Temperature Dependence Of Yield Strength Research Articles

Yield Limits of Plates at Extremely High Heat Flux

For heat fluxes ranging above 10 MW/m2 or so, solid surfaces usually experience large thermal stresses and degradation of mechanical properties. The resulting mechanical failure of such surfaces is a primary limitation to the design of thermal systems at extremely high heat flux. This investigation considers the elastic stresses in circular plates subjected to extremely high heat fluxes. A gaussian distributed heat load is applied to one surface of the plate and the heat flux at which yielding occurs is identified. Several candidate materials are examined, accounting for the temperature dependence of yield strength and other properties. The mechanical boundary conditions on the plate are varied. Figures of merit are given for the high flux performance of a number of materials.

Temperature Dependence of Yield Strength and Elongation of the Nickel-base Superalloy IN 738 LC and the Corresponding Microstructural Evolution

Tensile test data were obtained in the temperature range 20 C through 950 C with strain rates between 10{sup -2} and 10{sup -6} s{sup -1}. Extensive TEM investigations were performed after 0.2 % plastic strain and after fracture. Furthermore the fracture surfaces were studied in the SEM. Like in other superalloys, the yield strength decreased slightly between room temperature and 450 C. At higher temperatures it increased until it reached a maximum at about 750 C. Beyond 750 C a sharp decrease of strength was observed. These three extrema and the degradation of strength at high temperatures could be attributed to characteristic deformation mechanisms. With increasing temperature the following sequence of features was observed: faulted loops, anti-phase boundary-(APB)-coupled dislocation pairs, stacking faults bound by partial dislocations, and Orowan loops between {gamma}` precipitates. In a similar way tensile elongation exhibited a temperature dependence. In respect to fracture behaviour especially two aspects are important: (i) primary carbides crack after some few percent of plastic deformation and (ii) in principle every change in deformation mechanism was accompanied by inhomogeneous slip distribution. Hence poor ductility was caused by localized deformation in single slip bands leading to glide plane decohesion fracture. (orig.)

Elevated Temperature Tensile Properties of Ni3Al Alloyed with Chromium and Cerium Prepared from Elemental Powders.

Ni3Al matrix composites are potential materials for high temperature application because of their marvelous temperature dependence of yield strength. We have tried to make Ni3Al composites reinforced with alumina fibers that have excellent compatibility with Ni3Al matrix by means of powder metallurgy technique. As a result, we feel keenly that ductility of Ni3Al matrix composites at elevated temperature must be made better by alloying element. In this sence, chromium and cerium are added to Ni3Al with boron and tensile properties of these alloys were investigated.

The irreversible deformation of a duplex stainless steel under thermal cycling

Abstract In duplex stainless steels the constituent phases austenite and ferrite have different coefficients of thermal expansion. On varying the temperature a remarkable stress state develops which changes in sign from phase to phase. On pure thermal cycling the yield strength of the phases can be exceeded. Specimens of a forged duplex steel with uniaxial anisotropic microstructure were investigated in dilatometer experiments. Under repeated thermal cycles specimens deformed irreversibly with a monotonic accumulation of strains. In longitudinal specimens an increase in length was observed. Transverse specimens, however, decrease in length. A micromechanical model applying the finite element method is used to calculate the thermal properties of the duplex steel from the properties of its constituent phases. A mechanism driving the ratchet effect is proposed. The temperature dependent yield strength of the phases is responsible for the irreversible deformations. The model points out the importance of correct representation of the spatial distribution of the constituent phases in the micromechanical model. A systematic quantitative investigation of the effects of microstructure on the overall material behavior is based on a description of real and model microstructures by the same stereological parameters. Results from model calculations are in good agreement with results from experiments.

Molecular dynamics of silicon indentation.

We use nonequilibrium molecular dynamics to simulate the elastic-plastic deformation of silicon under tetrahedral nanometer-sized indentors. The results are described in terms of a rate-dependent and temperature-dependent phenomenological yield strength. We follow the structural change during indentation with a computer technique that allows us to model the dynamic simulation of diffraction patterns.

Effects of complex additions of Re or Ti with C on the strength and ductility of recrystallized molybdenum

The strength and ductility of a molybdenum alloy are influenced by carbon and its precipitates as well as the alloying element(s). In this investigation, the low-temperature tensile properties of MoRe and MoTi alloys with or without carbon addition were evaluated. Here the strength and ductility of the material were represented by two parameters, critical stress and critical temperature. Both Re and Ti additions increased the transgranular fracture strengths as well as the intergranular fracture strength, and consequently the critical stress increased and the critical temperature decreased. A small amount of carbon addition further increased the intergranular fracture strength and the critical temperature was substantially decreased. The critical temperature was approximately a function of critical stress for Mo-2% Re and 4%Re alloys and MoTi alloys. Very low critical temperatures for Mo-10%Re and 13%Re alloys are due to their characteristic temperature dependence of yield strength.

