Module 26
Common Binary Alloys
Lecture 26
Common Binary Alloys
1 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Introduction We are now familiar with binary and ternary phase diagrams. This tells us about the structure of alloys. This in turn determines its properties. We are also familiar with the limitations of phase diagram. However the entire concept was introduced with the help of hypothetical systems except for the case of iron – iron carbide system. Steel which is an iron carbon alloy is no doubt by far the most commonly used metallic material. However there are several other commercial alloys as well. In this lecture let us look at a few of these. We shall begin this lecture with the rules that govern the solubility limits in binary alloys. This to a great extent determines what kind of phase diagram a binary system is likely to have. There are several binary alloys belonging to either simple isomorphous or simple eutectics. We shall look at few of these. Apart from there are systems that have several intermediate phases. In some of the two metals are present in definite proportions as in a chemical compound. We shall learn the cases where we expect such intermediate compounds to form. Limits of solid solubility: There are several examples where the two metals may have unlimited solubility whereas there are cases where the solubility is very much restricted. There are certain rules that govern the limits of solubility. These are popularly known as the Hume Rothery rules. Two metals can have unlimited solubility if they satisfy the following four criteria. 1. The atomic diameters of the two metals should be within ±15%. This known as the size factor. 2. The two metals must have identical crystal structure. 3. The metals must have the same valance. 4. The difference between the electro‐negativity of the two atoms should not be greater than 0.4e.u. There are several examples of metals that satisfy the above set of rules. They have unlimited solubility in solid state. Such alloys are classified as isomorphous system. Cu‐Ni and Si‐Ge are common alloys within this system. 2 In terms of thermodynamic criteria ideal solutions tend to have unlimited solubility. It was illustrated earlier that if two metals A & B form ideal solutions in both liquid and solid state its phase diagram corresponds to that of an isomorphous system. In pure metals we only have like bonds (either AA or BB). When they dissolve or get mixed some of the like bonds are replaced by unlike bonds. Let the number of these bonds be represented as nAA, nBB and nAB. Consider a case where
. There is no preferential arrangement of atoms. This satisfies the condition for a random solid solution. In such a case the activity of A (or B) is likely to be equal NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | to its atom (mole) fraction and its phase diagram will be similar to that of an isomorphous . The deviation from ideality is positive. The system would exhibit a system. If
tendency to form clusters. If
. The system would exhibit a tendency to form compounds. Depending on the extent of deviation the type of phase diagram would change. Figure 1 illustrates how with increasing deviation from ideality an isomorphous system could transform into a eutectic system.
L L L +  L + 
T 
T L T 
A 
% B B A % B L + 2 L + 
Fig 1 


B A % B B The sketch on the extreme left of fig 1 shows a typical phase diagram for a system where both the liquid and the solid exhibit ideal behavior. The central sketch shows the effect of positive deviation leading to the formation of clusters. This results in a miscibility gap. With increasing deviation the minimum in the isomorphous portion of the diagram comes down and the peak temperature of the miscibility gap increases. Finally the two could meet resulting in a eutectic phase diagram. Examples of binary isomorphous system: Slide 1 presents the phase diagram of Cu‐Ni system. Both copper & nickel have the same crystal structure. Their lattice parameters are nearly the same. The atomic diameter of an elemental solid is directly proportional to its lattice parameter. Therefore size factor appears favorable for unlimited solubility. They have identical valence (2). The electro‐negativity is nearly the same. Therefore the two have unlimited solubility in the liquid state. 3 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Cu – Ni : Cupronickel
1455
Cu: fcc: 3.61
1083
UTS
Ni: fcc: 3.52
Slide 1
Cu:29 en: 1.90
Ni:28 en:1.91
Ductility
Cu
Ni
Good strength, ductility, low temp coefficient for resistance &
corrosion resistance (marine). Application: heat exchanger,
condenser tubes, thermocouple (Cu 46Ni : constantan)
The strength of metals increases with increasing alloy content. This is known as solid solution strengthening. The sketch in slide 1 also displays the effect of composition on the UTS (Ultimate Tensile Strength). It attains the highest value at an intermediate composition. The increase in strength is accompanied by loss of ductility. The alloy has good corrosion resistance, high strength, and good ductility. Its electrical resistance is less sensitive to temperature. It is commonly used in heat exchangers, condenser tubes. The constantan wire in thermocouples is made of Cu‐46Ni. Ge-Si
1402
Slide 2
940
Ge
Si
Ge: Diamond cubic: 5.66 Si: Diamond cubic: 5.43
Valence = 4 ( IV of periodic table) At No. 32 & 14
4 Ge & Si both belong to group IV in the periodic table. Their valence is four. Both have diamond cubic crystal structure. Their lattice parameters are 5.66 & 5.43 Angstrom respectively. Their NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | electro‐negativity is nearly the same. Therefore they too have unlimited solubility. Slide 2 gives phase diagram of Ge‐Si system. Cu-Au
1063
1083
L

