PROCESSOS DE PRECIPITAÇÃO POR REAÇÕES DE ORDEM NUMA SUPERLIGA À BASE DE NÍQUEL (59-23CR-16MO) / EFFECTS OF PRECIPITATION PROCESSES INDUCED BY ORDERING REACTIONS IN A NICKEL BASE SUPERALLOY (59-23 CR-16 MO)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

The nickel base superalloy, 59-23Cr-16Mo, was developed in the nineties and has since been used in chemical and petrochemical industries as well as in equipment for environmental control. The alloy has, occasionally, been presenting problems of pit corrosion in the heat affected zones (HAZ) of welded joints. With the purpose of elucidating the mechanisms associated with precipitation processes, specimens were taken from the as-received alloy and submitted to isothermal aging at two different temperatures of 700 and 900oC, for time intervals varying from 1 to 100 hours. A study of the microstructural evolution accompanying the aging treatment was then conducted by means of X-ray diffraction, optical and scanning electron microscopies, making use of the energy dispersion spectroscopy (EDS) and electron back scattering diffraction (EBSD) techniques. Conventional and high resolution electron microscopies were also used as additional tools in order to follow the nanoescale precipitation processes. The mechanical behavior of the alloy in its different microstructural conditions was evaluated by means of uniaxial tensile tests as well as by microhardness measurements. Variations in electrical resistivity accompanying microstructure evolution were followed and electrical resistivity of a TIG welded joint was mapped. The results obtained have indicated the presence of {1, 1/2, 0} reflections in the diffraction patterns of the majority of the microstructures in question. Such reflections, which are typical of short range ordered (SRO) state, indicate that the alloy is prone to precipitation mechanisms via ordering reactions, involving atomic stacking on {4,2,0} planes. A large population of nanoprecipitates, referring to the Ni2(Mo,Cr) phase, was detected in all of the microstructural conditions considered in this work. In addition, the aged specimens were also found to contain microscale Ni2(Mo,Cr)2 particles formed at the grain boundaries via ordering reactions occurring preferentially in the grain boundary area. The mechanical behavior of the aged alloy is strongly related to the precipitation of this microscale phase, which governs the ductility level as well as the transition in the fracture mode from intragranular to intergranular. Finally, the electrical resistivity variation within the HAZ was not associated with microstrutural alterations and therefore was attributed to variations in the quantity of the short range ordered structures.

ASSUNTO(S)

reacoes de ordem superligas ordering reactions superalloys

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