What Type of Material Undergoes Smooth Continuous Plastic Deformation

Plastic Deformation

Plastic deformation is the permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength and cause it to elongate, compress, buckle, bend, or twist.

From: Materials Enabled Designs , 2009

Introduction

F. Abe , in Creep-Resistant Steels, 2008

1.1 Definition of creep

Plastic deformation is irreversible and it consists of time-dependent and time-independent components. In general, creep refers to the time-dependent component of plastic deformation. This means that creep is a slow and continuous plastic deformation of materials over extended periods under load. Although creep can take place at all temperatures above absolute zero Kelvin, traditionally creep has been associated with time-dependent plastic deformation at elevated temperatures, often higher than roughly 0.4 T _m, where T _m is the absolute melting temperature, because diffusion can assist creep at elevated temperatures. For detailed description of mechanical equation of state, creep behavior of metals and alloys, dislocation motion during creep, mechanisms of creep, creep damage and fracture, the reader is referred to standard text books on creep. ^1-6

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Severe plastic deformation for grain refinement and enhancement of properties

A. Rosochowski , L. Olejnik , in Microstructure Evolution in Metal Forming Processes, 2012

5.2.3 Effects of SPD conditions

SPD can be carried out at room temperature or, in the case of harder and more brittle materials, at elevated temperatures. In practice, this means that only pure metals such as aluminium, copper and nickel can be subjected to SPD at room temperature. In all other cases, the billet material and/or the SPD tools have to be heated to an elevated temperature. Process temperatures in excess of 0.3T _m should be avoided, since they would suppress grain refinement by recovery and recrystallisation (Valiev, 2004). The typical minimum SPD temperatures are 200°C for AA5083, 250°C for AZ31, 400°C for CP Ti and 600°C for Ti–6Al–4V Lower temperatures might be possible, provided other SPD parameters help reduce the tendency to strain localisation and fracture.

SPD processes are often repeated several times in order to accumulate a high strain in the material. In this context, it is helpful to use large strain increments in each SPD operation to reduce the number of operations required. It was found that using a smaller strain increment results in a smaller strengthening effect in AA1070 even for the same value of the accumulated strain (Rosochowski et al., 2006a). On the other hand, smaller strain increments have been found to help CP Ti (grade 1) to be formed at room temperature without fracture (Zhao et al., 2010). In some SPD processes, the strain path can be changed between consecutive processing steps. It has been found that certain alterations of the strain path have a positive effect on the ability to form small equiaxed grains with high misorientation angles (Furukawa et al., 1998).

The strain rate has a negligible influence on grain refinement. However, it may affect the ability of materials such as magnesium and titanium to deform without fracture. Therefore, as shown for example by Semiatin et al. (1999) for CP Ti, it is recommended that the strain rate is reduced for such materials.

Applying a very high pressure (e.g. 6   GPa) during SPD enhances grain refinement; for example, the grain size obtained was reduced from the typical value of 0.35   μm to 0.17   μm for pure nickel (Zhilyaev et al., 2002). Lower values of pressure (e.g. 200   MPa) do not have this effect but can suppress material fracture during SPD, as shown by Lapovok (2006) for aluminium alloys.

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Fundamental aspects of creep deformation and deformation mechanism map

K. Maruyama , in Creep-Resistant Steels, 2008

8.8 Concluding remarks

Plastic deformation at room temperature is essentially time independent and finishes within a short period of time. At elevated temperatures, thermally activated migration of atoms and vacancies, namely diffusion, occurs extensively and time-dependent plastic deformation (creep) continues for a long period of time until fracture. Several deformation mechanisms, such as diffusion creep and dislocation creep appear, depending on testing stress and temperature. Deformation mechanism maps are useful in predicting the deformation mechanism operative under the creep condition of interest. In the case of heat-resistant steel, the stress and temperature ranges of interest are 1 × 10 ^‒4 E – 3 × 10^‒3 E (E: Young's modulus) and 0.4T _m – 0.55T _m (T _m: melting temperature). Under such conditions, the stress exponent for creep rate is usually greater than 3, suggesting that dislocation creep is the relevant deformation mechanism of engineered steels. ¹

Creep deformation occurs both above and below athermal yield stress. The athermal yield stress is the yield stress of a material at a high strain rate at a elevated temperature, for example during loading of the creep test. Below the athermal yield stress, creep (time-dependent plastic deformation) starts after elastic deformation upon loading. Above the stress, time independent plastic deformation (dislocation glide) occurs upon loading before time-dependent plastic deformation (creep). Since plastic deformation upon loading alters the major obstacle to creep deformation from the inherent obstacles to dislocation substructures, creep deformation behavior is different above and below the athermal yield stress. Engineering materials are used below the athermal yield stress. Their creep properties should be evaluated from short-term creep data obtained below their athermal yield stress.

