We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown Moreover, the combined processes developed recently are also explained Finally, the applications of the ultra-fine grained (UFG) metals are discussed

Severe Plastic Deformation (SPD) Processes for Metals

Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging

Ultrafine grained steels with grain sizes below about 1 μm offer the prospect of high strength and high toughness with traditional steel compositions. These materials are currently the subject of extensive research efforts worldwide. Ultrafine grained steels can be produced either by advanced thermomechanical processes or by severe plastic deformation strategies. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained bcc steels, the corresponding microstructures, and the resulting spectrum of mechanical properties.

Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

Nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequat. This article reviews recent progress in tailoring the nanostructure to achieve coexisting high strength and high ductility at room temperature. The focus is on a summary of the strategies currently being pursued as well as the outstanding issues that await future research.

Eight routes to improve the tensile ductility of bulk nanostructured metals and alloys

Mechanical properties of an ultrafine grained C-Mn steel processed by warm deformation and annealing

Microstructure and crystallographic texture of an ultrafine grained C-Mn steel and their evolution during warm deformation and annealing

Improvement of the work hardening rate of ultrafine grained steels through second phase particles

The effect of Mn content on the microstructure and mechanical properties of two ultrafine grained 0.2%C-Mn steels has been investigated. The ultrafine grained microstructure was produced by use of large strain warm deformation and subsequent annealing. The final microstructure consists of fine cementite particles within an ultrafine grained ferrite matrix. The increase in the Mn content leads to a decrease in the average ferrite grain size (from 1.3 to 0.8 μm for an increase in the Mn content from 0.74 to 1.52 mass%). This can be attributed to the enrichment of Mn in the cementite particles, which becomes finer in the steel with a higher Mn content. The increase in the Mn content results in an increase in strength at equal ductility and toughness.

https://www.jstage.jst.go.jp/article/isijinternational/45/11/45_11_1721/_pdf

Rongjie Song

Papers

Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

Mechanical properties of an ultrafine grained C-Mn steel processed by warm deformation and annealing

Microstructure and crystallographic texture of an ultrafine grained C-Mn steel and their evolution during warm deformation and annealing

Improvement of the work hardening rate of ultrafine grained steels through second phase particles

Influence of Mn Content on the Microstructure and Mechanical Properties of Ultrafine Grained C-Mn Steels