AuCu
Au
Au3Cu
Slide 3
AuCu3
Cu
Au: fcc (4.08) At. No. 79 Group IB
Cu: fcc (3.61) at. No. 29 Group IB
Au & Cu have the same crystal structure. They belong to group IB of the periodic table. They have identical valence. The difference in their atomic diameters is within 15%. Therefore they have unlimited solid solubility. However at lower temperatures it undergoes an order disorder transformation. It is an indicator of deviation from the condition for ideal solid solution. Crystal structure of Au‐Cu alloy is FCC. In the disordered state at higher temperatures the two atoms occupy lattice sites at random. For example in an alloy corresponding to Au3Cu one may assume that each lattice point is made of 75%Au and 25%Cu. In ordered Au3Cu, the gold atoms are located at face centers whereas the copper atoms are at the corner sites. The two sites are in the ratio 3:1. This satisfies the composition as well. There is an order disorder transformation at composition corresponding to AuCu3. In this case after the order disorder transformation Cu atoms occupy the face centers and Au atoms occupy the corner sites. In the case of AuCu there is a little difference in the way the two sites are occupied. We may assume that one the two occupies the corner sites and centers of the base. The other occupies the remaining face centers. 5 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | (c) (b) (a) Fig 2: Shows how atoms are arranged in disordered and ordered solid solutions. (a) Denotes disordered structure where each atom may be assumed to be made of partly Au and partly Cu. (b) Shows how the Au (red) and Cu (blue) atoms are arranged are arranged in ordered Au3Cu. (c) Shows how the Au (red) and Cu (blue) atoms are arranged are arranged in ordered AuCu. Binary eutectic: Pb-Sn: Eutectic: nAB < 0.5(nAA+nBB)
327
L
232
183
Sn
L+
L+
Slide 4

Pb
Sn: tetragonal (a=5.35 c=3.18) & Pb : fcc 4.95
Atomic no. Sn: 50 Pb: 82 (IV of periodic table)
6 One of the most common example of a binary eutectic is that of Pb & Sn. Both belong to group IV of the periodic table. However the crystal structure of lead is FCC (a= 4.95Angstrom) and that Sn is body centered tetragonal (a= 5.35 & c = 3.18 Angstrom). This is system where the number of unlike bonds is less than which is expected if atoms are randomly arranged. Therefore it has a tendency to form cluster. The sketch slide 4 gives a schematic representation of the Pb‐Sn phase diagram. Note that the composition of the binary eutectic is always nearer to the metal having lower melting point. Pb‐Sn eutectic is a very popular material for solder. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Al-Si Eutectic
1412
L
660
Slide 5 577
Al 11.6
Si
Al: fcc (4.05A) At. No. 13 group IIA : soft & ductile
Si: diamond cubic (5.43) At. No. 14 Group IVA Hard
& brittle
Aluminum is a soft and ductile metal. It has FCC structure. It belongs to group II of the periodic table. Si on the other hand is hard and brittle. It belongs to group IVA of the periodic table. Its crystal structure is diamond cubic. Therefore The two have very limited solubility is the solid state. The two form a simple binary eutectic system. Slide 5 shows a sketch of the Al‐Si phase diagram. The eutectic composition is around 11.8% Si. Si is amongst the few elements that expand of solidification. Therefore hyper eutectic Al‐Si alloys having around 12% Si does not shrink on solidification. Therefore these are easy to cast. It is one of the most popular cast aluminum alloys. Al-Si Eutectic
1412
L
660
577
Al
Si
Eutectic consisting of coarse Si plates in Al matrix is
brittle. This can be modified by adding Na or NaCl.
or rapid solidification. This improves its ductility.
Useful cast alloy: pump casing, engine manifolds,
piston ( with addition of a few other alloy elements)
7 Slide 6 Al–Si eutectic consists of coarse Si plates in Al matrix. Such a structure is brittle. This can be modified by adding Na or NaCl or by rapid solidification. As a result of modification eutectic temperature is lowered and the composition of the eutectic shifts towards higher Si content. This is illustrated with the help of a sketch given in slide 6. This improves its ductility. Useful NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | cast alloy: pump casing, engine manifolds, and pistons (with addition of a few other alloy elements). Pb-Sb: Eutectic
630
L
L+
327

