HOMEWORK 8. MID- SEMESTER LEARNING REFLECTION PROJECT WORK

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1 : materials science & slid state chemistry HOMEWORK 8. MID- SEMESTER LEARNING REFLECTION As yu have n-dubt nticed, self-directed learning is a big part f this curse. One f ur curse learning bjectives is aimed directly at life-lng learning skill building: Demnstrate a capacity fr self-directed learning, including gal-setting and selectin f learning strategies, time and effrt management, persnal mtivatin, resurce discvery, and selfreflectin and self-assessment. T supprt this gal, I attempted t design the prjects with rm fr yu t explre and take cntrl f yur learning, within a set f defined cnstraints. In additin, mst f yu specified persnal learning gals at the start f the term things that yu wanted t get ut f the experience. Hw are things ging s far? Are yu making prgress tward yur persnal learning gals? Are yu develping yur self-directed learning skills r attitudes? Are yu thinking differently abut yur learning? I m interested in hearing hw yu re ding s far, and hw the curse frmat may be cntributing t r hindering yur prgress as a self-directed learner. Please prvide a brief (ne-page maximum) written reflectin n yur experiences thus far in the curse. Sme aspects f the curse experience that yu may cnsider reflecting upn include the fllwing: Yur persnal learning gals. Did yu make prgress tward yur gals? Have yur gals changed? Hw are yu tracking and evaluating yur prgress? Yur attitudes, mtivatins, and behavirs: What have yu learned abut yur persnal beliefs r attitudes tward learning? What aspects f learning d yu find intrinsically mtivating, r persnally relevant r valuable? What are yur strategies fr maintaining perseverance, mtivatin, and interest? Hw are yu managing yur time and effrt? Yur interactins with thers: Hw have yur interactins with teammates affected yur engagement in learning? Did yur instructr interactins have any effects n yur learning appraches r utcmes? Did yu gain any insights with regard t interactins with thers? Yur thinking, and yur thinking abut yur thinking (cgnitive and metacgnitive skills): Did yu gain any insights int yur wn learning prcesses? Did yu gain skills in setting gals, selecting learning strategies, finding and using resurces, diagnsing prblems, mnitring and adapting yur appraches, r self-evaluating utcmes? This is yur reflectin, s please make it wrk fr yu and cmment n the things that yu feel are relevant t yu. I wn t be grading yur self-reflectin in the traditinal sense yu ll get full credit if yu submit a thughtful written reflectin. I just want t learn hw this curse is cntributing (r nt) tward yur develpment. Write yur reflectin as a wrd dcument r pdf. Please yur reflectin t me by midnight n Mnday, Nvember 03. Thanks! PROJECT WORK By nw, mst f yu have metallic physical artifacts in hand, and yu re ready t d sme testing and analysis. Yur materials science prject wrk this week will likely invlve sme sample preparatin, prperty testing, and micrstructural analysis, and perhaps sme additinal prcessing f yur allys. Hpefully, by the end f the week yu ll have sme labratry data as well as sme crss-sectinal samples that yu ve started t analyze via ptical r electrn micrscpy. Are yur test data and micrstructural analyses starting t cme tgether? Can yu begin t explain hw yur cmpsitinal r prcessing variables are affecting the ally prperties, and why the cmpsitinal r prcessing variables are having these effects n yur allys?

