Al-Si Phase Diagram from Cooling Curves

Microstructure of Al-Si Castings

 

Objectives:

1)      To introduce and demonstrate methods for the preparation of metallic alloys. It is important to understand some of the problems involved in the preparation of an alloy of desired composition and how to overcome or at least address them. Aluminum-Silicon alloys are used as an example.

 

2)      To use the cooling curves of Al(Si) alloys to determine the liquidus and solidus curves for the binary Al-Si system.

 

3)      To explore the microstructures of Al(Si) alloys of various compositions, e.g. hypo-, hypereutectic, the microstructural features (e.g., number of phases present, their volume fractions and correlate and morphologies etc.) to the cooling curve data.

 

Suggested Reference Texts:

1) R.E. Reed-Hill, Physical Metallurgy Principles, 2nd ed.,
  Chapters 12 "Phases", 13 "Nucleation and Growth Kinetics"
  and mostly 14 "Binary Phase Diagrams";

 

  2) L .F. Mondolfo, Metallography of Aluminum Alloys,
  Chapter 8 "Master Alloys" (pp 202-210) and
  Chapter 10 "Aluminum-Silicon Alloys" (pp226-236);

                                                   

I. Materials:

These are the target compositions and forms:

           

Form/cmp
Pure Al
Al+TiB
Al-1%Si+TiB
Al-4%Si+TiB
Al-7%Si+TiB
Al-13%Si+TiB
Al-20%Si+TiB

pot

X

--

X

X

X

X

X

rectangle

X

X

--

X

--

--

--

tensile

X

X

--

X

--

--

--

 

Cooling curves and microstructures will be generated from the 6 samples cast into small pots along the first line.  (Note: the TiB addition should not change the cooling curve, therefore Al+TiB will not be studied) These results will be used in write-up for Lab 1.  The rectangular and tensile samples are used in Labs II and III.

 

At the end of the Lab, each group should have in their possession..

1 slice from the 'pot' castings at 1%, 4%, 7%, 13% and 20%Si, cooling curve data from each 'pot' casting, one book mold casting for each of the 3 alloys and one tensile bar for each of the 3 alloys. Lab report 1 will only consider the 'pot' materials. The other shapes you will cast are used in Labs 2 and 3.

 

All the Si-Bearing alloys will also have a TiBor addition   Based on the values cited in Volume 15 of the ASM Metals Handbook (pg 7750-751), additions of Al-Ti-B alloy grain refiners are generally in the 0.01 to 0.03% Ti addition levels. The TiB master alloy we add contains about 5 weight percent Ti and 1 weight percent Boron.

 

II. Equipment:

Induction- Melter
Sand crucibles with thermocouples
Data-Acquisition Computer System
Weighing Scale
Sample Preparation Suite (Sectioning, mounting etc.)
Optical Microscopes
Transparent sheets with 2-dimensional10x10 cross-grid
Digital Image Analyzer
Book Molds
Tensile sample molds

III. Procedures:

We will basically work from left to right on the table of alloy compositions shown above. When you begin the lab about 30 pounds of commercial purity aluminum (99.7% pure, by weight) in the molten state will be watitng for you. You’ll remove alloy to produce articles and add material to change the composition. You will know the initial weight of the melt and the weight of the materials removed. You will have to calculate the appropriate masses of materials to be added. Details on the pot-molds for cooling curves follow.

Aluminum alloys of various target compositions will be prepared by induction-melting. The alloy melts will be cast into sand-crucibles. Thermocouples have been attached to the crucibles for probing the temperature of the alloys in-situ as they cool and solidify after pouring the melts. The cooling curves will be acquired via a computer-based data acquisition system and will be shown in real-time on a computer display. The data will be available on the web. The total weight of each alloy casing will be measured. Each student will prepare at least one sample of an Al(Si) alloy for metallographic examination (in week 2 of the ab). The sample preparation steps are similar to those outlined previously.

 

In the Foundry (room 050 in the basement):

 

1. The Al alloys will be melted with the induction melter and cast into the crucibles by the instructors present. The instructors will initially melt pure Al, and subsequnetly add the appropriate amounts of Si to the melt in order to produce alloys of the target compositions 1) to 4) as detailed on the previous page. Cooling curves will be acquired for each of the six compositions.
 
2. Weighing the alloy castings:
A scale is available outside of room 064 (the undergraduate teaching lab). One of the TA's and one representative of each of the groups will proceed to the location of the scale with the cast alloy and weigh it after (i) it has completely solidified, (ii) been removed from the crucible and (iii) has been quenched under running cold water.

 

The instructors present will remove the alloy castings from sand crucibles, handle the hot casting with an appropriate pair of foundry tongs or pliers and quench them under running water. Watch the spray produced by the water impacting on the hot metal!

The instructors will ensure that the casting has cooled sufficiently, so it can be handled safely by the TA's and students for weighing.

