Martensite
Martensite is a metastable structure in metals and non-metals, which arises without diffusion and athermal by cooperative shear movement from the initial structure. The material must be of the temperature of a high-temperature phase (for steel: γ - phase austenite ) at the equilibrium temperature to a low temperature phase (for steel: α - phase ferrite ), cooled (usually chilled ). The undercooling below the equilibrium temperature must be low enough to generate the necessary driving force for athermal phase transformation (see Figure 3 ), but must also be fast enough to prevent diffusion processes (see Time - temperature -transformation diagram ). The necessary undercooling and cooling rate are strongly on the considered material ( of steel: from the alloying elements ), varying over a wide range, so sometimes is a rapid quenching in water and if desired, subsequent freezing in liquid nitrogen (due to the Leidenfrost effect a direct quenching with liquid nitrogen possible) is not necessary, and sometimes is also sufficient slow cooling in air or in a hot bath.
The high-temperature phase is metastable at room temperature conserved, they can stress or strain induced to convert to martensite ( see austenite in steels ). Reversible martensitic transformations as a basic phenomenon of shape-memory effect, also belong to this category.
Martensitic transformations occur in unalloyed and alloyed steels as well as many non-ferrous metals, ceramics and polymers and are not purely limited to metals phenomenon. For steels, the martensitic transformation is a commonly used option of influencing property (see hardening and tempering ).
The structure is named after the German metallurgist Adolf Martens ( 1850-1914 ).
- 2.1 Maurer - chart
- 2.2 Schaeffler diagram
- 3.1 Formation of Ti - martensite
Martensite in iron-carbon system
In the steel martensite by a diffusionless folding-over from the face-centered cubic lattice of the austenite in a tetragonal body-centered lattice, during the rapid cooling to a temperature below the martensite start temperature MS ( martensite start). The conversion stops when the cooling is stopped. If the martensite MF ( martensite finish) reached, then increases with further cooling, the volume fraction of martensite no further.
This folding-over and this cooperative shear movement means that the Martensitgitter arises only through orderly angular and positional changes from the starting grid. The individual atoms move, this is only a fraction of the atomic spacing. The midrib of each resulting Martensitplatte, the so-called invariant habit plane does not participate ( see Figure 8) on the folding.
Depending on the proportion of the stored carbon is a part of the austenite is still not converted. This austenite can be explained by the high bias voltages, which exert the most recently formed martensite to martensite before incurred and thereby inhibit the further growth. The martensite plates show a lenticular or needle- shaped cross-section and pull up at the start of martensite formation from one side of the grain to the other, see Figure 1 more plates then grow at different angles, but mostly perpendicular to the already present in the grain.
The dissolved carbon in austenite remains by the rapid cooling during quenching even in the mixed crystal forcibly dissolved. Thus, the folded fcc lattice is distorted tetragonal, with a very hard structure is formed. The cooling rate, resulting from the first fractions of martensite ( in addition to ferrite, pearlite, bainite ) is called lower critical cooling rate. A result of the cooling for the first time only martensite, the upper critical cooling rate is reached.
Martensite in steels used in order to achieve a significant increase in hardness. The higher the carbon content of the martensite, the higher the hardness (from about 0.6 % C, but then sharply decreasing hardness, if no refrigeration - for example in liquid nitrogen - to convert the increasing austenite takes place ). Actual cause of the rising austenite and the associated hardness losses are the sloping with increasing carbon content martensite and Martensitfinishtemperaturen far below room temperature, as shown in Figure 2, the heat treatment for producing martensite is called hardening ( austenitizing and quenching martensite ). The curing is combined with the starting (first tempering stage to 200 ° C to remove the glass hardness). The hardenability of a steel can be given by the ideal critical diameter.
The martensite
The temperature at which the martensitic transformation begins, is below the equilibrium temperature T0, the same in the austenite and martensite have identical composition free enthalpies G. This is illustrated by assuming a linear relationship GT schematically in Figure 3.
The undercooling below T0 provides the free energy? G (T0 -MS) for the existing Gitterscherungen, for the newly created interfaces and lattice defects generated. The austenite - martensite transformation stops when it reaches the martensite finish Mf T0 and hence MS and Mf are strongly dependent on the alloying elements from.
Since the energetically more favorable cementite ( Fe3C ) can not arise because of the lack of diffusion, the formation of energy-rich martensite begins as soon as MS is not reached. On further cooling results in a proportional to the undercooling of martensite, which reached at Mf 100 %. The unconverted austenite is seen vividly increasingly deformed by the formation of martensite, so that an ever higher driving force and thus hypothermia is necessary to continue the conversion. Then also be explained that if Mf is below room temperature, an appropriate amount of residual austenite remains, which can be further converted only by freezing. From about 1.5% C, the austenite can no longer convert by freezing in liquid nitrogen. In contrast, a diffusion controlled transformation by tempering is always possible.
