ALBERTO BOLLERO REAL

5.1 Nanocrystalline powder processing
earths are chemically very reactive. The high thermal conductivity of copper and the high
surface velocity of the wheel, v s , are responsible for the high cooling or quench rate (about
10 6 K/s) attainable by this technique, and it can be varied by changing v s which, in a first
approximation, is directly proportional to the cooling rate. There is a strong dependence of
the coercivity on the wheel speed. The melt-spun ribbon is magnetically isotropic and
forms the basis of a range of commercial magnets (so-called “MQ-powder: MQ1”). There
are various production routes as, for example, hot-pressing (dense isotropic magnets:
MQ2) with subsequent die-upsetting or backward extrusion (dense anisotropic magnets:
MQ3) and polymer resin bonding techniques [89-93]. A more detailed explanation about
the hot deformation process will be given in Section 5.1.4.
Ribbon flakes in this study were prepared changing the parameters (v s = 30 - 40 ms -1 )
according to the composition to obtain a partly amorphous structure. Heat treatment of the
flakes was carried out at temperatures between 600 and 800 °C for 15 min under vacuum at
pressures lower than 10 -4 mbar.
5.1.2 High energy ball milling
Mechanical alloying was developed in the 1970s as a technique for dispersing
nanosized oxide inclusions into nickel based alloy powders [94]. Yeh et al [95] were the
first demonstrating an amorphisation by solid-state reaction using diffusion for the reaction
of hydrogen with crystalline Zr 3 Rh. Schwarz and Johnson [96] established the possibility
of applying this method to metallic systems. Schultz et al [97] demonstrated for the first
time that Nd-Fe-B magnets could also be produced by mechanically alloying Nd, Fe and B
and annealing the amorphous phase. During the last twenty years mechanical alloying has
been widely used in laboratories to prepare a variety of alloy powders because of its
versatility. It is performed in a high-energy ball mill, under argon, starting with powders of
the constituent elements. When the balls collide, the powder particles are trapped, being
heavily deformed and welded. In the beginning, powder with a layered microstructure is
produced with non-deformable powder particles embedded in the welded interfaces. In the
case of Nd-Fe-B powder, the layered microstructure will be formed of Fe and Nd with the
submicron undeformed B particles embedded in the Fe/Nd interfaces [98]. It is necessary
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