S. I. Chugunova

Plasticity characteristic obtained through hardness measurement

Abstract A characteristic of material plasticity δH is proposed. δH is determined as a part of plastic deformation in the total deformation during indentation. The following analytic expressions for the elastic deformation ϵe and for the total deformation ϵ on the contact area indenter-specimen in the direction of loading force are obtained, ϵe = 1.08(1 − ν − 2ν2)Hν/E, ϵ ≈ 0.076, where Hv is the Vickers hardness, E is Youngs modulus, ν is the Poisson ratio, and δH = 1 − (ϵe/ϵ). The δH value is calculated for various crystalline materials at different temperatures and in different structural states. δH is consistent with the concept of plasticity established before, and to characterize the influence of temperature, alloying and strain hardening on plasticity. The necessary condition for revealing ductility at tension and bending is δH ⩾ 0.9. δH can be used as a plasticity characteristic of brittle materials.

Temperature dependence of hardness in silicon–carbide ceramics with different porosity

Abstract The Vickers hardness HV of silicon–carbide ceramics (fully dense and with the porosity θ=5%, 16% and 20%) was investigated in the temperature range of 20–900°C. Athermal regions of hardness were detected at low temperatures and the length of these sections increases with the growth of porosity. The results obtained allow the postulation that the formation of the hardness print, in these sections, is conditioned by fracture (but not by plastic deformation as at higher temperatures), and the hardness, in these sections, corresponds to material fracture stress, but not to the yield stress of material. The dependence HV(θ) was described by the Ryshkevitch equation HV=HV c exp (−Bθ) ), where HVc is the hardness of compact material and the value of constant B is determined experimentally for all temperatures in the investigated range. The fracture toughness of ceramics (K1c) decreases monotonically with increasing θ. The diagram of the temperature dependence of hardness for crystals with high level of Peierls–Nabarro stress was proposed. In this diagram, the hardness can be determined by 3 processes: plastic deformation, brittle fracture and phase transformation during the indentation. This diagram may be extended to bcc refractory metals if there is twinning during low-temperature indentation.

Low and high temperature hardness of WC-6 wt%Co alloys

Abstract This paper reports hardness measurements on WC-6 wt%Co of three different grain sizes in the temperature range from −196 to 900 °C. Coarser grades have been found to soften with increasing temperature at a higher rate than finer grades. It has been confirmed that hardness decreases with increasing grain size over the whole range of temperatures and it has been shown that the decrease in hardness with increasing grain size follows a Hall-Petch-type relationship at all the temperatures tested.

Mechanical properties, indentation and dynamic yield stress of ceramic targets

Summary The dynamic yield Y D (which is determined usually in dynamic experiments from Hugoniot elastic limit) can be calculated for ceramic targets from static indentation experiments using Vickers hardness and plasticity characteristic δ H . The magnitude of Y D approximately equals to the static yield stress σ s , calculated by Marsh equation. The employment of a set of diamond indenters with different angles of sharpening makes possible to construct stress-strain curves for ceramic materials and to determine yield stress at different deformation degrees.

Resistance of covalent crystals to microindentation

ConclusionsA study was made of the resistance of various covalent crystals to microindentation, involving comparison of reduced and unreduced hardness values. A parameter p-“unit” indenter load necessary for the elastic-plastic penetration of an indenter to unit depth — has been introduced.A simple method is proposed for the determination of the elastic and plastic components of the displacement of an indenter during its penetration. By analogy with p, another two parameters, pp and pe — indenter loads necessary for the plastic and elastic penetration of an indenter to unit depth, respectively — have been introduced. It is shown that the ratio of the elastic to the plastic strain component varies from material to material, and is determined by the HV/E (or σs/E) ratio.The relative contributions from the elastic and plastic components to the total strain caused by the penetration of an indenter change with load, resulting in marked deviations from the law of similarity and in increases in hardness at small indenter loads.The elastic resistance of a material to microindentation, as characterized by the parameter pe, enables the elastic modulus of the material to be estimated, and a method of doing this is described. On the other hand, the plastic resistance of a material to microindentation, in particular the parameter pp determines the plastic deformation resistance of the material more accurately than does its hardness. At small indenter penetration depths the contribution from plastic deformation decreases compared with that from elastic deformation, resulting in a marked increase in hardness.

