Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 1994"

Hydrogen-enhanced localized plasticity—a mechanism for hydrogen-related fracture

Abstract: The mechanisms of hydrogen-related fracture are briefly reviewed and a few evaluative statements are made about the stress-induced hydride formation, decohesion, and hydrogen-enhanced localized plasticity mechanisms. A more complete discussion of the failure mechanism based on hydrogen-enhanced dislocation mobility is presented, and these observations are related to measurements of the macroscopic flow stress. The effects of hydrogen-induced slip localization on the measured flow stress is discussed. A theory of hydrogen shielding of the interaction of dislocations with elastic stress centres is outlined. It is shown that this shielding effect can account for the observed hydrogen-enhanced dislocation mobility.

A review of crack closure, fatigue crack threshold and related phenomena

Abstract: Published fatigue crack closure mechanisms are reviewed in the context of the following five ideas that have been developed over the past twenty years to explain the closure effects on near-threshold crack growth behaviour: (1) oxide, (2) asperity, (3) plasticity, (4) phase transformation and (5) viscous fluids. The first three have been considered as more important than the last two. Our analysis indicates that (a) there can be no contribution from plasticity to crack closure, (b) the crack closure contribution can be significant only if it is closed fully, which is reflected as an infinite slope in the load-displacement curve, (c) formation of oxide asperities from a fretting action is a random process and not a deterministic one, and therefore cannot explain the deterministic behaviour of the effect of load ratio on the threshold, and (d) the closure contribution from the asperities resulting from oxides or corrosion products,or surface roughness, it is less than 20% of what has been deduced based on the change in the slope of the load-displacement curves. Thus the analyses show that crack closure can exist, but its magnitude is either small or negligible. The critical evaluation of the literature data on (1) the threshold stress intensity variation with load ratio on many materials, and (2) and examination of the experimentally observed load-displacement curves confirm the above conclusions. Hence to rationalize the observed variations in the fatigue crack threshold with load ratio, we have proposed a new model. It postulates a requirement for two critical stress intensity parameters, namely ΔK th ∗ and K max ∗ , that must be satisfied simultaneously as the crack tip driving forces for fatigue crack growth. This requirement is fundamental to fatigue, since an unambiguous description of cyclic loads requires two independent load parameters. Several experimental results from the literature are presented in support of this postulation. Using these two critical parameters, the entire functional relationship between Kmax, ΔKth and R is explained without invoking an entrinsic factor, namely crack closure. In addition, for a given material and its crack tip environment, a unique relationship between ΔKth and Kmax exists that is independent of test methods used in determining thresholds. Finally, because of this two parameter requirement, all fatigue crack growth data need to be represented in terms of three-dimensional plots involving da/dN, ΔK and Kmax. For a two-dimensional representation, the data need to be transformed correctly, defining the net driving force involving both ΔK and Kmax parameters. The concepts presented are independent of whether crack closure exists or not, or even whether cracks exist or not.

A self-consistent viscoplastic model: prediction of rolling textures of anisotropic polycrystals

Abstract: The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in general non-homogeneous and, as a consequence, so is the local plastic deformation. We present in this work a model that describes the plastic behaviour of non-homogeneous materials composed of anisotropic regions (grains or phases). Our model is based on describing each region as a viscoplastic inclusion embedded in the effective medium represented by the other grains, and incorporates explicitly the grain interaction with its surroundings and the plastic anisotropy of grain and matrix. Within the model the grain response is coupled to the overall response of the polycrystal and the grain deformation may differ from the polycrystal's. A characteristic of our approach is that those deformation systems with lower critical resolved shear stress tend to be more active, and less than five systems per grain are sufficient to accomodate the imposed overall deformation. In this work we explore the consequences and the limits of the model, and its dependence on the assumed rate sensitivity as well. We combine the self-consistent formulation with a volume fraction transfer scheme for treating the reorientation due to twinning, and simulate rolling textures of brass (f.c.c.), Zircaloy (h.c.p.), calcite (trigonal) and uranium (orthorhombic). We compare the results with experimental measurements and Taylor-type predictions, infer information concerning the microscopic deformation mechanisms and discuss the limits of applicability of the approach.

