recovery recrystallization grain growth
TRANSCRIPT
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Recovery, Recrystallization & Grain Growth
Now let us consider some microstructural
transformations
MATERIALS SCIENCEMATERIALS SCIENCE
&&
ENGINEERINGENGINEERING
Anandh Subramaniam & Kantesh Balani
Materials Science and Engineering MSE!
Indian Instit"te o# Technology, $an%"r '()(*+
Email: [email protected], URL: home.iitk.ac.in/~anandh
AN INTR-.CTR/ E0$AN INTR-.CTR/ E0$
Part of
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1hen one %hase trans#or2s to another %hase it is called %hase trans#or2ation3
#ten the word%hase transition is "sed to descri4e trans#or2ations where there is no change in co2%osition3
In a %hase trans#or2ation we co"ld 4e concerned a4o"t %hases de#ined 4ased on5Str"ct"re6 e3g3 c"4ic to tetragonal %hase
7ro%erty6 e3g3 #erro2agnetic to %ara2agnetic %hase
7hase trans#or2ations co"ld 4e classi#ied 4ased on5
$inetic5Mass trans%ort 6 -i##"sional or -i##"sionless
Ther2odyna2ic5rder o# the trans#or2ation! 6 *storder, 'ndorder, higher order3
#ten s"4tler as%ects are considered "nder the %review o# trans#or2ations3
E3g3 i! ro"ghening transition o# s"r#aces, ii! coherent to se2icoherent transition o# inter#aces3
"e had noted the followin# in our introduction to phase
transformations
Phase Transformations
Microstructural Transformations
7hase trans#or2ations are associated with change in one or 2ore %ro%erties3
8ence #or 2icrostr"ct"re de%endent %ro%erties we wo"ld li9e to additionally :worry a4o"t; :s"4tler; trans#or2ations,
which involve de#ect str"ct"re and stress state $apart from phases%3
There#ore the 4roader s"4
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Cold wor9= dislocation density
= %oint de#ect density
Anneal
Material tends to lose
the stored strain energy
-ncrease in stren#th
of the material
+oftenin# of the material
Cold wor9
Recrystallization
RecoveryLow '
i#h '
Anneal
The cold wor9ed 2aterial is stronger harder!, 4"t is 4rittle $as noted !efore%.
8eating the 2aterial ty%ically 4elow (3 T2! is and holding #or s"##icient ti2e is a heat
treat2ent %rocess called annealing3
-e%ending on the te2%erat"re o# annealing %rocesses li9e Recovery at lower te2%erat"res!
or Recrystallization at higher te2%erat"res! 2ay ta9e %lace3 -"ring these %rocesses the
2aterial tends to go #ro2 a 2icrostr"ct"rally 2etasta4le state to a lower energy state towards
a sta4le state!3
>"rther :annealing; o# the recrystallized 2aterial can lead to grain growth3
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Cold wor9 Anneal
Recrystallization
Recovery
Grain growth
ri0in# force is free ener#) stored in
point defects and dislocations
ri0in# force is free ener#) stored in dislocations
ri0in# force is free ener#) stored in #rain !oundaries
10er0iew of processes takin# place durin# annealin# of cold worked material and the dri0in#
force for these processes
-t should !e noted that the dri0in# force cited a!o0e is a #lo!al $thermod)namic% dri0in# force. -f the
process will actuall) takes place will depend on the 2local3 conditions. -.e. !oth #lo!al and local criteria
ha0e to !e satisfied if these processes ha0e to take place.
