UNIVERSITEIT VAN PRETORIA DEPARTEMENT MATERIAALKUNDE EN METALLURGIESE INGENIEURSWESE MATERIAALKUNDE NMC 113 MATERIALS SCIENCE

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1 UNIVERSITEIT VAN PRETORIA DEPARTEMENT MATERIAALKUNDE EN METALLURGIESE INGENIEURSWESE MATERIAALKUNDE NMC 113 MATERIALS SCIENCE 20 JUNIE 2011 EKSAMEN / EXAMINATION TYD / TIME: 3 h EKSTERNE EKSAMINATOR / EXTERNAL EXAMINER Prof G Pienaar INTERNE EKSAMINATORE / INTERNAL EXAMINERS Mnr PCW Havemann, Mr. V Kurup VAN EN VOORLETTERS STUDENTNO. SURNAME AND INITIALS HANDTEKENING STUDENT NO. TELEFOON NR. SIGNATURE TELEPHONE NO. p2 p.3 p.4 p.5 p. 6 p.7 Totaal p.8 p. 9 p. 10 p. 11 p. 12 p Beantwoord alle vrae in die beperkte spasies voorsien. Nie-progammeerbare sakrekenaars mag gebruik word. Die blanko bladsy(e) agter in hierdie eksamenboek mag vir rofwerk gebruik word maar sal nie nagesien word nie. Geen vrae sal gedurende die eksamen beantwoord word nie. Maak aannames waar nodig en stel dit as sodanig. Tippex en Highlighters of soortgelyke produkte mag nie gebruik word nie Die vraestel bevat 14 geskrewe bladsye Answer all the questions in the limited spaces provided. Non-programmable calculators may be used. The blank page(s) at the back of this examination book may be used for rough work but will not be marked. No questions will be answered during the examination. Make assumptions where required and state it as such. Tippex and Highlighters or similar products may not be used. The paper contains 14 written pages. BYLAAG (p. 12) / APPENDIX (p. 12) DIE BYLAAG BEVAT INLIGTING WAT OP SEKERE VRAE UIT BEIDE AFDELINGS VAN TOEPASSING IS THE APPENDIX CONTAINS INFORMATION APPLICABLE TO CERTAIN QUESTIONS FROM BOTH PARTS OF THE PAPER 1

2 DEEL A ( 26 PUNTE) PART A ( 26 MARKS) Die volgende vrae het drie korrekte antwoorde per vraag. Gevolglik mag nie meer as drie blokkies per vraag gemerk word of die antwoord in die spasie geskryf word nie. (Drie punte per vraag.) The following questions have three correct answers per question. Therefore no more than three blocks may be marked per question or the answer written in the spaces. (Three marks per question) 1. Die volgende word nie as materiaaleienskappe beskou nie, en is dus ook afhanklik van eksterne faktore: The following are not regarded as materials properties, and are therefore also dependant on external factors: elektronmobiliteit magnetiese veldsterkte magnetiese permeabiliteit electron mobility magnetic field strength magnetic permeability diffusiekoëffisient (D o ) elektriese weerstand relatiewe permeabiliteit ( r ) diffusion coefficient (D o ) electrical resistance relative permeability ( r ) intrinsieke draerdigtheid (n i ) piesoëlektrisiteit geleidingsvermoë intrinsic carrier density (n i ) piezoelectricity conductivity 2. Die volgende eienskappe is afhanklik van die monstergrootte wat getoets word; The following properties depend on the size of the sample tested: Young s modulus (GPa) Brinell hardheid vloeigrens (MPa) Young s modulus (GPa) Brinell hardness yield strength (MPa) % koueverwerking slagenergie (J) ware vervorming % cold work impact energy(j) true strain breekkrag (N) ingenieursspanning (MPa) korrosietempo (g/dag) fracture force(n) engineering stress (MPa) rate of corrosion (g/day) 3. Die volgende materiaalprosserings-metodes is afhanklik van vastetoestand diffusie: The following materials processing methods depend on solid state diffusion: kruipvervorming creep deformation afblussing van n Al-4% Cu legering quenching of an Al-4%Cu alloy koudwalsing van n geelkoperplaat cold rolling of a brass sheet masjinering van n staalrat machining of a steel gear magnetisering van n ysterstaaf magnetization of an iron bar afblusverharding na martensiet van n staal beitel quench hardening to martensite of a steel chisel dotering van n silikonskyf doping of a silicon wafer HX van n koudverwerkte Al draad RC of a cold worked Al wire 4. Die volgende defekte in n materiaal kan as kristaldefekte beskou word: The following defects in a material can be classified as crystalline defects: korrelgrens MnS onsuiwerheid partikel leëruimte grain boundary MnS impurity particle vacancy blaasgat kantontwrigting presipitaat pore edge dislocation precipitate proeutektiese -fase in n legering magnetiese domein kraak proeutectic phase in an alloy magnetic domain crack 2

