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CHNG II - HP KIM - GIN PHA

II.1. KHI NIM V HP KIM

1) nh ngha:

Hp kim l mt loi vt liu c to thnh bng cch nu chy hay thiukt mt kim loi vi mt hay nhiu nguyn t khc.

Hp kim mang tnh kim loi: dn in, dn nhit cao, dbin dng, c nhkim.

Trong hp kim nguyn t chnh nguyn t l kim loi.

2) u vit ca hp kim:

Hp kim c s dng rng ri trong ch to c kh v n c nhng ctnh u vit hn hn kim loi nguyn cht v gi thnh thp hn.

Hp kim c c tnh tng hp tt hn kim loi nguyn cht: hp kim

c bn cao hn nhiu so vi kim loi nguyn cht, do thphn nhng c tnh chung ca n m bo tha mn y cc yucu ca ch to c kh.

Hp kim c tnh cng ngh a dng v ph hp: m bo tnh giacng ct gt, bin dng do, c thm ti ln... Mt s hp kimc bit c nhng tnh cht qu: khng r, c in trln, gin nc bit, chng mi mn ln, chu nhit cao... m kim loinguyn cht khng th c c.

Trong nhiu trng hp hp kim d ch to n gin v r tin hn:luyn thp c nhit chy thp hn luyn st, la tng bn v r

hn ng, ...3) Mt s khi nim:

a- Cu t (nguyn):

Cu t l nhng nguyn t (hoc nhng hp cht ha hc bn vng) cuto nn hp kim. Chng l cc thnh phn c lp.

b- H (h thng):

H l mt tp hp vt th ring bit ca hp kim trong iu kin xc nh.

c- Pha :

Pha l phn ng nht ca h (hp kim) c cng cu trc mng tinh th vcc tnh cht c, l, ha xc nh. Gia cc pha c b mt phn cch.

d- Tr ng thi cn bng (n nh):

H trng thi cn bng khi cc pha ca n u c nng lng t do nhnht trong cc iu kin v nhit , p sut v thnh phn xc nh. Tc lcc c tnh ca h khng bin i theo thi gian. Thng thng h vi cc

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pha trng thi cn bng bao gicng c bn, cng thp nht, khngc ng sut bn trong, x lch mng tinh th thp nht v c hnh thnhvi tc ngui chm.

e- Tr ng thi khng cn bng (khng n nh):

Khi thay i nhit v p sut lm tng nng lng t do v h trnntrng thi khng cn bng. Lc ny h c th chuyn bin sang trng thicn bng mi c nng lng t do nh hn. Ni chung trng thi khngcn bng l khng n nh, lun c xu hng t bin i sang trng thicn bng, n nh. Trong thc t mt s trng thi khng cn bng vn tnti lu di, do nhit thng chuyn bin xy ra rt chm hu nhkhng nhn thy c. Trng thi khng cn bng thng c bn, cng cao hn nn c s dng kh nhiu trong thc t (t chc mactenxitsau khi ti). Trng thi khng cn bng c hnh thnh vi tc nguinhanh.

f- Tr ng thi gi n nh:Trng thi gi n nh tn ti khi trng thi cn bng (n nh) tuyt i chtn ti trn l thuyt, tc l phi nung nng hay lm ngui v cng chmm trong thc t rt kh xy ra. Vy gi n nh thc cht l khng n nhnhng thc t li tn ti mt cch n nh ngay c khi nung nng hay lmngui trong mt phm vi no .

4) Cc dng cu to ca hp kim:

Trong thc t hp kim thng c cc dng cu to sau y: Hp kim c cu to mt pha l dung dch rn.

H p kim c cu to mt pha l h p cht ha hc (hay pha trunggian).

Hp kim c cu to bi hai hay nhiu pha.

II.2. CU TRC MNG TINH TH CC PHA CA HP KIM

1) Dung dch rn:

a- Khi nim:

nh ngha: dung dch rn l mt pha tinh th trong nguyn t dung migi nguyn kiu mng, cn nguyn t ca nguyn t ha tan c sp xp

vo trong mng tinh th ca nguyn t dung mi mt cch u n v ngunhin.

K hiu dung dch rn l A(B), trong A l nguyn t dung mi v B lnguyn t ha tan.

b- c im chung: Mng tinh th ca dung dch rn l mng tinh th ca nguyn t

dung mi.

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Nng cht ha tan thay i trong phm vi rng. Lin kt nguyn t l lin kt kim loi. V vy dung dch rn l mt

pha c do cao nht trong tt c cc pha cu to nn hp kim. Khi nng nguyn t ha tan cng tng th mt lch cng tng

dn n bn, cng, in tr cng tng, do, dai gim.

c- Phn loi:

Dung dch rn c chia ra lm hai loi : dung dch rn thay th v dungdch rn xen k.

