Nucleation of the Widmanstatten Pattern in Iron Meteorites

Introduction: The Widmanstatten pattern develops at low temperatures during the evolution of the asteroids. We have studied the origin of the Widmanstatten pattern in order to obtain metallographic cooling rates in the temperature range (~ 700 to 300 deg C). This paper summarizes our recent evaluation of the various mechanisms for the formation of the Widmanstatten pattern. All chemical groups of the iron meteorites are considered [1, 2]. We also propose a new mechanism for the formation of the Widmanstatten pattern in the low P metal phase of iron, stony-iron and stony meteorites. The results of this evaluation enables us to more accurately determine metallographic cooling rates particularly when incorporated with other recent advances in Fe-Ni and Fe-Ni (P saturated) phase diagrams and interdiffusion coefficients. Origin of the Widmanstatten Pattern in Meteorites: The formation of the Widmanstatten pattern depends on the bulk Ni and bulk P in a given meteorite. Five mechanisms have been proposed: (i) γ α+γ (mechanism I): This is the traditional mechanism which is based on the binary Fe-Ni phase diagram. This mechanism assumes that α can nucleate directly from γ phase within the α+γ two phase region when cooling from high temperature to low temperature. However, several experimental studies have shown that it is not possible to nucleate α phase directly within single crystal taenite, γ phase, in synthetic Fe-Ni alloys during cooling and before forming martensite, α2 phase [3, 4]. (ii) γ γ+Ph (phosphide, (FeNi)3P) α+γ+Ph (mechanism II): The important role of P in the formation of the Widmanstatten pattern in iron meteorites has been recognized, Goldstein et al. [5]. These authors showed, experimentally, that this mechanism can produce a Widmanstatten pattern in high P containing Fe-Ni-P alloys. (iii) γ α+γ α+γ+Ph (mechanism III): This mechanism was proposed by Moren et al. [6] in their cooling rate simulation of low P iron meteorites. However, under-cooling of 120-190 C below the equilibrium γ/(α + γ) phase boundary is necessary in many low P IVA iron meteorites. Narayan et al. [4] investigated, experimentally, the nucleation of intragranular α phase from γ phase in Fe-Ni-P alloys. They found that, in low P Fe-Ni alloys, α phase cannot nucleate when the alloy is cooled into the two-phase α+γ field. In fact, the α phase only nucleates when γ phase is saturated in P and enters the α+γ+Ph field. (iv) γ α2 α+γ (mechanism IV): This mechanism was initially proposed as an alternate to mechanism I, γ α+γ, for iron meteorites by Owen [7] and Buchwald [8]. Recently mechanism IV was used to determine the cooling rate of mesosiderites and low P IVA iron meteorites [9, 10]. However, the Widmanstatten pattern cannot form by this mechanism in low P meteorites [2], even though this mechanism is applicable to the formation of the plessite structure in taenite lamellae. (v) (γ α2+γ α+γ+(Ph)) (mechanism V): This mechanism was recently proposed by Yang et al. [1, 2] and can be used to explain the origin of the Widmanstatten pattern in low P meteorites. In this case, α2 forms once the martensite start temperature is crossed during cooling and before P saturation occurs. The kamacite phase nucleates near the Ms temperature by the reaction α2 α+γ.