Superplasticity of Alloys, Intermetallides and Ceramics

Progress in materials science depends to a great extent on new data on the relationship between materials’ structure and their properties. This conventional approach might seem rather unpromising, but the latest studies have confirmed that this old formula is still valid.

There are three conditions required for transforming a material into a superplastic (SP) state, viz., the presence of a stable microcrystalline structure (d < 10–15 μm), a deformation temperature exceeding 0.4 T _m , and a specific range of strain rates (usually from 10⁻⁴ to 10⁻¹ s⁻¹). In virtually all alloys the development of the SP effect has common features because of which it can be treated as a peculiar type of plastic deformation that is characterized by the specific phenomenology. Recent studies show that typical features of superplasticity can be observed not only in metals, but in intermetallic compounds and ceramics as well.

Numerous hypotheses to explain superplastic behaviour of materials have been proposed [1.1–9, 2.1]. The earlier theories presumed that the phenomenon is unique to only certain alloys and related to their specific features. However, superplasticity is a universal state of metallics that appears during deformation of microcrystalline materials within a certain range of temperature and strain rate. The similarity of the observed phenomena and the generally observed features of the structural changes occurring in various superplastic materials indicate the universal nature of the effect. However, a universal theory has not yet been developed. It is difficult to develop such a generalized theory due to a) the difficulties involved in studying the structure of the microcrystalline materials when the structural changes are not large; b) simultaneous operation of several deformation mechanisms during SP flow make it necessary to separate their individual contributions to the total deformation and mutual interaction; and c) although grain boundaries are known to play an important role in the deformation of fine-grained superplastic materials, the structure of these defective regions and the nature of the grain boundary processes, such as sliding and migration, are not completely clear.

SP deformation is a result of the macroscopic influence of grain boundaries on the properties of polycrystalline materials. Hence, it can be assumed that the refining of microstructure is accompanied by both quantitative and, probably, qualitative changes in the structure of these materials.

Magnesium alloys are distinguished by their greater stiffness and specific strength as compared to other metallic materials. Due to these characteristics, magnesium and its alloys can withstand higher impact loads than aluminium alloys. Consequently, magnesium alloys can be efficiently used in instruments and components which are subjected to such loads. The high chemical resistance of magnesium and its alloys to fuels, mineral oils, and alkalis and their characteristic physical properties such as low neutron absorption capacity and good thermal conductivity make them rather promising for applications in various instrumental components.

Superplasticity of aluminium alloys was first studied in the Al-33%Cu eutectic alloy [5.1]. After an appropriate treatment, i.e., hot processing, the alloy acquires a microduplex structure and a relative elongation of 2000% at 500°C. Later, superplasticity was observed in other eutectic and near-eutectic aluminium-based alloys, viz., Al-Si, Al-Cu-Mg, etc. [5.1]¹.

Among the hcp metals, polycrystalline titanium has the highest ductility. This is because slip along the prismatic and the pyramidal planes is possible in α-titanium, whereas in other hcp metals it occurs essentially along the basal planes. In addition, twinning plays an important role during the plastic deformation of α-titanium [6.1].

The ductility of steels can be increased dramatically by transforming them into the superplastic state. Due to polymorphic transformations in iron and a strong dependence of the microstructure and phase constitution of these alloys on the quantity of the alloying elements, additional possibilities for the refinement and stabilization of the microstructure arise. In the temperature range 0.4–0.7 T _m (which is characteristic of SP-deformation) in steels it is possible to obtain: (i) a matrix-type microstructure based on the α- or γ-phases and carbides (inter-metallics); (ii) a microduplex-type structure based on the α- and γ-phases; (iii) a mixture of the α- and γ-phases and carbides (intermetallics). A second phase stabilizes the microstructure and promotes SP flow. To analyze the conditions required for SP flow in various phase fields, it is most convenient to start with the iron-carbon alloys. Chemical compositions of the iron-based alloys which will be discussed and parameters of SP deformation are given in Table 7.1.

Nickel-based alloys are one of the most extensively used groups of heat resistent materials (superalloys) and are frequently found in engines operating at elevated temperatures up to 0.8 T _m . The practical significance of nickel-based alloys motivated intense studies on their structure and properties [8.1, 2]. These alloys consist of several structural constituents. The matrix of the alloy (γ-phase) is a nickel-based fcc solid solution containing a large fraction of an element that is soluble in nickel (cobalt, chromium, molybdenum or tungsten). The intermetallic γ′-phase, Ni₃(Al,Ti), having an ordered fcc structure, is usually the strengthening constituent. The coherence between the lattices of the γ and γ′ phases and the closeness of the lattice parameters (the mismatch between the parameters is less than 1%) create the possibility of developing phase a interface with a low surface energy. This situation imparts a high degree of stability to the γ′-phase.

Intermetallic compounds are superior to conventional metallic materials primarily because of their high-temperature strength parameters. The enhancement of these properties in the transition from metals to intermetallics is due to the change of the interatomic bonding as well as to a more complicated crystal structure. For example, in intermetallics, strong covalent bonds between the atoms provide a higher cohesive strength [9.1–5]. The more complicated crystal structure results in the growth of the elementary cell size, reduction of the symmetry and growth of the Burgers vector, which in turn lead to an increase in the Peierls stress and limit the number of active slip systems. These are the reasons why the majority of intermetallics are low-plasticity materials. In ordered intermetallic compounds, deformation is accomplished through the motion of superdislocations consisting of superpartial dislocations separated by a planar defect, i.e., an antiphase boundary [9.6]. Contrary to metals and disordered alloys where the slip plane is determined by the value of the Peierls forces or by the possibility of crystallographic splitting of the slipping dislocations, another criterion determines the slip plane choice. This is the energy of an antiphase boundary created during the motion of superdislocations in this plane. The anisotropy of the antiphase boundary energy can impede the process of cross slip and even slow down the motion of superdislocations [9.1–3,5]. These features provide for the enhanced high-temperature strength and heat and corrosion resistance of intermetallics and make their application as high-temperature structural materials rather promising.

Ceramics possess a number of unique properties, such as high-temperature strength, wear resistance and hardness, enhanced resistance to corrosion, and other species-specific physical and mechanical properties. These are determined by their electronic, atomic, micro- and macrostructure. In the electronic structure two types of bonds dominate — covalent and ionic. Such compounds can be semiconductors and dielectrics, have enhanced hardness and high elasticity moduli, stable mechanical properties in a broad range of temperatures, and low coefficient of thermal expansion. However, it is the nature of the bonding that also determines the principal limitation of ceramics, i.e., their absolute brittleness. When a load is applied, the bonding force hinders the motion of dislocations because of the high value of the potential Peierls-Nabarro barriers. Hence, under conventional conditions, SP deformation in ceramics is practically impossible. The brittleness of ceramics is caused by microscopic defects such as voids and inclusions that, being strong concentrators of stresses, become sources of cracking. Since the plasticity of ceramics is zero, no relaxation of stresses occurs in such places and under a load exceeding a certain threshold value the material immediately fails. Although ceramic materials possess many useful properties, the lack of plasticity considerably limits their application as construction materials.