Microstructural and Mechanical Aspects of High Nitrogen Steels at Cryogenic Temperature

Charpy V-Notch impact tests of N1, N2 and N3 steels from 77K to 293K are possessed in this paper. With increasing the nitrogen concentration, the ductile to brittle transition temperature (DBTT) increases. The toughness of the tested steels decreases rapidly with decreasing the temperature. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple → shallow dimple → mixture of quasi-cleavage facet and dimple → cleavage facet. Fracture facets with river patterns, with tear ridges, along annealing twin boundary and cross the annealing twin plane are observed in this investigation. Critical dislocation density of crack tips ρc=[6π(τp)2/(KIc)2]2 can affect ductile to brittle transition (DBT) behavior at cryogenic temperature. Deformation twinning is also frequently observed at cryogenic temperature. Crack forms along the coherent twin boundary between one twin and the matrix.

On the Microstructure and Fracture Surface of Bainite in JIS SK5 Steel

The microstructure and fracture surface of bainite in JIS SK5 steel have been investigated using transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) in this study. Specimens were austenitized at 880 for 30 min and then austempered at 300 and 400 for 1hr, as a result, lower bainite and upper bainite were formed, respectively. The lower bainite phase consists of plate-like bainitic ferrites and embedded cementite platelets, the cementite precipitated unidirectionally at an angle of 55 to 60 deg deviated from the long axes of the ferrites; in addition, the upper bainite phase consists of a parallel array of ferrite laths and discrete cementite layers sandwiched between them. Both the bainite structures have the same ferrite/cementite orientation relationship, which is close to that of the Bagaryatskii relation. The impact fractographs of lower and upper bainite structures exhibit brittle failure with cleavage facets, of which the size is correlated with the width of the bainite sheaves. Moreover, the crystallographic orientation of cleavage facets has been determined directly by EBSD. The results showed that the cleavage facet planes of lower bainite structure are close to the {001} plane of ferrite, and they were close to the {001}, {112} and {123} planes of ferrite for the upper bainite structure.

Dwell Sensitive Fatigue Response of Titanium Alloys for Power Plant Applications

The phenomenon of “dwell sensitivity” in the α+β and near α titanium alloys and the intrinsic relationship with quasi-cleavage facet formation is discussed. In the present paper, particular emphasis is placed upon the role of “cold creep” and ambient temperature strain accumulation under cyclic loading. A process of stress redistribution between microstructurally distinct regions that demonstrate different strengths is proposed as the fundamental cause of facet development and subsequent dwell failures. A model to describe the redistribution process is validated through a matrix of fatigue testing designed to assess the effects of microstructural form, stress axiality, and periods of dwell loading at peak stress on cyclic strain accumulation.

Dwell Sensitive Fatigue Response of Titanium Alloys for Power Plant Applications

The phenomenon of “dwell sensitivity” in the α+β and near α titanium alloys and the intrinsic relationship with quasi-cleavage facet formation is discussed. In the present paper, particular emphasis is placed upon the role of “cold creep” and ambient temperature strain accumulation under cyclic loading. A process of stress redistribution between microstructurally distinct regions that demonstrate different strengths is proposed as the fundamental cause of facet development and subsequent dwell failures. A model to describe the redistribution process is validated through a matrix of fatigue testing designed to assess the effects of microstructural form, stress axiality and periods of dwell loading at peak stress on cyclic strain accumulation.

Identification of Cleavage Planes in an Al3Ti-Base Alloy by Electron Channeling in the SEM

ABSTRACTSelected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al3Ti-base alloy having the L12 structure. In order to do this unambiguously in the scanning electron microscope (SEM), one needs to know that the cleavage facet from which any given channeling pattern is obtained is indeed normal to the electron beam. We accomplished this by utilizing a recently-developed technique in which an optical microscope with a short depth of focus is inserted in the SEM column and used to measure the elevations of several points on the cleavage facets. By appropriately tilting and rotating the sample, and using the optical microscope to measure elevations, it was possible to orient the facets normal to the beam. The cleavage planes in a cast and extruded alloy having an equiaxed grain structure were compared with those in a directionally-solidified (DS) alloy of the same composition. Of the eight cleavage facets examined in the DS material, six were of the {110} type and two were of the {111} type. Of the six facets examined in the cast and extruded material, two each were of the {110} and {111} types, and one each were of the {100} and {013} types. Although it cannot be said that all possible cleavage planes have been identified in this alloy, the availability of several low-strength cleavage planes apparently exacerbates its brittleness.