2) Dung dch rn thay th:

Dung dch rn thay th l loi dungdch rn m nguyn t ca cu tho tan thay th v tr nguyn tca cu t dung mi (hnh III.1).

Dung dch rn thay th c kiumng v s nguyn t trong khic s ng nh ca cu t dungmi. Tuy nhin s thay th t nhiuu gy ra s x lch mng, vkhng th c hai loi nguyn t cahai cu t c kch thc hon tonging nhau. Do vy s thay th chxy ra vi cc cu t c kch thc nguyn t khc nhau t (ng knhnguyn t sai khc ny khng qu 15%).

Ty thuc vo mc ha tan ngi ta cn chia ra dung dch rn ha tanv hn v c hn.

a- Dung dch rn thay th ha tan v hn:

L dung dch rn m trong nng ca cht ha tan c thbin i lintc, tc l vi nng bt k.

Trong loi dung dch rn ny khng thphn bit c cu t no l dungmi, cu t no l cht ha tan. Cu t no c lng cha nhiu nht ldung mi, cc cu t cn li l cht ha tan.

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iu kin hnh thnh dung dch rn ha tan v hn: C cng kiu mng tinh th. ng knh nguyn t khc nhau t, nh hn 8%. C cng ho tr. C tnh cht vt l v ha hc gn ging nhau (cu to lp v in

t, tnh m in, nhit chy, ...)b- Dung dch rn ha tan c hn:

L dung dch rn m trong cc cu t ch ha tan vo nhau vi gi trnht nh, tc l nng ca chng b gin on.

Cc cp cu t khng tha mn 1 trong bn iu kin trn s to thnhdung dch rn c hn.

c- Dung d ch rn trt t v khng trt t :

Nu s phn b nguyn t ca cu t ha tan trong mng dung mi mt

cch ngu nhin th c gi l dung dch rn khng trt t. Trong mt siu kin no (nhit , nng ) trong mt s h cc nguyn t thay thc tnh quy lut v gi l dung dch rn trt t.

3) Dung dch rn xen k:

Dung dch rn xen k l dungdch m nguyn t ca cu tha tan nm im trng trongmng tinh th ca nguyn tdung mi.

c im: Mt Mv trong dung

dch rn xen k tng vs nguyn t n tng.

Dung dch rn xen k ch ho tan c hn. Cc im trng trong mng u nh nn cc nguyn t ha tan B

mun lt vo cc im trng phi c ng knh kh nh vinguyn t dung mi dB/dA < 0.59

Dung dch rn xen k thng c to thnh bi dung mi l kim loi cng knh nguyn t ln nh Fe, Cr, W, Ti, v nguyn t ho tan lcc kim c ng nguyn t nh nh C, N, H, B.

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Gia cc kim loi vi nhau ch to thnh dung dch rn thay th, v chngc ng knh nguyn t khng khc nhau nhiu. Khi t l ng knhnguyn t trong khong 0.59-0.85, chng to nn hn hp c hc ca haikim loi.

4) Hp cht ha hc:

nh ngha: Hp cht ha hc l mt pha c to thnh bi cc nguyn tgia chng c xy ra phn ng ha hc.

c im: Mng tinh th ca hp cht ho hc khc kiu mng cc nguyn t

to thnh. Lin kt trong hp cht ho hc ch yu l lin kt Ion hoc Ion kt

hp ng ha tr. Hp cht ho hc c nng gn cao nht. Nng trong hp cht ha hc khng i v tun th theo quy tc

ha tr.Tnh cht: Nhit nng chy cao, cng cao, kh nng chng mi mntt v rt gin.

5) Pha trung gian:

a- nh ngha:

L mt pha c c im nm gia dung dch rn v hp cht ha hc.

b- c im: Mng tinh th ca pha trung khc hn mng tinh th cc nguyn t

to thnh. Nng pha trung gian c th thay i trong phm vi hp. Lin kt trong pha trung gian ch yu l lin kt kim loi. Pha trung gian c nhit nng chy, cng, kh nng chng mi

mn rt cao, tng i dn.

c- Mt spha trung gian:

i) Pha xen k :

Pha xen k l mt loi pha trung gian c to bi cc kim loi c ngknh nguyn t ln (nh W, Ti, Mo, Mn,) vi kim c ng knh

nguyn t b (nh H, N, C) Nu dKL/dAK < 0.59: pha xen k c kiu mng n gin (nh: WC,

TiC, Fe2N,). c im ca pha ny l nhit nng chy, cng, kh nng chng mi mn rt cao, tng i gin.