3 5. Die vloeigrens van n polikristallyne suiwer metaal bv. Cu kan verhoog word deur: The yield strength of a polycrystalline pure metal like Cu can be increased by: verhoging van K Ic verlaging van brosoorgangstemp. warmverwerking increasing K Ic lowering brittle transition temp. hot working toevoeging van legeringselemente koueverwerking interne spannings te verhoog addition of alloying elements cold working increase residual stresses korrelverfyning verlaging van HX temperatuur verhoging van toegepaste krag grain refinement decreasing RC temperature increasing applied force 6. n Toename in temperatuur van n metaal het die invloed om: An increase in temperature of a metal has the influence to: geleidingvermoë te verhoog Young se modulus te verhoog fasetransformasiereaksies te versnel increase conductivity increase Young s modulus accelerate phase transformation reactions digtheid te verhoog kruipvervorming te verlaag tyd vir herstel in HX te verkort increase density decrease creep deformation shorten time for recovery in RC leëruimte konsentrasie te verhoog aktiveringsenergie te verlaag diffusiekoëffisient (D o ) te verhoog increase concentration of vacancies decrease activation energy increase diffusion coefficient (D o ) Die volgende vrae het elk slegs een korrekte antwoord. Gevolglik mag slegs een blokkie per vraag gemerk of die antwoord in die blokkie geskryf word. (Twee punte per vraag) Each of the following questions has only one correct answer. Therefore only one block may be marked or the answer written in the block, per question. (Two marks per question) 7. Volgens die taksonomie van materiale, is die finale klassifikasie van die legering 89% Cu-9,25% Zn-1,75% Pb: The final classification of the alloy 89% Cu-9,25% Zn-1,75% Pb, according to the taxonomy of materials, is: familie van metale klas van sink legerings subklas van koper legerings family of metals class of zinc alloys subclass of copper alloys klas van koper legerings lid van sink legerings subklas van koper-sink legerings class of copper alloys member of zinc alloys subclass of copper-zinc alloys 8. Die resistiwiteit van n halfgeleier soos bv. germanium sal : The resistivity of a semiconductor like germanium will: liniër toeneem met toename in temperatuur increase linearly with an increase in temperature eksponensieel toeneem met toename in temperatuur increase exponentially with an increase in temperature liniër afneem met toename in temperatuur decrease linearly with an increase in temperature eksponensieel afneem met toename in temperatuur decrease exponentially with an increase in temperature 9. Hoe hoër die kantontwrigtingdigtheid in n metaal, hoe: The higher the dislocation density in a metal, the : laer die diffusiwiteit van n legeringselement lower the diffusivity of an alloying element hoër die werkverharding van die metaal higher the work hardening of the metal hoe kleiner die korrelgrootte van die metaal smaller the grain size of the metal hoër die HX temperatuur van die metaal higher the RC temperature of the metal hoër die smeebaarheid van die metaal higher the ductility of the metal 3

4 10. Die breuktaaiheid van keramieke is aansienlik laer as die van metale omrede: The fracture toughness of ceramics is significantly lower than that of metals because: keramieke is nie-kristallyn terwyl metale kristallyne struktuur het ceramics are non-crystalline whilst metals have crystalline structure die brosoorgangstemperatuur (BOT) van keramieke is laer as die van metale the brittle transition temperature (BTT) of ceramics is lower than that of metals keramieke is harder en sterker as metale ceramics are harder and stronger than metals die kritieke kraaklengte in keramieke is aansienlik langer as in metale the critical crack length in ceramics is significantly longer than in metals smeltpunt van keramieke is hoër as die van metale the melting point of ceramics is higher than that of metals DEEL B ( 92 punte) / PART B ( 92 marks) ( Toon alle berekeninge waar van toepassing / Show all calculations where applicable ) 1. Formuleer die Pauli-uitsluitingsbeginsel. / State the Pauli exclusion principle (2) 2. Die figuur hieronder toon die eenheidsel van kobalt met c/a = 1,62, atoomradius = 0,125 nm en atoomgetal = 27. The figure below gives the unit cell of cobalt with c/a = 1,62, atomic radius = 0,125 nm and atomic number = Die koördinasiegetal van Co is (2) The coordination number of Co is: 2.2 Die primêre bindingstipe in Co is (2) The primary bond in Co is: 2.3 Bewys deur berekeninge dat die waarde van c in die eenheidsel = 0,405 nm is (2) Prove with calculations that the value of c in the unit cell is = 0,405 nm. 4

