KMU 479

MATERIALS SCIENCE AND TECHNOLOGY II

Midterm Examination

Instructor: Dr. Selis Önel

Tuesday, November 25, 2008

 

Please give brief answers to the following questions. All answers should be in English.

Question 1. (12 pts)

One way of categorizing materials is done as:

1. Metals and alloys, 2. Ceramics and glasses, 3. Polymers, 4. Semiconductors, 5.Composites

Match the following properties with the appropriate category by putting M for Metals, P for Polymers and C for Ceramics next to each property

a) Strong, ductile  

b) Soft, ductile, low strength, low density

c) Brittle, glassy, elastic

d) High thermal & electrical conductivity

e) Thermal & electrical insulators

f) Non-conducting (insulators)

g) Opaque, reflective

h) Optically translucent or transparent

i) Compounds of metallic and non-metallic elements

j) Metallic bonding

k) Ionic bonding

l) Covalent bonding by sharing of e’s

 

Question 2. (10 pts)

Assume that during homogeneous nucleation, a spherical interface is created when a solid forms from the liquid. This solid may either grow into a stable nucleus or redissolve.

Derive the equation for the critical nucleus r*, which is the minimum size of a stable crystal. 

According to this equation, how does supercooling (undercooling) affect the critical nucleus required for nucleation?

(Note: Assume that the driving force for solidification, i.e. the free energy per unit volume, ΔGv is given by ΔHf(ΔT/Tm) where ΔT= Tm –T.  Tm is the melting temperature and ΔHf is the latent heat of fusion)

 

Question 3. (15 pts)

Below, atomic radius, crystal structure, electronegativity, and the most common valence are tabulated, for several elements; for those that are nonmetals, only atomic radii are indicated.

                                                                                     Crystal           ΔElectro-          

                   Element       ΔR%             Structure          negativity           Valence

 

                   Cu                                                              FCC                          2+

                   C                                         -44

                   H                                         -64

                   O                                         -53

                   Ag                                       +13                 FCC                   0     1+

                   Al                                        +12                 FCC                 -0.4   3+

                   Co                                        -2                   HCP                 -0.1   2+

                   Cr                                         -2                  BCC                 -0.3   3+

                   Fe                                         -3                  BCC                 -0.1   2+

                   Ni                                         -3                   FCC                 -0.1   2+

                   Pd                                         +8                  FCC                 +0.3  2+

                   Pt                                          +9                  FCC                 +0.3  2+

                   Zn                                         +4                  HCP                 -0.3   2+

 

I. Which of these elements would you expect to form the following with copper and why:

(a) A substitutional solid solution having complete solubility?

(b) A substitutional solid solution of incomplete solubility?

(c) An interstitial solid solution?

II. Suppose 1 atomic % of these elements is added to copper without exceeding the solubility limit. Which one would be expected to give the higher strength alloy?

 

Question 4. (13 pts)

Circle the correct word in the following sentence or fill in the blank with one of the words given below:

Composition, Freezing zone, Growth, Liquid, Microstructure, Nucleation, Solid, Temperature 

a)        In materials science and engineering applications, transformation of a                to                 is probably the most important and common phase transformation.

b)                                         is the temperature difference between the liquidus and solidus lines. Within this zone two phases exist at all times. The amounts of the two phases (the proportion of solid to liquid) vary, depending on the                                    and                                   .

c)        The two stages involved in the formation of particles of a new phase are:

                                    and                             .  The                                  process involves the formation of normally very small particles of the new phase(s) which are stable and capable of continued growth.  The                                       stage is simply the increase in size of the new phase particles.

d)       Three variables that determine the                                 of an alloy are i) the alloying elements present, ii) the concentrations of these alloying elements, and iii) the heat treatment of the alloy.

e)        For the condition of phase equilibrium the free energy is a maximum/minimum, the system is completely stable/metastable meaning that over time the phase characteristics are constant.  For stability/metastability, the system is not at equilibrium, and there are very slight (and often imperceptible) changes of the phase characteristics with time.

 

Question 5. (15 pts)

 

A cooling curve shows how the temperature of a material varies with time.

Using the figure above:

(I) Determine the cooling stages and explain what happens between indicated points

            a) A-B:

            b) B-C:

            c) C-D:

            d) D-E:

            e) E -  :

(II) Is this metal well inoculated or not? Why?

(II) What is the

a) Pouring temperature?               

b) Solidification temperature?       

c) Superheat?

d) Cooling rate just before solidification begins?

e) Total solidification time?

f) Local solidification time?

g) Supercooling (undercooling)?

h) Probable identity of the metal (Si, Zn, Sn or Pb) and why?

 

Question 6. (15 pts)

In the Aluminum-Neodymium phase diagram below:

 

a) Are any intermetallic compounds present? If so, identify them and determine whether they are stoichiometric or nonstoichiometric.

b) Identify the solid solutions present in the system.

c) Identify the three-phase reactions by writing down the temperature, the reaction in equation form, and the name of the reaction.

 

Question 6. (20 pts)

Consider a Pb–35% Sn alloy using the phase diagram below

Determine

(a) if the alloy is hypoeutectic or hypereutectic,

(b) the composition of the first solid to form during solidification,

(c) the amounts and compositions of each phase at 184°C,

(d) the amounts and compositions of each phase at 182°C, (e) the amounts and compositions of each microconstituent at 182°C,

(f) the amounts and compositions of each phase at 25°C.