What is Ductility?
Ductility is a capacity of a material to deform permanently (e.g., stretch, bend, or spread) in response to stress. Most common steels, for example, are quite ductile and hence can accommodate local stress concentrations.
Brittle materials, such as glass, cannot accommodate concentrations of stress because they lack ductility, and therefore fracture easily. When a material specimen is stressed, it deforms elastically (see elasticity) at first; above a certain deformation, called the elastic limit, deformation becomes permanent.
In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure.
Ductility is an important consideration in engineering and manufacturing, defining a material’s suitability for certain manufacturing operations (such as cold working) and its capacity to absorb mechanical overload. Materials that are generally described as ductile include gold and copper.
Malleability, a similar mechanical property, is characterized by a material’s ability to deform plastically without failure under compressive stress. Historically, materials were considered malleable if they were amenable to forming by hammering or rolling. Lead is an example of a material that is relatively malleable but not ductile.
Most metals are good examples of ductile materials, including gold, silver, copper, erbium, terbium, samarium aluminum, and steel have high ductility. Examples of metals that are not very ductile include tungsten and high-carbon steel. Nonmetals are not generally ductile.
How to Measure Ductility
Ductility is the ability of a metal to deform without fracturing. Metals that can be formed or pressed into another shape without any fracturing are considered to be ductile. Metals that fracture are classified as brittle (essentially the opposite of ductile).
Ductility plays a major role in formability. Metals that are excessively brittle may not be able to be formed successfully. For example, if a piece of metal is stretched into a thin wire, it is imperative that it has some ductility.
If the metal is too brittle, it will fracture as soon as the metal begins to stretch. Ductility is also a major safety consideration for structural projects. Ductility allows structures to bend and deform to some extent without rupturing when placed under heavy loads.
Percent elongation and percentage reduction are two ways to measure ductility:
- Percentage elongation measures the length that a metal deforms as a percentage of its original length, after it is pulled to failure during a tensile test.
- Percent reduction measures the narrowest part of the cross-section of a metal specimen following a tensile test-induced rupture.
Ductility can be dependent on temperature, so the temperatures the metal will be subjected to in an application should be taken into account. Most metals have a ductile-brittle transition temperature chart which can assist.
Which Metals Are Ductile?
There are many ductile metals, including:
- Low carbon steel
Metals that are considered brittle include cast iron, chromium, and tungsten. Examples of applications that require high ductility include metal cables, stampings, and structural beams.
Gold is extremely ductile. It can be drawn into a monatomic wire, and then stretched more before it breaks.
Ductility is especially important in metalworking, as materials that crack, break or shatter under stress cannot be manipulated using metal-forming processes such as hammering, rolling, drawing, or extruding. Malleable materials can be formed cold using stamping or pressing, whereas brittle materials may be cast or thermoformed.
High degrees of ductility occur due to metallic bonds, which are found predominantly in metals; this leads to the common perception that metals are ductile in general. In metallic bonds, valence shell electrons are delocalized and shared between many atoms.
The delocalized electrons allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.
The ductility of steel varies depending on the alloying constituents. Increasing the levels of carbon decreases ductility. Many plastics and amorphous solids, such as Play-Doh, are also malleable. The most ductile metal is platinum and the most malleable metal is gold.
When highly stretched, such metals distort via formation, reorientation, and migration of dislocations and crystal twins without noticeable hardening.
Factors that Affect Ductility of Metals:
Ductility is affected by intrinsic factors like composition, grain size, cell structure, etc., as well as by external factors like hydrostatic pressure, temperature, plastic deformation already suffered, etc.
Some important observations about ductility are given below:
- Metals with FCC and BCC crystal structure show higher ductility at high temperatures compared to those with HCP crystal structure.
- Grain size has a significant influence on ductility. Many alloys show super-plastic behavior when the grain size is very small in the order of few microns.
- Steels with higher oxygen content show low ductility.
- In some alloy’s impurities even in very small percentages have a significant effect on ductility. The ductility of carbon steels containing sulfur impurity as small as 0.018%, drastically decreases ductility at around 1040°C. This can however be remedied if Mn content is high. In fact, the ratio Mn/S is the factor that can alter the ductility of carbon steels at 1040°C. With the value of this ratio at 2, the percent elongation is only 12-15% at 1040°C while with a ratio of 14 it is 110 percent.
- Temperature is a major factor that influences ductility and hence formability. In general, it increases ductility, however, ductility may decrease at certain temperatures due to phase transformation and microstructural changes brought about by an increase in temperature. The effect of temperature on the ductility of stainless steel. It has low ductility at 1050°C and a maximum at 1350°C. Therefore, it has a very narrow hot working range.
- Hydrostatic pressure increases ductility. This observation was first made by Bridgeman. In torsion tests, the length of the specimen decreases with an increase in torsion. If the specimen is subjected to axial compressive stress in the torsion test it shows higher ductility than when there is no axial stress. If tensile axial stress is applied the ductility decreases still further.