# câu hỏi ôn tập VLH

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vt liu hc

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p n cng mn hc: Vt liu hc k thut

Cu 1 : Th no l mng tinh th? Trnh by cu truc tinh th in hnh ca cht rn co lin kt kim loai (kim loai nguyn cht)? ( 4)Tr li:

a/ Mng tinh th: c hiu l 1 m hnh khng gian m t quy lut hnh hc sp xp cc cht im th rn trong vt tinh th. Hiu theo cch khc, trong 1 n v tinh th xt trng thi rn, cc nguyn t (cht im) phn b theo mt quy lut hnh hc nht nh.

Ty thuc vo cc loi vt liu v iu kin bn ngoi nh nhit , p sut, mi n v tinh th c trng cho loi vt liu c cc nguyn t sp xp theo mt trt t ring di dng hnh hc xc nh.

b/cu trc in hnh ca kim loi nguyn cht:

Lp phng tm khi A2: c s l hnh lp phng cnh bng a,cc nguyn t(ion) nm cc nh v cc trung tm khi. nv= 8 nh.1/8+ 1 gia= 2 nguyn t dng t = s sp xp l 8 . mt th tch 68% c 2 loi l hng: + loi 4 mt: c kch thc 0.291 dng t nm trn cnh ni im gia cc cnh i din ca cc mt bn.

+ loi 8 mt: c kch thc 0.154 dng t nm tm cc mt bn v gia cc cnh a.

Mng A2 c nhiu l hng nhng kch thc cc l hng nh

Cc kim loi in hnh Fe ; crom ; molipden ; vonfram

Lp phng tm mt A1: khc vi mng A2, thay v nguyn t nm tm khi th nm tm cc mt bn. nv= 8 nh.1/8+ 6 mt. 1/2= 4 nguyn t dng t = s sp xp l 12. Mt th tch 74%c 2 loi l hng: + loi 4 mt: c kch thc 0.225 dng t nm ng cho khi tnh t nh

+ loi 8 mt: c kch thc 0.414 dng t nm trung tm khi v gia cc cnh

Mng A1 c t l hng hn nhng kch thc ln hn. Chnh iu ny l yu t quyt nh cho s ha tan di dng xen k.

Cc kim loi in hnh: Fe ; niken ; ng ; nhm ; ch ; bc ; vng;

Lp phng din tm A3: c s l khi lng tr lc gic, cc nguyn t nm trn 12 nh, tm ca 2 mt y v tm ca 3 khi lng tr tam gic. nv = 12. 1/6 + 2. +3 = 6 nguyn t

Chiu cao c ca ph thuc vo cnh a ca lc gic y m lun bng hay 1,633. trong thc t lun thay i; quy c

+ 1.57< ha bn hoc kt thc bin dng nhit dn ti mm

Ngc li : kt tinh li tip theo bng cch vi vo ct hay vi bt.

Cc c im

u im:

Kim li xp cht, do cao hn, t khi b nt, nng sut cao, gia cng c cc phi ln, tit kim nng lng.

Nhc im:

Kh ng u,t chc v c tnh, km chnh xc hnh dng, knh thc, oxy ha,...

Cht lng b mt khng cao: vy oxyt, thot cacbon.

V d minh ha:

To phi ch to trc khy u bng dp nng th tt hn ct t thi thp nguyn.

Tim thm v d t thc tCu 8: Trnh by cc c tnh ca dung dich rn. So snh dd rn xen k v rn thay th.

Cc c tnh ca dung dch rn: V mt cu trc dung dch rn ca hp kim c kiu mng tinh th vn l kiu mng ca kim loi dung mi. c tnh c bn ny quyt nh cc c trng c l ha tnh ca dung dch rn, v c bn vn gi c cc tnh cht c bn ca kim loi ch hay nn. Nh vy dung dch rn trong hp kim c cc c tnh c th nh sau:

Lin kt vn l lin kt kim loi, do vy dung dch rn vn gi c tnh do ging nh kim loi nguyn cht

Thnh phn ho hc thay i theo phm vi nht nh m khng lm thay i kiu mng.

Tnh cht bin i nhiu: do, dai, h s nhit in tr gim, in tr bn, cng tng ln.

Do cc c tnh trn nn dung dch rn l c s ca hp kim kt cu dng trong c kh. Trong hp kim ny pha c bn l dung dch rn, n chim xp x n 90% c trng hp n 100%.

