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Fiber Crystal Growth from the Melt reviews the growth, modelling, Part of the Advances in Materials Research book series (ADVSMATERIALS, volume 6).
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This article reviews published data on the mechanical properties of additively manufactured metallic materials. The additive manufacturing techniques utilized to generate samples covered in this review include powder bed fusion e. Read More. Figure 2: Summary of metal additive manufacturing processes, along with their commercial machine supplier names. There are eight different orientation and direction combinations.

Abbreviations: L, longitudinal; LoF defects are perpendic Adapted with permission from Reference Isolated defects dark spots are evident throughout the sample. Notch, fatig Figure The range of mechanical properties typically generated for structural materials.

The specific properties of interest depend on the intended application.

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Abbreviations: LEFM, linear elastic Figure Integrated multiscale approach for the development of additively manufactured alloys for structural applications. Preparation of monodisperse samples enables systematic As NCs grow with time, a size series of NCs may be is Energy diagrams E versus k show the complexity of the vale At low temperature, the energy cost to add charge to a NC gives rise to an onset to current flow, Coul The NCs are randomly close-packed in the solid with each NC separated from its nei C SAXS TEM images show that by tailoring the solvent The top of the image shows a glassy region formed at high growth rates.

e-book Fiber Crystal Growth from the Melt (Advances in Materials Research)

The sharp peaks that develop are from the inter-planar spacings of the deposited Ag su The disappearance of the sharp superlattice reflection upon heating indicates an order-to-disorder The triangles form spokes that extend radially outward from the center on the bottom of a glass vial. The red c Ordered rows of NCs form terraces, ledges, and kinks.

B Superlattice gr B At high magnification, the internal lattice structure of the NC building blocks is reso The periodic dot pattern running through the image arises from the coherent imaging of co The ordering and orientation of the NCs in the epitaxial thin film are evidenced by the A sintered, glassy CdSe NC solid provides a high refractive in The small NCs, having larger effective bandgap, are the donors of excitations; the larger The absorption and PL features of the small and large At large inter-particle distance D , NCs are electrically isolated and the superlattice is an insulator with a Coulomb bandgap.

As the inter It provides a convenient, effective, and low-cost method for the formation and manufacturing of The waveguides have different lateral dimensions and are Blaiszik, S. Kramer, S. Olugebefola, J. Moore, N. Sottos, S. White Vol. Approximation by a polynom of 3rd order with values of the constants shown. Experimental data are after Mileiko and Kazmin [27], Hurley [22] and Shahinian [21].

High temperature strength of some oxide fibres produced by ICM. Therefore, testing fibres in their mother matrix yields their virgin properties. Any further processing of the fibres changes their properties. An analysis of the bending tests performed under a varying load yields approximate creep characteristics of the composite in a particular specimen [ 42 ]. This allows excluding a number of the factors, which determine a usual scatter of the creep data of a batch of the material.

Performing such tests yields a characterisation of each specimen by its own set of the creep characteristics. The analysis is based on the following assumptions:. We call this value as creep resistance of a material on hours time base. For a beam of rectangular cross-section of height 2h and width b we have:. The solution was obtained neglecting a contribution of shear deformations to the displacement. The results of testing some fibres are presented in Figure 21 , Figure 22 and Figure In these Figures corresponding data for bulk single crystals are also plotted.

Creep resistance of bulk crystals of alumina-YAG eutectic obtained by using experimental data by Harada et al. Comparing creep resistance of ICM-fibres and bulk crystals for yttrium-aluminium garnet shows an excellent correspondence of creep resistance of bulk crystals and corresponding fibres grown from melt. A number of important conclusions can be drawn, in particular:. YAG fibre looks slightly better than the others. Its creep resistance differs essentially from the experimental point by Dokko et al. First of all, it should be noted that there are three most important characteristics of heat resistant composites, those being creep resistance, fracture toughness and oxidation resistance.

In the case of nickel alloys as matrices for oxide fibres composites sufficiently high oxidation resistance and fracture toughness are to be expected. That is why we will focus here on creep behaviour of nickel-based composites reinforced with ICM-fibres. On a fundamental study stage of developing composites the main bulk of creep tests are conducted by bending simple composite specimens and then calculating tensile creep characteristics of the composites according to the method developed in [ 42 ]. All other symbols are as in Eq.

Such an approach to the creep tests simplifies and speeding up the procedure. The experiments can be even more speeded up if the microstructural creep model developed in reference [ 51 ] is used in the analysis of experimental data [ 48 ].

Let us approximate Eq. An important point is a possibility to calculate the value of exponent q in the approximation.

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In the present context they are normally 10 -4 and 10 -1 h A comparison of exponent q calculated and the exponent values obtained in the experiments with 2-steps loading of specimens is presented in Figure One can see, that the calculations yield an acceptable result. The first one that is finding appropriate oxide fibres has been solved by inventing ICM. Calculated values of the exponent, q, and the values obtained in the experiments. The authors affiliated to NASA described what seemed to be the second problem. They found that sapphire fibres degraded during a liquid infiltration fabrication process.