The rectangular "bookmold" you will cast will have a target thickness of 0.25". After these are poured and solidified, the metal casting form will be removed and the cast aluminum can be cooled under running water. When casting book molds, the steel molds should not get in contact with water.

 
Record the weight of the cast alloys in your lab note book and calculate the current mass of the melt.

 

Go to room 259:

 

3. Remove the part that contains the thermocouple from the castings of alloy #1 to #4 by sectioning it appropriately using the cut-off wheel.
 
4. Section each casting further to produce 4-5 samples of dimensions suitable for subsequent optical metallography. Each group is required to prepare one sample from each of the 6 alloys. Groups of less than 4 members should mount sections from two different alloys into one bakelite mount. Careful book-keeping is required in this situation! Use the vibrating pen available in room 259 to identify the origin of your alloy sections after mounting. Correct identification will simplify your task of correlating the cooling curve data with your metallographic analysis.
 
5. Metallographic sample preparation: A reminder of the major steps is provided below.

 

. Obtaining a suitably dimensioned section of Al(Si) from the casting.
. Mounting the section in bakelite.
. Identification of mounted section using the vibrating pen.
. Grinding the mount with a series of SiC papers, e.g. 240,320,400 and 600.
. Bevel the bakelite mount.
. Careful polishing of the ground mount with 6 micron diamond paste followed by polishing with colloidal silica slurry (in room 248).
. Etching of the polished specimen.

6. Examine your sample with the optical microscope in room 248 to check that the etch reveals the microstructure well.
Examples of microstructures typical of commercial Al(Si) alloys for hypoeutectic, eutectic and hypereutectic compositions are shown for comparison and guidance in Fig. 2 of Appendix 1.

 

Go to room 252 - Optical Metallography:

 

7. Photograph the resulting microstructures of the alloys using the optical microscope.Take representative photographs of each alloy. Use proper magnifications between 100x and 500x. Determine which is the most suitable magnification for the volume fraction analysis of the microstructure of your sample. Take two photographs of two different areas representative of the microstructure. If your samples exhibit areas which differ from the "representative" microstructure, photograph these at the most suitable magnification.
 
8. Good quality xerox copies of all of the photographs each of you took should be made available to all of the students in your group. Each student will have at least 6 photo-micrographs, one for each of the alloys, at the most suitable magnification for the typical microstructure of the alloy. Also share any particular microstrucutural feature that you have found with your group.
 
9. Use the 2-dimensional grid of horizontal and vertical lines on a transparency (will be provided), e.g., overlaying the grid on each of the photographs or copies of photographs of the Al(Si) specimens, to determine the mean volume fractions of phases present in these microstructures. (The number of intersection points on the grid that fall on the photograph should be 100; that is, you should use a grid with 10 horizontal lines and 10 vertical lines, all of which fall within the size of the micrograph. See Appendix 2 regarding Quantitative Stereology)
 
10. For the Al(7at% Si) specimen, alloy 2), in particular, determine the standard deviation and the 95 percent confidence limit in the volume fraction of the Si-rich phase.
(See Appendix 2 regarding Quantitative Stereology and Statistical Analysis for details)

 

IV. Work for Future Labs

Another purpose of the experiments this week is to prepare materials for use in future labs. In addition to counting the small crucibles for this experiment the costs 0.25” (nominal) thick book molds and tensile samples from the following alloys

 

 

At the end of casting each lab, groups should have 3 tensile samples and 3 rectangular castings,each with one of these compositons.

 

V. Report

 

      In your report, you should include the followings:

 

1. Discuss the problems involved in the preparation of an alloy of desired composition and how to overcome or at least address them.
 
2. Discuss how the cooling/heating curves can be used to evaluate the equilibrium Al-Si phase diagram. Construct as much of the Al-Si phase diagram as possible from your data. (See Appendix 3 for information on Gibbs Phase Rule and Cooling Curve Analysis.)
 
3. Compare your results with the solidus and liquidus curves of the Al-Si phase diagram (Fig.1 in Appendix 1). Discuss the differences and explain why they are different?
 
4.   Compare the results of your microstructural metallographic analysis with the cooling curve analysis for each of the alloys we produced. Compare the microstructures that you obtained with the predictions from the Al-Si equilibrium phase diagram (Fig. 1 , Appendix 1) for the target compositions we aimed for. Discuss possible reasons for any difference (i.e., sources of error?).
 
5.   Determine the volume fractions of the phases present in the microstructures of the Al(Si) alloys of nominal target compositions all 6 compositions, using the cross-grids analysis as outlined in Appendix 2 on Quantitative Stereology. Compare your results with the equilibrium phase diagram and discuss any discrepancy.

 


 

 


Appendix 1: Microstructures of commercial Al(Si)-Alloys

 

 

Above: Typical microstructures of some commercial Al(Si) alloys of hypoeutectic compositions. Left: Cast in permanent mold, Al-5.64%Si-O.52%Fe-O.12%Cu-O.20%Zn, crystals of Si visible at grain boundaries of primary a-Al grains.