Slide 7 252
Pb 11.1
Sb
Pb: fcc 4.95 & Sb: rhomb 4.51A 57.1
Pb: At no. 82 Group IVASb: 51 Group VA
Pb‐Sb forms a binary eutectic. It is given in slide 7. Note the difference in their crystal structure & valence. Ni-Cr
1880
1455
L

1345

Slide 8 CrNi3
Magnetic
transformation
Cr
Ni
Cr: bcc (2.88A) At. No. 24 Group VIB
Ni: fcc (3.52A) At. No. 28 Group VIII
Ni has FCC structure whereas Cr is BCC. They belong to different groups in the periodic table. Slide 8 gives a sketch of the Ni‐Cr binary phase diagram. Ni‐Cr alloy has very good oxidation resistance. Nichrome is a 80Ni20Cr alloy. It is used as heating element. Addition of a few other alloy elements makes it a very attractive alloy for high temperature applications. 8 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Phase diagrams having intermediate phases: As-Ga system nAB > 0.5(nAA+nBB)
Inter-metallic compound formation
1238
810
29.8
817
Slide 9 29.5
Ga
GaAs
As
Ga (31): IIIA & As (33): VA of periodic table
Ga: Orthorhombic As: HCP
As‐Ga system is an excellent example of a binary alloy having an inter‐metallic compound GaAs. Note that the melting point of Ga is very low. But the inter‐metallic compound has a melting point which much higher than those of As & Ga. Slide 9 gives a sketch of the phase diagram. There is hardly any solid solubility. Here is an alloy where the number of bonds between unlike atoms is much more than those between like atoms. The slide also indicates the crystal structures of Ga & As and the groups of the periodic table they belong to. Mg-Sn: intermetallic
L
650



Slide 10 232

Mg
Mg2Sn
Sn
Mg: hcp (3.21, 5.21A) At. No. 12 group IIA
Sn: tetrag. (5.83, 3.18A) At. No. 50 Group IVA
9 Mg‐Sn is also an excellent example of a binary alloy having an inter‐metallic compound Mg2Sn. It has two eutectics one between Mg & Mg2Sn and other between Mg2Sn and Sn. Slide 10 gives NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | a sketch of the phase diagram. Note that both GaAs & Mg2Sn melt at fixed temperatures very much like pure metals. Cu-Zn: Brass
1083
902
834
700
600
560
62.5 Cu
37.5 Zn
Slide 11 

423
Cu



419

’
30
50 60
80
Zn
Cu: fcc (3.61A) At. No. 29 Group IB
Zn: hcp (2.66, 4.95A) At. No. 30 group IIB
Cu‐Zn alloys are known as brass. Slide 11 gives the phase diagram of binary Cu‐Zn system. Note that the melting point of Cu is much higher than that of Zn. The terminal solid solutions are called & . There are four intermediate phases. The phase  and  are the products of peritectic reaction. Unlike inter‐metallic compounds like GaAs or Mg2Sn, the and  phases do not have fixed composition or melting point. These are known as intermediate phases. All of them have single phase structure. Mechanical properties of brass
UTS
400
60%
UTS
MPa
% Elongation
Slide 12
0
% Zn
60
10 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | The copper end alloys have reddish color. The single phase alloys are known as alpha brass. They have good corrosion resistance. They are ductile. They can be cold worked. 70‐30 (70%Cu 30% Zn) and 60‐40 (60%Cu 40% Zn) are the two most common grades of brass. The former is a single phase alloy. It belongs to the class of alpha brass. As against this 60Cu40Zn is a two phase alloy. It consists of two phases  & . It is yellow in color. It has good machinability but it cannot be cold worked. It is also known as  brass. The sketch in slide 12 gives the effect of % Zn on % elongation and UTS of brass. Note that 70/30 brass has the highest ductility. Beta brass has the highest strength. Gamma brass is very brittle. It is of little use. Cu‐Sn alloys are known as bronze. Like zinc, tin too has a very low melting point. Its crystal structure is different from that of Cu. They also have different valence. This is why solubility of Sn in Cu is limited. Cu-Sn: Bronze
L
L