Dung dch rn thay th

cc dung dch rn thay th, cc nguyn t ca cht tan thng thng c phn b thng k trong mng dung mi. Mng khng gian xung quanh nguyn t cht tan xut hin nhng sai lch cc b. Nhng sai lch ny dn ti s thay i tnh cht v s thay i thng s mng trung bnh. S hnh thnh cc dung dch rn lun lun km theo vic tng in tr v gim h s nhit in tr. Cc kim loi dng dung dch rn thng km do, lun lun cng hn v bn hn so vi cc kim loi nguyn cht.

Dung dch rn xen kTrong kim loi, cc dung dch rn loi ny xut hin khi hp kim ha cc kim loi chuyn tip bng cc kim c bn knh nguyn t nh nh H, N, C, B. Nhng x lch mng xut hin khi to thnh dung dch rn xen k vt qu nhng x lch mng khi to thnh dung dch rn thay th, do vy cc tnh cht cng thay i mnh hn. Theo mc tng nng ca nguyn t ha tan trong dung dch rn m in tr, lc khng t, cng v bn tng, nhng do v dai gim i r rt.

Cu 9: Quan h gia dng gin pha v tnh cht ca hp kim? Cho v d minh ha?

Tr li:

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

-Hp kim c t chc 1 pha th tnh cht ca hp kim l tnh cht ca pha

-Hp kim c t chc bao gm hn hp ca nhiu pha th tnh cht ca hp kim l s tng hp hay kt hp tnh cht ca cc pha thnh phn ( khng phi l cng n thun) gm cc trng hp:

+hp kim l dung dch rn + cc pha trung gian

+quan h tnh cht- nng thng thng xc nh bng thc nghim

Quan h tuyn tnh ch ng khi cng c ht v pha phn b u n

Quan h phi tuyn : trong trng hp ht nh i hoc to ln, tnh cht t c s thay i ty theo tng trng hp : ht nh i th dai tng = bn

V d: xt hp kim ca St v Carbon gm c t chc 1 pha (Ferit, Austenit, Xementit) v t chc 2 pha ( Peclit, Ledeburit) tnh cht ca hp kim St v Carbon chnh l bao gm cc t chc 1 pha v 2 pha ca n ng vi mi pha th n c 1 trng thi nht nh

Cn b sung v ni dungCu 10: Cac t mt pha co trong gian trang thai Fe-CCo 3 t chc mt pha co trong gian :Ferit ,Austenit,Xementit.

Ferit: ky hiu(Fe):la pha tn tai nhit thng,do cha cacbon khng ng k nn c tnh ca Ferit chnh l ca st nguyn cht: do, dai mm v km bn. T chc t vi ca Ferit trnh by hnh sau c dng ht sng, a cnh.

Austenit: K hiu l ,n ch tn ti nhit cao hn 727oC, austenit c vai tr quyt nh trong bin dng nng v nhit luyn.Vi tnh do cao v rt mm nhit cao nn bin dng nng thp bao gi cng c thc hin trng thi austenit ng nht ( thng trn di 1000oC). V th c th tin hnh bin dng nng mi hp kim Fe-C vi C 4,3%. Do to thnh nhit cao nn xementit th nht c dng thng, th to i khi c th thy c bng mt thng.

+ Xementit th hai: c to thnh do gim nng cacbon trong austenit, thng thy rt r hp kim c >0,8%C n 2,14%C. Do to thnh nhit tng i cao >727oC, nn xementit th hai lm gim mnh tnh do v dai ca hp kim.

+ Xementit th ba: c to thnh do gim nng cacbon trong ferit, vi s lng t l rt nh nn thng c b qua.

+ Xementit cung tinh: c tao thanh do chuyn bin cung tinh peclit.Hnh v gin

Cu 11:

V gin trng thi Fe C (gio trnh VLKT trang 58).

Cc t chc hai pha :

+ Peclit

L hn hp cng tch ca F v X to thnh t phn ng cng tch.

Trong P c 88% F v 12% X phn b u

Peclit tm : F v X u dng tm nm xen k nhau.

Peclit ht : X thu gn li thnh dng ht nm phn b u trn nn F, peclit ht c bn, cng thp, do , dai cao hn i cht.

+ Lburit

L hn hp ca peclit tm trn nn xementit sng.

Lburit cng v gin v t l X cao v ch c trong gang trng. Hnh v gin Cu 12: Ti sao khi hm lng cacbon tng ln cng ca thp li tng ln?