 

Right: Sand-cast. Al-5.80%Si-l.80%Fe, in addition to a-Al grains with Si-crystals at boundaries (darkest) long needles of FeSiAl5 present (lighter).

 

 

 


 


Appendix 2: Quantitative Stereology

 

Definition:

Quantitative Stereology refers to methods used to obtain information describing 3-dimensional volumes derived from 2-dimensional cross-sections, such as photo-micrographs.

 

This type of analysis is used to determine

. average grain sizes

. volume fractions of phases

. average interlamellar spacings

. etc...

 

Important Assumptions:

. The examined sample area, cross-section is representative of the volume feature to be determined

      (e.g. multiple sampling, random sampling may be necessary).

 

. Metallographic sample preparation has not introduced a significant type and/or number of artifacts by causing distortions of the microstructural morphology, such as porosity, recrystallization,preferred corrosion of one phase over another etc...

 

. The magnification chosen for the micrograph to be analyzed is suitable, i.e. the magnification allows for sufficient resolution of the microstructural feature of interest and also for reasonable statistics by covering enough microstructural area.

 

Types of Analysis:

Areal Analysis:= determination of area fractions of phases
Lineal Analysis:= determination of fractal length of random lines within phase of interest          
Point Analysis:= determination of fractions of points that fall within phase of interest

 

Each of these methods can be equally valid for determining the volume fraction of a phase. Practicalities determine which method should be used in preference over the others. For instance, it will be very difficult to conduct an areal analysis for microstructures containing phases with very fine features. Furthermore, areal analyses are generally more cumbersome than the other methods when done manually.

Lineal and point analyses are relatively easy conduct manually. For this lab the fastest method, point analyses will be used to determined the volume fractions of the phases present in the microstructures of the Al(Si) alloy castings as follows:

 

1)  overlay the test grid on a suitable micrograph

 

2) count no. of points within the phase of interest (generally the lowest volume fraction phase) and divide by total no. of grid points

 

3) points that fall at a phase boundary count as 1/2

 

For microstructures that contain two phases in about equal volume fractions (≈50% each) using a grid with a small number of grid points (e.g. 25) will suffice. In other cases, such as for the Al(Si) alloy microstructures analyzed here, a much larger (i.e. 100) should be used to ensure reasonable statistics.           

 

Note:

 

Statistical Analysis:

 

The standard deviation, s, represents the averaged deviation of the sum of the individual measurements from the mean value, i.e

.    STANDARD DEVIATION, s:                                     

 

            

  where Xi= an individual measurement
  X  "= mean value of measurements
and N = total number of measurements.

                                                                    

                

Assuming that the test data can be described by a normal distribution about the average, then

 


Appendix 3: The Gibbs Phase Rule Definitions:

 

The Gibbs Phase Rule says:

 

f (total number of variables) - (number of conditions required for equilibrium)

 

Derivation of the Gibbs Phase Rule:

An Example:

Consider a 2-component system, A and B, without mutual chemical reactions between the species (i.e. C=N-R=2), containing 2 phases, a and b (i.e. P=2).

 

A total number of P(C + 1) = 6 variables must be defined to completely describe the thermodynamic state of the system, namely the temperatures, the pressures and the compositions of the phases, T(a) and T(b), Pea) and PCb) and XA(a) and XB(b). Here phase compositions are expressed in terms of (C-l) composition variables such as volume fractions or weight percentages (e.g. weight percentage of species A in phase a, XA(a)).

 

For thermodynamic equilibrium the relevant variables must have the same values for both phases which defines the following total number of equilibrium conditions, (P - l)(C + 2) = 4 , i.e.

(1) temperatures, T(a) = T(b),

(2) pressures, pea) = pCb), and the chemical potentials for a species must be the same in both phases

(3) mA(a) = mA(b),

and      (4) mB(a) = mB(b).

 

Thus, from Gibbs Phase Rule it follows for this system that f = 6 - 4 = 2.

 

If 2 variables, such as temperature and pressure for instance, are fixed then the equilibrium state of this system is completely defined.

 

 


A Simple Binary Eutectic System (another example):

Applying Gibbs Phase Rule to the equilibrium phase diagram:

 

At the eutectic temperature of a binary eutectic system at 1 atmosphere of pressure the number of degrees of freedom available to the system are given by

 

            f= C - P + 1= 2 - 3 + 1= 0,    the equilibrium is completely fixed.

 

If the two phases and the liquid are in equilibrium at a pressure of 1 atmosphere the temperature of such a binary system cannot change until the number of phases changes.

 

             For hyper- and hypoeutectic compositions, crystallization occurs over a range of temperatures (liquidus and solidus temperatures), i.e.

 

            f= C - P + 1= 2 - 2 + 1= 1,      at the equilibrium state one degree of freedom remains.

 

The lever rule can be used to determine the amounts of liquid and solid for a given temperature for such compositions.

 

Cooling Curve Analysis for such a system:

 

 

Have fun and enjoy the experience. Good luck with the report.