Neutron diffraction study of austempered ductile iron
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INTRODUCTION
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P U R E iron is dimorphic and is classified into two crystallographic phases: ferromagnetic o~-Fe (ferrite) with bodycentered cubic (bcc) symmetry, which is stable at temperatures up to 906 ~ and y-Fe (austenite) with facecentered cubic (fcc) symmetry, which is stable in the temperature range from 906 ~ to 1401 ~ Austempered ductile iron (ADI) is produced by heat treating cast ductile iron saturated with carbon. The main components of ADI are known to be ausferrite,tq containing acicular ferrite and thermodynamically stable carbon-enriched austenite, and segregated nodular graphite. Austempered ductile iron also contains small amounts of other precipitates from alloying elements. The austenite in ADI is stable, unlike the retained austenite in steel, because of its high carbon content. Austempered ductile iron is known to have many important advantages, such as easy castability, high strength and toughness, and a lower density than steel. The quantitative phase analysis of ADI is quite important, since the austenite content can have a pronounced effect on the physical properties of the ductile iron. Retained austenite in steel has usually been measured by optical metallography or by X-ray diffraction. The former is a destructive method, with relatively poor resolution. The Xray diffraction method is known to be more accurate, but it does not have sufficient penetration power to probe the entire volume of a sample. Neutrons, on the other hand, have much higher penetration power than X-rays, more than 1000 times for most metal samples. In this study, two neutron diffraction techniques were used for the characterization of ADI samples: high-resolution neutron diffraction for the microstructure and quantitative phase analysis; and small-angle neutron scattering (SANS) for the microstructural study.
C.S. CHOI, ResearchPhysicist,and W. SHARPE,MaterialsEngineer, are with the United States Army,ArmamentResearchDevelopmentand EngineeringCenter,PicatinnyArsenal,NJ 07806. J. BARKER,Materials Engineer, and R.J. FIELDS,Metallurgist,are with the MaterialsScience and Engineering Laboratory, National Institute of Standards and Technology,Gaithersburg,MD 20899. ManuscriptsubmittedApril 10, 1995. METALLURGICALANDMATERIALSTRANSACTIONSA
SAMPLE FABRICATION
Thick-walled cylindrical ingots of high carbon content ferrous materials were obtained by centrifugal casting, a standard industrial process used for pipe fabrication. The ingots were first heated to the austenitizing temperature (about 900 ~ to dissolve the carbon and were then quenched rapidly to the austempering temperature (about 350 ~ for about 2 hours of tempering. This tempering process was repeated twice. This austempered sample has been studied previously~2~with a low-resolution neutron diffractometer for quantitative phase analysis. The chemical composition of the ADI sample is given in Table I.
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RIETVELD PROFILE REFINEMENT OF A POLYCRYSTALLINE SOLID
Since most engineering materials are polycrystalline solids, their diffraction patterns may
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