Solid Solution Strengthening

9 Key excerpts on "Solid Solution Strengthening"

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  eBook - PDF

  Mechanical Behavior of Materials

  • Marc André Meyers, Krishan Kumar Chawla(Authors)
  • 2008(Publication Date)
  • Cambridge University Press

    (Publisher)

  A very versatile method of obtaining high strength levels in metals would be to restrict this rather easy motion of dislocations. We saw earlier that grain boundaries (Chapter 5) and stress fields of other disloca-tions (Chapter 6) can play this restrictive role at low temperatures and increase the strength of the material. When the dislocation mobility in a solid is restricted by the introduction of solute atoms, the result-ant strengthening is called solid-solution-hardening , and the alloy is called a solid solution . An example of the strengthening that can be achieved by solid solution is shown in Figure 10.2(a), in which we plot the increase in yield stress of steel as a function of the content of the solute. Note that solutes such as carbon and nitrogen, which go into interstitial positions of the iron lattice, have much larger strength-ening effects than substitutional atoms such as manganese. We shall explain this shortly. In order to analyze the phenomenon of harden-ing due to the presence of solute atoms, we must consider the increase in the stress necessary to move a dislocation in its slip plane in the presence of discrete barriers to the motion of dislocations. Conceptu-ally, it is useful and easier to think in terms of an energy of inter-action between the dislocation and the barrier (e.g., a solute atom or a precipitate). In the case of substitutional solutions, for a stationary dislocation, the interaction energy is the change in energy of the sys-tem consisting of a crystal and a dislocation when a solvent atom is removed and substituted with a solute atom. Knowing the interaction energy U , we can calculate the force dU / dx necessary to move a dis-location a distance dx normal to its length. In ceramics, solutes can also exercise a strengthening effect, as demonstrated by Figure 10.2(b) for monocrystalline alumina with additions of chromium. This increase manifests itself at high temperatures, where the ceramics become relatively ductile.

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Metallurgical and materials engineers are often called on to design alloys having high strengths yet some ductility and toughness; typically, ductility is sacrificed when an alloy is strengthened. Several hardening techniques are at the disposal of an engineer, and frequently alloy selection depends on the capacity of a material to be tailored with the mechanical characteristics required for a particular application. Important to the understanding of strengthening mechanisms is the relation be- tween dislocation motion and mechanical behavior of metals. Because macroscopic plastic deformation corresponds to the motion of large numbers of dislocations, the abil- ity of a metal to deform plastically depends on the ability of dislocations to move. Because hardness and strength (both yield and tensile) are related to the ease with which plastic deformation can be made to occur, by reducing the mobility of dislocations, the me- chanical strength may be enhanced—that is, greater mechanical forces are required to initiate plastic deformation. In contrast, the more unconstrained the dislocation motion, the greater is the facility with which a metal may deform, and the softer and weaker it becomes. Virtually all strengthening techniques rely on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. The present discussion is confined to strengthening mechanisms for single-phase metals by grain size reduction, solid-solution alloying, and strain hardening. Deformation and strengthening of multiphase alloys are more complicated, involving concepts be- yond the scope of the present discussion; later chapters treat techniques that are used to strengthen multiphase alloys. Tutorial Video: How Do Defects Affect Metals? Mechanisms of Strengthening in Metals Figure 8.13 For a single crystal subjected to a shear stress τ, (a) deformation by slip, (b) deformation by twinning.

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  Physical Metallurgy

  Principles and Design

  • Gregory N. Haidemenopoulos(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  8

  Strengthening mechanisms

  8.1 Introduction

  Pure metals are soft materials. For most engineering applications they cannot provide the required strength. This is the reason alloys were invented. The strength of metals can be increased by alloying and by certain thermal, mechanical or thermomechanical processing. In the previous chapter we have seen that the plastic deformation of metals proceeds by the glide of dislocations, a process we called slip. Strengthening can be achieved by creating obstacles to dislocation glide. It is these obstacles that are created with the alloying and suitable processing discussed above. The strengthening mechanisms, which is the subject of the present chapter, are simply mechanisms of interaction between dislocations and various obstacles. The major mechanisms are: lattice resistance, strain hardening, Solid Solution Strengthening, grain boundary strengthening and precipitation strengthening. In lattice resistance, the main obstacle to dislocation glide is the crystal lattice, the atomic bonds in particular. In strain hardening the main obstacle impeding dislocation glide is other dislocations. The interaction between dislocations forms sessile dislocations, which cannot contribute to plastic deformation. Strain hardening has been discussed in detail in the previous chapter and will not be considered further. In Solid Solution Strengthening the obstacles are solute atoms, either substitutional or interstitial, the strain field of which interacts with dislocations. In grain boundary strengthening, the obstacles are high-angle grain boundaries, but also sub-boundaries and interfaces as the interfaces between ferrite and cementite in pearlite or the interfaces between martensite laths. Finally, in precipitation strengthening, the obstacles are precipitates, second-phase particles or intermetallic compounds formed during thermal processing. In most alloys the yield strength is the result of a superposition of strengthening mechanisms. The effectiveness of each mechanism is characterized by the specific obstacle strength, which is related to the stress required to overcome the obstacles at T = 0K

