Solid Solution Hardening

6 Key excerpts on "Solid Solution Hardening"

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  eBook - ePub

  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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  Mechanical Behavior of Materials

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

    (Publisher)

  Simply put, the phenomenon can be regarded as one form of restrict-ing dislocation motion in crystalline materials, especially metals. We then extend this idea to precipitation and dispersion strengthening. Precipitates can be formed in certain alloys in the solid state. One starts with a solid solution at a high temperature, quenches it to a low temperature, and then ages it at an intermediate temperature to obtain a finely distributed precipitate. During aging, precipitates appear in a variety of sequences, depending on the alloy system under consideration. Precipitation strengthening has to do with the inter-action of dislocations with precipitates, rather than with single atoms 10.2 SOLID-SOLUTION STRENGTHENING 559 of solutes. A logical extension of this idea is to artificially disperse hard ceramic phases in a soft metallic matrix, instead of obtaining Solute atoms (zinc) Solute atoms (carbon) Solvent atoms (copper) Solvent atoms (iron) (a) (b) Fig. 10.1 The two basic forms of solid solutions. (a) Substitutional solid solution of zinc in copper to form brass. (b) Interstitial solid solution of carbon in iron to form steel. The interstitial solid-solution carbon atoms are shown in the face-centered cubic form of iron. them via a precipitation process. The mobility of dislocations is then restricted by these hard particles, and the alloy is strengthened. This process is called dispersion strengthening . 10.2 Solid-Solution Strengthening Dislocations are quite mobile in pure metals, and plastic deform-ation occurs by means of dislocation motion (i.e., by shear). 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.

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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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  Crystallography

  • Takashiro Akitsu(Author)
  • 2019(Publication Date)
  • IntechOpen

    (Publisher)

  The crystallography of precipitates is of interest not only for funda-mental materials science but also for engineering, in particular, structural materials engineering. The strength of metals and alloys is highly affected by a minor amount of precipitates such as a few percent. In the case of nonmetallic composite materials, their strength is determined by the volume fraction ratio of constituent phases. In other words, the strength of nonmetallic composites is expected not to exceed that of constituent phases. On the other hand, metals and alloys containing second-phase precipitate particles, say, 2% in volume fraction, can exhibit a strength sev-eral times greater than the matrix phase ( Figures 1 – 3 ). Such intense hardening is 33 achieved when precipitate particles are strong obstacles against the motion of dis-locations gliding on a slip plane in the matrix. They are strong obstacles in the case where dislocations are unable to cut through them ( Figure 4 ). In the classical theory of precipitation hardening (a.k.a. dispersion strengthening) established in the 1950s – 1960s, the obstacle strength is assumed to be determined by the shear modulus [1, 2]; those which are harder than the matrix are strong obstacles. In general, this condition is fulfilled by a combination of metallic matrix and nonme-tallic compound precipitates such as oxides and carbides whose strength is typically a few GPa, which is 10 times greater than the yield strength of metals. Recent experimental studies demonstrated that crystallography of precipitate particles is Figure 1. A model calculation of precipitation hardening in the hcp Ti, the hcp Mg, the fcc Cu, and the bcc Fe, as a function of the volume fraction of precipitates for the cases of precipitate diameter of 5 and 50 nm.

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  Gamma Titanium Aluminide Alloys

  Science and Technology

  • Fritz Appel, Jonathan David Heaton Paul, Michael Oehring(Authors)
  • 2011(Publication Date)
  • Wiley-VCH

    (Publisher)

  Compared with the large amount of mechanical data, little information is available about the work hardening of TiAl alloys. This is certainly in part due to the brittleness of the material, which persists up to relatively high temperatures. However, in compression it is possible to produce work hardening over large plastic strains, which provides a significant potential to strengthen the material. The competitive processes to work hardening are recovery and recrystallization. Thus, the rate at which the material hardens depends on the imparted mechanical strain energy and its release by diffusion-assisted processes. Taken together, these factors lead to a complex dependence of the work hardening on temperature. From the technical point of view work hardening is important because it is involved in various metallurgical processes, such as forming, shaping, diffusion bonding, and surface hardening. For all these processes detailed knowledge about the deformation-induced defect structures and their thermal stability is required. The available information about this subject will be assessed in the following sections.

  7.2.1 Work-Hardening Phenomena

  Work-hardening mechanisms will be characterized in terms of glide obstacles controlling the velocity and the slip path of the dislocations, analogous to the procedure that has been described in Section 6.4. Within this approach the flow stress σ (ε) beyond yielding may be described as [25]

  (7.3)

  ΔG is the Gibbs free energy of activation, k the Boltzmann constant and M T the Taylor factor. σ 0 represents a stress contribution from dislocation mechanisms operating at the onset of yielding and is considered to be independent of strain ε. Micromechanisms associated with σ0 have been discussed in Section 6.4.

  σμ

  (ε) is an athermal stress contribution to work hardening representing long-range dislocation interactions. σ *(ε) is an effective or thermal stress component due to thermally assisted overcoming of deformation induced short-range glide obstacles. V D (ε) and ΔF D

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