Sept_AMP_Digital

A D V A N C E D

M A T E R I A L S

P R O C E S S E S |

S E P T E M B E R

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eactor-based neutron radiogra-

phy is a nondestructive, noninva-

sive characterization technique

used extensively to characterize engi-

neering materials

[1]

. It is often used

to inspect

[2,3]

, evaluate porosity, and

to observe parts in-operando

[4]

. Neu-

tron radiography has been flourish-

ing at reactor facilities for more than

four decades, but is relatively new to

accelorator-based neutron sources

[5]

Recent advances in neutron sources

and detector technologies include the

spallation neutron source (SNS) at Oak

Ridge National Laboratory (ORNL) and

the microchannel plate (MCP) detec-

tor

[6-8]

, respectively. These advances

enable new contrast mechanisms using

neutron scattering Bragg features for

crystalline information, such as average

lattice strain

[9]

, crystalline plane orien-

tation, and identification of phases in a

neutron radiograph

[10]

This new capability can help vali-

date emerging additive manufacturing

(AM) processes. AM holds enormous

potential to revolutionize manufac-

turing by enabling new designs with

complex geometries

[11]

that are not

feasible with conventional manufac-

turing processes. However, the tech-

nique lacks process optimization and

control standards compared with tradi-

tional processes. Residual stresses that

commonly occur when materials are

machined, heat treated, hot and cold

worked, and welded significantly affect

mechanical properties and durability.

Residual stresses can also occur during

AM. For example, defects in AM parts

such as internal cracks can propagate

over time as the component relaxes

after removal from the build plate.

Because AM parts can only be

assessed after completion, it is impos-

sible to identify defects early enough to

minimize expensive reruns. Current val-

idation of AM materials and processes

occurs mainly through expensive tri-

al-and-error experiments at the compo-

nent level. By comparison, the level of

confidence in predictive computational

modeling in conventional processes

is high enough to allow process and

materials optimization through compu-

tational approaches. Thus, nondestruc-

tive characterization techniques and

processing-microstructure

databases

are needed to develop and validate pre-

dictive modeling tools for AM.

PRINCIPLES OF NEUTRON

RADIOGRAPHY

Neutron radiography (nR) mea-

sures neutron beam attenuation

caused by absorption and scattering

within a sample using a 2D, position-

sensitive detector that measures trans-

mitted neutron flux. Neutron computed

tomography (nCT) measures neutron

beam attenuation in 3D by rotating a

sample to record attenuation for mul-

tiple beam paths through an object.

The sample is usually rotated from 0

to 180

, assuming the beam is parallel.

Data quality is improved by scanning

over the full 360

range (super-sam-

pling technique). Computational recon-

struction enables virtual visualization

of the sample in 3D.

Neutrons can be used to study

defects, grain orientation and size,

residual stress, and phase distribution

using both imaging and diffraction

techniques. The principle of neutron

radiography is based on the Beer-

Lambert law given by:

(1)

where

(

) is the transmitted beam

intensity for a neutron of wavelength

;

(

)

is the incident beam intensity;

and

(

)

is the linear attenuation coef-

ficient of element

, of thickness

, as

seen by neutrons. The linear attenua-

tion coefficient,

is given by:

(2)

where

(

) is the neutron total cross

section (sum of scattering and absorp-

tion cross sections) of element

material density,

is Avogadro’s num-

ber, and

is the element molar mass.

At a time-of-flight (TOF) neutron source,

such as the SNS, neutrons are produced

in pulses after being kinetically cooled

or moderated by light atoms, resulting

in an uncertainty in the time of emis-

sion,

when the neutrons leave the

moderator. Neutron wavelength,

, is

determined by:

(3)

where

is the time of detection and

is the distance between the source and

detector. Such wavelength dependent

information is collected at a reactor

source, for example, by using a crystal

monochromator (a crystal that trans-

mits a selected band of neutron wave-

length). However, using this method

reduces incident intensity by almost

two orders of magnitude. All wave-

lengths within a pulse are collected at

an accelerator-based or TOF neutron

source, and wavelength (or energy)

is sorted by the time each neutron is

observed, reducing acquisition time

from several days to a few hours. This

enables acquiring neutron data at the

highest TOF resolution achievable and

post-processing the data in 2D and 3D

images. Because neutron cross sections

are wavelength dependent, the inten-

sity of each pixel of the radiograph,

obtained at different wavelength ranges

displays a different contrast. In non-

textured crystalline materials, narrow

dips and abrupt edges in pixel intensity

occur at precise neutron wavelengths

specified by Bragg’s law:

(4)

where

hkl

is the interplanar distance

for the (

hkl

) atomic planes and

hkl

are

the Bragg angles (angles between the

incident neutron beam and crystalline

planes). At a given (

hkl

) and for

hkl

90° or

hkl

> 90°, neutrons scatter/dif-

fract or transmit, respectively. At

hkl

90°, the neutron wavelength reaches

hkl

, neutrons reach the lowest

transmission, and the darkest pixels are

indicative of the

hkl

orientation, thus

effectively creating a map of preferred

crystalline plane orientations averaged

through the thickness of a material.

The wavelength resolution of a

neutron instrument, Δ

, and the

absence/presence of texture define

the shape of the Bragg edge. Assum-

ing sufficient wavelength resolution

[10]

sharp Bragg edges are indicative of

texture-free material. The position of

the Bragg edge is defined by the applied

or residual stress in the sample

[9,12]

. The

precise location of the Bragg edge as a