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Cold wor9= 8ardness
= Strength
Changes occ"r to al2ost all%hysicaland 2echanical%ro%erties
Ray di##raction5
J La"e %atterns o# single crystals show %rono"nced asterism
6 d"e to lattice c"rvat"res
J -e4yeScherrer %hotogra%hs show line 4roadening
6 Resid"al stresses K de#or2ations
= Electrical resistance
-"ctility
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Recovery
Recovery ta9es %lace at low te2%erat"res o# annealing
A%%arently no change in 2icrostr"ct"re
EDcess %oint de#ects created d"ring Cold wor9 are a4sor4ed5
J at s"r#ace or grain 4o"ndaries
J 4y dislocation cli24
Rando2 dislocations o# o%%osite sign co2e together and annihilate each other
-islocations o# sa2e sign arrange into low energy con#ig"rations5
J Edge 6 Tilt 4o"ndariesJ Screw 6 Twist 4o"ndaries
7L/GNIOATIN
verall red"ction in dislocation density is s2all
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7L/GNIOATIN0ent crystal
Low angle grain 4o"ndaries
7olygonization
Rando2disloca
tions$statisticall)stored%
4S
tr"ct"raldislocations
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Recrystallization
Trecrystallization(3? @ (3! T2
N"cleation and growth o# new, strain #ree crystals
N"cleation o# new grains in the "s"al sense 2ay not 4e %resent and grain 4o"ndary 2igratesinto a region o# higher dislocation density
5 $recr)stalli6ation% 7 5 $deformed material% 8 5 $undeformed material%
TRecrystallizationis the te2%erat"re at which ( o# the 2aterial recrystallizes in * ho"r
Region o# lower
dislocation densityRegion o# higherdislocation density
-irection o# grain
4o"ndary 2igration
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>"rther %oints a4o"t recrystallization
-e#or2ation = recrystallization te2%erat"re Trecrystallization!
Initial grain size recrystallization te2%erat"re
8igh cold wor9 K low initial grain size#iner recrystallized grains
= cold wor9 te2%erat"relower strain energy stored
= recrystallization te2%erat"re
Rate o# recrystallization P eD%onential #"nction o# te2%erat"re
TrecrystallizationP strong #"nction o# the %"rity o# the 2aterial
Trecrystallizationvery %"re 2aterials! (3? T2Trecrystallizationi2%"re! (3 @ (3+! T2
J TrecrystallizationHH3HHH %"re Al! QoC
Trecrystallization
co22ercial %"rity! 'QoC
The i2%"rity ato2s segregate to the grain 4o"ndary and retard their 2otion 6 +olute dra#
can 4e "sed to retain strength o# 2aterials at high te2%erat"res!
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The i2%"rity ato2s segregate to the grain 4o"ndary and retard their 2otion 6 Sol"te
drag can 4e "sed to retain strength o# 2aterials at high te2%erat"res!
Second %hase %articles also %in down the grain 4o"ndary d"ring its 2igration
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8ot 1or9 and Cold 1or9 8ot 1or97lastic de#or2ation a4ove TRecrystallization
Cold 1or9 7lastic de#or2ation 4elow TRecrystallization
Cold1or9
8ot1or9
Recrystallization te2%erat"re (3F T2!
1hen a 2etal is hot wor9ed, the conditions o# de#or2ation are
s"ch that the sa2%le is so#t and d"ctile3 The e##ects o# strain
hardening are negated 4y dyna2ic and static %rocesses which 9ee%
the sa2%le d"ctile!3
The lower li2it o# te2%erat"re #or hot wor9ing is ta9en as (3+ T2
3
The e##ects o# strain hardening is not negated3 Recovery 2echanis2sinvolve 2ainly 2otion o# %oint de#ects3
.%%er li2it(3? T23
9.
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9.;
'm
9.
'm
9.?'m
9.
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9.A
'm
9.B
'm
1ar2
wor9ing
1ften the ran#e is further su!di0ided into ot, (old and "arm workin# as in the fi#ure
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Grain growth
Glo4ally
J -riven 4y red"ction in grain 4o"ndary energy
Locally
J -riven 4y 4ond 2aDi2ization $coordination num!er maCimi6ation%
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0onded to
F ato2s
0onded to
? ato2s
-irection o# grain
4o"ndary 2igration
0o"ndary 2oves towards its
centre o# c"rvat"re
.M7
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Cold work Recovery Recrystallization Grain growth
Tensile strength
-"ctility
Electical cond"ctivity
Internal stress