TWIP steel

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Twinning-Induced Plasticity steel which is also known as TWIP steel is a class of austenitic steels which can deform by both glide of individual dislocations and mechanical twinning on the {1 1 1}γ<1 1 >γ system. [1] They have outstanding mechanical properties at room temperature combining high strength (ultimate tensile strength of up to 800 MPa) and ductility (elongation to failure up to 100%) based on a high work-hardening capacity. TWIP steels have mostly high content in Mn (above 20% in weight %) and small additions of elements such C (<1 wt.%), Si (<3 wt.%), or Al (<3 wt.%). The steels have low stacking fault energy (between 20 and 40 mJ/m2) at room temperature. Although the details of the mechanisms controlling strain-hardening in TWIP steels are still unclear, the high strain-hardening is commonly attributed to the reduction of the dislocation mean free path with the increasing fraction of deformation twins as these are considered to be strong obstacles to dislocation glide. Therefore, a quantitative study of deformation twinning in TWIP steels is critical to understand their strain-hardening mechanisms and mechanical properties. Deformation twinning can be considered as a nucleation and growth process. Twin growth is assumed to proceed by co-operative movement of Shockley partials on subsequent {111} planes.

Steel alloy made by combining iron and other elements

Steel is an alloy of iron and carbon, and sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons.

Crystal twinning when two separate crystals share some of the same crystal lattice points in a symmetrical manner

Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. The surface along which the lattice points are shared in twinned crystals is called a composition surface or twin plane.

Contents

History

First steel based on plasticity induced by mechanical twinning was found in 1998 which had strength of 800 MPa with a total elongation of above 85%. [2] These values vary with deformation temperature, strain rate and chemical composition. [3] [4]

Researchers have shown that increased work hardening attributed to the partitioning of the austenite grains is the main contributing factor to the overall elongation of TWIP steels in which the mechanical strain of twinning have a rather small contribution. [5]

Work hardening strengthening of a metal by plastic deformation

Work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material. Many non-brittle metals with a reasonably high melting point as well as several polymers can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at low temperatures, such as indium, however others can only be strengthened via work hardening, such as pure copper and aluminum.

Crystallite

A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. The orientation of crystallites can be random with no preferred direction, called random texture, or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter. Crystallites are also referred to as grains. The areas where crystallites meet are known as grain boundaries. Polycrystalline or multicrystalline materials, or polycrystals are solids that are composed of many crystallites of varying size and orientation.

Compositions

TWIP steels usually contain large concentrations of Mn because it is crucial to preserve the austenitic structure based on the ternary system of Fe-Mn-Al [6] and control Stacking Fault Energy (SFE) of the Iron-based alloys. [7] [8]

The addition of aluminium to Fe-high Mn TWIP steels is because it increases SFE significantly and therefore stabilizes the austenite against phase transformations which can occur in the Fe-Mn alloys during deformation. [9] Furthermore, it strengthens the austenite by solid-solution hardening. [10]

Properties

Total and uniform elongation of the Fe-55Mn-3Al-3Si wt% TWIP steel as a function of the test temperature; strain rate e=10 .s . Fe-55Mn-3Al-3Si Tensile properties - 1.svg
Total and uniform elongation of the Fe–55Mn–3Al–3Si wt% TWIP steel as a function of the test temperature; strain rate ε=10 .s .
0.2% proof and ultimate tensile strength of the Fe-55Mn-3Al-3Si wt% TWIP steel as a function of the test temperature; strain rate e=10 .s . Fe-55Mn-3Al-3Si Tensile properties - 2.svg
0.2% proof and ultimate tensile strength of the Fe–55Mn–3Al–3Si wt% TWIP steel as a function of the test temperature; strain rate ε=10 .s .

Austenitic steels are used widely in many applications because of their excellent strength and ductility combined with good wear and corrosion resistance. High-Mn TWIP steels are attractive for automotive applications due to their high energy absorption, which is more than twice that of conventional high strength steels, [3] and high stiffness which can improve the crash safety. [4]

Related Research Articles

Ductility

Ductility is a measure of a material's ability to undergo significant plastic deformation before rupture, which may be expressed as percent elongation or percent area reduction from a tensile test. According to Shigley's Mechanical Engineering Design significant denotes about 5.0 percent elongation. See also Eq. 2–12, p. 50 for definitions of percent elongation and percent area reduction. Ductility is often characterized by a material's ability to be stretched into a wire.

Martensite most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by diffusionless transformation.

Martensite is named after the German metallurgist Adolf Martens (1850–1914). The term most commonly refers to a very hard form of steel crystalline structure, but can also refer to any crystal structure that is formed by diffusionless transformation. Martensite includes a class of hard minerals that occur as lath- or plate-shaped crystal grains.

