Hufnagel abstract - Advanced Photon Source

puppypompAI and Robotics

Nov 14, 2013 (3 years and 9 months ago)

87 views

Evolution  of  complex  microstructures:  
 
Opportunities  for  new  experiments  with  hard  x

ray  sources
 
Todd  C.  Hufnagel
 
Department  of  Materials  Science  and  Engineering
 
Johns  Hopkins  University,  Baltimore,  Maryland
 
hufnagel@jhu.edu
 
 
The  goal  of  materials  science  
and  engineering  is  to  produce  materials  with  useful  
combinations  of  properties  at  affordable
 price
s
.  
Both  properties  and  costs  are  
influenced  by  microstructure,  so  understanding  and  controlling  the  development  of  
microstructure  is  important  for  virtually  a
ll  engineering  materials.  Although  
complex  microstructures  can  result  from  the  simplest  processing  

 witness  the  
i n t r i c a t e  g e o me t r y  o f  s n o wf l a k e s
 

 
precise  
control
 of  microstructure  usually  
requires  sophisticated  (and  therefore  expensive)  processing.  The  c
ost/benefit  
tradeoff
 can  be  
m a d e
 
s e v e r a l  w a y s;  f o r  i n s t a n c e,  w e  c a n  s t r i v e  t o  i m p r o v e  
properties,  usually  with  increased  processing  costs,  or  we  can  drive  down  costs  by  
more  economical  processing,  hopefully  while  maintaining  or  even  improving  
properties.
 
O
ne  r
oute  to  microstructural  control
 

 used  for  most  traditional  processing
 

 
is  to  
establish  global  conditions  of  important  process  variables  such  as  composi
tion,  
temperature,  and  pressure.  The  microstructure  then  evolves  according  to  
fundamental  physical  
processes  which  can  be  controlled  only  indirectly  and  
relatively  crudely
 by  selection  of  the  process  variables
.  
Typically
,  experiments  
designed  to  study  these  fundamental  processes  
are
 conducted  on  simplified  systems  
under  idealized  (
e.g.
 near  equilibrium)
 conditions.  Understanding  what  happens  in  
“real”  materials  under  
actual  manufacturing
 conditions  remains  a  significant  
challenge.
 
In  some  systems,  additional  control  over  microstructure  can  be  achieved  by  
tailoring  the  ways  in  which  the  fundamental  buildi
ng  blocks  of  the  material  interact.  
For  instance,  certain  block  copolymers  phase  separate  to  produce  
a  variety  of  
microstructures
,  
the  details  of  which  depend
 on  the  len
g t h  a nd  c he mi s t r y  o f  t he  
blocks
.  Similarly,
 a  wide  variety  of  colloidal  crystals  can  be
 produced  by  careful  
selection  
and  control  
of  interactions  among  
nano
particles
.
 
Even  more  precise  control  of  microstructure  relies  on  templates  for  organizing  the  
fundamental  building  blocks
 of  the  structure
.  For  i ns t a nc e,  i nf or ma t i on  c ode d  by  
base  pair  se
quences  in  DNA  allows  proteins  to  be  assembled  with  
remarkable
 
fidelity,  and  ultimately  the  production  of  complex  hierarchical  structures  such  as  
collagen.  Complicated  hierarchical  structures  are  also  assembled  during  
biomineralization,  which  can  involve  s
equential  deposition  of  organic  and  mineral  
layers,  including  control  over  the  nucleation  and  crystallographic  orientation  of  the  
mineral  component.  Taking  its  cues  from  nature,  the  field  of  biomimetics  strives  to  
understand  these  mechanisms,  with  the  goal
 of  producing  similarly  complex  
microstructures  to  produce  materials  with  useful  properties.  
 