Nu dKL/dAK > 0.59: pha xen k c kiu mng phc t p (Fe3C,Cr7C3, Cr2C6). c im: nhit nng chy, cng, kh nngchng mi mn cao.

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ii) Pha in t(pha Hume-Rothery):

Pha in t l pha trung gian c cu to phc tp, to nn bi 2 kim loi: Nhm 1: kim loi ha tr mt nh Cu, Ag, Au v kim loi chuyn

tip Fe, Co, Ni, Pd, Pt. Nhm 2: cc kim loi ha tr 2, 3, 4: Be, Mg, Zn, Cd(2), Al(3),

Si,

Nng in t (Ct) l t s gia tng s in t ha tr vi tng snguyn t ca hp cht. Hp cht AmBn c nng in t l:

C =(mA. HTA + nB. HTA)

(mA+nB)

Pha h p cht in t c nng in t bng mt trong cc gi tr 3/2,21/13, 7/4, m mi gi tr ng vi kiu mng xc nh. Nu:

Ct =3/2 l lp phng th tm, k hiu .

Ct =21/13 lp phng phc tp, k hiu . Ct =7/4 l mng lc gic xp cht, k hiu .

V d: Cu-Zn c cc pha in t sau: CuZnpha c Ct = (1.1 + 1.2)/(1 + 1) = 3/2 - mng lp phng th

tm. Cu5Zn8pha c Ct = (5.1 + 8.2)/(5 + 8) = 21/13- mng lp phng

phc tp. CuZn3pha c Ct =(1.1+3.2)/(1+3) = 7/4 -mng lc gic xp cht.

iii) Pha Laves :

Pha Laves l pha to nn bi hai nguyn t (A, B) c t l ng knhnguyn t dA/dB = 1,2 (t l ny c th bin i trong phm vi 1,1 1,6),c cng thc AB2, kiu mng su phng x p cht (MgZn2) hay lpphng tm mt (MgCu2).

Trong hp kim c th cn gp cc pha: , , , , ... Tuy nhin cc loi phany t phbin.

Mt c tnh quan trng ca cc pha trung gian l cng v dn. V vykhng bao gi ngi ta dng hp kim ch c mt pha l pha trung gian. Tl ca chng trong cc hp kim thng thng t hn 10% (c khi n 20

30%). y l cc pha cn trt lm tng bn, cng.6) Hn hp c hc:

Trong nhiu trng hp, hp kim c t chc hai hay nhiu pha: hai dungdch rn, dung dch rn v pha trung gian, ... Cu to nh vy gi l hnhp c hc.

Trn t chc t vi ta phn bit c rt r cc pha khc nhau trong hn hpc hc.

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II.3. GIN PHA CA HP KIM HAI CU T

1) Khi nim v gin pha:

a- nh ngha:

Gin pha l mt biu m t sph thuc trng thi pha ca hp kimvo nhit v nng . Gin pha cn gi l gin trng thi.

Gin pha c xy dng hon ton bng thc nghim, trong iu kinnung nng v lm ngui v cng chm, tc l trng thi cn bng.

b- Cng d ng

T gin trng thi c th xc nh c: Nhit chy, chuyn bin pha ca cc hp kim trong h khi nung

nng v lm ngui, nh c th xc nh d dng cc ch c,r, cn, hn, nhit luyn.

Trng thi pha (pha no, thnh phn pha, s l ng pha, t l) cahp kim trong h, t c th on c c tnh c bn v cngdng.

c- Cu to:

Gin pha ca h hai cu t gmhai trc: trc tung biu din nhit ,trc honh biu din thnh phn hahc (theo % khi lng). Trong htrc ngi ta v cc ng phnchia gin thnh cc khu vc c tchc v pha ging nhau.

Cc im trn ng nm ngang biuth cho cc h p kim c thnh phnkhc nhau nhng cng mt nhit. i t tri qua phi t l cu t Btng dn ln, cu t A gim i vngc li.

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Cc im nm trn ng thng ng biu th cho mt hp kim c thnhphn xc nh nhng cc nhit khc nhau. Nu hp kim c hai pha thim biu din ca chng phi nm v hai pha i din vi im biu dinhp kim.

d- Quy t c pha dng nghin cugin pha

Bc t do l s yu t c lp c th thay i c trong gii hn xc nhm khng lm thay i trng thi pha ca h, tc l khng lm thay i spha c.

