Influence of Morphology Pattern of Genuine Damascus Steel on the Nature of Relief of Fracture Surfaces

It has been revealed that the structure of the transverse and longitudinal parts of the genuine Damascus knife of the XVIII century consists of troos-tite-carbide of the banding. The microhardness of the troostite layers is 450 HV; the carbide layers reach 950 HV. It is shown that such a banding structure in hardness will have a huge impact on the formation of a naturally patterned genuine Damascus structure. It is established that the form of the main drawings of the genuine Damascus pattern consists of separate elements of the topographic contour: hollows, hills, saddles, crosses and wavy dunes. It is shown that the damask pattern is a set of individual elements of the macrostructure alternating in a disordered form on different local areas of the surface. It is established that the destruction of genuine Damascus steels occurs under the action of two or more mechanisms, during the implementation of which traces remain on the surface of the fractures in the form of relief details of a mixed type. The mechanisms of mixed destruction are a combination of intragrain cleavage, micropore fusion and intergrain cracking. It is established that in order to implement the mechanism of separation of the main crack in the direction of the chopping blow, an optimum is required in the dispersion of troostite (61.8%) and carbide (38.2%) layers in genuine Damascus steel, the ratio of their volumes obeys the golden section rule. It is established that the damask pattern, consisting of elements of sinuous lines of a topographic contour, does not significantly affect the level of mechanical properties. This factor is a category of the quality of banding structures in genuine Damascus steel.


Introduction
The morphology and quality of the patterned surface on antique Damascus blades raise many questions from technical specialists. Without comprehensive structural studies, the answers to these questions lead to an incorrect interpretation of the damask pattern. Confusion in the identification of Damascus structures contributes to the fact that any pattern obtained on the surface of plates made of modern alloy steels is classified as patterns of genuine Damascus steels.
A number of scientific articles are devoted to the study of patterns, chemical composition and mechanical properties of genuine Damascus steels [1]- [21].
However, these articles do not pay attention to the relationship between the genuine structure and the nature of destruction.
As a rule, the destruction surface of samples from Damascus steel was eva- Based on the data obtained, it will be possible to control the creation of a new variety of modern Bulat, achieving high-quality products. Taking into account the above morphological parameters, a new opportunity will appear for the systematization of Bulat varieties, both ancient genuine Damascus steels and modern Tools steels.

Materials and Methods
As the research material has selected a fragment of the Iranian Bulat knife  is visible on a fragment of the surface of the knife blade. The knife blade has a dark shade (troostite matrix), the pattern on the blade is light lines (carbide banding). The chemical composition of genuine Damascus steel was determined using an ARL 3460 optical emission spectrometer. According to the chemical analysis, the Iranian knife was assigned the Ds15P steel grade. In the marking, letters and numbers mean the following: Ds-genuine Damascus steel, containing less than 0.05% Manganese (Mn) and Silicon (Si), each separately; 15-the average mass fraction of Carbon (1.45%… 1.53%); P-Phosphorus with a mass fraction of at least 0.1%, but not more than 0.25% (0.171%… 0.192%). This indicator for phosphorus is hundreds of times higher than the threshold value for high-quality tools steels. All other alloying elements did not exceed thousandths of a percent. The studied fragment of the genuine Damascus steel according to the modern classification is unalloyed high-carbon steel with a high phosphorus content.
In modern industry, high-carbon steels with such a carbon and phosphorus content are not used. According to its chemical composition, it should be attributed to steels with special properties used in the manufacture of cold weapons in the Indo-Persian region in the XVII-XIX centuries.
For comprehensive studies of the fracture surface, a fragment from the tip of a 50 mm long Iranian knife was cut out. It has a characteristic damask macrostructure ( Figure 2). The morphology of the damask pattern was formed under natural causes of smoothing the peaks of hilly relief during forging. The structure of the transaction section of the blade has a characteristic banding structure ( Figure 2). The width of carbide layers on average does not exceed 40… 45 microns, and the width of troostite layers on average is at least 75… 80 microns.