An evaluation of the strain contributed by grain boundary sliding in superplasticity

Abstract: Grain boundary sliding is an important mode of deformation in superplasticity. Measurements of the contribution of sliding to the total strain generally give values of about 50–70% so that there is an apparent “missing strain” of about 30–50%. It is demonstrated that the experimental method used to measure the sliding contribution will lead to estimates in the range of about 45–90% even when all the deformation is by grain boundary sliding and the associated accommodation process. Since the problem of accommodation is less severe at the specimen surface, it is shown that estimates of the sliding contribution from surface marker lines will tend to lie at the lower end of this predicted range. It is concluded that grain boundary sliding accounts for essentially all the deformation under optimum superplastic conditions and there is no “missing strain” of about 30–50%.

The impact of grain boundary character distribution on fracture in polycrystals

Abstract: The importance of structural effects on intergranular fracture is discussed in order to understand, predict and control fracture in polycrystals. The heterogeneity of fracture in a polycrystal has been found to be due to the difference in structure-dependent propensity to intergranular fracture among grain boundaries. The grain boundary character distribution (GBCD) which describes the type and frequency of grain boundaries is shown to be one of important microstructural factors affecting fracture processes and characteristics. A recent prediction of GBCD-controlled toughness and brittle-ductile transition is introduced. The importance of the connectivity of grain boundaries, the so-called grain boundary correlation number, is also discussed. Recent successful achievement of the control of intergranular brittleness is presented as an application of the result of basic research on fracture to the control of intergranular brittleness by grain boundary design and control in advanced materials.

Fabrication of Al matrix in situ composites via self-propagating synthesis

Abstract: Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

Mechanical strength and thermal stability of Ti-based amorphous alloys with large glass-forming ability

Abstract: Ti-based alloy powders produced by the high pressure gas atomization technique were found to consist of an amorphous single phase in the particle size range below 32 μm for Ti 50 Zr 10 Cu 40 and below 25 μm for Ti 50 Zr 10 Ni 20 Cu 20 , though the cooling rate of their molten alloys for gas atomization is considerably lower than that for melt spinning. In addition, the TiZrCu amorphous powders exhibit a rather wide supercooled liquid region before crystallization. This is believed to be the first evidence for the appearance of the supercooled liquid region for atomized Ti-based amorphous powders. The significant increase in the glass-forming ability and the wide temperature range of the supercooled liquid region for the TiZrCu ternary alloys are presumably due to the simultaneous satisfaction of the two criteria of the significantly different atomic size ratios among the constituent elements and the difficulty of long-range redistribution of the constituent elements for the growth of crystalline phases.

Rapid solidification processing and microstructure formation

Abstract: An overview of rapid solidification is undertaken and various rapid solidification processes are grouped into three classes. This leads to a better understanding of the similarities and differences between the various methods in terms of solidification parameters. Microstructures (with specific attention to rapid laser treatment) are analysed and various microstructure selection maps are presented.

Analysis of hydrogen atom transport in a two phase alloy.

Abstract: The theoretical basis for analysing hydrogen atom transport in a two-phase alloy is evaluated. Experimental measurements of hydrogen atom transport in a duplex stainless steel, thermally treated to give a varying volume fraction of austenite, are described and analysed to ascertain the relative effect on the effective diffusivity of interfacial trapping and of the reduced diffusivity but enhanced solubility of the austenite phase. The effective diffusivity of the as-received duplex stainless steel is a factor of 400 less than that for the fully ferritic steel. It is demonstrated that diffusion through the austenite has no effect on the hydrogen transport, despite the higher solubility of hydrogen in this phase. However, the presence of austenite creates a more tortuous path for the hydrogen transport. The enhanaced solubility of hydrogen atoms in the austenite phase, relative to that in the ferrite phase, can be considered to induce a trapping effect on the hydrogen atom transport; however, the effect is small relative to the trapping associated with the austenite-ferrite interface. The binding energy of the interfacial traps is estimated to be about 52 kJ mol −1 .