Cu3Sn
Slide 13



Cu
~45% Sn
Cu: fcc (3.61A) At. No. 29 Group IB
Sn: Tetrag (5.83, 3.18A) At. No. 50 group IVA
The sketch in slide 13 gives a part of the Cu‐Sn phase diagram. Within this region itself there are several isothermal reactions involving 3 phase equilibrium involving several intermediate phases ( etc.). Only one of these () has a fixed composition. Cu‐10% Sn is a popular grade of bronze. It has a two phase structure. The matrix is ductile but the second phase is hard and brittle. It has good wear resistance. It makes this a good bearing alloy. 11 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Ti-Al
1720
1460

1340
1240

Slide 14
665

Ti
660
Al
Ti: hcp (2.95, 4.68A) / bcc (3.31) , At. No. 22 Group IVB
Al: fcc (4.05A), At. No. 13 group IIIA
Ti-V
1900
1720
L
1620, 0.3
882

600

Ti
Slide 15
V
Ti: hcp (2.95, 4.68A) / bcc (3.31) , At. No. 22 Group IVB
V: bcc (3.03A), At. No. 23 Group VB
12 Titanium has two crystalline forms. The room temperature form of Ti is HCP. However above 882 till its melting point it is BCC. The BCC form of Ti is more amenable to deformation. The room form has limited slip system. Therefore it has relatively poor ductility. Two of the most common alloy additions to Ti are Al & V. The sketch in slide 14 gives the phase diagram of Ti‐Al and the sketch in slide 15 gives the phase diagram of Ti‐V. Al is an alpha stabilizer. It extends the region in which  is stable. As against this V is a beta stabilizer. It extends the region in which  is stable. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Intermediate phases: We are now familiar with the existence of several intermediate phases in various binary phase diagrams. They may be classified into 4 groups. These are as follows: 1. Electrochemical compound: Mg2(Pb, Sn, Ge, Si), Mg3(Bi,Sb,As)2 2. Size factor compound: Fe3C 3. Laves phase: MgCu2, MgNi2 4. Electron compound: CuZn, Cu9Al4 , CuZn3 Electrochemical compound: It forms when one is electropositive & the other electronegative. Mg is a group II element. Metals like Pb(82) Sn(50) Ge(32) & Si(14) belong to group IV. The figures within brackets are the atomic numbers of the elements. The difference in valence makes Mg electropositive metals like Pb, Sn, Ge & Si electronegative. Therefore it favors formation of compounds such as Mg2Pb, Mg2Sn, Mg2Ge, & Mg2Si. Metals like Bi(83) Sb(51) & As(33) belong to group V. In view of the large difference in their valence with respect to that of Mg they form compounds such as Mg3Bi2, Mg3Sb2, and Mg3As2. Size factor compound: It forms when the solute atom is small enough to be accommodated within the interstices of metal. The most common examples are those of carbides and nitrides. If the ratio of the radius of the interstitial atom to that of the metal is between 0.41 & 0.59 it forms compounds like MX or M2X, where M stands for metal and X stands for carbon or nitrogen. Carbides and nitrides of metals like Ti, Zr, Hf, V, Nb, and Ta come under this category. If the ratio is greater than 0.59 there is more lattice distortion. The structure that forms is more complex that those of MX or M2X types of compounds. The most common example of such a compound is Fe3C known as cementite. Laves phase: This forms when atomic size difference is about 20‐30%. Each A atom has 12 B & 4 A atoms as its neighbor & each B atom has 6 A & 6 B atoms as its neighbor. Average co‐
ordination number is 13.33. This is greater than that of a close packed structure. MgCu2 and MgNi2 are two examples of such a phase. The former is cubic and the latter is hexagonal. 13 Electron compounds: This forms at specific electron to atom (e/a) ratio. Metals have free electrons. In mono‐valent metals the ratio is one. If a metal having higher valence is added the ratio keeps increasing. For example valence of copper is one. Its e/a ratio is one. When Zn whose valence is two is added to Cu the e/a ratio would increase. When the e/a ratio becomes 1.5 (3/2) a new intermediate phase called  brass forms. This occurs when the alloy has 50atomic % Zn. It corresponds to CuZn. The  brass is FCC whereas  brass is BCC. The electron NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | compounds that form at this e/a ratio is known to have b brass structure. Table 1 give a list of several other alloy system where such a compound can form. Table 1  brass brass  brass (e/a = 3/2) (e/a = 21/13) (e/a = 7/4)
CuZn Cu5Zn8
CuZn3
Cu3Al Cu9Al4
Cu3Sn
Cu5Sn Cu31Sn8
Cu3Si
NiAl Fe5Zn21
Ag5Al3
