Atom Probe Tomography Study of Magnesium in Ductile Iron
Principal Investigators
Jingjing Qing
Missouri University of Science & Technology
10/18/2016
PROGRAM OBJECTIVE(S)
The objective of this proposed study isto look for magnesium atoms at the graphite/matrix interface in ductile iron using atom probe tomography (APT).
TOTAL COST:
$19,025.73
TIME TO COMPLETION
Five months
ECONOMIC JUSTIFICATION AND PRACTICAL APPLICATION
The mechanism behind nodular graphite formation due to addition of magnesium is never clear. Finding Mg using novel high resolution high sensitivity technique is the first step to advance the understanding of the magnesium’s role in formation of nodular graphite. Moreover, this may advance the understanding on how to improve Mg treatment process.
POSSIBILITY OF SUCCESS
Once the concentration of nodulizing element at the graphite/matrix interface is higher than the detection limit of APT (10 at.ppm), it will be possible to determine the concentration profile of magnesium next to the noduelar graphite particle.
HISTORY AND THEORY
Production of ductile iron (DI) with consistent properties is generally accomplished by addition of nodulizing elements in the iron alloys. Nodulizing elements promote spheroidal graphite (SG) in iron, which include magnesium, cerium, calcium and rare earth element. Magnesium is the most commonly used nodulizing element in the production of DI. Production of compacted graphite iron (CGI) uses a combination of nodulizing elements and denodulizing elements. Denodulizing elements hinder the formation of SG structure which include sulfur, titanium and oxgen. It is well documented that an elevated magnesium/cerium/lanthanum content in the cast iron leads to FG-CG-SG transition. A high concentration of Mg (0.040~0.045%) promotes spheroidal graphite in cast iron alloys, and an intermediate level (0.015~0.035%) of Mg will promote formation of compacted shaped graphite, while flake graphite formation is only favorable at a low level of Mg concentration. Magnesium loss/fade or sulfur addition however reverse this process, i.e., loss of magnesium or addition of sulfur into cast iron promotes flake graphite. It is challenging to investigate the role of nodulizing element and denodulizing elements in graphite morphology transition since detection of trace amount of element is difficult. Sulfur is a surface-active element and it may be adsorbed on the graphite and affect the graphite growth. It has been shown in the literature that a single or two layers of sulfur atoms were detected on the graphite/matrix interface in gray irons using Auger Energy Spectrometry (AES). Similarly, a higher concentration of sulfur was detected at the flake graphite/matrix interface in the gray irons using Secondary Ion Mass Spectrometry (SIMS), compare to that in the flake graphite particle. A recent study has revealed that sulfur can promote flake graphite growth along prismatic direction by forming bonds with carbon atoms and opening up the folded graphene layers. This mechanism was revealed using DFT calculations. Nodulizing atoms are normally larger in sized, which are unlikely to form bonds with the carbon in graphite. It was mentioned in the literature that magnesium is reactive in solution which would slow down attachment of carbon atoms on graphite and thus produce an interface undercooling. However, the exact position of magnesium atom in the system and the exact mechanism are unclear. There was no detectable amount of magnesium found in the graphite lattice or at the interface of graphite/matrix for a SG particle or a CG particle. People were only able to find magnesium containing compounds either as nuclei for graphite particles or as inclusions in the iron matrix. The exact mechanisms for the nodulizing and denodulizing processes have not been fully understood. Atom Probe Tomography however allows one to detect small number of atoms (as low as 10at.ppm) and it reveals the 3-D distribution of atoms in the material, which may be promising to find Mg at the graphite/matrix interface orin SG in ductile iron.
WHAT IS PROPOSED THAT HAS NOT BEEN DONE BEFORE
Due to the detection limit of traditional techniques, such as energy-dispersive X-ray (EDX) and Auger energy spectrometry (AES), magnesium was not found at the graphite/matrix interface or in the spheroidal graphite particle. This work utilizing a high resolution and high sensitivity technique, APT, is unique and it has never been done in the past.
EXPERIMENTAL PROCEDURE
A ductile iron sample cast at MS&T foundry will be used to prepare APT sample. The sample will be prepared using Focused Ion Beam (FIB) and tested with APT at Northwestern University.
PROGRAM COSTS
Cost, $ / Fringe Benefit (35.37%), $Salary (1 month) / 6,667.00 / 2,358.00
Sectioning and polishing supply / 50.00
FIB and APT / 2,700.00
Travel / 500.00
Total before Indirect cost / 12,274.67
Indirect cost (MS&T) / 55%
Total / 19,025.73
BENEFITS TO MEMBERSHIP
Finding Mg using novel technique, ATP, may open a gate to understand the role of magnesium in the formation of nodular graphite. Moreover, this may advance the understanding on better control Mg treatment process in order to achieve higher quality ductile iron.
GANTT CHART
Dec.01-31, 2016 / Jan.01-31, 2017 / Feb.01-28,2017 / Mar.01-31, 2017 / Apr. 01-30,2017
Section and Polish Sample / X
FIB / X
APT / X
Analyze Results / X / X
Report / X
The estimated completion time is April 30, 2017 if starts at December 01, 2017. The project duration is five months.
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