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Volume II

N. Hansen , C.Y. Barlow , in Physical Metallurgy (Fifth Edition), 2014

17.3.1 Strain Using Dislocations

Plastic deformation of metals takes place predominantly by shearing: lattice planes in the material slide over each other, allowing macroscopic shape change without appreciably affecting the ordering and arrangements of atoms within the structure. The stress to cause plastic deformation can be reduced by a factor of 1000 if, rather than moving complete lattice planes simultaneously, deformation can be localized by the movement of line defects, which are dislocations (e.g. Hull and Bacon, 2011). For this reason, plastic deformation of metals depends on the generation and subsequent movement of dislocations. Metals, even in the annealed state, contain a statistical density of dislocations (which can be determined using thermodynamic principles) (Honeycombe, 1975), which is sufficient to allow plastic deformation to take place by this mechanism.

A characteristic intrinsic parameter of a dislocation is its Burgers Vector b, which is the amount of shear that it can produce by moving through the material. Dislocations can take a range of geometries (Hull and Bacon, 2011), and are classified into edge-type (movement resulting in deformation normal to the line defect) and screw-type (deformation parallel to the line defect). Mixed dislocations have intermediate character.

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RECOVERY AND RECRYSTALLIZATION

R.W. CAHN , in Physical Metallurgy (Fourth Edition), 1996

2.1. Recovery of electrical properties

Plastic deformation slightly increases the electrical resistivity. A great many investigations have been devoted to the stages by which the electrical resistivity returns to its fully annealed value. This is of interest both because it helps to disentangle the separate contributions made to the resistivity increase by dislocations and by deformation-induced vacancies, and because it helps to cast light on the complex mechanism of the damage caused by neutron irradiation in nuclear reactors; this damage also causes resistivity changes which anneal out in a different manner from those caused by plastic deformation. Further details will be found in ch. 18, §§ 2.2.3.3, 4.3.

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Machining: Wear of Tools

F. Klocke , ... T. Krieg , in Encyclopedia of Materials: Science and Technology, 2001

(d) Plastic deformation

Plastic deformation of the cutting edge occurs when the tool material softens at high temperature and begins to flow under the pressure of the cutting forces. Cutting edges made of tool steel or high-speed steel deform in inverse proportion to the difference between the temperature at the cutting edge and the yield temperature of the tool material.

Plastic deformation also occurs in cemented carbides and cermets, but only at higher temperatures (cutting speeds) and higher forces than is the case with tool steels and high-speed steels. With cemented carbides, the higher the percentage of the binding phase (usually cobalt), the greater will be the deformation.

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Volume III

Dierk Raabe , in Physical Metallurgy (Fifth Edition), 2014

23.2.4.2 Electromagnetic Properties

Plastic deformation slightly increases the electrical resistivity. Various studies have been devoted to the stages by which the electrical resistivity returns to its fully annealed value. This is of interest both because it helps to disentangle the separate contributions made to the resistivity increase by dislocations and by deformation-induced vacancies, and because it helps to elucidate the complex mechanism of the damage caused by neutron irradiation in nuclear reactors; this damage also causes resistivity changes that anneal out in a different manner from those caused by plastic deformation ( Beck and Hu, 1966; Haessner, 1978; Humphreys and Hatherly, 1995, 2004; Doherty et al., 1997; Doherty, 2005).

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Fracture of Ceramics and Glass

D. Munz , in Encyclopedia of Materials: Science and Technology, 2001

5 Plastic Deformation

Plastic deformation in a general sense can be defined as irreversible deformation. Different mechanisms may be responsible: dislocation motion, vacancy motion, twinning, phase transformation, or viscous flow of amorphous materials. In ceramics, dislocation motion requires high shear stresses due to the covalent atomic bonds. Therefore, under most loading conditions, ceramics fail by the extension of flaws, whereas the competing failure mechanism by dislocation motion would require higher stresses. Under specific loading conditions, however, plastic deformation and the formation of dislocations have been observed (see Nanocrystalline Materials: Mechanical Properties ).

Under contact loading by a blunt or sharp indenter, two failure modes are possible: the development of a cone crack by the tensile stresses outside the contact area, or damage by the compressive stress below the indenter. This damage is microcracking, which is triggered by "shear faults." These shear faults may be slip planes, twins, or weak interfaces. The onset of this damage can be found at the deviation from the straight line in a test with a spherical indenter, when plotting the contact pressure p ₀ versus the contact area in a log–log plot (see Fig. 4). From such tests the yield strength can be obtained. Examples are 7.3 GPa for silicon nitride, or 6.0 GPa for tungsten carbide. Plastic deformation may also occur during surface grinding (Pfeiffer et al. 1996). The results of this plastic deformation are residual stresses.