Table 1 also includes examples of two other set of electron compounds. They are known as gamma and epsilon brass. The former occurs at e/a = 21/13 and the latter occurs at e/a = 7/4. The table also gives a list of several other examples of electron compounds having the same crystal structures as those of gamma and epsilon brasses. 14 Summary: In this lecture we looked at several common binary alloys. Some of these are simple isomorphous or eutectic while others have several intermediate phases. Some of the solid solutions have tendency to form cluster whereas some exhibit ordering. Au‐Cu system is an excellent example where there is transition from ordered to disordered structure as it is heated. In ordered structures Au & Cu atoms occupy specific sites whereas in disordered state these are randomly located. We have also talked about four different types of intermediate phases and their characteristics. The structure of an alloy can be guessed if we know its phase diagram. There is strong correlation between structure and properties. Therefore looking at such diagrams we can have an idea about the properties of the alloys. We can say whether it can be cold worked or to what temperature the alloy is to be heated so that it can withstand plastic deformation. We shall learn more about these in subsequent lectures. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Exercise: 1. Which lead – tin alloy will be ideal for joining electronic assemblies? Give reason. 2. Why is Al‐12% Si alloy a very popular casting material for automotive applications? 3. Cartridge brass is easily cold worked but Muntz metal can not be cold worked. Explain why it is so. 4. What is the diffrence between disordered & ordered AuCu3 alloy? How is the presence of ordered structure detected? Answers: 1. Eutectic composition is ideal. It has 62Sn 38Pb. It melts at a fixed temperature. It flows easily into tiny gaps which is the essential criteria for joining electronic assemblies. 2. Apart from light weight it has excellent castability. By modification it can be made to solidify as complete eutectic structure. Most metals (inculdes Al) contract on solidification. Si exapnds on solidification. Al‐12Si is an optimum combination where there is little shrinkage on solidification. It is therefore easy to produce defect free casting with no shrinkage cavity. 3. Cartridge brass has 70%Cu & 30% Zn. A look at Cu‐Zn phase diagram shows that it is a single phase alloy. Therefore it can be deformed / shaped easily by cold work. Muntz metal on the other hand had 60%Cu40%Zn. It falls within  region of phase diagram.  is relatively brittle at room tem[perature. A two phase structure is always difficult to cold work. However if heated it goes to a single phase region. This is where it is amenable to working. (It can be hot worked) 4. In disordered state both Au & Cu atoms are distributed in both cube corners and face centres in proportion to their respective atomic percent. Virtually each atom could be assumed to be made of I part of Au & 3 parts of Cu. In ordered state all Au atoms are located in corner sites & Cu atoms occupy face centres. These sites are in ratio 1:3 in fcc lattice. This is best detected by powder X‐Ray diffraction technique. Disordered structure gives reflections corresponding to fcc lattice where as reflection form ordered structure corresponds to simple cubic lattice. Additional reflections are called super lattice reflection. 15 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Appendix This includes a binary phase diagrams of AlNi and AlFe system having a number of intermediate phases. AlNi system has an intermediate phase AlNi having a fixed melting point which is higher than that of pure Ni. However the others do not have a fixed melting point. Aluminides have attractive high temperature properties. Many of them have ordered structure. This is responsible for the anomalous temperature dependence of its yield strength. It increases with increasing temperature unlike most metals and alloys. A dislocation in an ordered structure splits into two pairs of partials separated by an anti‐phase domain. It is mobile as long as it remains in a plane. However at high temperature as a result of thermal activation a part of this may climb on to another plane making this immobile. Such a configuration is known as Kear – Wilsdrof lock. This is why the strength of an inter‐metallic like Ni3Al increases with temperature. It forms a major constituent in several Ni base super‐alloys used in gas turbines. Figure A1 gives a schematic binary phase diagram of Al‐Ni system. Table A1 gives the composition range and crystal structures of various phases that could be present in binary Al‐Ni system. 1638°C L 1455°C 1395 AlNi 1133°C 1385 
L T° C 854°C 700°C 660°C 0 Al 16 640°C at. % Ni Al3Ni Al3Ni2 Al3Ni5 AlNi3 100 Ni Fig A1: Gives a schematic (not to scale) binary Al‐Ni phase diagram. It has 6 intermediate phases. These are often called as Nickel Aluminide. One of these is a congruently melting alloy whose melting point (1638°C) is higher than that of both Al and Ni. Ni3Al has an ordered structure. It is a major constituent of several high temperature Ni base super‐alloys. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Table A1: Crystal structure & stability range of various phases in Al‐Ni binary alloy Phases Al rich solid solution Al3Ni Al3Ni2 AlNi Al3Ni5 AlNi3 Ni rich solid solution Crystal structure (Prototype) A1(Cu) FCC D011 (Fe3C) (Orthorhombic) D513 B2 (CsCl) Cmmm (Gs3Pt5) L12 (AuCu3) A1 (Cu) FCC Composition range (at. % Ni) Negligible solubility 25 37 ‐ 40 42 ‐ 69 64 ‐ 68 73 ‐ 76 80 ‐ 100 Figure A2 gives a schematic binary phase diagram of Fe‐Al system. Table A2 gives the composition range and crystal structures of various phases that could be present in binary Fe‐Al system. 17 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | 1538 L 1394 1310
1215
Fe Fe 1164
1171
1157