The following table gives an overview on the procedures for the computation of MS. Most approaches assume a linear- additive influence of alloying elements on the martensite start from. In fact, a coupled effect of alloying elements is available as attachment 8 included in the table for carbon.
Austenite
Figure 2 shows the dependence of MS and Mf on the carbon content of unalloyed steels. Falls below Mf the quenching temperature Tu ( eg, room temperature ), so remains in the austenite transformation structure that deals with the empirical relationship
Can be described. B is a temperature-dependent constant: B (20 ° C) = 1.1 x 10-2 (° C ) -1 and B ( 196 ° C) = 7.5 x 10-3 (° C) -1. In reality, MS, as Figure 4 shows, on the cooling rate dependent.
Microstructure
The martensitic transformation of the face-centered cubic ( fcc) high-temperature austenite phase ( γ - solid solution ) in the tetragonal centered metastable martensitic phase is via coordinated Gitterscherung, wherein the atoms move in the comparison with the atomic distance only small sections and to maintain their neighborhood relationships. This process can be formally explained by using Figure 5. In neighboring elementary cells of the Austenitgitters, with the lattice constants cA virtual Mart headquarters Ellen exist with the dimensions cM '= cA and aM ' = cA of √ 2 / second To obtain the correct lattice constants of the martensite cM and aM, cM must 'is decreased by about 20% and aM ' be increased by about 12 %.
In the octahedral interstices of the austenite martensite go into octahedral sites of the martensite, so that no diffusion of substances included in these gaps carbon atoms is necessary. The occupation of the so-called Z- positions of the Martensitgitters leads to a tetragonal distortion. The ratio aM / cM is shown in Figure 6 to the strong dependence on carbon content. After valid quantitatively
And
The tetragonality of the martensite is influenced by alloying atoms in a characteristic manner.
{111 } { 110} A → M
<110 > A → <111 > M
There. This " Kurdjumov -Sachs relationship " is confirmed experimentally with carbon contents above 0.5 mass percent. Since the products resulting from the conversion to the austenite - martensite interfaces stresses are reduced by adjusting distortions, the orientation of the habit plane can not be inferred clearly from Figure 5. According to the Christian habit planes { 111} A, { 225 } { 259 } A and A are observed depending on the carbon content.
Since the fcc γ - solid solution is packed atomically denser than the bcc α - solid solution or the trz - martensite occurs, the γ → α transformation with a volume increase, which has the dependence shown in Figure 7 on the carbon content. This volume increase resulted from a change in length perpendicular to the habit plane and a plane parallel to its shear. The macroscopic shear angle, as illustrated in Figure 8, on the basis of surface relief, caused on the polished surface is determined. It is about 10 °.
The mentioned adjustment deformations determine the forming Martensitmorphologie. The table below summarizes the observed at different carbon contents habit planes, orientation relationships and fine structures of unalloyed steels together. Arising in small carbon contents Massivmartensit consists of packets of parallel slats in former austenite grains. For larger carbon contents form increasingly beside the slats plate-shaped areas from which include Restaustenitgebiete.
When the martensite would be done about classical homogeneous or heterogeneous nucleation, then such a large amount of energy would be required to by Pitsch, as it can not be won at the low temperatures of the existing MS- thermal atomic motions. A thermally activated nucleation of martensite is not possible. It is believed, therefore, that in the austenite are already available so-called preformed germs. This can gradually grow with a smaller than the critical size by thermal fluctuation. After reaching the critical size unchecked growth takes place at speeds up to 5000 m / s
Another conversion mechanism which does not change the carbon content of austenite and ferrite, is the massive transformation. It occurs in very small carbon contents and is based on the rapid movement of incoherent interfaces. By heterogeneous nucleation, the growth is controlled by diffusion, whereby austenite can be exceeded. In this case, the diffusion path of one to two atomic distances required for the lattice atoms. The formation temperatures of massive ferrite is higher than that of the martensite. An increase in the cooling rate leads to a suppression of the conversion of solid benefit of the martensite. Depending on the formation temperature, one can distinguish a fast and a slow massive transformation. Near the equilibrium temperature occurs a diffusion of Interstitonsatome in the moving interface and thus determines its speed. At lower temperatures the diffusion ability of the Interstitionsatome decreases so much that the rate of conversion is limited only by the mobility of the boundary surfaces.
Structural modifications of the martensite
Depends on the temperature and the alloy content (particularly the carbon content ) are formed of different modifications of the martensite microstructure in the material.
