OVERVIEW Dynamic recrystallization (DRX) occurs when low to medium

ROLE OF GRAIN BOUNDARY CHARACTER ON DYNAMIC RECRYSTALLIZATION

Megan Frary, Boise State University, DMR 0642363

MCS=3k, DRX=0.5% MCS=10k, DRX=28%

MCS=13k, DRX=56% MCS=15k, DRX=76%

(left): No grain boundary layer. (right): Easily sheared GB layer.

50 GPa

0 GPa

GBs 10% (left) and 1% (right) bulk stiffness

(left) Experimental microstructure for OOF2. (right) GBs have same

stiffness as bulk.

MONTE CARLO SIMULATIONS on experimental microstructures consider anisotropic GB energy and preferential nucleation at triple junctions.

OOF2 SIMULATIONS are used to predict which triple junctions are most likely to act as nucleation sites for DRX. Different grain boundary characters are accounted for by assigning the GBs different stiffnesses relative to the bulk (i.e., making them more susceptible to GB sliding). OOF2 simulations show the resulting stress concentrations that would act as a driving force for nucleation. Results below are for a single triple junction and a small section of an experimental microstructure.

Orange line is stored energy vs. time and blue line is special boundary fraction. States a, b, c, d shown below.

APPROACHon nickel, Nil-based superalloys and stainless steel. Grain boundary engineering creates microstructures with different grain boundary character distributions and EBSD is used to map grain orientations and boundary misorientations.COMPUTATIONAL: Monte Carlo simulations are being developed to study the effects of changing the fraction of twin boundaries on the kinetics and flow behavior in DRX. Experimentally-determined microstructures are used as the initial structures for the model.

stacking fault energy materials undergo deformation at high homologous temperatures (e.g., during hot working). The project objective is to elucidate the role of grain boundary character on DRX, both locally (e.g., through nucleation at individual triple junctions) and globally (e.g., when the fraction of special boundaries changes).

EXPERIMENTAL: High temperature mechanical testing is being done

RESULTS

200 mm

MECHANICAL TESTING of grain boundary engineered stainless steel withdifferent initial special boundary fractions is

underway. The stress-strain curve below is for stainless steel tested at 1050°C and 10-5 s-1 with a special boundary fraction of 0.42. EBSD after testing shows dynamic recrystallization has occurred along some boundaries.

Undergraduate students Master of Science students

Podcasts: available soon

BEN ALBISTON: senior, processing and mechanical testing of stainless steel

CHRIS STIFTER: sophomore, processing and characterization of Ni

SHARLA HOPKINS: processing of Ni, now a high school teacher in Salt Lake City, UT

WADE LANNING: models of triple junctions, now a PhD student at Penn State

MICHAEL CARROLL: summer student from WSU

TRAM BUI: senior, testing and characterization of Ni

JARED STEIN is a third year graduate student in Materials Science and Engineering working toward a M.S. degree. Jared is developing Monte Carlo simulations to study dynamic recrystallization and the role that grain boundary character plays during deformation.

ROLE OF GRAIN BOUNDARY CHARACTER ON DYNAMIC RECRYSTALLIZATION

Megan Frary, Boise State University, DMR 0642363

To date, one graduate student and six undergraduate students have been involved in the project. The PI, advisor to the MSE Club, worked with club members to develop series of podcasts on processing of materials, available through the ASM webpage. In October 2008, the PI received the Bradley Stoughton Award for Young Teachers from ASM International.

http://www.materialsradio.org

Outreach Activities by PIDr. Frary has participated in the Women in Technical Careers Lunch series in Boise-area high schools for the last four years. These lunches are intended to recruit female students into science and engineering disciplines in college.