Effects of Initial Alloy on Microstructure and Magnetic Properties During Hot Deformation of Nd–Fe–B HDDR Powder

Abstract

High coercive Nd-Fe-B magnets are indispensable materials to traction motors in hybrid and electric vehicles. However, substitution of heavy rare earth element (HRE) such as Dy or Tb for Nd has been necessary for high coercive Nd-Fe-B magnets despite the critical problem of supply and demand with HRE. Therefore, HRE-lean and/or HRE-free high coercive Nd-Fe-B magnets have drawn a great attention to solve HRE resource problem in the automotive industry [1]. To achieve high coercivity with reducing HRE, the control of microstructures, such as grain size and grain boundary, is of significant importance. Melt-spinning and hydrogenation-disproportionation–desorption-recombination (HDDR) are known as quite suitable method to decrease grain size down to the single domain size $( \sim 250$ nm). It addition, hot-deformation is known as a useful method to obtain anisotropic magnets with magnetic powders produced by these methods. On the other hand, the coercivity of hot-deformed Nd-Fe-B magnets was too low to be used for motors of hybrid and electric vehicles even though they had submicron grain size. It is because of the presence of crystallographic defects, low anisotropic energy at the grain surface, and exchange coupling between neighboring grains, which could be improved by grain boundary diffusion process (GBDP) with HRE compounds or non-magnetic materials. However, the GBDP of ultrafine grained materials produced with melt-spun powders should be done at a temperature, lower than about $700 ^{circ}\mathrm {C}$. The GBDP above $700 ^{circ}\mathrm {C}$ could induce remarkable grain growth and low coercivity. On the other hands, HDDR powder has relatively large grains about $250 \sim 400$ nm compared to melt-spun powders, so the grain growth does not occur at temperature up to about $850 ^{circ}\mathrm {C}$. Therefore, it can be expected that hot-deformed magnets produced with HDDR powder have an advantage for subsequent GBDP compared to that of the melt-spun powder. However, there were only a few studies examining the hot-deformation behavior of HDDR powders and the reported magnetic properties were relatively poor. On the other hand, numbers of research reveal that rare earth-rich phase is critical factor to the texture formation during the hot-deformation process. So, it is expected that the microstructure of initial alloy could effect on the deformation behavior during hot-deformation process. Therefore, in this study, effect of initial alloy on microstructure and magnetic properties during hot-deformation of Nd-Fe-B HDDR powder was investigated.Alloy with composition of Nd 12.5 Fe $_{bal}$ Ga 0.3 Nb 0.2 B 6.4 prepared by strip-casting process was used as a starting material. The alloy was subjected to HDDR process after pre-annealing (SC-HT) at $1100 ^{circ}\mathrm {C}$ for 12 hours or without pre-annealing (SC). Produced HDDR powders were then hot-pressed at $700 ^{circ}$ Cunder 400 MPa in a vacuum. The cylindrical compact with 7 mm in diameter and 6 mm in height were then die-upsetted at $800 ^{circ}\mathrm {C}$ with deformation degree of $\varepsilon =1.5$ and deformation rate of $\varepsilon $'$= \sim 0.01 \mathrm {s}^{-1}$. Figure 1 shows magnetic properties and microstructure of each initial HDDR powder. The remanence of the powders is similar. However, their coercivity is diffenent about 1 kOe. This could be attributed to that the SCuHT HDDR powder have non-uniform and discontinuous Nd-rich phase distribution in grain boundary which can decrease coercivity by magnetic coupling between neighbor grains although it is not clear from SEM image as shown in Fig. 1(b) and (c).Figure 2 shows demagnetization curve ((Fig. 2(a)) and microstructure of hot-deformed magnets which is deformed using SCuHDDR powder (Fig. 2(b)) and SCuHT (Fig.2(c)), respectively. The remanence of hot-deformed magnet with SCuHT HDDR powder is lower than that with SCuHDDR powder. The difference in the remenance could be attributed to the fact that the distribution of the Nd-rich phase of the grain boundary may be affected by the hot deformation behavior which could be confirmed from Fig. 2(b) and (c). This non-uniformed Nd-rich phase of grain boundary can make it difficult to grain boundary sliding during the hot-deformation process, which induce poor grain alignment and low remanence. Based upon these results, effect of initial alloy on microstructure and magnetic properties during hot-deformation of Nd-Fe-B HDDR powder will be discussed. Fig. 1. Demagnetization curve (a) of obtained HDDR powders and FESEM images of SCuHDDR powder (b) and SCuHT HDDR powder.Fig. 2. Demagnetization curve (a) of hot-deformed magnet and FESEM images of hot-deformed magnet with SCuHDDR powder (b) and hot-deformed magnet with SC-HT HDDR powder (c).