Directional Migration Behavior And Mechanism Of Rare Earth In WC-Co Alloy Jointly Doped With Cr/V-RE/La₂O₃

The directional migration and enrichment phenomena of Ln (Ln: rare earth) on the sinter skin of Ln doped cemented carbide have been discovered for the first time by our research group. Similar report has not appeared worldwide. During the early stage research, we mainly concentrated on the study of the existing form of Ln both in the alloy and on the sinter skin, the conditions and mechanism of the enrichment of Ln on the sinter skin of Ln solo doped WC-Co alloy. It was shown that a Ln2O2S film can be formed in situ on the working faces of the as-sintered cemented carbide tool under some kind of service condition, when Ln can be confined in the alloy during the sintering process. As a result, the performance of the alloy can be effectively improved. This research mainly focus on the following aspects:the directional migration behavior of La, Ce, Pr and Nd in Cr/V-RE jointly doped WC-Co alloy during the sintering process, where RE represents mischmetal containing La, Ce, Pr and Nd with La and Ce as the main composition and is added in the form of RE-Co alloy; the directional migration behavior of La in the Cr/V-La2O3 jointly doped WC-Co alloy during the sintering process;the mechanisms for the formation of the Ln containing dispersed phase(s) and for the variation of the phase composition and structure, as well as the self lubrication function of the Ln containing phases. This research is aimed at the full controlling of the migration behavior of rare earth in the alloy. As is known, there are huge difference in the atomic radius between La/Ce and W/Co,29.36～48.97%. Therefore, how to stimulate and get control over the directional migration behavior is not only of great scientific significance, but also very important in the study of the mechanisms for the effect of Ln on the microstructure and properties, the performance improvement of cemented carbide in service, as well as the development of the self-lubricated function oriented ultrafine cemented carbide with high performance.Mischmetal and cobalt pre-alloyed powder with high activity, and high pure nano sized La2O3 with low activity were selected as Ln doping raw materials. SEM, EDS, X-ray diffractometer and optical microscope were used as the observation measures. The sinter kin observation and analysis method, coped with the inner microstructure observation and analysis, as well as the thermodynamic analysis were used in the study of the Ln atoms/ions directional migration behavior and the related mechanism.It is shown that during the sintering process of Cr/V-RE jointly doped WC-Co alloy, La, Ce, Pr and Nd can migrate directionally, the Ln containing dispersed phase (M) in the alloy with WC+β+M microstructure is thermodynamically unstable and part of the La, Ce, Pr and Nd atoms can reunion on the sinter skin, forming in situ a new Ln containing dispersed phase. During the sintering process of Cr/V-La2O3 jointly doped WC-Co alloy with WC+β+ηmicrostructure, La can migrate directionally, forming in site La containing dispersed phase. Whether the Ln atoms/ions is in the form of liquid or solid, the directional migration of Ln atoms/ions must meet both the internal and external environment conditions, i.e. there exist free carbon atoms with high activity in the alloy and there exist S and O impurity atoms in the sintering atmosphere. Therefore, the directional migration behavior of Ln atoms/ions is controllable. Carbide with high solubility in cobalt and with lower stimulation energy for the decomposition of high carbon type into low carbon type can be the stimulation primitive unit of the migration activity of Ln atoms/ions for its offering ability of free carbon atoms with high activity. Because of the existence of WC skeleton structure and the resulted migration barrier, even if the migration of Ln atoms is in the state of liquid, the polarization of Ln atoms is the requisite for the migration of Ln atoms through WC lattice and the resulted fast migration. Under the situation of the polarization, compared with La, Ce and Pr and Nd have the advantage of fast migration when the atomic size factor becomes the controlling one for the migration, because of the lanthanide contraction and the existence of the Ce4+ and Pr4+ irons. The activation energies of the Ln atoms polarization and migration are related to their original activity. For La2O3 doped WC-Co alloy with normal carbon content and the resulted WC+βtwo phase structure, the driving force for the decomposition of Cr3C2 or/and VC into the lower carbon type is decreased;as a result, the migration activity of La ions cannot be activated. With the increasement of the fugacity of S in the sintering atmosphere, LnCoO3, Ln2O2S and Ln2S3 dispersed phase(s) can be formed in turn on the sinter skin of Ln-Cr/V unitedly doped WC-Co alloy. Compared with the values ofΔfGv0 of La2O2S and La2S3, the respective ones of Ce2O2S and Ce2S3 are lower; therefore, the crystal structure of Ln2O2S and Ln2S3 on the sinter skin of Cr/V-RE jointly doped WC-Co alloy are Ce2O2S and Ce2S3 type, respectively. In the situation of relatively higher fugacity of S in the sintering atmosphere, the migration speed of S atoms into the alloy can be accelerated. As a result, parts of the Ce atoms in the alloy combine preferentially with the S atoms for the lower Gibbs free energy compared with the ones of La, Pr and Nd. The advantage of the migration of Ce atoms is thereby cut down and the migration total amount of Ce atoms is deduced. In the mean while, the opening of the migration channel facilitates the fast migration of La atoms; therefore, the Ln2S3 dispersed phase with higher La/Ce ratio than the one in RE-Co powder can be formed. There is good combining strength between the Ln containing dispersed phase and the WC+βmatrix and good wear resistance of Ln containing dispersed phase. The formation of Ln containing dispersed phase on the skin, no mater it is in the form of LnCoO3, Ln2O2S or Ln2S3 can endow the alloy with self lubrication and high heat resistance functions.