted with a rotating speed of 300 rmp, and the ball milling time is 12h.After HEBM, the passivation of powders is conducted under 0.2 atm for 30 min in an argon glove box to avoid self-burning due to the high activity of ball-milled powders.Then the ball-milled powder mixture is sintered in SPS apparatus at 1050 ºC under the pressure of 50 MPa for 5 min.
The microstructure of ball-milled powders before SPS and the Y2O3/K4169 composite after SPS are investigated by using scanning electron microscope (JSM7600F).The phases of the ball-milled powders before SPS and the Y2O3/K4169 composite after SPS are analysed by X-ray diffractometer (Bruker AXS D8), Cu target and a scanning speed of 0.2 °/s are employed for XRD tests.The energy dispersive spectrometry results of a ballmilled powder, as shown in Figure 2.Both oxygen and yttrium are uniformly distributed in the ball-milled powder, without obvious aggregation.The result confirms that high quality Y2O3 and K4169 powder mixture can be prepared after HEBM for 12h with the help of process control agent of stearic acid.

Results and discussion
Figure 3 shows the XRD pattern of the ball-milled Y2O3 and K4169 powders after HEBM.The characteristic diffraction peaks of Ni and Y2O3 are clear and sharp.Moreover, no other characteristic diffraction peak is found in Figure 3, indicating that no impurity is formed during the HEBM.
The XRD pattern of Y2O3/K4169 composite after SPS is shown in Figure 4.The characteristic diffraction peaks of γ phase and Y2O3 are observed, indicating that the Y2O3 was thermal stable during the SPS.Considering that the standard heat treatment is not carried out for Y2O3/K4169 composite after SPS, it is reasonable that the characteristic diffraction peaks of γ′ and γ″ phases are not found in the XRD pattern.It is believed that the nanoscale Y2O3 could enhance the material strength through the dislocation pinning mechanism.With the coarsening of γ′ and γ″ phases during long time service of Y2O3/K4169 composite, the strengthening effect from γ′ and γ″ is greatly decreased, but the Y2O3 could still work as reinforcement to strengthen the matrix, due to its high thermal stability.In the future, the deformation behavior and the microstructure evolution after heat treatment will be investigated to further optimize the microstructure of Y2O3/K4169 composite, aim to improve the hightemperature performance of Y2O3/K4169 composite.

Conclusion
The high quality Y2O3 and K4169 powder mixture can be obtained by HEBM which is performed with a rotational speed of 300 rmp for 12 h under argon protection, and the Y2O3/K4169 composite with uniformly distributed nanoscale Y2O3 particles is prepared by SPS of the ballmilled powders.It is believed that the nanoscale Y2O3 in Y2O3/K4169 composite could greatly enhance the material strength through the dislocation pinning mechanism during long time service due to its high thermal stability.The optimization of the microstructure of Y2O3/K4169 composite will be further investigated in the next research work.

Figure 1
Figure1shows the SEM images of the ball-milled powders.The size of the K4169 powders is greatly decreased from 45μm to 20μm.No obviously scattered Y2O3 particles are found in the ball-milled powders, indicating that the nanoscale Y2O3 powder and microscale K4169 powders are homogeneously mixed after HEBM.

Figure 2 .
Figure 2. The distribution of elements in the ball-milled Y2O3 and K4169 powders after HEBM.