Effect of grain size on properties of metals

          A grain is a crystal with almost any external shape, but with an internal atomic structure based on the space lattice with which it was born.

          The mechanical properties of metals that obtain maximum strength depends upon the arrangement of the grains, their shape and especially their size. Grain size directly controls the extent of slip interference by adjacent grains and strength, toughness, ductility and fatigue of metals thus affected. The grain size is controlled by several factors out of which temperature and time of heating are important.

The important effects of grain size are given below:

1. Fine grained structure has higher strength than coarse grained structure.
2. Fine grain provides better resistance to cracking, and better machine finish.
3. Coarse grains make surface rough and make metal less tough.
4. Coarse grained metal is difficult to polish.
5. Coarse grained structure has a better workability than fine-grained structure.
6. Coarse grained materials at high temperature exhibit better creep resistance than grained one.

TERMS USED IN EQUILIBRIUM DIAGRAMS

The following terms are frequently used in the study of solid phases and phase diagrams:

 i. Alloy

 ii. System

 iii. Components

 iv. Phase

 v. Phase diagram

 vi. Phase Equilibrium

 vii. Phase Transformation

 viii. Structural constituents

 ix. GIBB’S PHASE RULE OR THE PHASE RULE OR CONDENSED PHASE RULE 

Alloy

          A substance which is composed of two or more than two chemical elements such that metallic atoms predominates in composition and the metallic bond predominates is called an "Alloy". The element which is present in largest proportion is called "base metal" and all other elements present are known as "alloying elements".

         Alloys are classified as binary alloys, composed of two components; as ternary alloys, composed of three components; or as multicomponent alloys. Most commercial alloys are multicomponent. The composition of an alloy is described by giving the percentage (either by weight or by atoms) of each element in it.

          Metal alloys by virtue of composition, are often grouped into two classes : Ferrous and Non-Ferrous. Ferrous alloys are those in which iron is the principal constituent, include steels and cast irons. The non-ferrous alloys are all alloys that are not iron based.

          Alloys are widely used in industry because their physical and chemical properties can be easily varied to suit the exact individual requirement. One can achieve this by preparing alloys of different metals.

System

          A system may be composed of solids, liquids, gases or their combinations and may have metals and non-metals separately or in any combination. It is explained as the whole complex of phases of one or several components at different pressures and compositions. A system is so isolated from its surroundings that it is unaffected by solids, liquids, gasses or their combinations and is subjected to the change in the overall composition, temperature, pressure or total volume, only to the extent permitted by the investigator.

          A system is classified according to the number of components that constitute the system.

Components

          These are the substances, either chemical elements or chemical compounds, whose presence is essential and sufficient to make a system. A system consist of one or more components, A pure metal is called a one-component system, an alloy of two metals is called a binary or two component system, etc.,

Phase

          It is a homogeneous portion of a system that has uniform physical and chemical characteristics. The number of phases in a system is the number of different substances that exist in it in a homogeneous system. A homogeneous liquid solution is a single-phase system; a mixture of crystals of two types, differing in composition and structure separated by an interface, or the coexistence of the liquid alloy and its crystals comprise two-phase system.

Phase Equilibrium & Phase Transformation

Phase Equilibrium:

          The state of a system where the phase charecteristics remain constant over indefinite time periods. At equilibrium the free energy is a minimum.

        In an equilibrium diagram, liquid is one phase and solid solution is another phase.

Phase Transformation:

          A change in the number and/or character of the phases that constitute the microstructure of an alloy.

GIBB’S PHASE RULE OR THE PHASE RULE OR CONDENSED PHASE RULE

          This expresses mathematically the general relationships for the existence of stable phases corresponding to the equilibrium conditions (external conditions like temperature and pressure). It enables us to predict and check the processes that occur in alloys during heating or cooling. Using this rule, it is possible to determine whether the solidification process takes place at a constant temperature or within a certain temperature interval; it can also indicate the number of phases that can exist simultaneously in a system.

          The phase rule enunciated by J.W. Gibbs. The phase rule establishes the relationship between the number of phases P, number of components C, and number of degrees of freedom F. It is expresses mathematically as a simple form:

                                      P + F = C + n

(1)   In studying chemical equilibrium, temperature and pressure are regarded as external factors determining the state of the system so, 

P + F = C + 2                                               (1)

n = number of external factors = 2 (temperature and pressure)

          In applying the phase rule to metal systems the effect of pressure is neglected, leaving only one variable factor, -- temperature. Equation (1) reduces to

F = C + 1 - P                                              1(a)

          The number of degrees of freedom is essentially the number of independent variables, both internal (composition and phases) and external ones (temperature, pressure, concentration etc.,), which can be changed without changing the number of phases in equilibrium. The number of independent variables cannot be more than the number of variables, i.e., 

F = C - P + 2 £ P(C-1) + 2                     (2)

          Where P(C - 1) denotes the total number of compositional variables when P phases are there in the system. Including the two external variables (pressure and temperature), the total number of variables is P(C - 1) + 2 .

          In equilibrium all factors have definite values, hence the degree of freedom cannot be less than zero, C - P + 1 ³0

                                       then P £ C + 1

Obviously, the number of phases in a system cannot exceed the number of components plus one.

          The components of a system may be elements, ions or compounds. The components refer to the independent chemical species that comprises the system. In the ice-stream system, the component is H2O, in the Cu-Ni system the components are the elements Cu and Ni, whereas in the Al2O3-Cr2O3 system, one can take the two oxides to be components, nut it may be convenient to choose Fe and Fe3C (Iron carbide) as  the components.

          As follows from the phase rule, the number of phases existing simultaneously in a binary system cannot be more than three. These three phases can only exist at a definite phase composition and a definite temperature. If the number of phases in a binary system turns out to be more than three, this means that either the alloy is not in the equilibrium state or the number of phases and constituents has been determined incorrectly. In a ternary system no more than four phases may be in equilibrium. When only one phase is present in a system, the degree of freedom are equal to the total variable, with the increase in number of phases, the degree of freedom decrease. The degree of freedom cannot be less than zero. Obviously, we have an upper limit to the number of phases that can exist in equilibrium in a given system.

          At solidification temperature, a pure metal is a one-component system consisting of two phases of identical composition.

                                        F = 1 + 2 - 2 = 0

          Clearly, the number of phases and number of degrees of freedom equals zero, i.e., F = 0. This is known as non-variant equilibrium. When the number of phases is less than the maximum possible number by one, the number of degrees of freedom will also increase by one (F = 1). Such type of system is called as monovariant.

          An alloy of two metals is a two-phase and two-component system at solidification, F = 1. When F = 2, the system is said to be a divarient. Obviously, a system may be in equilibrium at different temperatures and concentrations.

          One can represent all transformations occurring in alloys and depending on temperature and concentration (C = 2) by equilibrium diagrams. The equilibrium diagrams are plotted with concentration as the abscissa and temperature as ordinate.