5 2.4 Bewys deur berekeninge dat die atoomdigtheid van Co 9,124 x atome / m 3 is Show by calculation that the atomic density of Co is 9,124 x atoms/m 3 (5) 2.5 Verduidelik kortliks maar volledig, o.a met verwysing na die elektronkonfigurasie van Co, waarom Co ferromagneties is. Stel Co se oksidasiegetal gelyk aan 2+ Give a brief but comprehensive explanation, amongst others with reference to the electron configuration of Co, why Co is ferromagnetic. Let the oxidation number of Co be 2+. (5) 3. Die ASTM korrelgrootte getal van n monster is 8. Bereken die oppervlakte in vierkante duim waarin 80 korrels by n vergroting van 200 X, waargeneem sal word. The ASTM grain size number of a sample is 8. Calculate the surface area in square inches in which 80 grains will be observed at a magnification of 200 X. (4) 5

6 4. n Staalrat gemaak van staal wat 0,2 % C bevat moet opgekool word by 903 o C sodat die koolstofkonsentrasie op n diepte van 1,5 mm vanaf die oppervlakte 0,6 %C is. Stel die oppervlak konsentrasie C is 1 %. Bereken die verlangde opkolingstyd in uur. A steel gear made from steel which contains 0,2 % carbon must be carburised at 903 o C to achieve a carbon concentration of 0,6 % at a depth of 1,5 mm below the surface. The surface concentration of C is 1%. Calculate the required carburising time in hours. (5) 5. Die figuur hieronder toon die trektoets resultaat op n 5 mm diameter staaldraad wat koudgetrek en toe getoets was. Die staal bevat 0,4 % C. The figure below shows the tensile test result of a 5 mm diameter steel wire which was cold drawn and then tested. The steel contains 0,4 % C. 5.1 Sal n motorvoertuig met n massa van 2000 kg aan die koudgetrekte staaldraad kan hang sonder dat die draad plasties vervorm. Motiveer die antwoord met die nodige berekening. Would it be possible to suspend a motor vehicle with a mass of 2000 kg from the cold drawn wire without the wire deforming plastically. Verify the answer with the applicable calculation. (4) 5.2 Trek op die figuur die verwagte trektoets resultaat in van dieselfde draad wat uitgegloei was. Draw the expected tensile test result on the figure for the same wire which was annealed. (3) 6

7 5.3 Bepaal die diameter waartoe n 5 mm dia. uitgegloeide staal draad koudgetrek moet word om `n smeebaarheid van 15% te hê m.b.v aangehegte Fig en berekeninge. Determine the diameter to which a 5 mm dia. annealed steel wire must be cold drawn to have a ductility of 15%. Use the attached Fig and applicable calculations. (3) 6. Die figuur hieronder toon die S-N kurwe vir `n staal monster waarvan die oppervlakte poleer was. The figure below shows the S-N curve for a steel sample of which the surface was polished Bepaal Basquin se wet vir die staal monster. Determine Basquin s law for the steel sample. (5) 6.2 Trek op die gegewe assestelsel `n S-N kurwe in vir die staal monster waarvan die oppervlakte masjineer was. Draw a S-N curve on the given axes for the steel sample of which the surface was machined (2) 7

8 7. Fig 2 in die Bylaag toon die kruipvervormingkurwes vir M200 staal. `n Spanning van 300 MPa word by 707 o C op `n monster van die staal toegepas en `n spanning van 186 MPa by 1100 o C op `n tweede monster. Fig. 2 in the Appendix shows the creep deformation curves for M200 steel. A stress of 300 MPa at a temperature of 707 o C is applied to a sample of the steel and a stress of 186 MPa at a temperature of 1100 o C on a second sample of the steel. 7.1 Wat is die gestadigde vervormingstempo s ss van die eerste en tweede monsters respektiewelik. What are the steady state creep rates ss for the first and second samples respectively (3) 7.2 Gee `n kort maar volledige verduideliking vir die verskil in die gestadigde kruiptempo s van die twee monsters met verwysing na o.a die meganismes van kruip wat van toepassing is. Give a brief but comprehensive explanation for the difference in the steady state creep rates of the two samples with reference also to the mechanism of creep which is applicable. (6) 8. `n Monster van geelkoper word 10 % koudverwerk en `n ander monster van dieseflde materiaal 35% koudverwerk. Beide monsters word nou in dieselfde oond geplaas om the HX. Trek m.b.v data verkry van Fig kurwes van Smeebaarheid teen HX tyd op onderstaande assestelsel. Benoem die asse. Dui toepaslike waardes van Smeebaarheid op die asse aan. A sample of brass is cold worked 10% and another of the same material cold worked 35%. Both samples are then placed in the same furnace and allowed to RC. Draw graphs of Ductility vs RC time on the axes below by using data obtained from Fig Name the axes. Show applicable values of Ductility on the axis. (5) 8

