# Presentation: 862912

## S. S. Babu
Oak Ridge National Laboratory
Oak Ridge, TN 37831

Microscopy and Microanalysis for phase transformation studies, May 30 to June 1, 1999, Fukuoka, Japan.

***Oak Ridge National Laboratory***
***Oak Ridge, TN 37831***
***Microscopy and Microanalysis for phase transformation studies, May 30 to June 1, 1999, Fukuoka, Japan.***

***Use of APFIM and the importance of thermodynamic and kinetic modeling techniques for studying non-equilibrium partitioning non-equilibrium partitioning in Nickel-base superalloy welds***

***Research Sponsored by  ***
***Basic Energy Sciences Division of the  Department of Energy***

## Why weld? Welding plays an important role in economical reuse and reclamation of used and failed Ni-base superalloy blades.

***Ongoing work has shown that it is possible to weld single-crystal Ni-base superalloys.***
***However, non-equilibrium microstructure development during solidification and solid-state decomposition may be expected.***
***Weld microstructure will control the weld properties and microstructural stability during service.***

## Experimental

***Alloys (2 - 3 mm single crystal thick sheets)***
_***PWA-1480***_***: Ni - 5.0% Al - 10.0% Cr - 1.5% Ti - 5.0% Co - 12.0% Ta - 4.0% W (wt.%)***
_***CMSX-4***_***: Ni - 5.5% Al -   6.5% Cr - 1.1% Ti - 7.0% Co -   7.0% Ta - 6.5% W (wt.%)***
***	******	******(small amounts of Mo and Re)***
_***CM247DS***_***: Ni - 5.5% Al -   8.0% Cr - 0.8% Ti - 9.0% Co -   3.2% Ta - 9.5% W (wt.%)***
***Welding procedure***
_***Electron Beam Welding (EB)***_***: (Slow weld cooling)***
***accelerating voltage: 100-125 kV, beam current: 7.5-10 mA, and welding speed: 4.2 x 10******-3  ******m s******- 1****** (Preheat: 500 °C)***
_***Pulsed Laser Beam(PL)***_***: (Rapid weld cooling)***
***average power: 90-240 W, pulse rate: 40 s******-1****** and welding speed : 2.1 ************10******-3****** (low) and 12.7 ************ 10******-3****** ms******-1****** (high)***
***Optical, Transmission electron and Atom-Probe Field-Ion microscopes were used for characterization.***

## TEM revealed  ́ precipitates inside  dendrite.

***Size of ************ ́ precipitates varied from ~0.05 to 0.5 μm. ***
***Average volume percentage of ************ ́ was 75%. ***
***Eutectic ************ + ************ ́ was observed along the inter-dendritic boundary.***

## FIM micrograph showing ,  ́, and - ́ interfaces.

***No indication of solute segregation at ************-************ ́ interfaces.***
***How do the elements partition between ************ and ************ ́?***

**PWA-1480**

## APFIM concentration profile from PWA-1480 welds shows no segregation at - ́ interface.

*** ******Not much difference in the composition of ************ and ************ ́phases was observed and were similar to the base metal values [3].***

## No eutectic constituents at the solidification cell boundaries were observed in the laser welds.

***Fine L1******2 ******- ordered ************ ́ phases (~75%) were observed.***
***Similar microstructure was observed in high-speed laser welds***

**low-speed laser welds**

## Shape of concentration profiles indicated a redistribution of alloying elements between  and  ́ phases by diffusion.

***Arrows indicate the direction of diffusion flux in between ************ and ************ ́ phases.  However, concentration gradients within ************ ́ phase are  not large. No solute segregation was observed at interfaces.***

## Comparison of calculated lattice misfit at - ́ interface in laser welds and in single-crystal base metal.

***Lattice misfit % = 100 ************  (a************ - a************ ́******)/[ (a************ + a************ ́******)/2 ]***
***a******************a************ ́******  are the lattice parameter of ************ and ************ ́ phases.***
***This was calculated with the measured concentration profile.***
***Lattice misfit in laser welds is positive compared to that of negative value in the single-crystal base metal.***
***will influence the microstructural evolution during post weld heat treatment and service***

## CM247DS samples were solutionized at 1573 K for 5 min and were continuously cooled at different rates.

***Microstructures were characterized with transmission electron microscopy and were compared with base metal microstructure.***

## Number density of  ́ precipitates in CM247DS increased with an increase in cooling rate during continuous cooling.

## How can we use the above result to design welding processes?The above data need to be used to extend thermodynamic and kinetic calculations to rapidly cooling weld metals.

## Ni-Al-Cr quasi-binary phase diagram suggests a eutectic reaction during weld solidification.  This is supported by experimental observations.

***Eutectic decomposition was observed in PWA-1480 EB welds.***
***This phase diagram was calculated with ThermoCalc software.***

## Diffusion controlled growth of primary  phase into liquid was calculated for Ni - 20 at.% Al - 5 at.%Cr alloy.

***During faster weld cooling ************ phase forms with less elemental partitioning into liquid.  Therefore, eutectic formation can be suppressed.  This was in agreement with CMSX-4 laser weld microstructure development.***

**10 °C/s**

**100 °C/s**

## Solid-state diffusion controlled growth calculations predicted concentration gradients in the  phase when cooled at 75 °C/s compared to 1 and 10 °C/s.

***The concentration profiles are due to incomplete redistribution of alloying elements.***
***These calculations were performed by using DicTra.***
***However, further work is necessary to describe the nucleation kinetics of ************ ́ during continuous cooling.***

**Al**

**Cr**

## Summary and Conclusions

***Weld thermal cycles affect the microstructure development significantly.***
***Number density of ************ ́ precipitates increases with cooling rate***
***Composition of ************ ́ precipitates different from base metal***
***Lattice misfit of ************ ́ precipitates with ************ different from base metal***
***It is possible to use the above changes so that one can modify the welding or post-weld heat treatment procedures.***
***However, we need to use the microanalysis results to improve the thermodynamic and kinetic calculations.  This is an important need so that one can extend these calculations to rapid materials processing.***

## Summary of Results

***In PWA-1480 electron beam welds,***
***Eutectic ************ ́ phase was observed along inter-dendritic boundaries. Coarse ************ ́ (> 100 nm) precipitates were found in the ************ matrix. Elemental partitioning between ************ and ************ ́ phase was similar to that in PWA-1480 base metal***
***In CMSX-4 laser welds, ***
***Negligible eutectic constituents were observed.  Fine  ************ ́ (10-50 nm) precipitates were found in the ************ matrix. APFIM analysis showed large concentration gradients within the ************ phase surrounding the ************ ́ precipitate***
***Transformation studies in CM247DS:***
***Number density of ************ ́ precipitates increased with an increase in cooling rate.***
***Thermodynamic and kinetic calculations agreed with experimental weld-microstructure development.***

## Extensive concentration variations were observed inside the  phase in CMSX-4 welds.

***This observation necessitates further investigation.***

## No evidence of solute segregation at /  ́ interface in CMSX-4 welds.

***Other FIM micrographs indicated fine distribution of ************ and ************ ́ phases.***

**CMSX-4**