Particle size, volume fraction and matrix strength effects on fatigue behavior and particle fracture in 2124 aluminum-SiCp composites

Abstract: The effects of particle size, volume fraction and matrix strength on the stress-controlled axial fatigue behavior and the probability of particle fracture were evaluated for 2124 aluminum alloy reinforced with SiC particles. Average particle sizes of 2, 5, 9 and 20/~m and volume fractions of 0.10, 0.20 and 0.35 were examined for four different microstructural conditions. Tensile and yield strengths and fatigue life were substantially higher in the reinforced alloys. Strength and fatigue life increased as reinforcement particle size decreased and volume fraction loading increased. The frequency of particle fracture during crack propagation was found to be dependent on matrix strength, particle size and volume fraction and on maximum crack tip stress intensity. Particle fracture can be rationalized, phenomenologically, by the application of modified process zone models, originally derived for static fracture processes, and weakest link statistics which account for the dependence of matrix yield strength and flow behavior and particle strength on the probability of particle fracture during monotonic fracture and fatigue crack propagation.

Amorphous (Ti,Zr, Hf)NiCu ternary alloys with a wide supercooled liquid region

Abstract: Ternary TiNiCu, ZrNiCu and HfNiCu amorphous alloys were found to exhibit a distinct glass transition combined with rather wide supercooled liquid region before crystallization. The largest temperature interval of the supercooled liquid region defined by the difference between TgTx, i.e. ΔTx (= Tx−Tg, is 55 K for Ti50Ni25Cu25, 60 K for Zr60Ni20Cu20 and 65 K for Hf60Ni20Cu20. These amorphous alloys with the largest ΔTx values also exhibit the highest Tx and the lowest melting temperature and crystallize through asingle-stage exothermic reaction leading to the simultaneous precipitation of more than two kinds of compound. It is therefore presumed that the appearance of the wide supercooled liquid region before crsystallization is due mainly to the necessary of the redistribution of the constituent elements for the precipitation of the compounds from the homogeneously mixed amorphous structure.

Stability and transformation to crystalline phases of amorphous ZrAlCu alloys with significant supercooled liquid region

Abstract: Crystallization from the supercooled liquid region was examined for an amorphous Zr65Al7.5Cu27.5 alloy with a significant supercooled liquid region before crystallization, with the aim of investigating the reason for the appearance of the wide supercooled liquid region. The crystallization takes place through the precipitation of a b.c.t. Zr2(Cu, Al) single phase which grows with a dendritic mode. The Avrami exponent (n value) and the activation energies for the nucleation and crystallization are 3.7, 260 and 230 kJ mol−1 respectively. Based on existing structural data as well as the present results, it is presumed that the appearance of the supercooled liquid region is due to the difficulty of growth resulting from the necessity of long-range redistribution of Al at the solid-liquid interface and to the difficulty of precipitation of Zr2(Cu, Al) caused by the large solid-liquid interfacial energy resulting from the highly dense random packed structure consisting of elements with significantly different atomic sizes and attractive bonding nature.

Correlation of microstructure variability and local stress field in two-phase materials

Abstract: Improved characterization of present-day materials that are manufactured to obtain optimal property values by microstructure effects necessitates a more thorough knowledge of the microstructure pattern. Methods of spatial statistics have been used to analyse patterns of second-phase inclusions as observed on planar sections. Several parameters and functions that discriminate between different dispersions of inclusions have been investigated. The effects of inclusion patterns on the interface stresses in unidirectional fibre reinforced composite material are discussed. The range of a local geometrical disorder and a physical range of interaction between stresses and positions of inclusions are obtained for investigated patterns.