2 MATERIALS SCIENCE AND SOLID STATE CHEMISTRY HOMEWORK 8, PAGE 2 OF 5 READING This week, I suggest fcusing yur reading n phase transfrmatins that are relevant t yur prject experiments. If yu chse t prcess metals by casting, fcus yur attentin n slidificatin kinetics and thermdynamics. If yur metals underwent slid-state phase transfrmatins (e.g. steel frging), then fcus n the kinetics and thermdynamics f slidstate reactins in steel allys.! Textbk Reading Fcus yur reading n sectins that help yu with yur prject analyses. Sme suggestins are prvided belw. Readings relevant t all teams: Ashby Engineering Materials 2: Chapter 6 (Driving Frce fr Structural Change) Ashby Engineering Materials 2: Sectins (Kinetics 1 - Diffusive Transfrmatins) Callister: (Kinetics f Phase Transfrmatins) and 11.9 (Precipitatin hardening) Readings relevant t specific teams: Steel teams. Callister: Sectins , , r Ashby Engineering Materials 2: Chapter 9 (Kinetics 3 - Displacive Transfrmatins) Casting teams. Ashby Engineering Materials 2: Chapter 8 (Kinetics 2 - Nucleatin)! Key Cncepts in Phase Transfrmatins There are a few key cncepts in kinetics, nucleatin, and grwth t cnsider as yu interpret yur prject results. Energy is everything! Why d materials change phases? Because the prduct phase(s) have a lwer Gibbs free energy than the parent phases(s). Hw d we knw what phases are lwest energy? The phase diagram is a great place t start simply find the pint f interest n the phase diagram (cmpsitin, temperature), and read which phases are suppsed t be present under thse cnditins. Nte the emphasis n the suppsed t here. Phase diagrams typically shw thermdynamic equilibrium cnditins, r what is suppsed t happen if the material is able t reach its lwest energy state. But mst phase transfrmatins in materials are diffusinal reactins, and diffusin takes time. Smetimes a lt f time. Smetimes far mre time than we prvide. Lw temperature slid-state reactins, in particular, can be quite sluggish due t slw mtin f atms in the slid state. When the reactin kinetics are slw, a few f different things can happen. First, yu culd simply wind up with the higher temperature phase(s) shwing up in yur allys, even thugh they are nt suppsed t. This happens when the driving frce fr the phase transfrmatin isn t high enugh, i.e., the higher temperature phase(s) are cmfrtable enugh t hang ut at lwer temperatures. Diamnd at rm temperature is a gd example f this. Secnd, the high-t-lw phase transfrmatin culd happen, but the prducts culd shw up in an unexpected mrphlgy. Maybe yu re expecting t see striped grains, but because the ally needs mre time t diffuse int stripes, it instead frms speckles. Third, yu culd frm entirely different phase(s) via a diffusinal reactin. The diffusinal frmatin f metastable phase(s) is surprisingly cmmn in allys: we frequently see this in wrught aluminum allys, and we can take advantage f these phases t strengthen aluminum allys. The aircraft aluminum allys, as well as 6061-T6 aluminum, are strng due t the frmatin f metastable phases. Finally, yu culd frm a metastable phase thrugh a diffusinless, r athermal, reactin. Diffusinless reactins ften frm upn fast quenching f a high temperature phase, and they invlve an incredibly fast shifting f atms int a different crystal structure. The mst cmmn diffusinless reactin is the frmatin f martensite during the fast quenching f high temperature (austenite) steel. Let s talk casting, r the nucleatin and grwth f a slid phase frm a liquid phase. Yur textbks present a cuple f different scenaris fr nucleatin: hmgeneus and hetergeneus. Hmgeneus nucleatin is abut at rare as gd Tex-Mex fd in Bstn, but it is interesting t cnsider frm a theretical perspective. The equatin fr Gibbs free energy f hmgeneus nucleatin has tw terms: The first term in this equatin is the vlume f a sphere times the vlume free energy term. Vlume free energy is the difference in Gibbs free energy between the parent (liquid) phase and the prduct (slid) phase. If

3 MATERIALS SCIENCE AND SOLID STATE CHEMISTRY HOMEWORK 8, PAGE 3 OF 5 yur metal is belw the melting temperature (r slidificatin temperature, if yu prefer), this vlume free energy term is negative. Yu want negative. Yu need negative. Withut negative changes in Gibbs free energy, nthing s ging t happen. The ther part f the equatin is what I call the surface energy term. Surface energy has an imprtant effect in phase changes. I like t cnsider this as the price yu have t pay in rder t get a new phase t frm. Fr example, t nucleate sme slid in a liquid, as in slidificatin during casting, yu have t create a new slid-liquid interface that was nt previusly there. This surface has a certain amunt f energy assciated with it. That means that this term is psitive. With ne negative and ne psitive term in this equatin, yu can imagine a situatin in which the surface energy term is higher than the vlume free energy term, right? If this is the case, yur slid wn t nucleate the price it has t pay is t high. This is why yu can have things like superheated water r supercled metal : the payff (vlume free energy) isn t yet high enugh t warrant the cst (surface energy). Slidificatin structures in cast allys. Nt much t discuss here, except that in cast allys yu will typically three slidificatin znes: the chill zne, the clumnar zne, and the equiaxed zne. If yu add nucleating agents (aka inculants), yu can reduce the clumnar zne and get mstly small equiaxed grains. If yu want t make me angry, ask me abut the time I spent hurs plishing a huge brnze ingt in rder t create a cl desk rnament that shwed the three slidificatin znes, nly t find that a nucleating agent was used in the metal. Painful memries. (image frm ASM Handbks, 8 th ed.) Dendritic grwth: What d the dendritic micrstructures lk like? Like trees, silly. Everybdy knws that. TTT diagrams. Given that mst transfrmatins in materials are diffusinal reactins, the transfrmatin time and transfrmatin temperature have dramatic effects n micrstructure and prperties. Nucleatin rate, grwth rate, and verall transfrmatin rate are all a functin f the transfrmatin temperature, since temperature determines bth the driving frce fr the phase change (hw badly a new phase wants t frm) and the rates f diffusin (hw lng it will take fr atms t change frm the ld phase t the new phase). In general, higher temperatures will give yu fewer, larger grains; and lwer temperature transfrmatins will prvide fr mre, smaller grains. With mre time, cmes mre diffusin, and this results in stuff either getting larger (if it is stable), r stuff heading tward the mre stable state. T turn thse fine grains int huge grains, just add time and temperature. Finally, I feel cmpelled t say a wrd r tw abut supersaturated slid slutins. If yu take a high temperature slid slutin and rapidly quench it, yu can ftentimes freeze the higher temperature phase by nt allwing sufficient diffusin time fr the lwer temperature phases t nucleate and grw. But here s where things get interesting. If yu want t change the prperties f the ally, yu can use time and temperature t encurage the lwer temperature phase(s) t frm in a specific shape r size r distributin. The best example f this is aluminum age hardening. If yu heat certain aluminum allys t the slutinizing temperature t disslve all f the ally additins in a single phase (Al) slid slutin, yu can easily quench the material in water t lck in this phase. But the prperties f this slid slutin are pretty lusy, s we dn t want t stp there. What we d like is t encurage the frmatin f tiny little precipitates (which happen t be metastable phases in mst Al allys) that give us an ally with high strength and hardness, thrugh a prcess called precipitatin hardening, r age hardening. We can d this in ne f tw ways. The mst cmmn apprach is called artificial aging, which invlves tssing the aluminum in an ven at a specified temperature and fr a specified time. Anther apprach is natural aging just leave it n a shelf, and yur aluminum will change prperties ver time. Crazy, huh? Almst as crazy as Brad Pitt in 12 Mnkeys.