The resistance of silicon carbide to static and impact local loading

The physical nature of the resistance of SiC crystals to static and local impact loading has been examined. Investigation of the temperature dependence of hardness for SiC crystals allows the determination of the characteristic deformation temperature (T * 1600 K), the parameter a that characterizes the degree of covalence in interatomic bonds (a 6) and the temperature range in which a phase transition under pressure during indentation is possible (T < 800 K). Indentation technique gives possibility to construct stress-strain curves for brittle materials and to determine Hugoniot Elastic Limit. During dynamic penetration of a kinetic projectile into a SiC target the phase transition takes place.

Plasticity determined by indentation and theoretical plasticity of materials

Characterization of the plasticity of materials by the part of plastic strain in the total elastic-plastic strain and application of this characteristic at indentation is considered. The dependence of the new plasticity characteristic on the structure and temperature is discussed. The concept of theoretical plasticity is introduced and the theoretical plasticity is calculated for a number of materials.

Pressure induced phase transition in ceramic materials during indentation

It was shown by authors that if there exists pressure-induced phase transition of ceramic material at the critical stress Pc so that Pc<H (H is the material hardness) the phase transition takes place during indentation in the thin layer under indenter. During kinetic energy projectile penetration into ceramics the stress must be of the same level as during indentation or more. X-Ray investigation of the powder from the crater of self-bounded silicon carbide destroyed by kinetic projectile has been carried out. In the initial specimen silicon carbide had the structure of the most abundant 6H hexagonal polytype with 6 layers in the lattice cell. But in the powder from crater there was the 33R rhombohedron polytype with 33 layers in the lattice cell together with initial 6H polytype. By this way phase transition 6H→33R was observed in the thin surface layer of silicon carbide. This transformation is usually connected with sharp increase in dislocation density and can be an effective mode of energy absorption.

Improved core model of indentation and its application to measure diamond hardness

A model of the indentation using conical and pyramidal indenters has been proposed, in which not only a sample but the indenter as well are elastoplastically deformed and their materials obey the Mises yield condition. These conditions are characteristic of the measuring of diamond hardness through a diamond indenter. The model that has been proposed generalizes and refines the known simplified Johnson’s model, which uses an elastically deformed indenter. The proposed model makes it possible to determine approximately the sizes of elastoplastic zones in the indenter and sample, the effective apex angle of the loaded indenter and effective angles of the indenter and imprint after unloading. Based on this model a procedure of the determination of the sample and indenter yield strengths (Ys and Yi, respectively) has been developed, in which the relations that use the experimental values of the effective angle of the sample imprint and measured values of the Meyer hardness, HM (mean contact pressure) are added to theoretical relations of the indentation model. The developed computational procedure was applied in indentation experiments on synthetic diamond at the temperature 900°C (at which diamond exhibits a noticeable plastic properties) using natural diamond pyramidal indenters having different apex angles. According to the proposed model, the stress-strain states of samples and indenters have been investigated and their yield strengths and plasticity characteristics were defined. The stress–strain curve of the diamond in the stress-total strain coordinates has been constructed. The strain hardening of diamond was also studied.

Effect of the granular composition of the initial powder on the structure and strength of reaction-sintered silicon carbide

ConclusionsThe strength of SSC is characterized by great structural sensitivity. With constant phase composition, only in consequence of variation of the dimensions and ratio of elements of the structure (grain size of silicon carbide and of silicon inclusions) can strength be changed by a factor of more than two. Strength increases with overall reduction of the size of elements of the structure. Materials with increased crack resistance form in the regions of two-fraction compositions adjacent to the apexes with the finest grain of SiC-I.The existence of a strong correlation between the systems in regard to strength, crack resistance, and content of Sifr indicates that the regularities we found are correct and of a general nature, independently of the sizes of the initial fractions taken for constructing the systems.Beginning at the melting point of silicon, the role of the load bearing structural element is assumed by the silicon carbide carcass whose strength is also strongly dependent on the granulometric situation developing in the system. This dependence is of a different nature than with SCC, which is confirmed by the lack of correlation between the strength of SCC and of material with free silicon distilled off.