The stability of antiphase boundaries on {111} planes in the l12compound fe3ge and its relation to the abnormal negative temperature dependence of yield strength

Abstract The instability of antiphase boundaries (APB's) on {111} is one possible explanation for the abnormal negative temperature dependence of yield strength (TOYS) in Fe3Ge but APB tubes are found in deformed Fe3Ge which suggests that APB{111}'s are not unstable. Actual measurement of the APB energy on {111} again indicates that APB {111}'s should be unstable and the APB{111}'s in the APB tubes are thought to be stabilized by the small APB widths and their end conditions. Thus an explanation of the abnormal negative temperature dependence of yield strength in terms of unstable {111} APB's still appears to be valid.

The negative temperature dependence of yield strength in the L1 2 compound Fe 3Ge

The flow stress anomaly in some L1 2 alloys is a well-documented phenomenon. There are, however, other L1 2 alloys which show a large and negative temperature dependence of yield strength (TDYS). The present work attempts to answer the question of why materials with the same L1 2 structure can exhibit different TDYS by investigating the L1 2 compound Fe 3Ge. Transmission electron microscopy studies show that 〈110rrang;{001} are the easiest slip systems in Fe 3Ge. The high Peierls stress can readily explain the large and negative TDYS. The absence of the more usual octahedral slip systems is thought to be due to the instability of APBs on {111} planes.

On the abnormal negative temperature dependence of yield strength in the L12compound Fe3Ge

Abstract Both the as-annealed and room temperature deformed microstructure of the L12 compound Fe3Ge have been studied using transmission electron microscopy. The as-annealed microstructure consisted of a dense array of stacking faults on {111} planes and a small concentration of thermal APB's on {100} planes. In the deformed microstructure, {001} cube slip has been identified. The asannealed stacking faults may also produce a strengthening effect on such slip. A combination of these two factors causes the large and negative temperature dependence of yield strength in Fe3Ge. The absence of octahedral slip is attributed to the fact that there is no dislocation configuration on {111} planes which is both glissile and allowed energetically.

Intermetallic/Metallic Polyphase In-Situ Composites

ABSTRACTTo evaluate various in-situ reinforcement schemes, a computer controlled containerless directional solidification system has been used to produce NiAl-based polyphase composites containing up to two intermetallic phases and at least one ductile phase. Systems evaluated include Ni-Al-Cr, Ni-Al-Mo, Ni-Al-V ternary systems that form NiAl/α-refractory metal eutectics and a three phase eutectic in the Ni-Al-Cr-Nb system. Initial screening of these in-situcomposites has included morphological characterization, four point bend testing, temperature dependent yield strength evaluation and compressive creep testing. Occasional growth defects termed “banding” currently interrupt the continuity of these composite structures and limit the attainment of optimum properties. However, both the creep strength and toughness of NiAl were improved by in-situreinforcement.

Compression ductility and fracture of boron-free and highly boron-doped Al 5CuTi 2 intermetallic compound

This paper reports on the D0{sub 22} crystallographic structure of Al{sub 3}Ti intermetallic compound transformed into the L1{sub 2} structure by alloying with Ni,Cu,Zn and Fe. These modified intermetallics are very attractive as potential high-temperature structural materials. Since they have similar Al content as Al{sub 3}Ti they are expected to exhibit an excellent oxidation resistance. Also, their densities are relatively low ({lt}4.5 g/cm{sup 3}) and it has recently been reported that the L1{sub 2} Al{sub 22}Fe{sub 3}Ti{sub 8} and Al{sub 5}CuTi{sub 2} (both modified Al{sub 3}Ti) intermetallics exhibit a positive temperature dependence of yield strength (0.2% offset) in compression. The L1{sub 2} crystallographic structure of the transformed Al{sub 3}Ti-based compounds has five independent slip systems sufficient for a generalized plastic flow according to von Mises criterion. However, contrary to expectation the transformed L1{sub 2} intermetallics are still brittle and at room temperature fracture occurs by transgranular cleavage.

