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The particle size of the starting powder, like the grain size of sintered specimens, does not have a pronounced effect on the mechanical properties of sintered iron because the effective length or a slip plane is determined by the size of the smaller fragments bounded by the pores in the particles and interfaces. The strength and ductility characteristics of sintered iron in a wide range of porosity variation are satisfactorily described by the expression obtained in [3]. The strain-strengthening of sintered porous iron, like that of polycrystalline nonporous iron, has a parabolic character, with the exponent of strainstrengthening being independent of porosity and the coefficient of strain-strengthening decreasing with, increasing porosity according to the same law as the limit of proportionality.

Clearly, therefore, it is important to group diamond processing conditions according to one of the key properties of diamond, viz., abrasivity. This can be done by cluster analysis and by the principal components method. These methods make it possible to combine a rigorous formal approach eliminating subjective errors in the model construction stage (classification) with the experience and intuition of the investigator in the interpretation of results. The authors carried out such an analysis on 108 batches of micropowders ranging in grain size from 60/40 to 5/3 ~m (eight size groups). In each size group determinations were made of abrasivity and it was this characteristic that was employed as an attribute. Thus, the physical object under examination was described by eight attributes. Any multimensional observation (in our case eight-dimensional) can be geometrically interpreted as a point in multidimensional space. Geometric proximity of two or more points in this space indicates proximity of the physical states of the corresponding objects, i.e., their uniformity.

In this work an attempt was made, by lowering the temperature in the detonation zone, decreasing the particle flight velocity, and employing starting powders in which the WC particle size 20-40 mum predominated, to obtain detonation-deposited VK20 alloy coatings approaching closely in structure and phase composition hard-metal composites produced by the powder metallurgy method.

The authors employ x-ray diffraction and Vickers hardness testing in order to assess the effect of sintering on the microstructure, lattice parameters, porosity, and hardness of Mo-Ni alloys as well as the metallurgical influence of varying nickel content on these properties.

On the basis of results of our own and other authors' investigations into the variation of the strength of tungsten with temperature, a chart of fracture mechanisms of tungsten is proposed, which differs from the chart published in [1] in that it has two additional regions with characteristic mechanisms of fracture: partially tough fracture and tough fracture along grain boundaries. The concept of partially tough fracture is introduced, which is observed in a wide range of operating temperatures. The desirability is demonstrated of determining two upper cold-brittleness temperatures — T br u1 , which corresponds to a transition from quasibrittle to partially tough fracture, and T br u2 , representing a transition from partially tough to tough fracture. The positions of the cold-brittleness temperatures have been indicated on the chart of fracture mechanisms. It shows that the scientific classification of fracture mechanisms proposed in [2] makes it possible to analyze the effect of temperature on the mechanism of fracture and mechanical properties of tungsten and provides a convenient basis for the construction of a chart of fracture mechanisms.

1. In the investigated range of temperatures (800\2-1000\dgC) the variation in the mechanical properties of deformed low-alloy molybdenum as a result of annealing is determined mainly by the annealing temperature. Further increase in the annealing time from 10 to 500 h and the application of a load reaching 0.75 σ500 during the annealing process and little effect on the mechanical properties of molybdenum. 2. If the annealing of deformed molybdenum is carried out in a range of temperature in which primary recrystallization occurs, the additional application of a load during the annealing process somewhat increases the recrystallization rate and reduces the strength characteristics. 3. Under the influence of the applied load a cellular structure is again formed in part of the recrystallized grains, and a dynamic equilibrium is established between the deformation and recrystallization processes. With the selected level of stresses (σ≤0.75σ500) the hardening resulting from the formation of substructure under the influence of a load during the annealing process is small, since together with the grains in which the cellular structure is formed a large amount (50% or more) of recrystallized grains without the cellular structure remains. 4. The influence of a load during the annealing of deformed molybdenum does not give rise to a significant increase in the temperature corresponding to the transition to the brittle state. 5. By introducing disperse particles of a second phase it is possible to reliably stabilize the cellular structure in molybdenum at 1000\dgC under conditions involving the action of considerable loads for prolonged periods. Under these conditions the stress-rupture strength increases by several times.

The parameter α = H/kTm characterizes the resistance of a crystal lattice to dislocation motion. For crystals with a covalent component of the interatomic bond the parameter α increases with the degree of the interatomic bond orientation. At high temperatures α determines the value of the critical shear stress and the temperature t* at which a sharp change of the temperature dependence of the critical shear stress begins. It is shown that t* = , where c characterizes the slope of the curve τcr = f(T) at the temperature t*. Passage of the temperature t* is accompanied by a change in the mechanism of plastic deformation. The correlation of strength and plastic properties with the degree of the interatomic bond orientation is discussed. [Russian Text Ignored].