4 MATERIALS SCIENCE AND SOLID STATE CHEMISTRY HOMEWORK 8, PAGE 4 OF 5 SCI1410 Brad Pitt, artificial aging: Brad Pitt, natural aging: PROBLEMS 1. What rles d nucleatin and grwth play in determining the prperties f the ally(s) yu are studying fr yur secnd prject? What prcessing parameters affect the rates f nucleatin and grwth in yur ally(s)? Hw des varying these parameters prvide fr different micrstructures in yur material(s)? 2. Pure titanium has tw alltrpic frms: β-ti (high temperature) and α-ti (lw temperature). Fr equilibrium heating cnditins, the lw temperature α phase transfrms t the high temperature β phase at 883 C (1156K). Let s cnsider what size f a β-ti nucleus we wuld have t frm in rder fr it t be stable at a temperature just abve the transfrmatin temperature. a. Calculate the critical radius r* and the activatin free energy ΔG* fr a bdy-centered cubic beta phase nucleus in a superheated alpha titanium matrix at 1165 K. Assume a spherical nucleus (hmgeneus nucleatin), ΔGv (1165 K) = J/m3, and σsl = 0.2 J/m2. b. Given yur answer in part (a), explain why nucleatin f the beta phase is unlikely t ccur if yu heat the titanium t the 1165 K temperature. What can yu d t make nucleatin mre likely t ccur? 3. Fr inspiratin nly lk at these cl micrstructures frm ASM Handbk! Pretty cl, huh? Why dn t my micrstructures ever turn ut lking this awesme?

5 MATERIALS SCIENCE AND SOLID STATE CHEMISTRY HOMEWORK 8, PAGE 5 OF 5 4. The fllwing tw micrgraphs are frm a hypeutectic (less than 12.6% Si) aluminum-silicn ally that was melted and pured int tw different mlds a sand mld, and a metallic mld. a. Which pht left r right is frm the sample that cled in the metallic mld? Hw d yu knw? b. Based n the micrstructure, estimate the cmpsitin f the Al-Si ally. The phase diagram is prvided belw fr reference. 5. Pure titanium underges a crystallgraphic transfrmatin at 883 C. Belw this temperature, pure titanium has a hexagnal clse packed structure knwn as alpha (α); abve it, titanium exists as the bdy centered cubic beta (β) phase. Additin f certain allying elements t pure titanium results in significant changes in the phase transfrmatin temperatures and in the stability f the alpha and beta phases. The mst cmmn titanium ally is Ti-6-4, which is cmpsed f titanium with 6 weight percent aluminum and 4 weight percent vanadium. A schematic, pseud-binary phase diagram fr Ti-Al-V allys is shwn belw. c. Based n the schematic Ti-Al-V phase diagram, what phases d yu expect t see in rm temperature allys f Ti-6Al-4V? d. In the fur bxes, sketch the expected micrstructure in the Ti-6Al-4V ally. Label the phases in yur sketches. Assume furnace (very slw) cling. e. Wuld the Ti-6Al-4V ally prvide any prperty advantages ver single-phase alpha allys? Briefly explain yur answer.