Figure 4. Relation between indentation stress (load/contact area) and indentation strain (contact radius/sphere radius) for silicon nitride; dashed line: elastic response (after Fischer-Cripps and Lawn 1996).

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Effect of surface treatments on the fatigue life of magnesium and its alloys for biomedical applications

R.A. Antunes , M.C.L. de Oliveira , in Surface Modification of Magnesium and its Alloys for Biomedical Applications, 2015

9.3.1.4 Severe plastic deformation processes

SPD processes are based on metal-forming operations under extensive hydrostatic pressure. Very high strains can be achieved without imposing significant dimensional changes on the processed solid part. The main goal of SPD processes is to produce ultrafine-grained (UFG) bulk metallic components with superior mechanical strength (Valiev et al., 2006). Several different methods have been developed throughout the years, such as equal-channel angular pressing (ECAP), high-pressure torsion, accumulative roll-bonding, cyclic extrusion–compression, and others. An excellent overview of SPD methods is provided by Azushima et al. (2008).

Increased mechanical strength is based on the traditional strengthening mechanism of grain refinement of crystalline metallic materials, which can be described by the Hall–Petch relationship:

(9.6) ${\sigma }_{\text{y}}={\sigma }_0+k·D^{-1/2}$

In this equation, σ _y is the yield stress, σ ₀ is the friction force, k is a constant (locking parameter), and D is the grain size (Dieter, 1988). The development of SPD processes has made it possible to achieve grain sizes classified as ultrafine, typically in the range 100   nm   < D  <   1   μm. In addition to the monotonic mechanical response of SPD-processed metals, their fatigue behavior is also of prime importance to allow for a reliable performance in engineering applications. Mughrabi and Höppel (2010) gave a deep analysis of the fatigue properties of UFG metals and alloys. UFG metals have greater strength and lower ductility than conventional coarse-grained (CG) materials. This leads to differences in high-cycle and low-cycle fatigue regimes according to the total strain fatigue life predicted by Eqn (9.1). These differences can be summarized from the schematic representation shown in Figure 9.9. The parameters shown in Figure 9.9 were defined earlier, in Section9.2. It is clear that UFG materials are expected to exhibit longer fatigue lives than CG alloys in the high-cycle regime, whereas the opposite occurs in the low-cycle regime.

Figure 9.9. Schematic representation of fatigue lives of ultrafine-grained and coarse-grained metallic materials.

From Mughrabi and Höppel (2010).

SPD of magnesium alloys has been developed by several authors (Hamu, Eliezer, & Wagner, 2009; Wang, Chen, Lin, Zhang, & Zhai, 2007). Kim, Lee, and Chumg (2005) showed that the ECAP-processed AZ31 alloy exhibited a greater fatigue threshold and a lower crack growth rate than its CG counterpart as a result of its greater ductility, which increases the ability to accommodate plastic strain during fatigue. Kulyasova et al. (2009) studied the fatigue behavior of the AM60 alloy processed by ECAP at 350   °C, 230   °C, and 150   °C. They observed that the microstructure was characterized by a more uniform and smaller grain size with the reduction of ECAP processing temperature, thus leading to an increase of the fatigue limit. Nevertheless, plastic deformation of magnesium alloys at room temperature is difficult because of the reduced ductility of the hexagonal, closely packed crystalline structure. Thus, SPD processes are conducted more easily at progressively increasing temperatures. Recently, Akbaripanah, Fereshtech-Saniee, Mahmudi, and Kim (2013) showed that the fatigue behavior of the AM60 alloy in the low-cycle regime is strongly related to both ductility and texture developed during the ECAP process. The ductility of the alloy increases with the number of passes during ECAP, leading to an improved fatigue life in the low cycle regime. However, texture effects resulting from increased plastic deformation can decrease the fatigue life. The strong relationship between mechanical properties and texture of ECAP-processed magnesium alloys has been recognized by many authors (Balogh, Figueiredo, Ungár, & Langdon, 2010; Figueiredo et al., 2010). Different fiber orientations can be produced depending on the alloying elements, affecting the plasticity and mechanical behavior of magnesium alloys (Agnew, Mehrotra, Lillo, Stoica, & Liaw, 2005). In this regard, the potential of SPD processes to improve fatigue properties of magnesium alloys is still highly unexplored. The correlation between microstructure, texture, plasticity and the fatigue behavior of SPD-processed magnesium alloys and the role of specific alloying elements is not yet understood. The influence of SPD processes on the corrosion resistance of magnesium alloys should be not disregarded. Alvarez-Lopez et al. (2010) showed that the corrosion behavior of the AZ31 alloy in biological fluids can be improved by the grain size reduction attained after ECAP. However, Song et al. (2011) reported that ECAP decreased the corrosion resistance of the AZ91D alloy as a result of the introduction of strain-induced defects, which increased the action of the magnesium matrix. The study of these effects and the interaction of fatigue and corrosion of SPD-processed biomedical magnesium alloys are not seen in the literature.

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Precipitation and solute clustering in aluminium: advanced characterisation techniques

G. Sha , ... S.P. Ringer , in Fundamentals of Aluminium Metallurgy, 2011

12.6 Precipitation in Al alloys under severe plastic deformation

Severe plastic deformation (SPD) is effective for producing metals and alloys having ultrafine grained (UFG) and/or nanocrystalline structures ( Valiev et al., 2000; Valiev and Langdon, 2006; Zhilyaev and Langdon, 2008). Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are two well-known SPD techniques having different capacities on grain refinement. ECAP can refine the grain size of material to ~   100   nm in Al alloys, whereas HPT can refine the grain size to a few ten nm. According to the Hall–Petch relationship (Hall, 1951; Petch, 1953), the grain refinement can cause significant strengthening of the material. Understanding grain refining effects and texture development under SPD has been a focus of many researchers, especially for single phase materials. More recently, there has been growing interest in understanding precipitation in order to control the precipitate microstructures of Al alloys and to achieve a combination of strengthening from both grain refinement and precipitation hardening (Murayama et al., 2001).

The evidence to date suggests that processing by SPD has a relatively complicated effect on precipitate microstructures. The precipitations under SPD are significantly different from those obtained by conventional ageing treatment, as shown in Fig. 12.8. The SPD processing has a marked effect on the size and morphology of precipitates. The large amount of shear deformation introduced by SPD will first initiate a rotation of pre-existing platelet or rod precipitates, as indicated by the arced reflection from precipitates in a circumferential direction, such as θ′ in an Al-Cu-Mg alloy sample (Murayama et al., 2001) and η in an Al-Zn-Mg-Cu alloy sample (Sha et al., 2009a) processed after the first ECAP pass. The shear deformation can further cause the fragmentation of pre-existing large precipitates and the formation of smaller spherical particles (Murayama et al., 2001; Xu et al., 2005). Careful TEM characterisation confirms that the SPD is effective in altering the orientation relationship between precipitates and the matrix (Murayama et al., 2001; Sha et al., 2009). As a result, the initial low energy interface associated with the preferential orientation relationship between the precipitate and the matrix will disappear. The higher surface energy at the interface between the precipitates and the matrix in the sample processed by SPD will promote the isotropic growth of the precipitates rather than anisotropic growth. The alternation of orientation relationship between precipitate and the matrix induced by SPD is responsible for the precipitates having lower aspect ratios or spherical morphology.

12.8. TEM bright field images of Al-Zn-Mg-Cu alloy samples processed by ECAP at 200   °C after one pass (a) and four passes (b) and samples only thermally aged at 200   °C for 5 minutes (c) and 20 minutes (d) which are equivalent to the thermal durations of one pass and four passes ECAP processing, respectively.

Temperature is another processing parameter affecting precipitation under SPD. SPD at room temperature generally suppresses precipitation in Al alloys. No precipitation was reported in as-quenched samples during ECAP (Zhao et al, 2004) and there was dissolution of pre-existing θ′ precipitates in the matrix after eight passes of ECAP (Murayama et al., 2001). In contrast, when SPD is conducted at higher temperatures, such as 200   °C, precipitation is promoted in an Al-Zn-Mg alloy (Sha et al., 2009b). A recent investigation on an Al-Zn-Mg-Cu alloy confirms that precipitation under ECAPaccelerates the precipitation kinetics to about 50 times faster than that in conventional ageing treatments. SPD affects the precipitation thermodynamics by promoting isotropic growth and forming equiaxed η precipitates in the alloy. The high density of mobile dislocations produced by ECAP promotes the dissolution of small metastable precipitates and the formation of large η precipitates by coalescence.

Precipitation under SPD is a relatively new area, which demands more research to understand fundamentals. High volumes of grain boundaries are introduced into the materials due to the grain refinement under SPD. The segregation of solutes at grain boundaries can have a significant influence on the solute balance in the material (Sha et al., 2009a). Many basic questions regarding thermodynamics and kinetics of precipitation have yet to be answered in order to control the precipitation under SPD. For example, what is the phase equilibrium for each alloy system under SPD? How do mobile dislocations affect precipitation at different temperature? Better fundamental understanding will help to establish the threshold temperature at which precipitation can be activated under SPD.

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