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  Steels: Microstructure and Properties

  • H.K.D.H. Bhadeshia, R.W.K. Honeycombe, Harry Bhadeshia, Robert Honeycombe(Authors)
  • 2011(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  However, it should be noted that the relative strengthening may alter with the temperature of testing, and with the concentrations of interstitial solutes present in the steels. The strengthening achieved by substitutional solute atoms is, in general, greater the larger the difference in atomic size of the solute from that of iron, applying the Hume-Rothery size effect. However, from the work of Fleischer and Takeuchi it is apparent that differences in the elastic behaviour of solute and solvent atoms are also important in determining the overall strengthening achieved. In practical terms, the contribution to strength from solid solution effects is superimposed on hardening from other sources, e.g. grain size and dis-persions. Also it is a strengthening increment, like that due to grain size, which need not adversely affect ductility. In industrial steels, solid solution strengthen-ing is a far from negligible factor in the overall strength, where it is achieved by a number of familiar alloying elements, e.g. manganese, silicon, nickel, molyb-denum, several of which are frequently present in a particular steel and are additive in their effect. These alloying elements are usually added for other reasons, e.g. Si to achieve deoxidation, Mn to combine with sulphur or Mo to promote hardenability. Therefore, the solid solution hardening contribution can be viewed as a useful bonus. 2.5 GRAIN SIZE 2.5.1 Hall–Petch effect The refinement of the grain size of ferrite provides one of the most important strengthening routes in the heat treatment of steels. The first scientific analysis of the relationship between grain size and strength, carried out on ARMCO 28 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS Fig. 2.7 Solid Solution Strengthening of α -iron crystals by substitutional solutes. Ratio of the critical resolved shear stress τ 0 to shear modulus µ as a function of atomic concentration (Takeuchi, Journal of the Physical Society of Japan 27 , 929, 1969).

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  292 • Chapter 8 / Deformation and Strengthening Mechanisms Metallurgical and materials engineers are often called on to design alloys having high strengths yet some ductility and toughness; typically, ductility is sacrificed when an alloy is strengthened. Several hardening techniques are at the disposal of an engineer, and frequently alloy selection depends on the capacity of a material to be tailored with the mechanical characteristics required for a particular application. Important to the understanding of strengthening mechanisms is the relation be- tween dislocation motion and mechanical behavior of metals. Because macroscopic plastic deformation corresponds to the motion of large numbers of dislocations, the abil- ity of a metal to deform plastically depends on the ability of dislocations to move. Because hardness and strength (both yield and tensile) are related to the ease with which plastic deformation can be made to occur, by reducing the mobility of dislocations, the me- chanical strength may be enhanced—that is, greater mechanical forces are required to initiate plastic deformation. In contrast, the more unconstrained the dislocation motion, the greater is the facility with which a metal may deform, and the softer and weaker it becomes. Virtually all strengthening techniques rely on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. The present discussion is confined to strengthening mechanisms for single-phase metals by grain size reduction, solid-solution alloying, and strain hardening. Deformation and strengthening of multiphase alloys are more complicated, involving concepts be- yond the scope of the present discussion; later chapters treat techniques that are used to strengthen multiphase alloys. Tutorial Video: How Do Defects Affect Metals? Mechanisms of Strengthening in Metals Figure 8.13 For a single crystal subjected to a shear stress τ, (a) deformation by slip, (b) deformation by twinning.

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  Impurities in Engineering Materials

  ImPatt, Reliability, & Control

  • Clyde Briant(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  9Solid Solution Hardening by Impurities

  Tetsuo Mohri and Tomoo Suzuki*

  Hokkaido University Sapporo, Japan

  I Introduction

  Solid solution hardening is a typical hardening behavior observed commonly in all classes of metallic alloy systems including off-stoichiometric intermetallic compounds. The origin of solid solution hardening is usually ascribed to the interaction between a dislocation and impurities contained in a solid solution that act as obstacles to a moving dislocation. Besides substitutional and interstitial foreign elements, one may include even a vacancy, a forest dislocation, and other defects as impurities that have the potential to cause hardening. The magnitude and extension of the interaction force depend on the kind of impurities. The primary mission of the study of solid solution hardening is to clarify the elementary interaction mechanism and to estimate the magnitude of the interaction force for each obstacle on the basis of dislocation theory.

  In general, the critical resolved shear stress (CRSS) of a solid solution cannot be given as a simple sum of an elementary interaction force over all impurities encountered by a dislocation line. This is because a non-linear dislocation line, due to a finite magnitude of line tension, is subject to various averaging processes during multiple interactions with impurities on a slip plane. Thus, the study of solid solution hardening requires a statistical treatment to clarify the functional dependence of CRSS on the concentration and distribution of an impurity and the extension of its interaction field, the line tension of a dislocation line, etc., without going into the details of the mechanism of interaction with an impurity.