Curie temperature 910 2 1092
770 660
Coherent equilibrium 652 
FeAl2 

Fe2Al5 Fe Fe3Al at. % Al FeAl3 Al Fig A2: Gives a schematic (not to scale) binary Fe‐Al phase diagram. It has 7 intermediate phases. These are often called as Iron Aluminide. One of these () is not stable at room Table A2: Crystal structure & stability range of various phases in Fe‐Al binary alloy Phases  Fe solid solution  Fe Fe3Al FeAl (2) Fe2Al3 FeAl2 Fe2Al5 FeAl3 Al solid solution 18 Crystal structure BCC FCC D03 BCC (Ordered)
Complex cubic Triclinic Orthorhombic Monoclinic FCC Composition range (at. %) 0 ‐ 45 0 ‐ 1.3
23 ‐ 34 23 ‐ 55
58 ‐ 65 66 ‐ 67 70 ‐ 73 74.5 – 76.5 99.998 ‐ 100 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | Representation of crystal structure: We are familiar with unit cell and Bravais lattice. These are frame work used to represent the way atoms are arranged in a crystal. In pure metals a lattice point may denote the location of an atom. However in intermediate phases or in compounds a lattice point may represent a group of atoms. Most minerals and inorganic chemicals have well developed crystal faces. Often these are large enough to find their symmetry elements (mirror plane, 2‐6 fold axis of symmetry, centre of symmetry etc). There are 32 different combinations of symmetry elements around a point. These are known as point groups. All of them are easily identifiable from the shape of the crystal. They can be classified into 7 crystal system. However with the introduction of X‐ray diffraction technique it is now possible to indentify precise locations of atoms in a unit cell of a crystal. Therefore in order to describe a crystal structure it became necessary to introduce the concept of space group representing an array of symmetry elements in three dimensions. A large number of the possible space groups, consists of an array of point groups around the 14 Bravais lattices. However apart from these, it is necessary to consider additional microscopic symmetry elements that are not possible in point groups. The inclusion of both macroscopic and microscopic symmetry elements at all lattice points, gives rise to 230 space groups to which all crystals must belong. In order to describe the crystal structure several notations have evolved over the years. These are Strukturbericht symbol, Pearson symbol, and Space group. Table A3 gives the Stukturbericht designation for a few selected types of crystal structures. It consists of a letter describing the type of structure followed by a number denoting specific type within this category. (For details see books on crystal structure: J D Tilley Crystals and Crystal Structures, John Wiley & Sons Ltd 2006) Table A3: Strukturbericht designation and names for a few selected crystal structures Type Chemical symbol A Elements B AB : compound C D L AB2 : compound AmBn : compound Alloys Example A1: FCC (Al, Cu); A2: BCC (Fe, Mo); A3: HCP (Mg, Zn); A4: Diamond (C) B1: Halite (NaCl); B2: CsCl; B3: Zinc Blende (ZnS); B4: Wurzite (ZnO) C1: Fluorite (CaF2); C2: Rutile (TiO2) D03 : Fe3Al L12 : AuCu3 19 Pearson symbol gives successively the crystal structure, the Bravais lattice and the number of atoms in a unit cell. For example crystal structure of FCC Cu is denoted as cF4 and that of rock salt (NaCl) is given by cF8. Note that ‘c’ stands for ‘cubic crystal’ and ‘F’ denotes ‘Face centered lattice’. The numbers 4 & 8 denote the number of atoms in unit cells of Cu and NaCl respectively. In this nomenclature ‘a’ means triclinic, ‘m’ means monoclinic, ‘o’ means NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | orthorhombic, ‘t’ means tetragonal, ‘h’ means hexagonal and ‘c’ means cubic. A capital letter is used to represent Bravais lattice. ‘F’ denotes face centered, ‘I’ denotes body centered, ‘C’ denotes base centered, ‘R’ denotes rhombohedral and ‘P’ denotes primitive. Space group notations give an idea of the lattice and the symmetry elements. For example P denotes simple lattice whereas F denotes a lattice having a point at the centre of all the 6 faces. The notation begins with a capital letter representing the type of the lattice followed by a set of symmetry elements. The representation is a little more complex. It is beyond the scope of an elementary course of physical metallurgy. Interested readers may refer to a book on crystallography. Table A4 gives a comparison of the three different ways of representing the structures of various crystalline materials. There are instances where only one of these may not be sufficient to describe the structure of a crystal. Table A4: Representation of crystal structure using the above notations Strukturbericht A1 A2 A3 B1 B2 D03 L12 Prototype Cu W Mg NaCl CsCl BiF3 AuCu3 Pearson cF4 cI2 hP2 cF8 cP2 cF16 cP4 Space group Fm3m Im3m P63/mmc Fm3m Pm3m Fm3m Pm3m Many of the intermediate phases or inter‐metallic compounds have attractive properties. For example aluminides of Ni, Ti & Fe have very good oxidation / corrosion resistance, low density and high strength & stiffness at elevated temperature. Some of these like Al3Ni have the exact stoichiometric composition whereas other may be stable over a range of composition. Most of these have ordered structure. This is responsible for its high strength. A major limitation of these is poor ductility. Considerable efforts have gone in, to improve its ductility. One of the possible ways is alloying. The two of the most widely studied inter‐metallics are Ni3Al and NiAl. 20 The crystal structure of Ni3Al is L12. The corresponding Pearson symbol is cP4. It suggests that the crystal structure is cubic. P stands for primitive Bravais lattice. ‘4’ denotes the number of atoms in a unit cell. It is a derivative of face centered cubic (fcc) structure. Figure A3 (a) gives a sketch of a unit cell of Ni3Al. In an ordered structure Al occupies the corner sites whereas Ni occupies face centers. The number of corner and face centre sites are in the ratio 1:3. This corresponds to its exact stoichiometry. It is a major constituent of several commercial Ni base super‐alloys. Some of these may have as high as 70% Ni3Al. It is often referred to as gamma prime phase (’). NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | | The crystal structure of NiAl is B2. The corresponding Pearson symbol is cP2. It suggests that the crystal structure is cubic. P stands for primitive Bravais lattice. ‘2’ denotes the number of atoms in a unit cell. It is a derivative of body centered cubic (bcc) structure. Figure A3 (b) gives a sketch of a unit cell of NiAl. In an ordered structure Al occupies the corner sites whereas Ni occupies body centers (or vice versa). Al Ni Ni3Al NiAl Fig A3: The atomic arrangement within a unit cube of (a) Ni3Al where Al atoms are at the 8 corners and Ni atoms are at the 6 face centers of a unit cube and (b) NiAl where Al atoms are at the 8 corners whereas Ni atom is at the centre of a unit cube. 21 NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering | |