Byung Sup Rho

The effect of δ-ferrite on fatigue cracks in 304L steels

Abstract Continuous low-cycle fatigue tests of 304L stainless steel at 300 and 600°C and Δ e t =±2.0% in an air environment were conducted, and the effects of the amount and direction of δ -ferrite stringers on fatigue crack initiation and fatigue life were investigated. Fatigue crack initiation was observed at the interface between δ -ferrite and a particulate matrix. The effect of δ -ferrite in 304L stainless steel on fatigue life in low-cycle fatigue was investigated. 304L stainless steel with δ -ferrite perpendicular to the loading axis has a much shorter fatigue life than that with δ -ferrite parallel to the stress axis. These phenomena are thought to be caused by the strain incompatibility at the interface between δ -ferrite and the matrix.

Effects of nitrogen on low-cycle fatigue properties of type 304L austenitic stainless steels tested with and without tensile strain hold

A quantitative analysis of the effects of nitrogen on high temperature low-cycle fatigue without and with tensile strain hold at 600 °C has been conducted for type 304L stainless steels. For better understanding of the role of nitrogen on grain boundary precipitation, the grain boundary segregation of nitrogen was analyzed by Auger electron spectroscopy. The nitrogen addition is found to give relatively better resistance to creep-fatigue than continuous low-cycle fatigue. This in turn improves the fatigue life. This is due to the retardation of the precipitation of carbides at the grain boundary and reduction in the density of grain boundary cavitation sites which are the main factor of grain boundary damage under creep-fatigue test.

A study on the crack initiation and growth from δ-ferrite/γ phase interface under continuous fatigue and creep-fatigue conditions in type 304L stainless steels

Abstract The extensive microstructural observation has been conducted to investigate the effect of δ-ferrite on the low cycle fatigue properties in terms of its content and directionality, using type 304L stainless steels. Under continuous fatigue test, the crack initiation and growth mechanisms are varied depending on the directionality of the δ-ferrite fibers with respect to the loading direction. In the case of parallel δ-ferrite fibers to the loading direction, the fatigue cracks nucleate from the δ-ferrites due to the impingement of slip bands and propagate along the slip lines, whereas in the case of perpendicular δ-ferrite fibers to the loading direction, the debonding of the δ/γ interface provides the crack nucleation site. The nucleated cracks propagate along planes of maximum tensile stress. Under creep-fatigue conditions, cavities considerably nucleate at the perpendicular δ/γ interface to the loading direction, resulting in the accelerated growth of grain boundary cracks. However, the parallel δ-ferrite fibers have little influence on the creep-fatigue property.

The effect of test temperature on the intergranular cracking of Nb-A286 alloy in low cycle fatigue

The continuous low cycle fatigue behaviors of a Fe-base superalloy, Nb-modified A286 alloy, have been evaluated at the test temperatures of 650°C and 350°C under various total strain ranges. It was found that the change of the slope in the Coffin–Manson plot was closely related to the fatigue cracking with the test temperature. In the high temperature low cycle fatigue (HTLCF) of Nb-A286 alloy, the fatigue cracking exhibited the intergranular mode at 650°C and the transgranular mode at 350°C. The intergranular fatigue cracking at 650°C was due to the precipitation of the η phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the η precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. This is confirmed by the results of low cycle fatigue at 25°C after heat treatment which forms the η phase at the grain boundary.

Fatigue-induced precipitates at grain boundary of Nb-A286 alloy in high temperature low cycle fatigue

A quantitative analysis of the effect of the total strain range on the η precipitates at grain boundary under high temperature low-cycle fatigue has been interpreted for Nb-A286 alloy. The value of the activation energy of η phase formation with total strain range was analyzed by using the in situ X-ray diffraction (XRD) technique and the differential thermal analysis (DTA) technique. The applied strain during the low-cycle fatigue decreases the activation energy of η phase formation at grain boundary. The trend of decreasing the temperature of η phase formation with the applied strain was found to be due to the accommodation of deformation near grain boundary.

The effect of applied strain range on the fatigue cracking in Nb-A286 iron-base superalloy

Abstract The low cycle fatigue tests of Nb-A286 iron-base superalloy was conducted to investigate the effect of the applied strain range on the cracking at a given temperature of 550°C. The experimental results strongly indicate that the slope change of Coffin–Manson plot is definitely related with the deformation mode that controls the fracture mode. The fatigue cracking mode changes from the transgranular to the predominantly intergranular modes with the applied strain range. The reason for this change of the fatigue cracking with strain range is induced by the combination of mutually competing factors involving the grain boundary precipitate, the density of the slip band, and the concentration of stress at points where the slip bands impinge upon the grain boundaries.

Effect of alloyed Ti:Zr ratio on phase stability of Al66Mn9(Ti, Zr)25 intermetallic compounds

The tetragonal Al3(Ti1 − xZrx) phase is transformed by the addition of 9% Mn to the cubic L12-type Al66Mn9(Ti1 − xZrx)25 with a small amount of second phases such as Al8Mn5 and Al2Zr still existing in the as-cast condition. Second phases disappear when intermetallic compounds are homogenized at 1000 °C for 24 h, and alloys are of the monolithic L12 phase with compositional variations still existent. The combined addition of Ti and Zr is found to reduce the hardness of intermetallic alloys compared to the addition of Ti or Zr alone. The Al66Mn9(Ti, Zr)25 phase is found to be structurally stable but compositionally unstable.