Quy tc pha xc nh mi quan hgia s pha P, bc t do F v s cut. Ta c: F = C P + 1

e- Quy t c n by:

Quy tc n by c dng xc

nh nng ca cc pha (hnh II.7).H p kim x% B nhit T c haipha l L v : pha lng (L) v tr N,pha v tr M. T l gia L v c xc nh nh sau:

L =OM

MN. % ; =

ON

MN. %

2) Gin pha hai cu t khng ha tan vo nhau (gin loi I):

S cu t: A v B (C = 2) Cc pha c th to thnh: lng (L) ha tan v hn ca A v B, A v

B. Spha ln nht PMAX = 3.

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im A l nhit nng chy ca nguyn t A. im B l nhit nng chy ca nguyn t B.

AEB l ng lng: ti nhit ng vi ng lng hp kim btu kt tinh. cao hn ng lng hp kim hon ton trng thilng.

CED l ng c: ti nhit ng vi ng c hp kim lngkt thc kt tinh. Thp hn nhit ny hp kim trng thi rn.

Trong khong nhit gia ng lng v ng c l qu trnh nngchy hay kt tinh ca hp kim, tn ti ng thi c pha rn v lng. Viloi gin ny, CED cn gi l ng cng tinh, E gi l im cng tinh(eutectic), hp kim c thnh phn ti E gi l hp kim cng tinh. Cc hpkim c thnh phn nm bn tri im E gi l hp kim trc cng tinh(hypoeutectic). Cc hp kim nm bn phi im E gi l hp kim sau cngtinh (hypereutectic). Trong thc t h Pb - Sb thuc loi gin ny.

b- Qu trnh kt tinh ca hp kim trc cng tinh: cao hn nhit ng vi im 0: hp kim hon ton trng thilng (L).

Lm ngui t 0 n 1: y l qu trnh ngui ca hp kim lng (L). Ti nhit ng vi im 1: t hp kim lng kt tinh ra tinh th A

(thnh phn ti 1). Lm ngui t 1 n 2: tinh th A sinh ra ngy cng nhiu, hp kim

lng cng ngy cng t i v thnh phn ca n bin i theo ngt 1 -> E (giu B hn).

Ti nhit ng vi im 2: phn hp kim lng cui cng c thnh

phn ti E s kt tinh ng thi ra A v B cng mt lc. T chcny gi l t chc cng tinh. Qu trnh ny gi l chuyn bin cngtinh (eutectic), xy ra ti nhit khng thay i.

Lm ngui t 2 n 3 l qu trnh ngui ca hp kim rn, khngxy ra chuyn bin no khc. T chc nhn c nhit thngl A + (A +B). Cc tinh th A kt tinh ra trc c kch thc th tohn cng tinh (A + B).

c- Qu trnh kt tinh ca hp kim sau cng tinh:

Qu trnh kt tinh ca hp kim sau cng tinh hon ton ging nh hp kim

trc cng tinh, nhng ch khc l t hp kim lng s kt tinh ra tinh th Btrc v phn lng cn li s ngho B i khi nhit tip tc gim xung.

T chc nhn c nhit thng l B + (A + B).

d- Qu trnh kt tinh ca hp kim cng tinh:

Hp kim ny c thnh phn ng vi im E. Khi lm ngui n nhit ng vi im E hp kim lng s kt tinh ng thi ra (A + B) cng mt lcv sn phm cui cng l cng tinh (A + B).

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Nhn xt: Cc hp kim c gin loi 1 kt tinh theo th t sau : trc tinpha lng kt tinh ra mt trong hai cu t nguyn cht trc, lm cho phalng ngho cu t ny v bin i thnh phn n im cng tinh E. ny pha lng cn li s kt tinh ra hai cu t cng mt lc.

3) Gin pha hai cu t ha ta v hn vo nhau, khng to nn phatrung gian (gin loi II): S cu t: A v B (C = 2). Spha c th to thnh: hp kim lng ha tan v hn ca A v B,

dung dch rn ha tan v hn ca A v B l (Pmax = 2).

ng AmB gi l ng lng, ng AnB gi l ng c. Hp kimng niken (Cu-Ni) c gin loi ny.

b- Xt qu trnh kt tinh ca mt hp kim c th (hp kim 1):

Ti nhit ng vi 0 hp kim tn ti trng thi lng. Lm ngui t 0 - 1: qu trnh ngui ca hp kim lng. Ti nhit ng vi im 1: t hp kim lng kt tinh ra dung dch

rn ha tan v hn ca A v B l . Lm ngui trong khong nhit t 1 n im 2 dung dch rn

sinh ra ngy cng nhiu, thnh phn ha hc ca n bin i theong t 1 - 2', hp kim lng ngy cng t i v thnh phn ha hcbin i t 1' - 2.

