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  • Študij peritektične reakcije L + [alfa]-AlB2 <->AlB2
    Zupanič, Franc ...
    In this work the peritectic reaction L + [alpha]-AlB2 <-> AlB2 in the system Al - B was investigated. It was not possible to determine the peritectic temperature by means of the differential thermal ... analysis (DTA) of the alloys AlB3 and AlB1. Therefore the isothermal annealing of the alloys between 650 and 1000 oC was carried out, and the results show that this reaction takes place at about 900 oC. This temperature is considerably lower than that published in the Binary alloy phase diagrams /90Mas/. In Al-B alloys containing more than 1 wt.% B the solidification sequence begins with the primary crystallisation of [alpha]-AlB12. It was observed that the peritectic reaction does not take place during continuous cooling even at a cooling rate as low as 10 oC/min, since primary [alpha]-AlB12 particles do not reperesent a convenient substrate for heterogeneous nucleation of AlB2, which should nucleate somewhere in the remaining liquid. Thereafter both phases [alpha]-AlB12 and AlB2 grow competitively until the solidification of the remaining liquid stops with the nonvariant eutectic reaction. It was observed that the high temperature [alpha]-AlB12 phase pertectically decomposes during isothermal annealing under 900 oC. The characteristic feature of this reactionis that both phases are faceted and that they try to keep faceted interfaces with the liquid during growth and dissolution. Due to the predminant growth of AlB2 in preferable directions it is not able to completely envelop the primary [alpha]AlB12 and the peritectic transformation takes place with the simultaneous growth of AlB2 and the dissolution of [alpha]-AlB12. When small [alpha]-AlB12 particles (up to 10 micro m) are present, AlB12 grows almost independently from dissolving [alpha]-AlB12, which may occasionally become engulfed into the growing AlB2. On the other hand, when larger (> 50 microm) and faceted [alpha]-AlB12 particles are present, then several AlB2 particles are nucleated or more possibly attached to their surface, and thereafter they grow in the faceted manner at the expense of the high temperature phase. The peritectic reaction in the direction L + [alpha]-AlB12 is quite slow, since even after 70 h of exposure at temperatures under 900 oC only a part of [alpha]-AlB2 decomposes. The peritectic decomposition of [alpha]-AlB12 is observed also in the solid-state. In this case AlB2 particles, which have been nucleated during cooling,grow at the expense of metastable [alpha]-AlB12 and the reaction rate is possibly controlled by the solid-state of boron. The peritectic reaction in the opposite direction is appreciably faster, because after 15 min of exposure at 940 oC most of AlB2 particles become completely dissolved, but the surviving particles exhibit a spheroidal and no longer faceted morphology due to the relatively fast dissolution rate.
    Source: Livarski vestnik : glasilo Društva livarjev Slovenije. - ISSN 0024-5135 (45, št. 3/4, 1998, str. 86-93)
    Type of material - article, component part
    Publish date - 1998
    Language - slovenian
    COBISS.SI-ID - 4254486

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