  The two subjects described above concern the interactions of a single dislocation line with impurities. In order to analyze and understand experimentally observed solid solution hardening behavior, however, one also needs to take the dislocationdislocation interactions into account, since the actual deformation process is driven by dislocations that have a density on the order of 106 – 1012 cm/cm3

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  Metals and Materials

  Science, Processes, Applications

  • R. E. Smallman, R J Bishop(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 8 Strengthening and toughening 8.1 Introduction The production of materials which possess con-siderable strength at both room and elevated temperatures is of great practical importance. We have already seen how alloying, solute-dislocation interaction, grain size control and cold-working can give rise to an increased yield stress. Of these methods, refining the grain size is of universal application to materials in which the yield stress has a significant dependence upon grain size. In certain alloy systems, it is possible to produce an additional increase in strength and hardness by heat-treatment alone. Such a method has many advantages, since the required strength can be induced at the most convenient stage of produc-tion or fabrication; moreover, the component is not sent into service in a highly stressed, plastically deformed state. The basic requirement for such a special alloy is that it should undergo a phase transformation in the solid state. One type of alloy satisfying this requirement, already considered, is that which can undergo an order-disorder reaction; the hardening accompanying this process (similar in many ways to precipitation-hardening) is termed order-hardening. However, conditions for this form of hardening are quite stringent, so that the two principal hardening methods, commonly used for alloys, are based upon (1) precipitation from a supersaturated solid solution and (2) eutectoid decomposition. In engineering applications, strength is, without doubt, an important parameter. However, it is by no means the only important one and usually a material must provide a combination of properties. Some ductility is generally essential, enabling the material to relieve stress concentrations by plastic deformation and to resist fracture. The ability of materials to resist crack propagation and fracture, known generally as toughness, will be discussed in this chapter.

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  Materials Corrosion and Protection

  • Yongchang Huang, Jianqi Zhang, Yongchang Huang, Jianqi Zhang(Authors)
  • 2018(Publication Date)
  • De Gruyter

    (Publisher)

  (metal elements). These elements generally gather partially in the grain boundary. In addition, among the high-temperature alloys, lead, tin, arsenic, antimony, bismuth, etc., are harmful impurity elements with low melting point, which should be decreased as much as possible or be completely eliminated; silicon, phosphorus, sulfur are common impurity elements, whose dissolubilities are small, and they generally gather partially on the grain boundary and are not favorable for strengthening of the grain boundary. As shown from the above analysis, the most common way of strengthening iron- base, nickel-base, and cobalt-base high-temperature alloy may be Solid Solution Strengthening, precipitated phase strengthening, and grain boundary strengthening. (1) Solid Solution Strengthening Increasing heat strengthening through alloy elements’ solid dissolution into the matrix is called Solid Solution Strengthening. It is a matrix strengthening method widely applied in high-temperature alloys. For example, in Ni-Cr base alloy, we gen- erally add high-melting-point metals like W, Mo, Ta, Nb, etc. (V, VI family), to com- monly act with Cr. A larger amount of this type of metal dissolves into γ-Ni, which will decrease the stacking-fault energy (SFE), increase the width of diffusion disloca- tion, increase the activation energy of diffusion, and retard the slip and intersection slip of dislocation in high temperature creep condition, thereby increasing the heat strengthening of heat-resisting alloy. The heat strengthening enhancement of alloy through solid solution strength- ening is mainly realized by increasing the atom binding force and lattice divergence change of alloy element. When temperature T ≤ 0.6T melting point (absolute tempera- ture), the increase in atom binding force and divergence change in lattice increase the slip resistance in the solid solution, making it difficult to produce slip deformation, thereby reaching the objective of strengthening.

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  Materials Science and Engineering

  An Introduction

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  There is a discontinuity of slip planes within the vicinity of a grain boundary. • A metal that has small grains is stronger than one with large grains because the former has more grain boundary area and, thus, more barriers to dislocation motion. Strengthening by Grain Size Reduction • The strength and hardness of a metal increase with increase of concentration of impurity atoms that go into solid solution (both substitutional and interstitial). • Solid-solution strengthening results from lattice strain interactions between impurity atoms and dislocations; these interactions produce a decrease in dislocation mobility. Solid-Solution Strengthening • Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is deformed plastically. • Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work (Figures 7.19a and 7.19b); ductility decreases (Figure 7.19c). • During plastic deformation, dislocation density increases and repulsive dislocation– dislocation strain field interactions increase; this leads to lower dislocation mobilities and increases in strength and hardness. Strain Hardening • During recovery: There is some relief of internal strain energy by dislocation motion. Dislocation density decreases, and dislocations assume low-energy configurations. Some material properties revert back to their precold-worked values. Recovery • During recrystallization: A new set of strain-free and equiaxed grains form that have relatively low dislocation densities. The metal becomes softer, weaker, and more ductile. • For a cold-worked metal that experiences recrystallization, as temperature increases (at constant heat-treating time), tensile strength decreases and ductility increases (per Figure 7.22). • The recrystallization temperature of a metal alloy is that temperature at which recrys- tallization reaches completion in 1 h.

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