Ti nhit ng vi im 2 hp kim lng ht. Lm ngui t 2 - 3 l qu trnh ngui ca dung dch rn .

c- Thin tch nhnh cy (thin tch trong bn thn ht):Hp kim c gin loi 2 khi kt tinh ti mi nhit khc nhau, thnhphn ha hc ca dung dch rn cng khc nhau. Do , bng cch lmngui thng thng (ngui nhanh) ht kim loi c to thnh s khngng nht v thnh phn ha hc. Hin tng ny gi l thin tch nhnhcy (hay thin tch trong bn thn ht). Khc phc bng cch lm nguichm hay khuch tn sau khi c.

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4) Gin pha hai cu t ha tan c hn vo nhau, khng to thnhpha trung gian (gin loi III): S cu t: A v B (C = 2) Spha c th to thnh: ( PMax = 3) Hp kim lng ha tan v hn ca A v B

- dung dch rn ca B ha tan c hn trong cu t A, A(B) - dung dch rn ca A ha tan c hn trong cu t B, B(A)

ng AEB l ng lng, ACDB l ng c, CED l ng cng tinh.CF l ng gii hn ha tan ca B trong cu t A trng thi rn, DG lng gii hn ha tan ca A trong B trng thi rn. im E l imcng tinh.

H hp kim Ag - Cu v Pb - Sn c gin loi ny.

Cng tng t nh gin loi 1, nhit chy ca cu tbt k s gim

i nu c thm cu t th hai vo. Gin lai III gm nh tng hpca hai gin loi I v loi II. C th chia cc hp kim ca h thnh banhm sau:

a- Nhm cha rt t cu t th hai (c thnh phn nm bn tri F v bnphi G), qu trnh kt tinh ging gin loi II, sn phm nhn c ldung dch rn a v b.

b- Nhm cha mt lng hn ch cu t th 2 (thnh phn nm trongkhong F C' v G D'), ban u kt tinh ra dung dch rn, nhng khi nhit tip tc gim i thp hn ng CF v DG th do lng cu t ha tanl qu bo ha nn tit ra lng cu t tha di dng dung dch rn thcp ( tha B tit ra II, tha A tit ra II).

Ta xt qu trnh kt tinh ca hp kim I: Ti nhit ng vi 0: hp kim trng thi lng. Lm ngui t nhit ng vi im 0 n 1 l qu trnh ngui ca

hp kim lng. Ti nhit ng vi im 1, hp kim lng kt tinh ra dung dch rn

, c thnh phn xc nh ti im 1'.

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Lm ngui t nhit ng vi im 1 n im 2, dung dch rn sinh ra ngy cng nhiu, thnh phn ca n bin i theo ng t1' - 2, hp kim lng ngy cng t i, thnh phn ca n bin i theong t 2 - 2'.

Ti nhit ng vi im 2 hp kim lng ht.

Lm ngui t 2 n 3 l qu trnh ngui ca dung dch rn . Ti nhit ng vi im 3, lng ha tan ca B vo A l qu bo

ha nn B tha c tit ra di dng dung dch rn II ( II). Ti nhit thng sn phm nhn c l + II

c - Nhm cha mt lng ln cu t th hai (c thnh phn nm trongkhong C' D') ban u kt tinh ra dung dch rn (a hay b ), pha lng cn libin i thnh phn theo ng lng n im E v kt tinh ra t chccng tinh. Khi nhit h xung thp hn ng CF v DG cng c qutrnh tit ra cu t ha tan tha di dng II v II .

Qu trnh kt tinh ca nhm ny ging gin loi I. Xt qu trnh kt tinhca hp kim II: Ti nhit ng vi im 0 hp kim trng thi lng. Lm ngui t nhit ng vi im 0 n dim 1, l qu trnh

ngui ca hp kim lng. Ti nhit ng vi im 1 t hp kim lng kt tinh ra dung dch

rn c thnh phn ti 1'. Lm ngui t nhit ng vi im 1 n im 2 dung dch rn

sinh ra ngy cng nhiu, thnh phn ca n thay i t 1' - C. Hpkim lng ngy cng t i, thnh phn ca n thay i t 1 - E.

Ti nhit ng vi im 2, hp kim lng cn li c thnh phn tiE s kt tinh ng thi ra hai dung dch rn C v D cng mt lc.Qu trnh ny din ra ti nhit khng i LE -> (C + D)

Lm ngui t nhit ng vi im 2 n 3 do lng cu t hatan l qu bo ha nn c qu trnh tit ra II t v II t . Tuynhin II c tit ra t trong cng tinh, nm ln ln vi C nnkhng nhn thy c. Do vy t chc nhn c nhit thngl + II + ( + ).