Austenite metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in stainless steel.

Stress–strain curve curve which represents stress value against strain value of the given material ,when the material is subjected to increasing pull

The relationship between the stress and strain that a particular material displays is known as that particular material's stress–strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). These curves reveal many of the properties of a material.

Bainite

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it no longer is thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

Carbon steel steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Magnetic shape memory alloys (MSMAs), also called ferromagnetic shape memory alloys (FSMA), are particular shape memory alloys which produce forces and deformations in response to a magnetic field. The thermal shape memory effect has been obtained in these materials, too.

Maraging steel steel

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt.% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt.% Ni steels to which small additions of Al, Ti, and Nb were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being practiced over tool steels, high-carbon, and high-chromium steels to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

Superalloy alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures; good surface stability; and corrosion and oxidation resistance

A superalloy, or high-performance alloy, is an alloy that exhibits several key characteristics: excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation. The crystal structure is typically face-centered cubic austenitic. Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then cooling.

Austenitic stainless steel

Austenitic stainless steel is a specific type of stainless steel alloy. Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic,martensitic and duplex. These stainless steels possess austenite as their primary crystalline structure. This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements nickel, manganese and nitrogen. Due to their crystalline structure austenitic steels are not hardenable by heat treatment and are essentially non-magnetic.

Alloy steel steel that is alloyed with a variety of elements

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

Dual-phase steel

Dual-phase steel (DP steel) is a high-strength steel that has a ferritic–martensitic microstructure. DP steels are produced from low or medium carbon steels that are quenched from a temperature above A1 but below A3 determined from continuous cooling transformation diagram. This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content. For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement). The desire to produce high strength steels with formability greater than microalloyed steel led the development of DP steels in the 1970s.

TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for "Transformation induced plasticity," which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility.

Mangalloy

Mangalloy, also called manganese steel or Hadfield steel, is an alloy steel containing an average of around 13% manganese. Mangalloy is known for its high impact strength and resistance to abrasion once in its work-hardened state.

References

  1. ↑ Harshad Kumar Dharamshi Hansraj Bhadeshia, Sir Robert Honeycombe, Steels, Microstructure and Properties, Third edition, Butterworth-Heinemann publications, Great Britain, p 229. ISBN   0-7506-8084-9
  2. ↑ Oliver Grässel and Georg Frommeyer, Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–Al steels, Material Science and Technology, Vol. 14 (1998) No. 12, pp. 1213-1216. doi : 10.1179/026708398790300891
  3. 1 2 3 4 Georg Frommeyer, Udo Brüx and Peter Neumann, Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes, ISIJ International, Vol. 43 (2003) pp. 438-446.
  4. 1 2 Oliver Grässel, Lars Krüger, Georg Frommeyer and Lothar Werner Meyer, High Strength Fe-Mn-(Al,Si) TRIP/TWIP Steels Development -Properties-Application, International Journal of Plasticity, Vol. 16 (2000), pp. 1391-1409. doi : 10.1016/S0749-6419(00)00015-2
  5. ↑ Bo Qin and Harshad Kumar Dharamshi Hansraj Bhadeshia, Plastic strain due to twinning in austenitic TWIP steels, Materials Science and Technology, Vol. 24 (2008) No. 8, pp. 969-973. doi : 10.1179/174328408X263688
  6. ↑ Sato K, Tanaka K & Inoue, Determination of the a/g Equilibrium in the Iron Rich Portion of the Fe-Mn-Al System, ISIJ International, Vol. 29 (1989), pp. 788-792.
  7. ↑ P.Y. Volosevich , V.N. Grindnev and Y.N. Petrov, Manganese Influence on Stacking-Fault Energy in Iron-Manganese Alloys, Physics of Metals and Metallography, Vol. 42 (1976), pp. 126 -130.
  8. ↑ Y.K. Lee and C.S. Choi, Driving Force for γ→ε Martensitic Transformation and Stacking Fault Energy of γ in Fe-Mn Binary System, Metallurgical and Materials Transactions A, Vol. 31A (2000), pp. 355-360. doi : 10.1007/s11661-000-0271-3
  9. ↑ Jianfeng Wan, Shipu Chen, T.Y. Hsu and Xu Zuyao, The stability of transition phases in Fe-Mn-Si based alloys, CALPHAD, Vol. 25 (2001) , pp. 355-362. doi : 10.1016/S0364-5916(01)00055-4
  10. ↑ J. Charles, A. Berghézan and A. Lutts, Structural and Mechanical Properties of High-Alloy Manganese-Aluminum Steels, Journal de Physique Colloques, Vol. 43 (1982), pp. C4-435. doi : 10.1051/jphyscol:1982466

See also

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