S. M. Zhu

Microstructural characteristics and creep rupture behavior of electron beam and laser welded AISI 316L stainless steel

Abstract AISI 316L stainless steel was welded by the electron beam (EB) and laser techniques. Microstructural characteristics, hardness profile, creep rupture properties and creep damage of the welds were investigated. Fully austenitic microstructure was obtained in the two welds. The solidification structure of the welds consisted of the cellular and equiaxed dendrites. The creep rupture lives of the two welds were almost the same, and they were reduced by a factor of about two compared to the base metal. Moreover, the rupture elongation of the welds was lower than that of the base metal. Creep damage was observed in the “parting” region of the welds and in the heat-affected zone (HAZ), respectively. Final creep fracture occurred in the “parting” region of the welds.

Laser surface alloying of Incoloy 800H with silicon carbide: microstructural aspects

Abstract Incoloy 800H was laser surface alloyed with SiC powders. Transmission electron microscopy examination of the alloy surface revealed that M 7 C 3 carbides were precipitated in the melted zone. Such precipitation was caused by the decomposition of the SiC particles during the laser melting. Furthermore, it was found that the hardness of the surface-modified layer shows a pronounced increase after laser treatment.

Serrated flow in Fe-28Mn-9Al-xC alloys in a temperature range of 573-873 K

The serrated flow phenomenon or Portevin-Le Chatelier (PLC) effect during plastic deformation of solid solutions has long been an area of interest. The PLC effect in dilute alloys has been well understood both from the experimental and theoretical viewpoints. The micromechanism is generally recognized to be associated with dynamic interaction between mobile dislocations and diffusive vacancies (for substitutional alloys) or solutes (for interstitial alloys), widely known as dynamic strain aging (DSA). The Fe-Mn-Al-C alloy system has attracted considerable attention in the past decades as a promising replacement for the conventional Fe-Ni-Cr stainless steel in view of its high strength, good toughness, light weight and low cost. Serrated flow for Fe-28Mn-9Al-xC is studied in this paper.

Effects of nitrogen implantation on low cycle fatigue behavior of ferritic Fe-24Cr-4Al stainless alloy

The effects of nitrogen implantation on cyclic deformation response, near-surface dislocation sub-structure, surface slip band formation, crack initiation, and fatigue life under low cycle fatigue of the ferritic Fe-24Cr-4Al stainless alloy were investigated. Implantation was carried out at an energy of 65 keV and at a fluence of 2 × 1017 ions/cm2. Nitrogen implantation resulted in a substantial cyclic hardening in the alloy. Homogeneous planar dislocation arrangements were formed in the near-surface region of implanted specimens after fatigue, while dislocation loop debris and patches were developed in the un-implanted specimens. Moreover, formation of persistent slip bands (PSBs) was greatly suppressed in the surface of the implanted specimens. Nitrogen implantation also resulted in an alteration of the crack initiation mode from the grain boundary to the surface penetration of the PSBs nucleated below the surface layer. Fatigue life improvements after nitrogen implantation could only be obtained when the PSBs were not only suppressed but also homogenized in the implanted surface layer.

Effect of nitrogen implantation on fatigue crack initiation in ferritic Fe-24Cr-4Al alloy

Nitrogen implantation in a ferritic Fe-24Cr-4Al alloy results in cyclic hardening behavior and a slight improvement in fatigue life. Optical and SEM observations reveal that the slip bands are more homogeneous in the implanted alloy than those of the un-implanted specimen and the density of slip bands is significantly lower at the surface of the implanted alloy. For the un-implanted alloy, impingement of persistent slip bands (PSBs) at the free surface leads to the initiation of intergranular cracks. However, the emergence of PSBs at the free surface is retarded in the implanted alloy due to the interaction between PSBs and nitrides of the implanted layer. The surface cracks exhibit a brittle appearance for the ion beam modified specimen.