5) Gin trng thi hai cu tkhng ha tan ln nhau nhng tonn pha trung gian n nh (gin loi IV): Cc cu t: A v B (C = 2) Cc pha c th to thnh: Hp kim lng, Cu t A, B Hp cht ha hc ca chng l AmBn (k hiu l H).

Hp cht ha hc H c nhit nng chy ring, thnh phn xc nh vkhng th ha tan thm cu t A, B. H hp kim Mg - Si thuc loi ny.

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Nghin cu qu trnh kt tinh ca hp kim c gin loi ny ta a vnghin cu hai gin pha loi 1 l A-H v H-B (H c xem l mt cut c lp)

Gin loi IV l gin pha hai cu t tng tc phn ng ha hc vinhau to ra pha trung gian AmBn v c h in hnh l h magie-canxi(Mg-Ca). C dng ghp hai gin loi I l A-AmBn (Mg - Mg4Ca3) vAmBn-B (Mg4Ca3 - Ca). y pha trung gian AmBn c nhit chy cnh, khng bphn hu trc trc khi nng chy c coi nh mt cut.

6) Quan h gia dng gin pha v tnh cht ca hp kim:

Cc loi gin pha khc nhau c mi quan h gia dng ca gin vtnh cht ca hp kim hon ton khc nhau. Ta bit rng pha thnh phn lpha to nn t chc ca hp kim. Khi hp kim c t chc mt pha th phathnh phn duy nht ng nht vi h p kim, tc l tnh cht ca nchnh l tnh cht ca hp kim. Trng hp c t chc gm nhiu pha thtnh cht ca ca hp kim la s tng hp tnh cht ca cc pha thnh phn.Ta s xem xt c th mi quan h gia dng ca gin pha vi tnh chtca hp kim nh th no.

a- Gi n loi 1: C l tnh: Tnh cht ca hp kim l trung gian gia tnh cht ca

tinh th A v tinh th B, tc l tinh th no c t l cng ln th snh hng cng nhiu n tinh cht tnh cht ca n. Tnh cht ca

hp kim ph thuc vo thnh phn theo quy lut bc nht, c thnh sau: Tnh cht hp kim = %A X t/c A + %B X t/c B Tnh cng ngh : Tnh c ca hp kim ni chung tt v chy long cao, nhit

nng chy thp, kt tinh trong mt khong nhit t gy co ngt. Tnh cht gia cng p lc khng cao. Tnh gia cng ct gt tt, phoi d gy.

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b- Gi n loi 2: C l tnh: Mi quan h gia tnh cht v thnh phn theo quy lut

bc hai. ng cong biu din c cc i ti 50% thnh phn, bn v cng u cao hn cu t thnh phn. in tr tng mnhtheo thnh phn cht ha tan.

Tnh cng ngh: Tnh c xu v kh nng in y khun khng cao, khong

ng thng ln. Tnh gia cng p lc tt v kh do dai. Tnh gia cng ct gt xu v bn cng cao, do phoi kh

gy.

c- Gi n loi 3:

Mi quan h ny l tng hp ca hai loi trn.

d- Gi n loi 4:

Mi quan h gia tnh cht v thnh phn c dng ng thng v imcc i ng vi thnh phn ca hp cht ha hc H.

II.4. GIN HP KIM Fe-C

1) Cu t st (Fe):

St l nguyn t c kh nhiu trong t nhin. Hin ti ngi ta luync st vi sch 99,99999% Fe. Trong thc t sn xut ngi tathng nghin cu vi st c lng cha 99,8 - 99,9%. St ny gi l stnguyn cht k thut (st am k).

a- C tnh:

St l nguyn t c c tnh kh cao, c th nh sau: Gii hn bn ko: b = 250 MN/m

2 (MPa) Gii hn chy quy c: 0.2 = 120MN/m

2

gin di tng i: % = 50 tht t i: % = 85. dai va p: ak= 3000 kJ/m

2

cng HB = 80

b- Tnh a hnh ca st:

St l kim loi c tnh a hnh, n c hai kiu mng tinh th cc khongnhit khc nhau:

i) Mng lp phng th tm tn ti nhit : Nh hn 9110C gi l st anpha (Fe) c a = 2,68 Kx. Di 7680C

c t tnh, cao hn nhit ny mt t tnh. T nhit cao hn7680C gi l st b ta, c a = 2,90Kx.