This was observed in testing fibres extracted from the matrix [ 49 ]. They concluded that the sapphire fibre is not an appropriate reinforcement for nickel alloys. The experiment was correct, but the conclusion was not completely correct. It will be shown below that the matrix melt interacts with the fibre and causes the fibre degradation but after melt crystallised the matrix heals the defect resulting from the interaction provided there is a good adhesion between the fibre and matrix.

Moreover, preexisting defects can also be healed [ 42 , 50 ]. In fact, the conclusion mentioned can be a kind of the delusion. The real problem is to find ways to organize a strong interface between an oxide fibre and nickel-based matrix. Three commercially available Russian superalloys have being used as matrix materials. Since Ni-based alloys do not normally wet oxides, pressure casting is used to produce the composites. To fabricate specimens for mechanical testing, a special simple mould was designed that is composed of external quartz tube with one end closed, internal alumina tube with open ends, porous alumina plug at the top of the fibre bundle, and alumina bed at the bottom of the quartz tube [ 51 ].

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Crystallisation of the matrix was performed in the axial temperature gradient to make either a single crystalline or columnar microstructure of the matrix. Creep experiments were carried in 3-point bending by either step-wise loading and in such case the value of the exponent in a creep power law is determined directly or by a single load, in which case the exponent is calculated as shown above.

Then tensile creep characteristics of the composites are calculated according to a procedure also described above. It is important to note that the density of the composites under consideration is between 6. A compilation of the data presented in Figure 26 is plotted in Figure 27 as a correlation of the maxima of creep resistance of the composites reinforced with various oxide fibres on fibre volume fraction. First, the curves reveal non-brittle behaviour of the composites even when volume fraction of the brittle constituent reaches 0. Such behaviour is to be related to a special fracture toughness properties of MMCs referred to in the Introduction.

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Secondly, a sequence of the curves for 0 — 0. An invention of high entropy alloys HEAs , which are normally composed of five or more elements each with concentration between 5—35 atomic percent [ 53 , 54 ] stimulated the opening of a new chapter in the high-temperature metal matrix composites because i among such alloys there are some with both high melting points and sufficiently high oxidation resistance [ 55 ] and ii equiatomic compositions of HEAs promises an easier solution of the wetting problem while dealing with oxide reinforcements.

Specimens were again produced by pressure liquid infiltration of a fibre bundle located in quartz casting mould with the matrix melt. In these experiments, a molybdenum foil 25 microns thick covered the inside surface of the mould to prevent the interaction between the HEA melt and silica. The necessary gas pressure in this case, 1 — 1.

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The microstructure of the composite is illustrated in Figure The results of The X-ray microanalysis of the matrix shows that the base phase grey phase in the Figure contains all the elements in nearly equi-atomic ratios, tungsten being an exception. Temperature dependence of the strength is presented in Figure The experiments yields two interesting observations. First, the room temperature strength values are relatively low and this is to be the case since the specimens have large fibre volume fractions. The crack size arising because of fracture of a fibre cluster can be too large for the matrix to dissipate the elastic energy and arrest the crack.

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The probability of fibre cluster formation increases with the fibre volume fraction increases. A corresponding model of the composites failure behaviour was developed by Mileiko [ 1 ]. A particular value of V f max depends on fracture toughness of the matrix, fibre strength characteristics and a degree of non-homogeneity of fibre packing. Two factors contribute to this effect. The second factor is a shift of critical value V f max of fibre volume fraction to higher values as a result of an increase in the matrix fracture toughness with temperature. Composites of such a type are to be developed for high temperature use.

Hence, creep properties of the composites are of importance.

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  • At a prelimary stage of the work described in [ 60 ] no creep tests have been performed yet, the results of strength tests together with a previous experience of the present author in studying creep of fibrous composites [ 51 , 52 , 58 ] allow formulating some expectations. If a matrix melt wets a fibre then the matrix heals the surface defects that yields an increase in the effective fibre strength by a factor as high as 5 in the case of a fibre obtained by the internal crystallisation method see above Figure The effective fibre strength determines creep resistance of composites at temperatures, at which fibre creep is negligible.

    Also it is important to note that good wetting and, consequently, a strong interface make the basis of high creep resistance of the composites with large fibre volume fractions. As was mentioned in Section 1 the development of refractory metal alloys for high temperature usage meets a problem in reaching an appropriate balance of three main characteristics of such alloys those being creep resistance, fracture toughness damage tolerance , and oxidation resistance.

    Hence, the developments of fibrous composites with refractory alloy matrices can obviously be considered as a way to overcome a problem of reaching the balance between creep resistance and fracture toughness. Recent results show a possibility to obtain such composites with an enhanced oxidation resistance [ 61 , 62 ].

    Measurements of oxidation resistance and creep properties were performed on specimens composed of unalloyed molybdenum matrix containing 0. Three sets of the experiments were performed to evaluate oxidation resistance of the composites. A decrease in the mass of the specimens with time is plotted in Figure A significant difference in the oxidation kinetics of molybdenum reinforced with the fibre containing yttrium compared to both pure molybdenum and molybdenum reinforced with sapphire fibre is obvious.