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Mechanical and Technological Properties of Metals

Example of Metal

A picture of metal which illustrates its mechanical strength

A picture of metal which illustrates its mechanical strength

Introduction

What are the mechanical and technological properties of metals? Metals are of great importance to human race even before the birth of Christ (BC). Their great importance is the reason why there is multiplication of metallic industries in every year. Steel, which is the most used metal alloy, in many industries have been playing lots of good roles in many departments of technological developments.


Their applications (metals) can be found in medical, aerospace, agricultural, electronics, automobile, building, and even in educational departments of the world. In medical sector, most medical equipments are products of metal. One which is not farfetched is the needle which is attached with the syringe for injection purposes. Most thermometers, microscopes, inlays, and other instruments used by doctors and their co-health workers are made of metal. Almost all the parts of aircrafts are made of metals. Agricultural and electronics machines cannot work efficiently without metals having their functions to perform in one or two areas. The same thing appears in automobile, building and also in education sectors.


Before any metal is used for any important application, there are certain properties they must possess. These properties that they are expected to posses in other to function properly in any areas of their applications is termed mechanical properties of metals. In this article, both the mechanical and technological properties of metals are to be explained. The features (properties) make metals to have high strength, corrosion resistance, ability to withstand stress and others.



Mechanical Properties of Metals

It is very good to know the technological or physical properties of metals for research and other purposes.What are the mechanical properties of metals?

The mechanical or physical properties of metals are:

  • Hardness;
  • Hardenability;
  • Toughness;
  • Tenability;
  • Stiffness;
  • Creep; and
  • Endurance.


Hardness

It simply implies the ability of any material to resist indentation. Any metal with low hardness receives high indentation depth while that with higher hardness is likely to possess lesser indentation distance into the metal. Hardness can also be defined as the ability of a metal to resist notch by any object. It is the property of any metal that enables it to resist indentation.


Where can hardness be tested on any metal? Hardness can either be tested on either the surface or edge of any metal. It is a physical or mechanical property of metals.


The hardness or degree of hardness of any metal can be determined by the nature of heat treatment given to the metal. The hardness of a metal quenched in furnace cannot be the same when that metal is quenched in brine or air. Quenching is rapid cooling of hot metal in any medium. The medium in this context can be oil, air, furnace, or brine.


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What is or are the application of hardness of any metal? The hardness of metals determines suitable places the metals can be used. There is specific hardness required of metals that can be used for construction of bridges when compared with for metal used in construction of gathers for just six bedrooms flat located in any timid area where there is no attack by criminals.


Hardness of metals with low strength can be improved by alloying, strain-hardening and heat treatment processes. Almost all metals which are used for construction functions are not used in their pure state but as alloys. The reason for alloying, which is the mixture between a base element and other minors is to improve the mechanical strength of the metal. Strain-hardening process is also done to increase strengths of metals. It hinders dislocation movement in materials. In heat treatment, metals are subjected to high temperature and slowly or rapidly cooled to improve strength. Heat treatments include quenching, annealing, tempering, normalizing and others.


Hardenability

It is another mechanical property of metals, which is the depth to which metals can be hardened. According to William D. Callister, Jr, and David G. Rethwisch in their book entitled Materials Science and Engineering, they defined hardenability as the depth of hardness penetration.


There are factors that determine hardenability and also tests which is applied in determining the hardenability of materials. The tests used in determining hardenability of metals are called hardenability tests. These tests are Jominy end-quench test, hot brine test, and Grossman’s method. Among all these three, the easiest and most frequently used is the Jominy end-quench method. Again, factors that determine the hardenability of metals are amount of alloying elements, austenitising temperature and time, nature of coolants, criterion of hardenability, grains size, and amount of carbon content. Note that metals with very high hardenability can fracture by means of cracking is the metal is very brittle.


Toughness

It is the property of metal that enable it to absorb maximum energy before it fractures. The mechanical term, toughness can be used in many contexts. Before any structural material is produced, its toughness must be taken into consideration.