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T 1.3920C n 1.5390Cgi l st enta (Fe) c a= 2,93Kx.

ii) Mng lp phng dintm:

Tn ti nhit 9110C< t0 < 1.3920C gi l stgamma (Fe), c a = 3,56Kx.

2) Cu t cc bon (C):

Trong t nhin cc bon tn tidi ba dng:

V nh hnh: mi loithan g, than khongsn, c s dng chyu lm nhin liu vmt phn lm nguynliu.

Graphic: c kiu mnglc gic xp lp, thngs mng a v c; a=2.5, t s c/a = 2.74. Tnh cht: lc lin kt ho tr trong mi lp kh ln, cn gia

cc lp l lc ht phn t Vanderwaal rt yu.

b =1-2 N/mm2 rt thp, coi nh khng c bn. H s ma st b cho nn kh nng chng mi mn tt (cn gi lcht bi trn kh).

Kim cng: tn ti di kiu mng kim cng (t din a gicu), c cng cao nht.

3) Tng tc gia st v cc bon:

a- To dung dch rn:

i) Dung dch rn xen k ca C trong Fe (ferit = Fe(C) = = F): C kiu mng lp phng th tm (tm khi) c mt xp thp,

c hai dng im trng: im trng 4 mt v im trng 8 mt. im trng 4 mt: c v tr nm 1/4 ng thng ni hai im

gia hai cnh bn trn cng mt mt bn, kch thc l hng li rtnh =0.221d (d l ng knh nguyn t st), nn khng th changuyn t C.

im trng 8 mt: c tm im nm gia cc mt bn v giacc cnhbn. C kch thc qu nh (0.154d) nn khng cha ninguyn t C.

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ha tan cc i ca C vo Fe l 0.02% 7270C, nhit thng l 0.006%. C ch yu nm bin gii ht.

Tnh cht: cng thp, bn thp, do cao.

ii) Dung dch rn xen k ca C trong Fe (auxtennit, Fe(C) = ): C kiu mng lp phng din tm, c t im trng nhng cc

im trng c kch thc ln. Trong mng ny c im trng 4 mtv im trng 8 mt.

im trng 4 mt: c tm im nm trn 1/4 cc ng cho khic kch thc l 0.225d.

im trng 8 mt: c tm im l trung tm khi v gia cccnh bn, kch thc l kh ln 0.41d. C th ha tan C dng xenk cc l hng ca khi 8 mt.

Trong thc t ha tan ca C trong Fe cc i l 2.14% 1.1470C. 7270C ho tan ln nht l 0.8%.

Tnh cht: ch tn ti nhit ln hn 7270C. bn cao, dokh cao, cng thp.

b- To Xementit(Fe3C): Khi lng C vt qu gin hn ha tan k trn, C s kt hp vi Fe

to thnh Fe3C (Xementit). Xementit l pha khng n nh, d to thnh nhng trong mt s

iu kin bphn ha thnh Fe v C. Tnh cht: cng 800 HB, chng mi mn tt, gin kh cao,

c mu ca x c (ngc trai).

4) Gin pha Fe - C:

a- Dng gin : (hnh II.17)

b- Gi i thch gin :

i) Cc im c bit:

im Nhit t0C %C im Nhit t0C %C

A 1539 0 H 1499 0,10

B 1499 0,50 J 1499 0,16

C 1147 4,30 K 727 6,67

D 1250 6,67 L 0 6,67

E 1147 2,14 N 1392 0

F 1147 6,67 P 727 0,02

G 911 0 Q 0 0,006

S 727 0,8

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Cc ng: ABCD l ng lng. AHJECF l ng rn (ng c). ECF: t =1.1470C khng i l ng cng tinh, vi im C (4.3%C,

1.1470C) l im cng tinh.

Ti t0 = 1.1470C th Lc ( E F ) + Xe =LeI (Lburit mt) l hn hpc hc cng tinh ca auxtennit v xementit. Tn ti nhit 7270C < t0 2.14%C

(ng ECF): LC E + Fe3CF.

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Phn ng cng tch xy ra 7270C hu nh vi mi h p kim(ng PSK): S P + Fe3CK.

c- Cc t chc cbn:

i) Cc t chc mt pha:

Ferit (c k hiu l hay F hay Fe(C)) l dung dch rn xen k caC trong Fe vi mng lp phng tm khi (a=0.286-0.29 ).Lng ho tan rt nh (ln nht l 0,02%C 7270C - ti im P;v nh nht l 0,006%C - im Q).

