Assignment Engineering Materials (Microstructures of Alloys )

BY :

Mirza Humza Begg

Me131056

Me-4A

ALLOYS OF IRON CARBON (STEEL):ASTM A470:

An example of a low alloy steel that is used for its high-temperature properties is ASTM A470 turbine rotor steel. These steels are used in steam turbines for electric power generation and usually contain combinations of nickel, chromium, molybdenum, and/or vanadium. The microstructure of ASTM A470 rotor steel consisting of tempered upper bainite is shown in the figure given below.

AISI/SAE 1040:

The figure given below shows the microstructure of a typical medium-carbon steel (AISI/SAE 1040) showing ferrite grains (white etching constituent) and pearlite (dark etching constituent).

Mechanical properties of medium-carbon steels can be increased by heat treatment (by austenitizing or quenching & tempering). The rule of thumb is that 0.30 percent is the lower limit for hardening steel by heat treatment. However, plain carbon steels can be hardened only in thin sections with rapid quenching. Often there is distortion and cracking on quenching. They have poor impact resistance at low temperatures. To improve heat treating capabilities alloying elements like Cr, Ni, Mo, etc. are added to carbon steels and they are called alloy steels.

With 0.45 to 0.75 percent carbon, steels can be challenging to weld. Preheating, post heating (to control cooling rate), and sometimes even heating during welding become necessary to

produce acceptable welds and to control the mechanical properties of the steel after welding.

ALLOYS OF ALUMINUM:

Aluminum 6063:

In the below picture we can observe that in the microstructure there are numerous amount of grain boundaries. This tells us that the cracks will easily propagate and because of this it will not have high tensile strength but plasticity will be there due to high density.It can also be seen that there is dislocation because there are large numbers of dark black lines which tells that at high temperature the grain will grow and hence easily form a crack in the structure causing the material to fail.

ALUMINUM ALLOY 7075-T651:

From the picture of 7075 Aluminum alloy we can observe that the grain size is quite small. That is the reason it has high strength. If we compare it to 6061 Alloy we can see that these grains are indeed very small compared to 6061. Also there are very few dark lines which indicate that dislocation is very minimal but as a matter of fact it does not have very small grains so it is also not good for elevated temperatures.

Magnesium Alloys:

MAGNESIUM ALLOY Mg8Li:

Mg8Li alloy reinforced by 7 vol.% SiC particles was processed by a powder metallurgical method. Samples were deformed in tension and compression at temperatures from room temperature up to 300 °C. The yield stress as well as the maximum stress decrease with increasing temperature. Decreasing stresses detected at temperatures higher than 150 °C indicate possible

presence of recovery process. Estimated activation enthalpy is close to the activation enthalpy for the grain boundary sliding. Strain rate sensitivity was estimated at elevated temperatures. Enhanced plasticity was estimated at 300 °C. Light and scanning electron microscopy revealed the cavitation during the high temperature deformation. Li is soluble in hcp α-phase up to 4 wt. %, while Mg alloyed with greater than 12 wt. % Li has a bcc structure (β-phase). Ductility of the hcp α-phase are worse in comparison with the bcc alloys that are very good machinable and weld able. Disadvantages of Mg-Li alloys with bcc structure are a high chemical activity and poor corrosion resistivity. Some compromise would be an alloy with 8 wt. % of Li (a mixture of phases α+β) that might exhibit both improved mechanical properties as well as a good corrosion resistance. In light micrograph, light α-phase and darker β-phase may be visible. The alloys were produced by pressure infiltration under an argon pressure (up to 6 MPa) at temperatures of 615-635 K. Among Mg alloys, magnesium-lithium alloys, as the lightest metallic materials, are attractive for a large amount of applications. They are of great importance also for medicine purposes. The density of Mg-Li alloy decreases with an increase of lithium content. The addition of Li increases ductility.

Microstructure of the Mg-Al-Sr alloys(AJ91):

Special industrial applications require improvement of the high temperature properties. For these elevated temperature applications, alloys containing rare earth elements have been developed. New Mg-Al-Sr alloys are being developed with the aim to find cast alloys withgood creep resistance and good strength and replace expensive rare earth alloying elements with some cheaper one. Pekguleryuz [14] reported that Mg-Al-Sr alloys show different microstructures based on the Sr/Al ratio. For Sr/Al ratio below about 0.3, only Al4Sr interme‐tallic phase is present as the second phase in the structure. When the Sr/Al ratio is higher, a second intermetallic phase, a new, ternary Mg-Al-Sr compound, is observed. When the Sr/Al ratio is very low, there is insufficient amount of Sr to bind all Al atoms and the excess Al wouldform the Mg17Al12 phase. Figure below shows light micrograph of the squeeze cast AJ51 alloy. The primary Mg grains are surrounded by the interconnected network of the grain boundaryphase. This phase is formed during solidification process

and it has lamellar type morphology.The γ phase (Mg17Al12) is accompanied with Al4 Sr phase.