Microtubule nucleation is regulated by the γ-tubulin ring complex (γTuRC) and related γ-tubulin complexes, providing spatial and temporal control over the initiation of microtubule growth. Recent structural work has shed light on the mechanism of γTuRC-based microtubule nucleation, confirming the long-standing hypothesis that the γTuRC functions as a microtubule template. The first crystallographic analysis of a non-γ-tubulin γTuRC component (γ-tubulin complex protein 4 (GCP4)) has resulted in a new appreciation of the relationships among all γTuRC proteins, leading to a refined model of their organization and function. The structures have also suggested an unexpected mechanism for regulating γTuRC activity via conformational modulation of the complex component GCP3. New experiments on γTuRC localization extend these insights, suggesting a direct link between its attachment at specific cellular sites and its activation.

Microtubule nucleation by γ-tubulin complexes

Large strain work hardening and textures

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M
 s temperature induced by carbon enrichment.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

Solid solution softening and solid solution hardening

Recently published experimental data demonstrate that the strength of mixed microstructures of tempered bainite and martensite can peak at an intermediate volume fraction of martensite. In the present work, a quantitative interpretation of these observations is achieved by modelling the mechanical properties of bainite and martensite in their tempered states. It is found that the peak in the curve of the strength as a function of the volume fraction of martensite can be attributed to two factors. When bainite forms it enriches the residual austenite with carbon, so that the strength of the subsequent martensite increases. In addition, during its deformation, the strength of the bainite is enhanced via plastic constraint by the surrounding stronger martensite. Taking these effects into account, it is possible to predict accurately both the trends and the absolute values of published experimental data on the strength of mixed microstructures.MST/1901

Strength of mixtures of bainite and martensite

The new results of an extensive study of the effects of alloying elements upon the properties of iron are presented, including lattice parameters, elastic constants, the effect of temperature on strength, solid solution strengthening and softening, work hardening, strain aging, hot working and toughness.

W. C. Leslie

Papers

Iron and its dilute substitutional solid solutions