γ/γ′: The Key to Superalloy Behavior

ABSTRACTGamma prime (Ni3Al), with its unusual temperature dependence of yield strength, is the strengthening phase in Ni-base superalloys. Many facets of γ/γ′ coherent precipitation strengthening have been well studied: coarsening kinetics, volume fraction vs. strength, yield strength vs. temperature, dislocation dynamics, etc.This contribution deals with several important aspects of two phase behavior in coherent systems (for high temperature application) which are either relatively new or have received inadequate attention. The most desirable γ/γ′ misfit, in sense and magnitude, is dependent on the temperature of application. It must be recognized that the sense and size of misfit are not only a function of alloying, but temperature as well. Sufficient data are presented for various alloying additions to γ and γ′ to allow one to roughly compute the misfit at room temperature or 1000C. The evolution and significance of the ′′ morphology are discussed. Stability of particle size and shape are related to misfit, temperature and stress. It is demonstrated that for high temperature creep resistance, a fine rafted structure is the most desirable. Finally, some data are presented which demonstrate the dependence of γ′ strength on stoichiometry as well-as temperature.A portion of this work was sponsored by AFOSR, with A. Rosenstein as contract monitor.

Temperature dependence of strength in a Cu3Au-5% Ni alloy and its relevance to the APB morphology

Temperature dependence of yield strength in a Cu3Au-5%Ni alloy is investigated by compression tests over a temperature range between 77 and 700 K. The ordered alloy having L12 crystal structure exhibits swirl-like antiphase boundary morphology and was found not to possess the anomalous positive temperature dependence of strength. Nickel free Cu3Au alloy has been reported to have the mechanical anomaly like Ni3Al and to exhibit maze-like APB morphology. Such relevance between APB morphology and the temperature dependence of strength in L12 ordered alloys is discussed in terms of the relative phase stability between L12 and long period superlattice structures.

Void-strengthening in aluminum and its nature

Temperature and strain rate dependence of yield strength were used to analyze the nature of aluminum strengthened by the formation of voids. Aluminum rods 99.999% pure were quenched and heat treated to form voids with an approximate density of 10 13–10 14 voids/cm 3. Voids in selected samples were observed by electron microscopy. The yield strength of the void strengthened samples was measured at various temperatures from 77 to 593 K and at two strain rates, 3.33 × 10 −2/s and 1.67 × 10 −3/s. Tests at room temperature and at 77 K were made at various strain rates. The similarity of Coulomb's approach to void strengthening and that of Orowan stress with Ashby's dipole criterion was observed, and it was determined that the extent and effectiveness of void strengthening depends primarily on void density and much less on void size. The amount of strengthening obtained at room temperature was found to be consistent with the estimates. Compared to annealed aluminum, void strengthened aluminum is more susceptible to the instability of plastic flow at low temperatures; its temperature dependence of yield strength varies in different temperature ranges and shows a higher strain rate sensitivity of yield strength.

溶接継手の低温変形

The temperature dependence of deformation properties of welded joints was investigated by means of the photoelastic coating technique. A smooth tensile specimen with a buttwelded joint was used to study the micro-structural dependence of plastic behaviour of a mild steel. From successive developments of photoelastic fringe patterns obtained in tension tests at various low temperatures, the structural and temperature dependence of yield strength of such parts as parentmetal, heat-affected zone and weld metal was determined. It was found also that the yield strength of the heat-affected zone was higher than that of the parent metal or the weld metal in the range from room temperature to -196°C. The existence of such a harder metal as the heat-affected zone produces a plastic constraint in a softer metal, on which the former borders, and then it hinders the trarsverse contraction of the latter and leads to the embrittlement in theconstrained regions. The temperature dependence of plastic constraint was also discussed.

Thermal‐Mechanical History and the Strength of Magnesium Oxide Single Crystals: I, Mechanical Tests

Various dislocation impurity interactions in magnesium oxide and their effect on mechanical properties between room temperature and 1000°C are discussed. (1) Impurity precipitate particles in single crystals can be redistributed by heat treatment. Their size and density affect the lattice resistance to the motion of fresh dislocations and thus affect the yield strength at room temperature. Above 1200°C impurities go into solid solution and the strength decreases to 0.7 times the as‐received fully‐precipitated value. (2) Fresh dislocations can be locked by heating above 600°C to produce yield phenomena at room temperature. The upper yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Locking is attributed to the diffusion of impurities and point defects to dislocations. (3) The basic temperature dependence of yield strength contains two regions: (a) Up to approximately 500°C the strength decreases rapidly, suggesting a dislocation cutting mechanism during flow. Material preheated above 1200°C shows a stronger dependence than as‐received material. (b) Above 500° C strength decreases linearly at a rate equal to the decline in elastic modulus. Superimposed on this basic temperature dependence curve there is, for material preannealed above 1200° C, a hump due to reprecipitation between 500° and 1000°c.