9 9. Die figuur hieronder toon die Mg-Pb fasediagram. The figure below shows the Mg-Pb phase diagram. 9.1 Beskou die intermetaalverbinding wat by M voorkom (81% Pb) en bereken die waardes van x en y in Mg x Pb y Consider the inter-metallic compound occurring at M (81% Pb) and calculate the values of x and y in Mg x Pb y (4) 9.2 Beskou `n legering wat 50 m% Pb bevat Consider an alloy containing 50 wt% Pb Teken `n ewewigsafkoelkurwe vir die legering vanaf 700 o C tot 100 o C op die asse langs die fasediagram. Dui alle fases ( nie reaksies) en toepaslike temperature op die afkoelkurwe aan. Draw a equilibrium cooling curve for the alloy from 700 o C to 100 o C on the axes next to the phase diagram. Indicate all phases (not reactions) and applicable temperatures on the cooling curve. (5) Teken `n mikrostruktuur vir die legering by `n temperatuur net onder 466 o C. Benoem al die fases teenwoordig in die mikrostruktuur en bereken die % eutektiese komponent in die legering. Sketch the microstructure of the alloy at a temperature just below 466 o C. Name all the phases present in the microstructure and calculate the % eutectic component in the alloy. (5) 9

10 10. Fig. 3 toon die Fe-C fasediagram. Gee `n kort maar volledige bespreking van die meganisme van perlietvorming onder ewewigsafkoeling van `n eutektoïde staal. Maak gebruik van toepaslike sketse. Fig. 3 shows the Fe-C phase diagram. Give a brief but comprehensive discussion of the mechanism of pearlite formation on the equilibrium cooling of an eutectoid steel. Make use of applicable sketches. (4) 11. `n P-tipe ekstrinsieke halfgeleier met `n geleidingsvermoë van 1,4 x 10 5 [.m] -1 by kamertemperatuur moet produseer word deur silikon met gallium te doteer. A p-type extrinsic semiconductor with a conductivity of 1,4 x 10 5 [.m] -1 at room temperature must be produced by doping silicon with gallium Bereken die verlangde konsentrasie meerderheidsdraers per m 3 in die halfgeleier Calculate the required concentration majority carriers per m 3 in the semiconductor (4) 11.2 Bereken die verlangde konsentrasie minderheidsdraers per m 3 in die halfgeleier Calculate the required concentration minority carriers per m 3 in the semiconductor (2) 11.3 Maak `n benoemde skets van die energieband-model vir die halfgeleier Make a named sketch of the energy-band model for the semiconductor (3) 10

11 12. Die figuur hieronder toon die demagnetiseringskurwes vir twee magnetiese materiale. The figure below shows the demagnitization curves for two magnetic materials 12.1 Watter materiaal is die mees geskikte om te gebruik om `n permanente magneet van te maak. Motiveer die antwoord m.b.v toepaslike berekeninge. Which material is the most suitable to manufacture a permanent magnet from. Motivate the answer with applicable calculations. (5) 12.2 Bereken die benaderde volume in m 3 van die monster van material A wat gebruik was om die demagnetiseringskurwe op te stel as n drywing van 2,5 Watt vir 3,6 sek. toegepas was. Calculate the approximate volume of the sample of material A in m 3 which was used to construct the demagnitization curve if a power of 2,5 Watt was applied for 3,6 seconds. (4) 11

12 BYLAAG / APPENDIX R = (J/molK) g = 9,81 m/s 2 N A = 6,023x10 23 atm/mol q = 1,6 x C 1 duim / inch = 25,4 mm Giga = 10 9 Mega = 10 6 nano = 10-9 M Mg = 24,31 g/mol M Pb = 207,19 g/mol Area = r 2 Area = ½bh Vol. = 4/3 r 3 = M/V H =ni/l %Red. = (A o A i ) x 100 / A o Watt = J/s b N f = C N = 2 n-1 = E = F/A = l / l o C s C x / C s C o = erf (x/2 Dt) D = D o e Q/RT J = -D dc/dx [kg/m 2.s] σ = 1/ = q(n n + p p ) n p = n 2 i / N a B = H RC = Recrystallization. HX = Herkristallisasie Fig. 1 Diffusiwiteitsdata / Diffusivity data 12

13 Fig Ductility vs % Cold work for 1040 steel, brass and copper. 13

14 Fig. 3. Fe-C phase diagram 14