Ferit c tnh st t tn ti t0 7680C . Do cha t C nn c tnh ca Feritchnh l ca st nguyn cht: do, dai, mm v km bn. Trong thc tFerit c th ho tan c Si, Mn, Cr

T chc t vi ca Ferit c dng cc ht a cnh, ht sng.

Auxtennit [k hiu bng , A, Fe (C)] (hay pha do v dai) l dungdch rn xen k C trong Fe c mng lp phng tm mt ( a 0.364 ) vi lng ho tan C ng k (cao nht 2.14%C) 1.1470Cti im E.

Auxtennit khng c tnh st t, c tnh thun t, ch tn ti nhit cao(>7270C), khng s dng trc tip ch to cc chi tit my nhng c vai trquan trng khi nhit luyn.

Xementit (k hiu Xe, Fe3C) l pha xen k vi kiu mng phc tpc cng thc Fe3C v c nng C l 6.67% .

Xementit c tnh st t yu ch n 2100C, rt cng nn chng mi mn ttnhng rt gin.

XeI c to thnh do gim nng C trong h p kim lng theong DC khi h nhit , n ch c hp kim ln hn 4.3%C v

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nh hn 6.67%. Do to nhit cao ( 11470C)nn kch thc ln, cth thbng mt thng.

XeII c to thnh do

gim nng cacbontrong auxtennit theong ES khi h nhit, loi ny c hp kim>0.8% cho ti 2.14%. Doto thnh nhit tng i cao (>7270C)nn tp trung bin gii ht.

XeIII c to thnh do gim nng C trong Ferit theo ng PQ

khi h nhit , vi s lng rt t, kh pht hin trn t chc t viv thng c b qua. Xementit cng tch c to thnh do chuyn bin cng tch

austenit peclit ([F + Xe]). Loi xementit ny c vai tr rtquan trng.

ii) Cc t chc hai pha. Peclit (k hiu P = [F+Fe3C] = [F+Xe]) l hn hp c hc cng tch

ca ferit v xementit c to thnh t auxtennit 0,8%C 7270C.Trong peclit c 88% ferit v 12% xementit phn b u.

Peclit l t chc kh bn, nhng cng do, dai p ng rt tt cc yu cuca vt liu kt cu v cng c.

Peclit tm thng gp hn c, c cu trc tm (hay phin), hai pha F v Xenm an xen u nhau. Trn mt ct ngang li cc vch theo cng mthng hay a hng, trong cc vch ti mng l xementit, vch sngdy l ferit.

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Peclit ht t gp hn, c cu trc xementit dng thu gn nht, ht xementitphn b u trn nn ferit.

So vi peclit ht, peclit tm c cng cao hn, do, dai thp hni cht, bn thp hn. Auxtennit ng nht d to peclit tm, cnauxtennit khng ng nht to ra

peclit ht. Lburit [k hiu l Le hay

( + Xe ), (P + Xe)]: l hnhp c hc cng tinh caauxtennit v xementit tothnh tpha lng c 4.3%C 1.1470C. Khi lm ngui tiptc li c phn ng cng tch auxtennit chuyn hathnh peclit. Lburit cng

v gin (v c ti 2/3 lxementit). T chc Lburitch c trong gang trng.

d- Phn bit thp v gang trng:

i) Thp:

nh ngha: Thp l hp kim ca Fe - C m nng 0.02% < C 2.14%.

Cc loi thp: Thp trc cng tch vi lng C

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Gang trng trc cng tinh vi thnh phn C4.3%C, bn phi im

C, c t chc lburit + xementit I: LeII+XeI.

e- Cc im (nhit ) ti hn tronggin pha Fe C:

i) Khi nung nng v lm ngui rt chm:

Gin trng thi Fe - C c xy dng bng thc nghim trong iu kinnung nng hay lm ngui chm.

A1 =PK=7 2 70C.

A3 =GS=9 1 00 - 7270C.

Acm= ES = 1.147 7270C.

Gi l cc im ti hn trong gin trng thi Fe-C.

ii) Khi nung nng thc t (c thm chc):Nhit lun cao hn cc nhit ti hn.

AC1 A1 = TC1. AC3 A3 = TC3. ACcm Acm = TCcm.

Trong TC1, TC3, TCcm gi l qu nung. qu nung ph thuctc nung. Tc nung cng ln, qu nung cng ln.

iii) Khi lm ngui thc t (c thm chr):

Nhit lun thp hn nhit ti hn: A1 Ar1= Tr1 A3 Ar3 = Tr3. Acm Arcm = Trcm

Trong Tr1, Tr3, T rcm gi l qu ngui. qu ngui ph thucvo tc ngui. Tc ngui cng ln, qu ngui cng ln.