Results and Discussions
When conducting structural studies, it was found that the structure of the trans-  According to electron microscopy data, it is confirmed that the eutectoid matrix is a pure quenching troostite with an interplate distance of no more than 100 nm. The volume fraction of lamellar troostite is about 80% of the total fraction of matrix structures (Figure 4(b)).
In the process of structural studies, troostite sites were found that differ from   Another part of the excess carbides is large elongated particles of an oblong shape. In places, you can observe thickening, tightening, kinks and accretions.
Large oblong carbides have sizes from 6 microns to 12 microns. The volume fraction of such carbides in the Damascus structure is at least 85% of the total excess carbide phase ( Figure 5(c)).
After conducting structural studies, it was found that the oblong carbide consists of separate isolated particles, outwardly resembling sea pebbles with a thickening in the middle part. Thus, large oblong carbides are not monolithic formations.
Large oblong carbides make the main contribution to the formation of carbide banding and, accordingly, the morphology of the details of the fracture relief. The relief of the damask surface is formed both for natural reasons during the forging process and with artificial relief application during drilling and stuffing dents. The natural relief combines sweeping undulating irregularities with hilly convex local areas. Let's decipher the fundamental concepts that lay the foundation of the Damascus structure.
The natural wavy relief is characterized by the absence of pronounced irregularities. It is formed during the final forging of the Damascus blade by the rounded surfaces of the hammer strikers. Due to the difference in the hardness of the layers, the banded structure of the metal seems to roll over one another in the longitudinal and transverse directions, creating wavy folds of higher-strength layers. As a rule, the crest of the wave with such deformation does not exceed 500 microns. The wave-like shape of the projection lines is one of the elements of the damask structure, an arbitrary combination that gives an elegant pattern in its beauty, giving the impression of sea ripples or sand dunes. Such a pattern in the East is called Kum Hindi, which means Indian wave.
Natural hilly relief is a large variety of wavy relief. It is formed mainly from a set of local bulges separated by longitudinal saddles and hollows (Figure 6(a) and Figure 6(b)). According to the shape and structure of convex and concave surfaces, this relief is characterized by irregularity along the length and significant fluctuations in the heights of convex hills, sometimes exceeding 1000 microns. The forms of the projection lines of the damask pattern represent elements of a topographic contour such as saddles, hollows and hills, an arbitrary combination of which gives the impression of flowing jets and whirlpools. Such a pattern in the East is called Taban or Kara Taban, which means resplendent or black-resplendent.
Artificial relief is formed in strictly defined places. Artificial alteration of the natural relief of the surface is used to enhance the contrast of the damask pattern. It is obtained by pressing dents or cutting (chopping, drilling) various irregularities on a wavy or hilly surface of a forged billet. When these artificial irregularities are smoothed with a hammer, after grinding and etching patterns typical of genuine Damascus steels of the ancient East are formed.   The element of the damask structure, which looks like eyes, with relatively wide spherical horizontals and a spot in the center in the form of a pupil, is a topographic element of the figure "Basin", that is, a closed cup-shaped dent with gentle slopes (Figure 8(a)).
The element of the damask structure, which is horizontal sinuous lines superimposed on each other after a certain step in the form of slopes or steps, is a topographic element of the figure "Hollow", that is, a linearly elongated depression descending in one direction (Figure 8(b)).
The element of the damask structure, which is a concavity located between two adjacent peaks of the bulges in the form of hills, is identified as a topographic element of the figure "Saddles" (Figure 8(c)).
Moreover, the term "Hilly," from topography, should be interpreted as a domed or conical elevation on local areas of the surface. Irregularities in the form of hills can be rounded or oval in shape with gentle slopes. This element of the damask pattern is represented in the form of closed irregularly shaped horizontals encircling each other.
Another picture is observed when considering an element of a damask structure of the "Crosses" type, which is a cruciform concavity between four hilly bulges (Figure 8(d)). This complex element of the damask structure is sur-  The most common element of the damask structure should be considered "Dunes wavy", that is, an irregularity linearly elongated along the blade in the form of elevations and deflections (Figure 8(e)). In this case, the "Dunes" are a chain of oblong bulges and concavities in the form of wave crests. In the longitudinal section, the crests of the dunes are undulating horizontal. On the damask pattern, the image of "Dunes" has a sinuous appearance, which gives the pattern a wavy character.
The typical element of the damask structure of the "Knottiness" type was found for layered structures, which is a sinuous arrangement of a cluster of striped structures (Figure 8 And most importantly, the overlay of the topographic map on the surface of the damask pattern showed that it coincides with its main parameters. All forms of projections of the pattern lines are described mathematically; therefore, when using a specially shaped tool, it is possible to control the damask macrostructure.
However, it should be understood that the damask pattern, consisting of elements of sinuous lines of a topographic contour, does not significantly affect the level of mechanical properties. This external structural factor is rather a category of the quality of banded structures in genuine Damascus steel. Thus, most of the studied samples made of genuine Damascus steel have a layered structure, differing from each other either by external features obtained using shaped forging. For example, Bulat varieties with a fibrous structure will not be inferior elasticity parameters to Bulat varieties with a mesh structure if identical thermomechanical treatment is properly carried out in them.
As described above, the damask structure has a characteristic carbide banded with minima and maxima in hardness; therefore, in the process of impact destruction, the main crack will leave different traces on the fracture surface in the form of characteristic relief details.
The destruction of anisotropic materials such as Damascus steel will occur under the action of two or more mechanisms that cause the destruction of a mixed type. The mechanisms of mixed destruction are a combination of intragrain cleavage and micropore fusion in a small local area, or a combination of chipping and interlayer cracking, etc. The reasons for the occurrence of destruction by a mixed mechanism are enormous, and its consequence may be the details of the fracture surface relief visible on the fractogram, which are difficult to identify.
When examining the fracture surface of genuine Damascus steel, characteristic details of the fracture relief were revealed in troostite and carbide layers (Figures 9(a)-(c)).
On the fractogram of the fracture surface of the troostite layer of the damask structure, relief details consisting of cleavage facets are found (Figure 9(a)). Such a relief fracture by the nature of the gloss is sometimes called a silky fracture. Cleavage is a type of destruction is splitting along crystallographic planes in places where slipping deformation is difficult. One of the main features of the destruction of troostite layers by chipping is relief details in the form of cleavage facets with a wavy pattern, which are steps directed along the same general slip plane. Striving to minimize the internal energy of the fracture of the second and third-order cleavage steps unite like waves in the direction of crack propagation.
Individual facets of cleavage with a wavy pattern are separated by ridges of the tears-off, on which the forms of the relief are identified in the form of small cups. The Cup-and-cone fracture formed when micropores merge. Thus, the destruction of the troostite layer in the damask structure occurs by a mixed mechanism, which is a combination of facets of cleavage and fusion of micropores on the ridges of tears-off. When studying the fractogram of the fracture surface of the carbide layers of the damask structure, randomly distributed carbides embedded in the troostite matrix are observed (Figure 9(b)). The destruction of the carbide layers occurs by a mixed mechanism combining relief-free cleavage and intergrain cracking.
The details of the relief of the fracture surface of the carbide layers consist of smooth shiny facets of the cleavage which do not have a wavy relief and ridges of tears-off.
The size factor of the layered structure in the presence of a brittle carbide phase uniquely affects the nature of the fracture. For example, the details of the relief of the fracture surface of the carbide layer indicate the absence of a reserve of plasticity. In these layers, stress relaxation does not occur in front of the moving crack front. It is possible only with a large stratification of the damask structure. If the genuine Damascus steel consists exclusively of finely dispersed carbide banding, it will be brittle and will break under small bending loads. Therefore, it is necessary to have an optimum thickness of both brittle carbide layers and more viscous troostite layers.
It is desirable that high-purity troostite layers always exceed high-strength carbide layers by volume fraction. Ideally, the optimum in the ratio of carbide and troostite layers in genuine Damascus steel obeys the golden section rule and makes up a troostite matrix with a volume fraction of 61.8%, and carbide layers-38.2%.
As a result, in the direction of the chopping blow on the end part of the knife blade, the main front of the crack will grow along the direction of the layers. The hardness of the carbide layers here is 950 HV, and the hardness of the troostite Materials Sciences and Applications layers is 450 HV on average. In carbide layers, the rate of destruction will prevail over troostite layers. The crack begins to branch along with the layers, reducing the propagation energy due to the separation of its front. In the process of destruction, a wave front of the main crack is formed. The wave's topography of the fracture surface indicates the mechanism of dividing the main crack front into several parts. In general, catastrophic destruction does not occur. The impact resistance of the genuine Damascus knife will increase.
It is in the implementation of the mechanism of separation of the main crack front in the layered damask structure that the uniqueness of the ancient Eastern technology of manufacturing bladed weapons consists. The damask structure has a good margin of survivability in case of shock destruction. With the elasticity of a spring with good impact resistance, the genuine Damascus blade has become a legend of the Indo-Persian region.

