Thermomechanical Processing Effects

In terms of practical use, one of the most important features of phase equilibria can often be the effect of composition on some critical temperature. This can be a liquidus or solidus or a solid-state transformation temperature, such as the /3-transus temperature, (T ), in a Ti alloy. The solidus value can be critical, as solution heat-treatment windows may be limited by incipient melting. In some materials a solid-state transformation temperature may be of prime importance. For example, in Ti alloys it may be specified that thermomechanical processing is performed at some well-defined temperature below the / -transus temperature. The CALPHAD route provides a method where such temperatures can be quickly and reliably calculated. 

Fig. 5. Effect of thermomechanical processing on fracture toughness of Fe-12Ni-0.5 A1 alloy at -196°C.

Thermomechanical processing and the addition of about 150 ppm carbon were additional effective methods of strengthening, while maintaining high toughness. 

C. L. Briant The effect of thermomechanical processing on the microstructure of tungsten rod, Proc. 13th Plansee Seminar, Vol. 1 (Plansee AG, Reutte 1993) p.321... 

Steam explosion is a thermomechanical process. At high pressure, steam penetrates to cellulose fiber through diffusion and when the pressure suddenly releases, creates shear force, hydrolysis of the glycosidic and hydrogen bonds and leads to formation of nanofibers [126]. In 1927, Mason introduced the steam explosion method to defibril-late wood to fiber for board production [127]. The effective parameters of this process are pressure, temperature and time of material being autoclaved. The steam explosion process can be used solely or in combination with other processes. For instance, cellulose nanofibers from banana at 20 lb pressure, 110-120°C, for 1 hour [16] and from pineapple at 20 lb pressure [128] were produced just using steam explosion. 

Variations in resistivity with temperature are complex functions of composition and thermomechanical processing (see figure). Note also the pronounced effect of the R-phase on resistivity. 

Electrical resistance vs temperature curves for a 71-50.6Ni (at.%) alloy that was thermomechanically treated asindicated. A Quenched from 1000 C (1830°F). B Quenched from 1000 C (1830 °F), aged at400 C (750 F). C Directly aged at400 C (750 F). Tr is the transition temperature from austenite to the rhombohedral R phase. Tr is the shifted transition temperature from processing effects. Arbitrary units for electrical resistance. 

Previous thermogravimetric analysis had established that under such processing conditions no appreciable thermomechanical degradative effects were detected. To perform tensile and fracture tests the sheets were successively cut by a mill. 

Thermal or thermomechanical processes (e.g. extrusion or hot press molding) are used to form materials under low moisture conditions. The glass transition temperature (Tg)—because of their hydrophilic nature (which varies between proteins)—is highly affected by moisture (160-200°C decrease in Tg in the dry state and around 60-100 C for material with 10% moisture content). In practice, when protein materials contain about 15% water (i.e. which generally occurs when they are at equilibrium with 85% relative humidity at ambient temperature), their Tg is close to the ambient temperature. This effect is even more obvious in the presence of plasticizers. 

The microanalytical methods of differential thermal analysis, differential scanning calorimetry, accelerating rate calorimetry, and thermomechanical analysis provide important information about chemical kinetics and thermodynamics but do not provide information about large-scale effects. Although a number of techniques are available for kinetics and heat-of-reaction analysis, a major advantage to heat flow calorimetry is that it better simulates the effects of real process conditions, such as degree of mixing or heat transfer coefficients. 

As the loading rate increases, thermal effects need to be accounted for and the analysis is extended to a coupled thermomechanical framework. Evidence of a temperature effect in glassy polymer fracture is found (e.g., in [2,3]) with a temperature increase beyond the glass transition temperature Tg. The influence of thermal effects on the fracture process is also reported. 

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