recovery recrystallization grain growth

7/24/2019 Recovery Recrystallization Grain Growth

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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

http://home.iitk.ac.in/~anandh/E!ook.htm

A Learners GuideA Learners Guide
http://home.iitk.ac.in/~anandh/E-book.htmhttp://home.iitk.ac.in/~anandh/E-book.htm


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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.

'm

9.;

'm

9.

'm

9.?'m

9.

'm

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

recovery recrystallization grain growth

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