The  development  of  new  (third
-­‐
 and  fourth
-­‐
generation)  hard  x
-­‐
ray  sources
,  
together  with  advances  in  x
-­‐
ray  detectors,
 provides  many  opportunities  for  
observing  
mic
rostructure  and  its  evolution.  Some  of  these  are  quantitative  
improvements  that  make  use  of  the  brilliance  of  modern  sources  to  enable  
experiments  that  were  previously  impractical.  For  instance,  three
-­‐
dimensional  x
-­‐
ray  
diffraction  (3DXRD)  uses  a  microfocus
ed  x
-­‐
ray  beam  to  map  crystalline  
microstructures  on  micron  length  scales.  X
-­‐
ray  microfocusing  combined  with  fast  
x
-­‐
ray  area  detectors  gives  temporal  and  spatial  resolution  sufficient  to  study  rapid,  
localized  transformations  (Figure
 
1).
 
 
Even  more
 exciting
 opportunities  arise  for  entirely  new  techniques
 based  on  the  
brilliance  of  modern  x
-­‐
ray  sources,  their  coherence,  or  both.  For  example,  Figure
 
2  
shows  an  x
-­‐
ray  phase  contrast  image  of  a  rapidly  progating  exothermic  reaction  
front  in  a  reactive  metallic  mu
ltil
a
yer.
 The  reaction  front  advances  via  sequential  
growth  of  steps  in  a  direction  transverse  to  the  overall  propagation  direction.  Notice  
that  the  phase  contrast  images  reveal  information  about  the  internal  structure  of  the  
advancing  step.
 
The  extremely  
short  pulses  produced  by  x
-­‐
ray  free  electron  lasers  (XFELs)  hold  
promise  for  studying  dynamics  on  femtosecond  time  scales.  In  most  cases  this  will  
be  in  the  context  of  multi
-­‐
shot  pump
-­‐
probe  studies  of  transitions  
that  can  be  
reversibly  excited.  Although  mi
crostructural  development  is  irreversible,  it  is  likely  
that  useful  information  about  fundamental  processes  such  as  grain  boundary  
migration  can  be  obtained  from  multi
-­‐
shot  experiments.  It  may  also  be  possible  to  
devise  single
-­‐
shot  experiments  for  studying
 irreversible  processes.
 
Techniques  enabled  by  the  coherence  of  modern  sources  include  coherent  
imaging,  x
-­‐
ray    photon  correlation  spectroscopy,  and  fluctuation  electron  
microscopy.  These  hold  great  pro
mise  for  revealing  details  of  structure  and  
dynamics  
of  complex  systems,  particularly  those  without  long
-­‐
range  crystalline  
order.
 
 
 
 
Figure  1  

 Time  resolved  x

ray  microdiffraction
.  
 
Al/Ni  multilayers  with  nanoscale  layering  can  sustain  
exothermic  reactions  as  localized  fronts  ~100  µm  
wide  that  propagate  w
ith  velocities  of  1

10  m/s.  In  
this  example,  diffraction  reveals  that  in  the  earliest  
stages  of  the  transformation,  Al  melts  and  the  cubic  
intermetallic  AlNi  (B2  structure)  nucleates.  At  longer  
times  (30

40  ms)  these  phases  undergo  a  peritectic  
reaction  to
 form  the  stable  intermetallic  Al
3
Ni
2
.
 
 
1700
1600
1500
1400
1300
1200
1100
1000
900
800
Temperature (K)
10
2
10
3
10
4
10
5
Time (µs)
1.0
0.8
0.6
0.4
0.2
0.0
Integrated peak area (arb.)
Temperature
AlNi
Al
3
Ni
2
Al
3
V
Amorphous
Al/Ni
Ni
Al
AlNi
 
Figure  
2
 

 
X

ray  phase  contrast  imagine.  In  Al/Zr  
multilayers,  
self

pro
p
agating  
reactions  described  in  
Figure  1  
in  some  cases  propagate  by  lateral  growth  of  
steps  transverse  to  the  overall  direction  of  
propagation.  Here,  the  overall  reaction  front  moves  
at  1.6  m/s,  but  the  velocity  of  the  steps  is  much  
higher,  9.4  m/s.  Note  also  that  the  imaging  reveals  
information  
about  the  internal  structure  of  the  
advancing  steps.
 
 
!"#$%&
'()$&*+$,-./01223
4250
6(7$&*+
,218/0123
'99$%&