Conclusions
1) Layer-by-layer chemical analysis of the genuine Damascus steel showed that the dark areas matrix corresponds to high-purity eutectoid steel with a troostite structure, microhardness 450 HV. The volume share of lamellar troostite forms is 80%, divorced troostite forms are 15%, and globular troostite forms are 5% of the total share of matrix structures. The destruction of the troostite layers occurs by a mixed mechanism, which is a combination of relief details in the form of facets of cleavage with a wavy pattern, framed by ridges of tear-off formed by the fusion of micropores.
2) Layer-by-layer chemical analysis of the genuine Damascus steel revealed that the light areas are carbide-troostite structures corresponding to white eutectic whit cast iron in terms of carbon content and impurities. The microhardness is at least 950 HV. The destruction of carbide layers occurs by a mixed mechanism combining relief-free cleavage and intergranular cracking. The details of the fracture surface relief consist of a scattering of carbide phases against the background of smooth facets of the cleavage without ridges of tear-off.
3) It is shown that the shape of the damask pattern on the ancient Eastern blades does not appear by chance. The regularity and repeatability of the main projections of geometric shapes on different local areas of the blade surface are traced. Together they create a complete picture of the damask pattern. All forms of projections of the pattern lines are described mathematically, so when using a special tool, you can control the damask structure.
4) It is assumed that in order to implement the mechanism of separation of the main crack in the direction of the chopping blow, an optimum is needed in the dispersion of troostite (61.8%) and carbide (38.2%) layers in genuine Damascus steel, the ratio of their volumes obeys the golden section rule. The impact resistance of the Damascus knife is increased by reducing the energy intensity of the crack front when it is divided into separate parts.
5) It is established that the damask pattern, consisting of elements of sinuous lines of a topographic contour, does not significantly affect the level of mechan-

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.