Thermal Residual Stress Evolution in a TiC-50vol.%Ni 3 Al Cermet J. Wall a, H. Choo a,b, J.W. Richardson c, T.N. Tiegs b, P.K. Liaw a a Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN USA b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN USA c Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL USA

Acknowledgement s The authors wish to thank the technical staff at the Intense Pulsed Neutron Source (IPNS), a division of Argonne National Laboratory, Argonne, IL. A facility operated by the University of Chicago for the U.S. Department of Energy This work is supported by the NSF International Materials Institutes (IMI) Program under contract DMR , with Dr. Carmen Huber as the program director

Outline Introduction and Preliminary Discussion Microstructural Analysis Neutron Diffraction Results Finite Element Modeling Conclusions and Future Work

Introductio n TiC-Ni 3 Al Cermets: Exhibit yield strength anomaly (Plucknett et.al. 1998) Oxidation / corrosion resistant (Becher et.al. 1997) CTE can be tailored to closely match that of steel (Tiegs et.al. 2000) Ni 3 Al Yield Stress (Liu et.al. 1996) TiC-20vol.%Ni 3 Al Yield Stress (Plucknett et.al. 1998)

Introductio n Thermal Residual Stress (TRS) of Particulate Cermets – Driving force – thermal expansion coefficient mismatch of constituent phases (matrix in tension, inclusions in compression) CTE-TiC ~ 6.4  m/m K CTE-Ni 3 Al ~ 14.1  m/m K – Approximate hydrostatic strain state in both phases – Stress computed from volumetric strain using Hooke’s Law – TRS is known to effect bulk mechanical properties of cermets TRS Evolution – At processing temperature – cermets at zero- stress state – At temperature just below metal matrix solidus – thermal residual stresses develop – At room temperature – thermal residual stresses typically maximized

Introductio n TiC-Ni 3 Al Cermets - Well suited for application in automotive and aerospace industries - Good chemical stability to high temperature - Relatively high strength to high temperature - Wear resistant - Relatively lightweight Motivation of Research - Will the anomalous yield strength of Ni 3 Al binder cause atypical TRS development in cermets?

Microstructure: TiC- 50vol.%Ni 3 Al Cermet 50  m Ni 3 Al Cell Dense TiC Network

5m5m Microstructure: TiC-50vol.%Ni 3 Al Cermet Dense TiC Network (Backscattered Micrograph) Ni 3 Al Ti C

Neutron Diffraction Experiments Experiments performed at IPNS using the General Purpose Powder Diffractometer (GPPD) One thermal cycle from R.T. – 1250K – R.T. - Diffraction data collected at ~100K temperature increments Time resolved experiments at 1250K - Diffraction data collected over 15 minute intervals to 12 hours. - TiC Data was resolved using Rietveld refinement. - Ni 3 Al strain data was assessed from the TiC data using 2-phase strain balance

Neutron Diffraction Experiments Experimental Setup GPPD (V band heater not shown) Evacuated V Sample Well V Sample Can Specimen Transmit ted Neutrons Positioning Stem Thermocouple Polychro matic Neutron Beam To Bank 1 To Bank 2

Results: TiC lattice parameter converges with that of stress-free reference standard (heating)

Results: Some path dependence of TiC lattice parameter during cooling

Results: TiC lattice strain shows relatively elastic behavior during heating

Results: TiC lattice strain shows two regions of TRS relief at intermediate temperature

Results: Ni 3 Al lattice strain as assessed form TiC data

Results: TRS evolves relatively linearly during heating/cooling cycle to 1250K

Results: Time resolved ND showed no evidence of creep relaxation at 1250K

Results: Time resolved ND showed no additional phase formation at 1250K

Finite Element Modeling (FEM) FEM Considerations - Microstructural phase segregation - Primarily elastic TRS evolution during heating - Intermediate temperature path- dependence during cooling - High resistance to creep relaxation

Finite Element Modeling (FEM) Unit Cell 1 - Cube inclusion imbedded in cube matrix (50:50 vol.) - Does not consider microstructural segregation - Predicted TRS < Measured TRS by a factor of 3

Finite Element Modeling (FEM) Unit Cell 2 - Spherical Ni 3 Al cell shrouded with TiC structural shell - More representative of microstructure - Good agreement with measured TRS data

Finite Element Modeling (FEM) Elastic Model - Effective in predicting unidirectional thermal residual strain (  ii ) - Suggests TRS evolution is highly dependent on microstructure Ni 3 Al Cell TiC Network 2-D FEM Mesh

Results FEM: Elastic FEM shows approximate TRS prediction

Finite Element Model - Allow plastic flow of TiC structural shell followed by severe strain hardening in these regions. - Elastic-Quasaiplastic (E-QP) model is approximately representative of measured TRS evolution Path dependence of TRS evolution - Slight plastic deformation of Ni 3 Al in dense TiC network. - Plastic accommodation of TRS at 900K and 600K immediately followed by severe strain hardening. - Low mean free path / Kear Wilsdorf locks in Ni 3 Al phase Finite Element Modeling (FEM)

Results FEM: E-QP Model shows path dependance at intermediate temperature

Conclusions Segregated microstructure was found to direct TRS development in TiC-50vol.% Ni 3 Al cermet TRS evolution was found to be slightly path-dependent in the temperature range studied Cermet was found to exhibit a high resistance TRS creep relaxation at 1250K An finite element model based on phase segregation was developed to predict TRS evolution in the temperature range studied

Future Work TEM studies to determine deformation mechanisms in plastic relaxation regions. Thermal residual stress evolution in cermets with different phase volume fractions Strain anistropy during thermal cycling

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