alloys are used to optimize phase compositions, sizes, and distribution of phases.
Figure 2.19 Schematic of primary and secondary α‐phases within a β‐phase matrix in Ti alloys.
Titanium alloy Ti‐6Al‐2Sn‐4Zr‐6Mo (or Ti‐6246) is a solution‐strengthened heat‐treatable α‐β alloy with higher β‐stabilizer content. It was introduced in 1966 (Eylon et al. 1984), but has more recently been optimized and designed for use as forgings in the intermediate temperature sections of gas turbine engines, particularly in compressor disks, impellers, and fan blades (Wood and Favor 1972; Boyer et al. 1994). The morphology of the alloy can be changed by thermomechanical processing (Uginet 1994). Ti‐6246 processing routes have been shown to generate three different types of microstructures: a β‐annealed (“lamellar”) microstructure, an α‐β‐recrystallized microstructure (“bimodal”), and a through β‐transus‐processed (“necklace”) microstructure (Sauer and Lütjering 2001). The as‐received microstructure commonly used is a lamellar structure containing primary‐α (grains) at prior β‐grain triplets and secondary‐α (lamellae of α) in a transformed β‐matrix (Figure 2.19). It can consist of acicular (needle‐like) or platelet microstructures. The grain boundaries and platelet boundaries in materials with acicular microstructure contain thin layers of softer β‐phase (BCC). The mechanical properties of this alloy depend primarily on the colony size of secondary‐α lamellae. Recently, novel friction stir processing has been used to optimize the microstructure to reduce primary grains and produce a very fine structure with acicular orthorhombic or α″‐laths with nanoprecipitates within the laths, which has promising strength characteristics at least under low heat input (Dutt et al. 2019).
2.8.4 Mechanical Metallurgy
The field of mechanical metallurgy deals with the behavior and response of metals and alloys to applied forces. This specific area within metallurgy delves into the structural responses to mechanical processes, including rolling, extruding, deep drawing, and bending. Dieter (1961) presented an excellent introductory textbook on mechanical metallurgy as part of the metallurgy and metallurgical engineering series.
Mechanical metallurgy is important because the structural response to mechanical forces can cause a huge impact on macroscopic properties. Processes, such as rolling and extruding, can impart strong texture within a polycrystalline material. This preferred orientation can lead to anisotropic properties on the macroscale. Understanding the effects of forming processes on final component properties is increasingly being recognized as a tool to obtain properties of interest, as well as avoiding unwanted impacts.
When dealing with alloys at high temperatures, an additional point to consider is the definition of the melting point. For pure elements, T m can be determined precisely, but complications arise for engineering materials with complex compositions for which the melting point cannot be defined well. For metallic alloys, liquidous point – the temperature at which an alloy is completely melted – is convenient to use.
2.9 Classification of Solids Based on Mechanical Response at High Temperatures
At low homologous temperatures, “elastic” response dominates the deformation in most crystalline solids. Very little inelastic strain occurs during load application time until the stress reaches a limiting level called yield strength or after a reasonably long time of sustained load. If a polycrystalline specimen is uniformly loaded to a uniaxial tension or compression, the specimen deforms elastically to a limiting strain, known as elastic limit. Simplest assumption is that all the grains also suffer the same strain as that of the specimen as a whole. Most solids, with very low porosities, exhibit linear elastic behavior. Unless specifically mentioned, linear elastic response is assumed to be the norm for all solids of engineering importance. Customarily, the porosity is assumed to be very low. However, the compaction of materials with high porosity subjected to load results in nonlinear response. Porous materials, such as firebricks and ceramic foams (used as insulations and protective covers), exhibit stress‐wise nonlinear elastic response, but we will not address such materials in this book.
With increase in temperature, two other mechanisms start to contribute to the deformation: “delayed elastic” and “viscous.” The delayed elastic response may also be called “anelastic,” but we will refrain from using it as per the suggestion of British Standard Institution (1975). Mechanical response of materials at high temperatures can be described as “Elasto – Delayed‐Elastic – Viscous” or simply response (see Chapter 5). Irrespective of loading conditions, monotonic or cyclic, total strain can be described generally as:
Equation (2.3) is a descripted representation of the EDEV model. EDEV response can be divided into three distinct types, depending on stress dependence of delayed elastic and viscous characteristics. Solid materials can be classified broadly into three distinct groups based on phenomenological deformation mechanism. This trinity of classification can be described in a simple manner only if the elastic response is assumed to be independent of stress. Most engineering materials fall into this category. However, porous materials exhibit nonlinear elastic behavior as mentioned earlier. Nonetheless, the room is open for further subdivision to take into account, if necessary, materials with high porosity exhibiting nonlinear elastic response.
Trinity of High‐Temperature Deformation Mechanisms
EDEV‐1
EDEV‐2
EDEV‐3
EDEV‐1: Materials exhibiting “linear elastic, linear delayed elastic, and linear viscous” behavior. Ordinary soda‐lime–silica glass used as structural materials in buildings and cars belongs to this class.
EDEV‐2: Materials exhibiting “linear elastic, linear delayed elastic, but nonlinear viscous” response. As an example, natural water ice (H2O) on Earth's surface belongs to this class.
EDEV‐3: Materials exhibiting “linear elastic, nonlinear delayed elastic, and nonlinear viscous” response. Most metals and alloys belong to this class of materials.
It should be noted that, in this book, we will use the term “viscoelasticity” in a general sense to refer to time‐dependent mechanical response, irrespective of linear or nonlinear dependence of flow on stress.
There is confusion as to the use of the term “viscoelastic.” Traditionally, this term is often used without paying any critical attention to the details on the mechanisms involved in producing the inelastic (other than linear elastic) deformation. Consequently, terms such as “viscoplastic” appeared frequently in the engineering literature. The classical viscoelastic material (obeying linear elastic, linear delayed elastic, and linear viscous response) is equivalent to EDEV‐1 type of material. Naturally, the term “viscoelasticity” is loosely used without any clarifications to the stress dependence of delayed elastic and viscous components