Toughness can also be defined as the ability of any material to resist fracture when cracks are noticed on the material. For notch toughness, it is tested by what is called impact test. What is impact test? Impact test or testing is a test done on materials to determine their fracture characteristics under high loading rates. Impact test is done because of inability of laboratory tensile test to correctly give accurate result. Tensile testing is conducted under low loading rates.


Note that toughness of any material can be determined using either laboratory tensile test or impact test. The kind of test that is employed is dependent on the strain rate of the material. It is important to have in mind that for practical purposes, toughness decreases with rise in temperature of the materials.


Tenability

It is a physical property of metals, which enables them to resist attack on metals by any substance. This property is very important to prevent corrosion in metallic materials. Care is taking when manufacturing materials to be used in corrosive environment. Corrosion resistant elements, like chromium, are used during manufacturing of steel product to reduce the materials susceptibility to corrosion.


Stiffness: What is stiffness? It is the property of metal by which it resists deformation. The word, stiffness, is used in most cases with strength of materials. As the stiffness of any material is increased, it strength is also increased. It is the mechanical properties of metals related to applied load or force.


If a material has a load of 2000kg loaded on it and it takes many years for the material to deform, the material is said to have high stiffness. Applying Young’s modulus:

E = σ/ϵ

The symbol, E, stands for the stiffness of any metal, and is known as the Young’s modulus. Young Modulus states that whenever stress is applied on elastic material, the material is forced to undergo plastic deformation and extends. As the stress is increased, the extension of the material increases. The symbol, ϵ, stands for the extension which is caused by the stress, σ.



Creep

It is the property of metal by virtue at which it deforms continuously under steady load. It is a long time deformation process. Creep occurs in metallic materials and it is dependent on temperature. Before creep is being discussed in detail, the metal must first be subjected to high temperature and under static mechanical stress.


Creep can also be defined as undesirable phenomenon in materials, which is time-dependent and permanent deformation of materials when subjected to a constant load or stress.


Creep becomes important for metals whose temperature is greater than 0.4 Tm. Tm stands for absolute melting temperature of the metal. Any material whose temperature is 0.4Tm and below does not really need its creep considered or its consideration is of no importance. Plastic and rubbers, which are amorphous polymers, are sensitive to creep.

Diagram explaining creep behaviour

Diagram explaining creep behaviour

Creep Characteristics or Behaviour

Typical creep curve of strain versus time at constant temperature is shown above. The minimum creep rate, Δϵ/ Δt, is the slope of the linear segment in the secondary region. Rapture lifetime, tr, is the total time to rupture. It is the total time taken for rapture to occur on metallic material subjected to static tensile stress at constant temperature. It is also called rupture lifetime. Creep test conducted to determine rupture time is called creep rupture tests. When a metallic material is subjected to constant temperature, the strains are obtained and then plotted against the individual time intervals. The test done to achieve the result is called constant-stress test.


After plotting of the strains against the time, the slope can be obtained, and there are three major divisions of the curve. The slope obtained from this experiment is called the minimum creep rate. The three regions of the creep curve are the primary, secondary, and tertiary strain-time regions.


The primary creep is also called the transient creep. It is the first region as shown in the curve. What happens at primary creep? In primary creep, there is continuous decreasing creep rate. It means that the slope of the curve at this region diminishes with time. The material undergoing creep experiences increase in creep resistance and strain hardening.


Secondary creep can also be called steady-state creep. This region experiences the longest creep in the curve and the creep rate at this point is constant. The plot of strain against the time at this region gives linear result. In secondary region, the material becomes more elastic.


The last of the three main regions is the tertiary creep. There is accelerated rate and ultimate failure of the metallic sample at this stage. The failure is termed rupture and it happens because the load acting on the material is much to the extent that the specimen could not withstand it again. The rupture can result from internal crack, grain boundary separation and even voids in the metallic structure.


The slope of the secondary creep is the most important parameter in creep test. This is identified in the strain-time creep curve. It is an engineering design parameter applicable in nuclear power plant.


What is the application of creep characteristics? Creep characteristics enables design engineers to find out the stability of any material use. In other words, the characteristic helps design engineers to know the best place where the material can be used.