Brian Lin1, Yuan Jin2,

Marc Bernacki2, Nathalie Bozzolo2,

Anthony Rollett1, Gregory Rohrer1

Evolution of Microstructure in Nickel During Processing for Grain

Boundary Engineering

1 Carnegie Mellon University 2 Centre de Mise en Forme des Matériaux

2

Outline

• Analysis of special CSL GBs population with GBE processing

– Observing CSL population changes with annealing

– Almost all changes in microstructure arise in the recrystallizing region

…covered in conference proceedings

• Σ3 evolution with recrystallization

– Σ3 boundaries are formed in a continuous manner as the recrystallization front/interface migrates

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Motivation

Grain boundary engineered (GBE) materials enable improved material properties by obtaining high fractions of desired or “special” boundaries

P. Lin and S. Wright, "Grain Boundary Engineering for Improved Resistance to Intergranular Degradation."

Special boundaries are those with a CSL of 3 or 3n (where n = 1,2,3).

We focus on the Σ3, which is related to the annealing twin, so really we’re interested in the twinning mechanism

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Introduction

Grain Boundary Engineering

• Thermo-mechanical Processing (TMP)

– Low (<10%) or medium (20-30%) strain

– Strain-anneal:

• low deformations / high, long time temperature anneal

– Strain-recrystallization:

• medium deformation / low, short time temperature anneal

• High Fraction of Special Boundaries

– After processing, populations may increase from 20-30% in traditional FCC materials with low stacking fault energies to 60-70% in GBE materials

The majority of GBE studies have focused on comparing the initial and final microstructure special boundary fraction as a measure of success of GBE

We wish to elucidate what happens in between

V. Randle: Mater. Sci. Technology, Vol. 26 (2010), p. 253.

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Experimental Procedure

99.999% pure Nickel

• Sample Preparation:– 2 mm thick sectioned from 5 mm diameter rod

– Homogenize microstructure: 410°C in dry Ar/H2 atmosphere for 24 hours

• Thermo-mechanical Processing:– Samples are cold-rolled to 25%

• Hikari EBSD detector on a Quanta 200 FEG SEM– Crystal orientations in the RD and TD

– 450 µm x 450 µm area of a 1.5 µm step size

– Orientation maps after:

• Deformation

• Each anneal

• Anneal Procedure– 490°C in dry Ar/H2 atmosphere

– 5 min intervals to a total time of 30 min

H.M Miller, C.S. Kim, J. Gruber, V. Randle, G.S. Rohrer: Mater. Sci. Forum, Vol. 558-559 (2007), p. 641.

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Results

Recrystallization is based on area fraction of map recrystallized partitioned by grain orientation spread…

Increasing Time

Grain Orientation Spread• measure of the

difference of the avg. orientation with all other orientations in same grain

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Special Boundary Analysis

Recrystallized Partition (right) exhibits very different behavior

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Σ9 Σ27a Σ27b

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Triple Junction Analysis

Σ9 arises from interaction of Σ3 boundaries due to geometric constraints

Boundary Type Time (minutes)

0 5 10 15 20 25 30

ΣΣΣ 1.6% 4.3% 6.2% 8.2% 11.1% 15.7% 19.3%

à 3/3/9 1.5% 3.9% 5.3% 6.7% 8.9% 11.5% 14.7%

The decrease in the RRR TJs is expected along with a pronounced increase of ΣΣΣ types of TJs

This is associated with the increase of special boundary fractions, which are connected to one another

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Summary

• The evolution of Σ3 boundaries in the GBE microstructures can be carefully monitored throughout the recrystallization process of annealing

• The increase in Σ3 boundaries, which appears linear when considering the overall microstructure, actually stagnates when considering only the recrystallization partition

• This suggests that fewer Σ3 boundaries are generated during the later stages of grain growth regime

• The identical behavior the Σ9 and Σ27 in both cases suggests that these are simply a result of geometric constraints of special boundaries at the triple junctions [1]

This strengthens our understanding of how GBE is achieved

[1] M. Frary, C.A. Schuh: Philosophical Magazine, 85:11 (2005), p.1123-1143

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Outline

• Analysis of special CSL GBs population with GBE processing

– Observing CSL population changes with annealing

– All changes in microstructure arise in the recrystallizing region

– …more in conference proceedings

• Σ3 evolution with recrystallization

– Σ3 boundaries are formed in a continuous manner as the recrystallization front/interface migrates

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More Partitioning

We took this one step further and compared the fraction of special boundaries in the overall microstructure and each partitioned region.

The behavior of the two partitioned regions appear independent of each other, which suggests everything is occurring at the interface!

Recrystallized

Deformed

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Discussion

An interesting observation is made when we consider the number of grains appearing in the recrystallized partition

The occurrence of twinning assists in the progress of growth during recrystallization [1]

4% 14% 23% 34%

54% 64% 70% 74%

Qualitative observations agree that the Σ3 boundaries (white lines) are added to an initial cluster over time increasing the number of grains if Σ3s

are counted

[1] D.P. Field, L.T. Bradford, M.M. Nowell, T.M. Lillo, Acta Materialia, Vol. 55, Issue 12, 2007, p. 4233-4241.

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Growth Accident Model

Mechanism proposed by H. Gleiter (1969)

Modified by:C.S Pande (1990)S. Mahajan (1997)

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Localized Environment

Qualitative observations from time sequence of IPF maps

– Σ3 formation occurs parallel to the boundary migration

– Migrating boundaries do not move uniformlyProgressing Recrystallization

I. Grain Boundary Velocity

– Is not consistent throughout the recrystallization process

– Y. Jin (Wednesday B9)

II. Misorientation

– Initially: 60.8°

misorientation (not about a {111})

– Twinned boundary causes a 50.2°

misorientation

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Local Misorientation

• 1D Misorientation Parameter

1 2

3

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Boundary Angle

1 42.2

2 22.1

3 21.5

Boundary Angle

1 35.2

2 22.3

Boundary Angle

1 18.9

2 24.9

3 22.3

4 28.2

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3

4

The 1D misorientation angle is insufficient in providing any information on the probability for a Σ3 to form

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Misorientation Factor

Grain Boundary Plane Distribution (GBPD): 2-parameter description of the misorientation, describing the variation in frequency of GB normals

Distribution is more or less random, but does show some inclination towards and around the (111).

In AGG Ni from a nanocrystalline matrix, there is the lack of annealing twins This is suspected due to the lack of {111} faces

In AGG Ni from a nanocrystalline matrix, there is the lack of annealing twins This is suspected due to the lack of {111} faces

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Conclusions

• The evolution of special boundaries in the microstructures can be observed throughout the recrystallization process. In the recrystallized regions of the maps, the fraction of Σ3 increases rapidly at first and then somewhat decreases.

• Consideration of the recrystallization fraction provides new information on the formation of new Σ3 boundaries

• Misorientation of the interface has been shown to possibly play some role on the twinning mechanism occurrence

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Acknowledgments

Acknowledgements:

The work was supported primarily by a Materials World Network grant from the National Science Foundation under the Award Number DMR-1107896. Y.J. and N.B. are grateful for support from ANR.

Thank you for your attention