Phase Transformations in Alloys
The metallurgical phases present in an alloy have a huge impact on the properties of a metal component. Phases are distinct materials that are comprised of the elements in the alloy. These distinct materials have distinct properties that have an impact on the overall properties of the entire alloy. Additionally, the size, shape, and location of the phases within the alloy also effect on the overall properties of an alloy.
Within many common alloys it is possible to alter the phases present with heat treatment. Forming one or more phases from a different phase is called a phase transformation. Phase transformations occur when in an alloy is heated or during cooling from an elevated temperature.
There are several types of phase transformations that can occur when an alloy is cooled from an elevated temperature. The type of transformation that can occur depends on the specific alloy. Not all types of transformations occur in all alloys, and in some alloys no transformations are possible, other than the solid-liquid transformation.
Two of the most common phase transformations encountered with common alloys are eutectoid and precipitation. For both types of transformation, the transformation involves the movement of atoms through the metal to rearrange themselves to form the new phase or phases.
A eutectoid transformation involves a change from a single phase to two other phases when the initial phase is cooled form an elevated temperature. The most common alloy in which this transformation is encountered is steel. The transformation occurs when steel is cooled from the austentizing temperature. During slow to moderate cooling, the austenite transforms to ferrite and cementite. The microstructure consists of cementite plates with ferrite between the plates. This is commonly referred to as pearlite. A micrograph of a steel alloy with 0.6% carbon is shown here.
During faster cooling of some alloys, the ferrite forms in the shape of needles or plates and the cementite forms as particles. This structure is referred to as bainite. A micrograph of a steel with bainite is shown in Figure 2.
The reverse transformation occurs when steel with ferrite and cementite is heated. When the temperature is high enough, the ferrite and cementite transform to austenite. So, the austenite to ferrite + cementite phase transformation is reversible, and repeatable.
Precipitation transformations involve the formation of particles of one phase within an already existing phase. These particles are called precipitates. This transformation occurs when an alloy is cooled from an elevated temperature. At the elevated temperature the phase present consists of the main element in the alloy with the alloying elements in solid solution. When the alloy is cooled the solid solution is not able to hold all the atoms of the alloying elements in solution, so precipitates form that consist of the solute atoms and possibly the atoms of the main element in the alloy.
For engineered metal components, precipitation during cooling is undesirable because of the resulting size and location of the precipitates. So, the process is modified by first quenching the alloy to room temperature to suppress the atom motion. Then the alloy is either allowed to transformation at room temperature, if room temperature transformation is possible, or the alloy is reheated to an intermediate temperature to speed up the transformation. An example of a common alloy system in which precipitation is used is the aluminum-copper system. This figure shows a micrograph of Al2Cu precipitates in an aluminum matrix.
The precipitation transformation occurs in a number of alloys including aluminum alloys (Al-Cu, Al-Mg-Si, Al-Zn-Mg, and Al-Zn-Mg-Cu), precipitation hardened steels (e.g. 17-4 PH, 15-5 PH, and 13-8), some copper alloys (Cu-Be and Cu-Cr), and Zn-Al alloys.
Regardless of the particular transformation, control of the heating temperature, heating time, cooling rate, and, if necessary, reheating temperature and time are all important factors for controlling whether the desired transformation is complete and the shape, size, and location of the phases that form. These in turn have a big impact on the properties of a metal component. The relationship between heat treating process conditions, final microstructure, and properties is discussed in our Metallurgy of Steel, Metallurgy of Steel Heat Treating, and Precipitation Strengthening courses.