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Title: Microstructural Characterization of Nodular Ductile Iron

Abstract

The objective of this study is to quantify the graphite particle phase in nodular ductile iron (NDI). This study provides the basis for initializing microstructure in direct numerical simulations, as part of developing microstructure-fracture response models. The work presented here is a subset of a PhD dissertation on spall fracture in NDI. NDI is an ideal material for studying the influence of microstructure on ductile fracture because it contains a readily identifiable second-phase particle population, embedded in a ductile metallic matrix, which serves as primary void nucleation sites. Nucleated voids grow and coalesce under continued tensile loading, as part of the micromechanisms of ductile fracture, and lead to macroscopic failure. For this study, we used 2D optical microscopy and quantitative metallography relationships to characterize the volume fraction, size distribution, nearest-neighbor distance, and other higher-order metrics of the graphite particle phase. We found that the volume fraction was {Phi} = 0.115, the average particle diameter was d{sub avg} = 25.9 {mu}m, the Weibull shape and scaling parameters were {beta} = 1.8 and {eta} = 29.1 {mu}m, respectively, the (first) nearest neighbor distance was L{sub nn} = 32.4 {mu}m, the exponential coefficients for volume fraction fluctuations was A{sub {Phi}} = 1.89 andmore » B{sub {Phi}} = -0.59, respectively. Based on reaching a coefficient-of-variation (COV) of 0.01, the representative volume element (RVE) size was determined to be 8.9L{sub nn} (288 {mu}m).« less

Authors:
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1034481
Report Number(s):
LLNL-TR-522092
TRN: US201204%%28
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; DISTRIBUTION; FLUCTUATIONS; FRACTURES; GRAPHITE; IRON; METALLOGRAPHY; METRICS; MICROSTRUCTURE; NUCLEATION; OPTICAL MICROSCOPY; SHAPE

Citation Formats

Springer, H K. Microstructural Characterization of Nodular Ductile Iron. United States: N. p., 2012. Web. doi:10.2172/1034481.
Springer, H K. Microstructural Characterization of Nodular Ductile Iron. United States. doi:10.2172/1034481.
Springer, H K. Tue . "Microstructural Characterization of Nodular Ductile Iron". United States. doi:10.2172/1034481. https://www.osti.gov/servlets/purl/1034481.
@article{osti_1034481,
title = {Microstructural Characterization of Nodular Ductile Iron},
author = {Springer, H K},
abstractNote = {The objective of this study is to quantify the graphite particle phase in nodular ductile iron (NDI). This study provides the basis for initializing microstructure in direct numerical simulations, as part of developing microstructure-fracture response models. The work presented here is a subset of a PhD dissertation on spall fracture in NDI. NDI is an ideal material for studying the influence of microstructure on ductile fracture because it contains a readily identifiable second-phase particle population, embedded in a ductile metallic matrix, which serves as primary void nucleation sites. Nucleated voids grow and coalesce under continued tensile loading, as part of the micromechanisms of ductile fracture, and lead to macroscopic failure. For this study, we used 2D optical microscopy and quantitative metallography relationships to characterize the volume fraction, size distribution, nearest-neighbor distance, and other higher-order metrics of the graphite particle phase. We found that the volume fraction was {Phi} = 0.115, the average particle diameter was d{sub avg} = 25.9 {mu}m, the Weibull shape and scaling parameters were {beta} = 1.8 and {eta} = 29.1 {mu}m, respectively, the (first) nearest neighbor distance was L{sub nn} = 32.4 {mu}m, the exponential coefficients for volume fraction fluctuations was A{sub {Phi}} = 1.89 and B{sub {Phi}} = -0.59, respectively. Based on reaching a coefficient-of-variation (COV) of 0.01, the representative volume element (RVE) size was determined to be 8.9L{sub nn} (288 {mu}m).},
doi = {10.2172/1034481},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jan 03 00:00:00 EST 2012},
month = {Tue Jan 03 00:00:00 EST 2012}
}

Technical Report:

