Nucleation of martensite

The homogeneous nucleation of martensite in typical solids is too slow by many orders of magnitude to account for observed results. Calculations of typical values of AQc using the classical nucleation model of Section 19.1.4 (see Exercise 19.3) yield values greatly exceeding 76 kT. Furthermore, nearly all martensitic transformations commence at very sparsely distributed sites. Small-particle experiments [14] have yielded typical nucleation densities on the order of one nucleation event per 50 pm diameter Fe-Ni alloy powder particle.3 Thus, nucleation of martensite is believed to occur at a small number of especially potent heterogeneous nucleation sites. 

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G denotes the shear modulus and a is the specific interfacial energy. In the sense of Eqn. (6.8), we can use Eqn. (12.5) to calculate the activation energy for the nucleation of martensite. Normally, AGtr >RT, which implies that martensite nucleation is unlikely to be induced by thermal fluctuations. We conclude that the nucleation is heterogeneous and dislocation arrays are the nucleation sites. 

Szklarska-Smialowska, Z., Electron Microprobe Study of the Effect of Sulphide Inclusions on the Nucleation of Corrosion Pits in Stainless Steels , Br. Corros. J., S, 159 (1970) Weinstein, M. and Speirs, K., Mechanisms of Chloride-activated Pitting Corrosion of Martensitic Stainless Steels , J. Electrochem. Soc., 117, 256 (1970)... 

G.B. Olson and M. Cohen. Theory of martensitic nucleation A current assessment. In Proceedings of an International Conference on Solids Solid Phase Transformations, pages 1145-1164, Warrendale, PA, 1982. The Metallurgical Society of AIME. 

R.E. Cech and D. Turnbull. Heterogeneous nucleation of the martensite transformation. Trans. AIME, 206 124-132, 1956. 

At Ms temperature TiNi initiates a uniform (inhomogeneous) distortion of its lattice — through a collective atomic shear movement. The lower the temperature, the greater the magnitude of shear movements. As a result, between Ms and Mr temperature the crystal structure is not definable. In sharp contrast, other known martensitic transformations initiate a nonuniform (heterogeneous) nucleation at Ms and thereafter the growth of martensite is achieved by shifting of a two dimensional plane known as invariant plane [28] at a time. Thus, between Ms and Mr temperature the crystal structure is that of austenite and/or martensite . 

The relationship of brittle fracture to plastic deformation has, of course, been elaborated in various ways with the aid of dislocation theory, e.g. nucleation of microcracks has been discussed in terms of piling-up of dislocations [124]. Davies [145] has shown that embrittlement requires the presence of islands of martensite (about 1 pm in size) and has suggested that cracks are initiated in the martensite or at the martensite-ferrite interface. 

In the shape-memory transformation described, only the shape of the parent phase is remembered . It is called the one-way shape-memory effect. It is also possible to produce alloys that display two-way shape-memory effects. In these materials, both the shape of the parent phase and the martensitic phase is remembered . This reversible effect is caused by the fact that the nucleation of the martensite is very sensitive to the stress field. Introduction of lattice defects such as precipitates can restrict the number of variants that form and the positions where they nucleate. Such materials generate the martensitic shape on cooling below the temperature Mf. Cycling between higher and lower temperatures causes the alloy to switch alternately between the two shapes. There is considerable research interest in developing and exploiting two-way shape-memory effect alloys at present. 

While the nucleation scenario under an applied shear stress that we are about to develop below closely resembles that of nucleation of a martensite embryo (Orowan 1954), it contains no corresponding interface energy, since in an amorphous structure the border of the shear transformation has no distinguishable feature that is different from the atomic packing in the surrounding amorphous structure. 

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

Martensitic traasfonnation Master ec[uations Mean field crossover to Ising Mechanical properties Metallic alloys Metallic glasses Metastable alloys Microhardness test Microscopic theory of nucleation... 

Solids undergoing martensitic phase transformations are currently a subject of intense interest in mechanics. In spite of recent progress in understanding the absolute stability of elastic phases under applied loads, the presence of metastable configurations remains a major puzzle. In this overview we presented the simplest possible discussion of nucleation and growth phenomena in the framework of the dynamical theory of elastic rods. We argue that the resolution of an apparent nonuniqueness at the continuum level requires "dehomogenization" of the main system of equations and the detailed description of the processes at micro scale. 

Phase transitions in solids are also fruitfully classified on the basis of the mechanism. The important kinds of transitions normally encountered are (i) nucleation-and-growth transitions (ii) order-disorder transitions and (iii) martensitic transitions. 

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