Tr li:nh hng ca hm lng cacbon n c tnh v cng ngh nhit luyn:T gin pha Fe-C ta thy khi hm lng cacbon tng ln t l Xmentitl pha gin trong t chc cng tng ln tng ng (c thm 0,10%C s tng thm 1,50% xmentit ) do lm thay i t chc t vi trng thi cn bng ( ). Tc l hm lng cacbon cng cao thp cng cng, cng km do dai vcng gin.Tim thm v d t thc tCu 13:Nhit luyn l g? Qu trnh nhit luyn c c trng bi nhng thng s no?nh hng ca chng n qu trnh nhit luyn nh th no?Cho v d?

Tr li:

_ Nhit luyn l qu trnh nung nng kim loi n nhit cn thit,gi nhit trong thi gian nht nh ri lm ngui trong mi trng thch hp.

_ Nhng thng s c trng cho qu trnh nhit luyn:nhit nung nng(Tn);thi gian gi nhit(Tgn) v tc lm ngui(Vng).

_ Nhng thng s ny nh hng n s thay i t chc t vi, bn, cng, do, dai, cng vnh,bin dng ca kim loi.

_ V d: Nhit ti cng cao th cng kim loi cng cao.

(thp C45 ti t=740C -21.8HRC t=900C -25HRC).Tim thm v d t thc tCu 14: Chuyn bin ca Austenit khi lm ngui nhanh - chuyn bin Mactenxit (khi ti)?

Tr li:

Khi lm ngui nhanh chuyn bin t Fe sang Fe, m khng c s khuych tn cacbon. L chuyn bin t Austenit thnh Mactenxit.

Xy ra nhit 250oCtr xung.

Tc ngui nh nht chuyn bin ny xy ra gi l tc ti hn Vth (tc ngui ti hn).B sung ni dungCu 15:Trnh by bn cht v cu trc ca mactenxit?

Tr li:

Bn cht v cu trc ca mactenxit:

A - nh ngha :

Mactenxit l dung dch rn xen k qu bo ha ca cacbon trong Fe c nng cacbon bng nng ca austenit ban u.

Khi lm ngui rt nhanh cacbon trong austenit khng kp khuch tn to thnh xmentit. Lc t nhit tng i thp ch c s chuyn mng t Fe sang Fe. Lng cacbon trong hai t chc ny bng nhau.

B Cu trc ca mactenxit :

- Mactenxit c kiu mng chnh phng tm khi vi hai thong s mng l a v c. T s c/a gi l chnh phng. Thng thng t s

c/a = 1,001 1,06.

Mactenxit c dng hnh kim, mt u nhn, cc kim ny to vi nhau gc 1200 hay 600 .

Cc nguyn t cacbon chui vo cc l hng trong mng ca Fe .

C Cc tnh cht ca mactenxit :

Do hm lng cacbon qu bo ha nn gy ra x lch mng ln, do vy cng cao v tnh chng mi mn ln. Hm lng cacbon cng ln cng cng caoMactenxit c tnh dn cao ph thuc vo kch thc ht ca n v ng sut bn trong. Ht cng nh, ng sut cng thp tnh dn cng thp.Cu 16:Ram l g? trnh by chuyn bin khi nung nng thp ti (khi ram)?

Tr li:

- Ram l phng php nhit luyn nung nng thp ti c t chc Mactenxit qu bo ha v stenit d chuyn thnh cc t chc n nh hn ph hp vi yu cu t ra.

- Chuyn bin khi ram: Noi chung cac nguyn t hp kim hoa tan trong mactenxit u can tr s phn hoa cua cac pha nay khi ram hay noi cu th la lam tng cac nhit chuyn bin khi ram. Nh vy dn n cac hiu ng nh sau:+ Nng cao tinh chiu nhit cao, tinh bn nong, tinh cng nong.+ Do khuych tan kho khn cacbit tao thanh rt phn tan va nho min, lam tng tinh cng va tinh chng mai mon, c goi la hoa cung phn tan. S tng cng khi ram thep hp kim nhit thich hp lam cho austenit d -> mactenxit va cacbit tit ra dang phn tan, nho min c goi la cung th hai.+ Cung ram hay lam vic mt nhit , thep hp kim bao gi cng co cng, bn cao hn. iu nay cung co nghia cung at cng bn nh nhau, phai ram thep hp kim nhit bao hn nn kh bo c ng sut bn trong nhiu hn vi th thep co th am bao dai tt.Tim thm v d t thc tCu 17: Trinh bay cac phng phap ram thep cacbon? ng dung trong thc t?