Gregory B. Olson

Bio: Gregory B. Olson is an academic researcher from Northwestern University. The author has contributed to research in topics: Nucleation & Diffusionless transformation. The author has an hindex of 58, co-authored 314 publications receiving 13278 citations. Previous affiliations of Gregory B. Olson include Massachusetts Institute of Technology & Lawrence Livermore National Laboratory.

Kinetics of strain-induced martensitic nucleation

Abstract: Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.

Computational Design of Hierarchically Structured Materials

Abstract: A systems approach that integrates processing, structure, property, and performance relations has been used in the conceptual design of multilevel-structured materials. For high-performance alloy steels, numerical implementation of materials science principles provides a hierarchy of computational models defining subsystem design parameters that are integrated, through computational thermodynamics, in the comprehensive design of materials as interactive systems. Designed properties combine strength, toughness, and resistance to impurity embrittlement. The methods have also been applied to nonferrous metals, ceramics, and polymers.

A mechanism for the strain-induced nucleation of martensitic transformations*

Abstract: The previous work of Professor W. G. Burgers and Dr. A. J. Bogers is used to develop a mechanism of strain-induced martensitic nucleation, involving two intersecting shear systems. We distinguish between strain-induced nucleation and stress-assisted nucleation, the latter involving the same sites and embryos as does the regular spontaneous transformation. The strain-induced nucleation, on the other hand, depends on the creation of new sites and embryos by plastic deformation; this phenomenon may also contribute in a major way to autocatalytic nucleation during the course of martensitic transformation. For the case of strain-induced nucleation, it is possible to focus on specific intersecting-shear systems when the austenitic stacking-fault energy is low and e (h.c.p.) martensite can form as a part of the shear displacements. It then becomes feasible to extend the intersecting-shear mechanism from this special case to alloys of higher stacking-fault energy, where e is no longer stable relative to the austenite. It should be noted, however, that these events are very early stages in the formation of martensitic plates and relate primarily to the genesis of embryos; the actual growth start-ups which determine the operational (measured) nucleation rates may be controlled by subsequent processes.

A general mechanism of martensitic nucleation: Part I. General concepts and the FCC → HCP transformation

Abstract: Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites.

A constitutive model for transformation plasticity accompanying strain-induced martensitic transformations in metastable austenitic steels

Abstract: We propose a constitutive model which describes the transformation plasticity accompanying strain-induced martensitic transformation in nonthermoelastic alloys. The model consists of two parts: a transformation kinetics law describing the evolution of the volume fraction of martensite and a constitutive law defining the flow strength of the evolving two-phase composite. The Olson-Cohen model for martensite volume fraction evolution is recast in a generalized rate form so that the extent of martensite nucleation is not only a function of plastic strain and temperature, but also of the stress state. A selfconsistent method is then used for predicting the resultant stress-strain behavior. The model describes both the hardening influence of the transformation product, and the softening influence of the transformation itself, as represented by a spontaneous transformation strain. The model is then implemented in a finite element program suitable for analysis of boundary value problems. Model predictions are compared with existing experimental data for austenitic steels. We present results from a few simple analyses, including tensile necking, illustrating the critical importance of stress state sensitivity in the evolution model.

经验与教育 = Experience and education

Abstract: Experience and Educationis the best concise statement on education ever published by John Dewey, the man acknowledged to be the pre-eminent educational theorist of the twentieth century. Written more than two decades after Democracy and Education(Dewey's most comprehensive statement of his position in educational philosophy), this book demonstrates how Dewey reformulated his ideas as a result of his intervening experience with the progressive schools and in the light of the criticisms his theories had received. Analysing both "traditional" and "progressive" education, Dr. Dewey here insists that neither the old nor the new education is adequate and that each is miseducative because neither of them applies the principles of a carefully developed philosophy of experience. Many pages of this volume illustrate Dr. Dewey's ideas for a philosophy of experience and its relation to education. He particularly urges that all teachers and educators looking for a new movement in education should think in terms of the deeped and larger issues of education rather than in terms of some divisive "ism" about education, even such an "ism" as "progressivism." His philosophy, here expressed in its most essential, most readable form, predicates an American educational system that respects all sources of experience, on that offers a true learning situation that is both historical and social, both orderly and dynamic.

Mechanical Behavior of Materials

Abstract: A balanced mechanics-materials approach and coverage of the latest developments in biomaterials and electronic materials, the new edition of this popular text is the most thorough and modern book available for upper-level undergraduate courses on the mechanical behavior of materials To ensure that the student gains a thorough understanding the authors present the fundamental mechanisms that operate at micro- and nano-meter level across a wide-range of materials, in a way that is mathematically simple and requires no extensive knowledge of materials This integrated approach provides a conceptual presentation that shows how the microstructure of a material controls its mechanical behavior, and this is reinforced through extensive use of micrographs and illustrations New worked examples and exercises help the student test their understanding Further resources for this title, including lecture slides of select illustrations and solutions for exercises, are available online at wwwcambridgeorg/97800521866758

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.