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  • The objective of this study is to characterize the strength and fracture response of nodular ductile iron (NDI) and its underlying ferritic matrix phase. Quasistatic and split Hopkinson pressure bar (SHPB) compression tests were performed on NDI and a model material for the NDI matrix phase (Fe-Si alloy). Smooth and notch round bar (NRB) samples were loaded in tension until fracture to determine strain-at-failure with varying stress triaxiality. Multiple tests were performed on each small and large smooth bar samples to obtain fracture statistics with sample size. Fracture statistics are important for initializing simulations of fragmentation events. Johnson-Cook strength modelsmore » were developed for the NDI and the Fe-Si alloy. NDI strength model parameters are: A = 525 MPa, B = 650 MPa, n = 0.6, and C = 0.0205. The average SHPB experimental strain-rate of 2312/s was used for the reference strain-rate in this model. Fe-Si alloy strength model parameters are: A=560 MPa, B = 625 MPa, n = 0.5, and C = 0.02. The average SHPB experimental strain-rate of 2850/s was used for the reference strain-rate in this model. A Johnson-Cook failure model was developed for NDI with model parameters: D{sub 1} = 0.029, D{sub 2} = 0.44, D{sub 3} = -1.5, and D{sub 4} = D{sub 5} = 0. An exponential relationship was developed for the elongation-at-failure statistics as a function of length-scale with model parameters: S{sub f1} = 0.108, S{sub f2} = -0.00169, and L{sub m} = 32.4 {mu}m. NDI strength and failure models, including failure statistics, will be used in continuum-scale simulations of explosively-driven ring fragmentation. The Fe-Si alloy strength model will be used in mesoscale simulations of spall fracture in NDI, where the NDI matrix phase is captured explicitly.« less
  • Design criteria for preventing ductile and brittle failure are presented for spent-fuel shipping containers fabricated from ferritic nodular cast iron. A data base was assembled to determine the applicability of NRC Regulatory Guide 7.6 to ferritic nodular cast iron. In addition, fracture toughness acceptance criteria for this material were investigated using both fracture arrest and fracture initiation approaches. The study includes a review of the brittle fracture sensitivities of nodular cast iron to various processes involved in its fabrication. 19 figures, 7 tables.
  • Zircaloy-2 becomes susceptible to nodular corrosion in high-temperature, high-pressure steam when the total solute concentration of the {beta}-stabilizing alloying elements Fe, Ni and Cr in the {alpha}-zirconium matrix falls below a critical value C{sub c} that is characteristic of the test conditions. C{sub c} for typical commercial Zircaloy-2 in a 24hr/510 C/10.4MPa steam-test is the precipitate-free a-matrix concentration in equilibrium with solute-saturated {beta} phase at about 840 C, the corresponding critical temperature T{sub c}.Thus, immunity to nodular corrosion is a metastable condition for {alpha}-Zircaloy that requires fast cooling from above T{sub c} to achieve adequate solute concentration throughout the matrix.more » Annealing Zircaloy at any temperature below T{sub c} for a sufficiently long time makes it susceptible to nodular corrosion. In the ({alpha}+{chi}) phase field, where {chi} collectively designates the Fe-, Cr-, and Ni-containing precipitate phases, lowering the solute concentration to less than C{sub c} by Ostwald ripening can require many hundreds of hours. Above about 825 C, the temperature of the ({alpha}+{chi})/({alpha}+{beta}+{chi}) transus, solute-saturated {beta} phase surrounds each precipitate and a strong inverse activity gradient promotes equilibration with the much lower solute concentration in the {alpha} matrix. Sensitization to nodular corrosion occurs most rapidly at about 835 C between the ({alpha}+{chi})/({alpha}+{beta}+{chi}) transus and T{sub c}. Annealing Zircaloy at temperatures above T{sub c} for a sufficiently long time will raise the solute concentration above C{sub c} and, with rapid cooling, heal any degree of susceptibility. Annealing within the protective coarsening window between T{sub c} and about 850 C, the temperature of the ({alpha}+{beta}+{chi})/({alpha}+{beta}) transus, achieves rapid precipitate growth in a matrix immune to nodular corrosion.« less
  • Nanocomposites of iron in sapphire ({alpha}-Al{sub 2}O{sub 3}) prepared by ion implantation have been studied as a model to investigate the potential of such materials for applications in high technology areas. The implantation was performed with 160 keV ions at several doses; the nanocomposites were then annealed at selected temperatures between 700 and 1,400 C in an Ar-4%H{sub 2} atmosphere for 1 hour. Rutherford backscattering spectroscopy and high resolution transmission electron microscopy (TEM) were used to characterize the structure of these nanocomposites. Measurements showed that damage depth extended to about 300 nm and the embedded iron extended to about 200more » nm. This region became amorphous when the fluence reaches 2 {times} 10{sup 17} Fe/cm{sup 2} at this energy. Thermal annealing could be used to restore the crystallinity to the damaged near-surface region, to form the metallic colloids, and also to coarsen the precipitates. In the case of high dose implantation, oriented precipitates with diameters of 2 to 3 nm were identified by TEM techniques as {alpha}-Fe which had the following orientation relationship with the sapphire matrix: <111>{sub Fe} {parallel} <310>{sub Sapphire} and {l_brace}01{bar 1}{r_brace}{sub Fe} {parallel} {l_brace}006{r_brace}{sub Sapphire}. The optical density and luminescence spectra were also measured. The predominant defects were oxygen vacancies with two electrons (F center) at the known absorption peak of 200 nm.« less
  • To understand the underlying phenomena when characterizing material performance, we must know the chemistry and physics of the early stages of oxidation, chemistry, and bonding at the substrate/oxide interface, effect of segregants on the strength of bonding, transport processes through the scale formed during corrosion, mechanisms of residual stress generation and relief, and fracture behavior at the oxide/substrate interface. Specific objectives of the program described here are to (a) systematically investigate the relationships among substrate composition and properties and scale/coating adherence, damage tolerance, and micromechanical properties; (b) use results from the investigation to prevent scale/coating failure at elevated temperatures; andmore » (c) identify conditions that lead to coatings that are more damage tolerant and scales that are amenable to legitimate synthesis routes. This report presents experimental data on the microstructural characteristics of alumina scales that have been thermally developed on several Fe-based alumina-forming intermetallic alloys. In addition, data are presented on scale adhesion, along with determinations of strain from data obtained by ruby fluorescence.« less