With the assumption that the ultimate stress σut at the tensile test with constant rate corresponds to the maximum in the curve load P–strain ϵ from the equation dP/dϵ = 0 and from the model developed by Hart [1] for the homogeneous deformation the condition was deduced for attaining (m is the strain rate sensitivity index, E is the true strain). This equation is solved with respect to σut and the corresponding strain Eut. Use is made of current ideas of work hardening of metals with b.c.c. lattice. The structural sensitivity of σut is discussed as well as the conditons under which the dependence of σut on the grain size is given by an equation analogous to the Hall-Petch equation for flow stress. Formation of the cell dislocation structure in course of the deformation decreases both the work hardening and the value σut. Conditions for unstability of plastic flow and for formation of “the neck” differ from that for attaining σut by the sign of the index m. An equation for the uniform deformation Ec and the corresponding stress σc may be deduced from the equations for σut and Eut by substitution of m for −m. Unter der Annahme, das die Grenzspannung σut beim Dehnungstest mit konstanter Geschwindigkeit dem Maximum in der Last (P)–Dehnungs(ϵ)-Kurve entspricht, wurde mit der Gleichung dP/dϵ = 0 und mit dem Modell von Hart [1] fur homogene Deformation die Bedingung fur das Erreichen von wobei m der Empfindlichkeitsindex der Dehnungsgeschwindigkeit und E die wahre Dehnung ist. Diese Gleichung wird bezuglich σut und der entsprechenden Dehnung Eut gelost. Dabei wird von gegenwartigen Vorstellungen uber die Verfestigung von Metallen mit k.r.z. Gittern Gebrauch gemacht. Es werden die strukturelle Empfindlichkeit von σut diskutiert sowie die Bedingungen unter denen die Abhangigkeit von σut von der Korngrose durch die der Hall-Petch-Gleichung analogen Gleichung fur die Flusspannung gegeben wird. Die Bildung der Versetzungsstruktur der Zellen wahrend des Verlaufs der Deformation erniedrigt sowohl die Verfestigung als auch den Wert σut. Bedingungen fur eine Instabilitat des plastischen Flusses und fur die Bildung des „neck” unterscheiden sich von denen zur Erreichung von σut duch das Vorzeichen von m. Eine Gleichung fur die gleichmasige Deformation Ec und die entsprechende Spannung σc konnen aus den Gleichungen fur σut und Eut durch Substitution von m fur –m abgeleitet werden.

The mechanism of microhardness print forming in diamond-structure crystals is considered to be the result of a local phase transition under the indenter with the formation of a metallic phase due to the high hydrostatic pressure generated in this case. It is shown that at the temperature T < 0.3 to 0.4 Tml, where the microhardness Hv is weakly temperature-dependent, one has Hv ≈ Pc(Pc transition pressure). In GaAs crystals, where Hv ≪ Pc, the athermal part in the hardness-temperature curve was not observed. It is shown as well that at the moment when the indenter entering the crystal is stopped there is a thin layer around it which still contains the metallic phase with high electrical conduction. The estimated thickness of the layer is ≈ 0.05 μm. The plastic deformation is supposed not to proceed at this moment because of the friction forces which oppose the squeezing-out of the metallic layer. [Russian text ignored].

An approximate solution is given for the dependence of the dislocation track length, l, around a microhardness indentation in single crystals on the load applied to the indenter (P), the temperature (T), and the duration under load (t): l ∼ Pm/(2m+1)t1/(2m+1) [exp −U/kT(2m+1)]. The assumption is made that dislocations move from the indenter in plane arrays and that the dislocation velocity is v ∼ τm exp (− U/kT), where τ is an effective stress, U is the activation energy. In a definite range of parameters the equation agrees satisfactorily with the literature data and those obtained in the present work, and may be used to estimate the dislocation mobility from the dislocation track length. The value of the ratio l/d (d is the indentation diagonal) is constant for ionic crystals but depends on the temperature and the load in the case of covalent crystals. l is connected with the yield strength τs as lτs ≈ const for covalent crystals (m ≈ 1), but l2τs ≈ const for ionic crystals (m ≫ 2). Es wird eine Naherungslosung fur die Abhangigkeit der Versetzungslange l um einen Mikroharteeindruck in Einkristallen von der angewandten Last auf den Stempel (P), der Temperatur (T) und der Dauer der Last (t)angegeben: l∼pm/(2m+1)t1/(2m+1) exp [−U/kT(2m + 1)]. Es wird die Annahme gemacht, das Versetzungen sich vom Stempel in ebenen Anordnungen bewegen und das die Versetzungsgeschwindigkeit v ∼ τm exp ( — U/kT) ist, wobei τ eine effektive Spannung und U die Aktivierungsenergie ist. In einem definierten Bereich von Parametern stimmt die Gleichung befriedigend mit Literaturwerten uberein und solchen, die in der vorliegenden Arbeit erhalten werden, und kann zur Berechnung der Versetzungsbeweglichkeit aus der Lange des Versetzungspfades benutzt werden. Der Wert des Verhaltnisses l/d (d ist die Eindruckdiagonale) ist-fur Ionenkristalle konstant, hangt jedoch von der Temperatur und Last im Fall der kovalenten Kristalle ab. l ist mit der Flusstarke τs wie lτs = const fur kovalente Kristalle (m ≈ 1), jedoch wie l2τs ≈ const fur Ionenkristalle (m ≫ 2) verknupft.

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.

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.

The effect of additions of transitional refractory metals on the structure and properties of Al–Zn–Mg alloys, made by ingot and PM routes, was investigated. The strength of the ingot alloys especially is increased by scandium and zirconium. The modifying action of scandium inhibits recrystallisation and precipitation of the fine-grained coherent Al3(Sc1–xZrx) phase. The effect is weaker in PM alloys where the ultra-high cooling rate during high pressure water atomisation produces the fine-grained structure. PM semi-products of the base composition Al–5Zn–3Mg and alloys without scandium are not recrystallised during heating to 500°C, whereas cast alloys of similar composition recrystallised on the hot extrusion stage at 400–450°C. Of the Sc alloys, Al–5Zn–3Mg–0·5Mn–0·7Zr–0·3Sc showed the highest strength (UTS = 651 MPa, YS = 596 MPa), whereas of the PM alloys without scandium Al–5Zn–3Mg–0·85Zr–0·22Cr–0·17Ni–0·15Ti alloy showed UTS = 618 MPa and YS = 553 MPa. At melt cooling rates of 105–106 K s–1 the total...