Temperature strengthening mechanisms

Table 2. Strengthening Mechanisms in High Temperature Alloys...

Because pure aluminum is n picaUy too soft to be drawn into a fine wine, it is often alloyed with 1° o sihcon or 1° o magnesium to provide a sofid solution-strengthening mechanism. The resistance of Al-1° o Mg wine to fatigue failure and to degradation of ultimate strength after exposure to elevated temperatures is superior to that of Al—1° o Si wine. 

To strengthen mechanical properties and vary the transition temperature, hydrophobic monomers such as BMA were introduced into poly(IPAAm). The unique swelling properties with varying BMA composition are shown in Figure 11. 

TABLE 2. STRENGTHENING MECHANISMS IN HIGH TEMPERATURE ALLOYS... 

The active strengthening mechanism at room temperature of the studied material was identified as solid solution strengthening, grain boundary strengthening, and Al3Zr precipitate strengthening. 

The flexure bar subjected to 1200°C under an applied stress of 200 MPa failed after 58 hours. The failure was completely brittle, since there was no fiber-pull out and the crack propagated in a perpendicular manner through the entire bar. This brittle failure indicates that at this temperature there is a strengthening of the fiber/matrix bond which reduces the likelihood of fiber pull-out or crack bridging as possible toughening/strengthening mechanisms. 

It was interesting to note that the specimens which survived the 100-hour exposure time fractured at stress levels higher than the fast-fractured tensile strength measured at 1400 C. It is believed that the improved tensile strength is due to some strengthening mechanism which occurs as a result of the tensile stress and temperature exposure of the specimen in air for 100 hours (surface flaw healing, etc.). 

The term superalloy is used for a group of nickel-, iron-nickel-, and cobalt-based high-temperature materials for applications at temperatures > 540 °C. It is useful to compare the main subgroups in terms of the strengthening mechanisms applied and stress-rupture characteristics achieved, as shown in Fig. 3.1-127. In this section iron-nickel- and nickel-based superalloys are covered whereas cobalt-based superalloys are dealt with in Sect. 3.1.6.3. Nickel-based superalloys are among the most complex metallic materials with numerous alloying elements serving particular functions, as briefly outlined here. 

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