In-situ laser ultrasonic grain size measurement in superalloy

INCONEL 718Thomas Garcin1, Jean Hubert Schmitt 2, Matthias Militzer 1

1 The University of British Columbia, 2 Ecole Centrale Paris, Laboratoire Mécanique des Sols, Structures, et Matériaux – UMR CNRS 8579

Acknowledgement: Aubert & Duval, member of Eramet Group

.

2nd Workshop on Laser Ultrasonics for Metallurgy

(April 26-27th 2016, Vancouver, Canada)

1

Motivations

• Inconel 718 used in aviation industry

• High strength material obtained by forging process

• Control the microstructure evolution during forging

2

Ten

sile

Str

engt

hs

(Ksi

)

Temperature (°F)

±1 Gpa at 650°C

Goals

• Control the grain growth + dissolution of second phase particles prior to forging

• Starting structure has 20 µm polygonal grain

• + 2 to 3 % of delta phase precipitates

• Real monitoring of grain growth during soaking

3

Thermo-mechanical processing lab

4

Gleeble jaws assembly

5

LUMet laser probe

6

Real time sensing at high temperature

7

Principle of the technique

8

FEM simulation

Up to 50 waveforms measured per second

Broadband ultrasound pulse (2 to 30 MHz)

Analysis software

9

Measured ultrasonic parameters

10

• Time of arrival of echoes -> Velocity 𝑉

• Amplitude of echoes -> Attenuation α(𝑓)

𝑉 =2(𝑒 + 𝜖)

𝜏

α(𝑓) =20

2𝑒log𝐴𝑒𝑐ℎ𝑜(𝑖)

𝐴𝑒𝑐ℎ𝑜(𝑗)

Filtered signal

𝜏

Velocity of ultrasonic wave

11

𝐶𝑖𝑗𝑘𝑙 = 𝑐′𝑖𝑗𝑘𝑙𝑓(𝑜𝑑𝑓)

What can be investigated ? Phase transformationSecond phase/Precipitation Recrystallization

EBSD to Velocity map (mm/µs)

Pro

pag

atio

n (𝑛

)

Velocity Distribution 𝑉 =

𝑃ℎ𝑎𝑠𝑒𝑠

𝐾(𝑜𝑑𝑓)

𝜌Pure Titanium

Rotated Elastic Tensor

Attenuation and scattering by grain

12

V1 V2<V1

Scattering in bi-crystalPro

pag

atio

n (𝑛

)

Scattering depends on ultrasonic wavelength

Scattering depends on grain boundary disorientation and

incidence angle and grain volume

Nicoletti et al. 1994

Large grain size = High attenuation

Other sources of attenuation

13

𝛼(𝑓) = 𝑎 + 𝑏𝑓𝑛 + 𝑐𝑓𝑚

Grain Scattering Diffraction(sample geometry)

Internal Friction/Gain

Thic

knes

s e

TWO ECHOES METHOD

D = 2e D = 4e

How to measure grain size ?

14

𝛼(𝑓) = 𝑎 + 𝑏𝑓𝑛

𝑏 = 𝐶(𝑇) 𝐷𝑖𝑛−1(𝑡) − 𝐷0

𝑛−1(𝑡0 𝑓𝑛

1) Reference sample 𝐷02) ONE ECHO METHOD

Isolate only grain scattering

Measurement precision < 10 %

Frequency dependant grain size parameter

Reference

Current

Reference

Current

High Scattering

Low Scattering

Experiments

15

• Isothermal holding at 1050°C for various time

• Laser ultrasound measurement of mean grain size

• Validation with metallography and modeling

LUMet measurement

Microstructure evolution

16

• Grain growth during isothermal holding

• Second phase is almost fully dissolve after 175 s

Stage of heterogeneous grain growth

17

• Local Nb microsegregations affect the stability of the second phase leading to heterogeneous grain grow

Fraction of large grain

18

• Grains larger than the maximum diameter in initial state

• Conserve grain with aspect ration < 0.3 , i.e. twins

• Fraction = Ratio the threshold area by the total area

• Renormalization

Metallographic analysis

19

• Evaluation of the mean grain size EQAD =

• Maximum 1% largest grain diameter 4 𝐴 𝜋

Attenuation measurement

20

• Single echo method, i.e. with a reference sample

• Measure of the relative change in grain size

Correlation at 1050°C

21

• Evolution of the scattering parameter b with the relative change in mean grain size.

• Direct measurement of the coefficient C*

Linear regression coefficient C* = 0.022

Grain grow tests

22

• Insight into the grain growth behavior.

• Different grain growth stages

• 1) Zener

• 2) Rapid grain growth

• 3) Parabolic

Criteria for abnormal grain growth

23

• Normalization procedure

• Time at the onset of abnormal grain growth

Conclusions

24

• The δ-phase precipitates dissolve in the sample at 1050 C leading to heterogeneous grain growth.

• Ultrasonic attenuation monitored in situ the occurrence of this heterogeneous growth stage

• Direct quantification of the onset and completion of heterogeneous grain growth.