Neutron Diffraction Study of Austempered Ductile Iron C.S. CHOI, W. SHARPE, J. BARKER, and R.J. FIELDS Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, a-Fe (ferrite), y-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal, 14/mmm, with a c/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20/xm, was observed by optical micrography. I. INTRODUCTION PURE iron is dimorphic and is classified into two crys- tallographic phases: ferromagnetic o~-Fe (ferrite) with body- centered cubic (bcc) symmetry, which is stable at temperatures up to 906 ~ and y-Fe (austenite) with face- centered cubic (fcc) symmetry, which is stable in the tem- perature 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 el- ements. The austenite in ADI is stable, unlike the retained austenite in steel, because of its high carbon content. Austem- pered ductile iron is known to have many important advan- tages, 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 X- ray 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-resolu- tion neutron diffraction for the microstructure and quanti- tative 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,ArmamentResearchDevelopment and EngineeringCenter,Picatinny Arsenal,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. II. 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 dif- fractometer for quantitative phase analysis. The chemical composition of the ADI sample is given in Table I. III. RIETVELD PROFILE REFINEMENT OF A POLYCRYSTALLINE SOLID Since most engineering materials are polycrystalline sol- ids, their diffraction patterns may be described in a similar way to those of powder samples. In the neutron diffraction profile, the intensity at the ith position, Yi, may be expressed by the method of Rietveld: [3J Table I. Chemical Composition of the ADI Sample in Weight Percent Element Wt Pet C 3.77 Si 2.60 Ni 1.03 Cu 0.968 Mn 0.230 Cr 0.034 A1 0.016 B 0.001 S 0.008 P 0.016 Ti 0.0068 Sn 0.004 Mo 0.001 Mg 0.058 U.S.GOVERNMENT WORK METALLURGICAL ANDMATERIALS TRANSACTIONS A VOLUME 27A,APRIL 199~-923 NOTPROTECTED BYU.S.COPYRIGHT