This article presents data on the mechanism underlying the nucleation and growth of a-Sn crystals in mono-and polycrystal B-Sn tin. It is demonstrated that the low temperature deformation and thermocycling (subjection to the cyclic changes of temperature) of the samples of tin (99.9995 yo Sn) brings to the increase in the a-Sn crystal nuclestion rate.Kinetics of the nucleation is characterised by the presence of the non-steady state period t,.On analysing the temperature dependencies of the rate of the a-Sn centre growth obtained for the planes (loo), (001) and (110) it was shown that the mechanism of movement of the interphase boundary depends on the value of the motive force A P . @a3HOn I'paHIlUbI 3aBHCHT OT BeJIHqMHbI nBIIlKYWefi CMJIM dp.
This article presents data on the impurities influencing the , ! ? 4 a transformation of high purity tin (99.9995%): temperature of the a ~, 9 transition-To, nucleation and acrystal growth. A detailed discussion of the problem of the nucleation and a-crystal growth of highpurity tin (99.9995% Sn) was given in a previous review (LARIKOV, BRICE). This article deals with the question of impurities influencing some parameters of the j3 -+ OL transforniation of high-purity tin. B HacTomueik pa6ol.e npencTasneHHThe data on the impurity effect on the process of the p -a transformation of tin obtained for up to -60 "C are put together in the reviews by BUSCH, KERN, BY-KHOVSKY. Supplementary information concerning this problem may be found also in the work of LOHBERG, PRESCHE ; MACINTOSCH. On the whole, the data on the effect of impurities on the p + a transformation of tin may be divided into three groups: those concerning the effect of impurities on the p a transition temperature, on the formation of the a-Sn nuclei and on the growth rate of the a-Sn crystals. polymorphic transformation To, there are essential disagreements in literature. Thus, COHEN and EIJK give the values of 20" and 18 "C; COHEN, DECKER give 12" to 14.3 "C (see reviews by BUSCH and BYKHOVSKY). Further, RAYNOLD, SMITH established for 99.997% pure tin with lead as the main impurity (0.003~0 Pb) that To = 13.2 "C; if the main impurity is iron then To = 10.4 "C. As it was noted already, the last determination of To = + 1.5 "C was made for 99.9995% purity tin (BYKHOVSKY, LARIKOV, FALCHENEO). The given result implies that the presence of small amounts of impurities leads, as a rule, to To increase.The question about the effect of impurities on the rate of the a-Sn centre nucleation under the j3 -+ a transformation of tin was considered in many works.It should be noted, first of all, that similarly t o deformation (CEERTOCK; LOHBERO, PRESCHE) of semiconducting compounds such as InSb, CdTe isomorphic t o tin, lead to the essential shortening of the incubation period preceding the appearance of the a-Sn nuclei, that is, bring to the acceleration of the j3 --t a transformation of tin (GORYUNOVA). There are also data which show that the particles of the second phase, crystallized from the supersaturated solid solution a t low temperature, accelerate the nucleation and , ! l -+ OL transition on the whole (ROGERS, FYDELL).The effect of soluble impurities on the nucleation and growth of the centres of a new phase is determined by the character of their distribution, that is, the effect As to the temperature of the oc 10
The analysis of dynamic recrystallization influencing the crystal growth of the new phase in high purity tin and thallium (99.9996%) is carried out. It is shown that in tin, in which the p --f a transition proceeds on the dislocation or the normal mechanisms (depending upon a supercooling degree, A T ) a superposition of dynamic recrystallization results in an increase of the growth velocity of a-Sn crystals. The most distinct of acceleration is observed at -21 f -23 "C, when the growth velocity of recrystallization centers and the maximal growth velocity, Vmax, of the crystals are comparable values. The dynamic recrystallization superposition in thallium offers to transform the transformation machanism. Thereby an erase of the features of the martensite a @ transition mechanism and an appearance of the features of the normal crystal growth mechanism of the new phase are observed.Es wird eine Analyse einer dynamischen Rekristallisation vorgenommen, die das Wachstum der neuen kristallinen Phase in hochreinem Zinn bzw. Thallium (99.9995%) beein-fluBt. Es wird gezeigt, daB im Zinn, in dem der p + a Ubergang an Versetzungen oder..iiber normale Mechanismen vor sich geht (abhangig vom Ubersiittigungsgrad A T), eine Uberlagerung einer dynamischen Rekristallisation in einer Zunahme der Wachstumsgeschwindigkeit der a-Sn-Kristalle resultiert. Der groBte Beschleunigungseffekt wird bei -21 bis -23 "C beobachtet, wenn die Wachstumsgeschwindigkeit der Rekristallisationszentren und die maximale Wachstumsgeschwindigkeit, V,,, der Kristalle vergleichbare Werte erreichen. Die dynamische Rekristallisationsiiberlagerung im Thallium zeigt eine Wandlung des Transformationsmechanismus. Dabei verschwinden die Merkmale des a z p Martensitubergangsmechanismus, und Erscheinungen des normalen Wachstumsmechanismus der neuen Phase werden beobachtet.
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