Lath martensite
The lath (also called lath, block or low carbon Massivmartensit, martensite lath in English ) is formed at higher temperatures ( at temperatures closer to the martensite start temperature ) and lower carbon contents of about 0.4-0.5 % C, in hypoeutectoid steels. It consists of flattened lancets, which are tightly packed next to one another to form layers, and then layered on solid blocks. It is predominantly found in unalloyed and low-alloy steels with less than 0.4 % C, but also in alloys of iron with <25 % nickel. Characteristic is the training in the form of packets of parallel < 1 micron wide lancets, without leaving residual austenite. A fabric made from 100 % lath martensite arises only when the carbon content is less than 0.3 %.
Lath martensite has a high dislocation density (up to 1012 cm -2) and is much more malleable than the Plattenmartensit because it arises at higher temperatures and thus created through the Gitterumklappen elastic strains can degrade better by sliding and recovery mechanisms.
Plattenmartensit
The Plattenmartensit (also called needle-shaped, acicular, twinned martensite in the English plate martensite or twinned martensite ) occurs at lower temperatures and higher carbon contents of about 0.8-1 % C, ie for example in hypereutectoid steels. The martensite does not grow in this lancet, but in sheet form, wherein the stacked plates are not parallel, but are at different angles to each other. In the spaces remains austenite.
The plates are prevented on the one hand through the grain boundaries of the austenite, and the other by the plates already arisen at higher temperatures on growth, so that the newly created disks are getting shorter and shorter with time, and wet the area more dense. The mean length of the plates varies between a quarter and a third of the original austenite grain size.
The Plattenmartensit is less deformable than the lath martensite, because at lower temperatures the primary mechanism of plastic deformation are not sliding and recovery operations, but the formation of twins.
Mischmartensit
In the area between the lancet and the Plattenmartensit, ie between about 0.5-0.8 % C, produced an intermediate form, the Mischmartensit.
Martensite in chrome - nickel steels
In the martensitic transformation of stainless steels, the following ways are possible:
In case a), the martensite as the plain carbon steel is formed, described by a cooperative Gitterscherung.
In case b) the bcc ( body-centered cubic is ) lattice by shearing of individual atomic layers in the hcp ( hexagonal close-packed ) lattice converted. The orientation relationship of the two gratings is:
For the case c) then includes a shear direction normal martensite lattice. The following orientation relationship exists between the ε and the α' - lattice:
Mason diagram
For chromium -nickel steels, the quenched after solution annealing structure is described by the Maurer - diagram (Figure 9). With increasing chromium and nickel contents, the structure is first Pearlitic - Ferritic, Martensitic then - Pearlitic, then Austenitic- Martensitic and finally purely austenitic.
Schaeffler diagram
At higher chromium contents delta ferrite occurs on that one, especially when welding is undesirable microstructure, whose possible formation can be assessed in Schaefflerdiagramm (Figure 10). Here also the stabilizing effect of other alloying element on the ferrite and austenite is considered by them to be grouped into classes according to their effect on Cr or Ni- equivalents.
Cr equivalent =% Cr % Mo 1.5% Si 0.5 % Nb 2% Ti
Ni equivalent = % Ni 30% C 0.5% Mn
Increasing Cr equivalents result in an immediate austenitic - martensitic structure and then to austenite with very high proportions of delta ferrite. Increasing Ni equivalents opposite effect and reduce the delta-ferrite to a purely austenitic structure is formed.
Martensite in titanium and titanium alloys
Pure titanium can exist in two different crystal modifications. Above 882 ° C as the high-temperature phase β - titanium in the bcc crystal lattice and below 882 ° C as the hexagonal ( hcp ) α - titanium. When addition of alloying elements, a mixed crystal region forms. A distinction being made elements, which stabilize the α - region ( Al, Sn, Zr, O, N) and those that stabilize the β - region ( Mo, Fe, V, Cr, Nb). Refer to Figure 11
Formation of Ti - martensite
When quenching from the β - region in water or oil to temperatures of the α - region, it can come to the martensite - typical diffusionless Gitterscherung. Since the Titanmartensit, in contrast to steel, no forced dissolved alloy elements, there is no solidification. However, the material properties of titanium alloys can be influenced by adjusting the microstructure. For example, can be quenched from the α ß region and a fine structure with rounded β - blades are adjusted by subsequent annealing, which has favorable strength values .
Application Examples
Today, plates are used in the automotive industry, containing martensite. Generally one speaks of multiphase steels. In effect, this is the dual-phase steels, TRIP steels, TWIP steels and referred the Martensitic steels. These are characterized by high strength and can still transform relatively well.
Another use of martensite in tool steels, especially in maraging steels.
The formation of martensite can be nicely seen in the Katanas differential hardening of Japanese Tamahagane steel.