Frontiers in Aerospace Engineering, Vol. 4 No. 1‐May 2015 1 2325‐6796/15/01 001‐13, © 2015 DEStech Publications, Inc. doi: 10.12783/fae.2015.0401.01 Strain Hardening in Aerospace Alloys R. K. Gupta 1 *, Christy Mathew 2 , P. Ramkumar 1 1 Materials and Mechanical Entity, Vikram Sarabhai Space Centre, Trivandrum‐22 2 Mar Athanasius College of Engineering, Kothamangalam‐686666 *rohitkumar_gupta@vssc.gov.in Abstract Strain hardening is one of the important strengthening mechanisms, which plays significant role in processing and application of metals and alloys. For non‐heat treatable alloys, it becomes more important. Its effect is different in different metals and alloys and accordingly specific process and application regime are selected. A large variety of metals and alloys from the family of light alloys (Al, Ti based), high strength steels and high temperature alloys (Co, Ni and Nb based) are used in aerospace systems. This paper analyses importance of strain hardening phenomenon in these alloys. Attempts are made to explain the differential behaviour of various alloys in governing the tensile to yield strength ratio along with % elongation. Role of temperature in this behaviour is also included. Keywords Strain Hardening; Aerospace Alloys; Ti6Al4V; Maraging Steel; Al Alloys Introduction Strain hardening or work hardening is one of the most commonly used means of improving strength of an alloy. In a simple way it is the use of permanent deformation to increase the strength of the metal. It is either denoted by tempers (as half hard, full hard, spring temper, etc.) in case of steels or by % cold work in case of light metals/ alloys (aluminium/ titanium). Strain hardening in metals, i.e. the capacity of the material where flow stress increases with increasing plastic strain, is being studied since the discovery of dislocations, and is still a matter of current interest. Understanding the strain‐hardening capability of structural alloys is of practical importance also since it controls their fracture properties and deformability. Strain hardening is an important industrial process that is used to harden metals or alloys that do not respond to heat treatment. For alloys strengthened by solid‐solution addition, rate of strain hardening may be either increased or decreased as compared to pure metals. Dieter explained that the final strength of a cold worked solid solution alloy is mostly greater than that of pure metal cold worked to the same extent at the same temperature. Increasing temperatures reduces strain hardening and accordingly strength. The rate of strain hardening can be assessed from the slope of the true stress‐true strain graph (flow curve). Generally, the rate of strain hardening is lower for hcp metals than for cubic metals. Increase in strength can be seen from schematic Fig. 1 explaining strain hardening phenomenon. It also explains that material shows increase in strength in unloading and reloading. The degree of cold working determines the strength of a metal. As the amount of cold work increases, so does the strength. However, the total elongation also changes with amount of cold work. Harder tempers like spring and super spring have high strength and low ductility (bad formability), whereas softer tempers like annealed and 1/4 hard have low strength and high ductility (very good formability). When choosing a material for specific application, it is best to use the highest strength material that still meets the formability requirements of the design. As the percentage cold work increases, there is a diminishing increase (increase with lower rate) on strength. Generally it is seen that increase in strength with increase in cold work results in lowering of % elongation. This is due to the fact that at higher amount of reductions, there are fewer free dislocations to become entangled. Since the material is less able to plastically deform, fracture becomes much more likely. At high levels of cold work, the material becomes very difficult to further process or form. If it must be formed, or reduced further in thickness, then annealing becomes necessary. Extent of reduction/ changes in ductility due to cold work is different for