Phase stability, constitution and precipitation effects in Fe-Ni-Cr alloys.

A study of the constitution, transformation and precipitation effects in ternary Fe-Cr-Ni alloys and quaternary Fe-Cr-Ni-X alloys containing Mo, Nb, Ti, and Si was carried out. A systematic approach was adopted so that the microstructural effects observed as a result of ageing the selected iron base ternary alloys could be directly compared to the quaternary alloys.A series of ageing curves were plotted for the six ternary alloys over the temperature range 400°-900°C and for the ten quaternary alloys in the temperature range 650°-850°C. Optical and electron metallography were used to study the transformation and precipitation of intermetallic phases and carbides in the aged microstructures.The transformation of delta ferrite to sigma phase in a duplex (gamma+delta) ternary alloy was studied. The transformation was preceded by the precipitation of a cellular structure which formed on the delta/gamma grain boundaries and consisted of M23C6 and new austenite. A complex transformation product, which resembled a region of imiscibility, then formed at the delta/(gamma + M23C6) interface, this product consumed the delta ferrite grain, eventually transforming to sigma phase and new austenite. The alpha' phase, more commonly referred to as 475°C embrittlement, was also observed precipitating in the delta ferrite grains in two of the iron based ternary alloys in the temperature region 400°-500°C. Overageing of the alpha' precipitates after 1000 hours at 500°G was accompanied by the precipitation of a rod like austenite. The elements 2%Mo, 1%Nb, 1/4%Ti and 1% and 2%Si were added totwo Fe-Ni-Cr base alloys. One alloy was an austenitic (20Cr,23Ni) and the other was a transformable alloy (18Cr, 7Ni). The quaternary element additions had no effect on the constitution of the austenitic ternary alloy. However the addition of Mo to the 18Cr, 7Ni ternary alloy caused the transformation of delta ferrite to sigma phase over ageing temperature range 650°-850°C. The addition of Si moved the constitution of the ternary alloy further into the gamma+delta phase field, a small amount of sigma phase was observed in the 2%Si quaternary alloy on ageing at 650°C for 1000 hours. Irradiation damage studies were performed using High Voltage Electron Microscope and Variable Energy Cyclotron. The effect ofthe quaternary alloying additions Mo, Si, and Nb on the voidswelling behaviour of a 20Cr, 23Ni alloy were studied using 46MeV Ni6 ions in the Variable Energy Cyclotron, irradiating to a total dose of 10dpa at 550°C. All additions reduced void swelling, the largest reduction was observed in the Si containing allov. The Insert A Thermal ageing showed the presence of M23C6 the amount of which increased with increasing ageing temperature. No delta ferrite was observed in these alloys.