Interface control for resistance to intergranular cracking

Abstract: Theoretical and experimental results are presented, with the primary objective of improving the resistance of convectional polycrystalline alloys to intergranular degradation phenomena, through the application of grain boundary design and control. Geometric considerations are discussed, which show that, as a consequence of both energetic and crystallographic constraints associated with twinning, a grain boundary character distribution (GBCD), consisting entire of low σ grain boundaries, is attainable. A geometric model of crack propagation through active intergranular paths is used to evaluate the potential effects of σ grain boundary fraction and grain size on intergranular cracking. The effect of the GBCD on intergranular stress corrosion cracking and intergranular corrosion in a nickel-based alloy 600 (Ni16Cr9Fe) is determined. Important factors in achieving microstructural optimization of alloy 600 are presented. These results provide direct experimental support for the model of intergranular crack propagation, and demonstrate the importance of grain boundary structure control for enhancing the resistance of a material to intergranular degradation.

Evolution of microstructure in semi-crystalline polymers under large plastic deformation

Abstract: The plastic properties of semi-crystalline polymers up to large strains are reviewed in terms of the particular mechanisms activated in these materials. Also, a specific point is made on the transformations affecting the microstructure under the effect of the deformation (density, viscoelastic response and macromolecular orientation). The case of orthorhombic polyethylene and poly(ether ether ketone) are investigated for the illustration of the general principles emphasizing crystalline defects. The constitutive plastic behaviour is determined by means of a novel technique. The microstructural evolution is modelled in terms of glide in the crystalline lamellae, the development of damage and the modelling of the amorphous chains in the interlamellar regions.

Recent progress in amorphous metallic materials in Japan

Abstract: A review is given of recent work concerned with the production methods and the characteristic properties of amorphous metallic materials newly developed in Japan during the last 10 years. Some of the topics covered include the following: (1) high-strength, light-weight alloys with an amorphous structure or with nanocrystals dispersed in an amorphous matrix; (2) soft magnetic alloys with nanocrystalline structures; (3) highly stable amorphous alloys with a wide supercooled liquid region, and their application to working processes.

Advanced metals for aerospace and automotive use

Abstract: Aerospace systems and automobiles with improved performance require materials with enhanced characteristics compared to conventional state-of-the-art materials. For automobiles, this is particularly true for power-train use as opposed to body use. These improvements will come from advances in synthesis/processing rather than from the traditional frenzy to define microstructure-property relationships. For both types of applications, characteristics such as strength, temperature capability, ductility and “forgiveness” (fracture toughness, fatigue crack growth rate, etc.) must be as high as possible, while parameters such as density and cost must be minimized; the latter is of general concern for automobile use and relatively inexpensive light aircraft. Because of their low density, the light metals based on aluminum, magnesium, titanium, and the intermetallic titanium aluminides are particularly attractive for both types of applications. In this paper, advanced materials in general are discussed, followed by a consideration of advanced materials specifically in aerospace and automobile applications. Various methods of synthesizing and processing the lightweight metals with enhanced physical and mechanical properties are then presented. The processes discussed include ingots and castings; rapid solidification; mechanical alloying; production and use of nanostructured materials; spray deposition; vapor deposition processes; thermochemical processing; metal matrix composites; and fusion joining.

Dislocation structures in 316L stainless steel cycled with plastic strain amplitudes over a wide interval

Abstract: Dislocation structures in two heats of 316L austenitic stainless steels, cyclically strained at room temperature with constant plastic strain amplitude over a wide interval, were studied. The spatial arrangement of dislocations in the grains was determined using the technique of oriented foils. The main types of dislocation structures, characteristic of amplitudes corresponding to three regimes of cyclic stress-strain curves, and their proportions as a function of the plastic strain amplitude and the number of cycles, were evaluated. In each regime of cyclic stress-strain curve a dominant dislocation structure was found: planar dislocation arrays arranged in sheets in regime I ( ϵ ap −4 ), imperfect vein structure with persistent slip bands in the plateau regime II (1 × 10 −4 ϵ ap −3 ), and cells and wall structures in regime III ( ϵ ap > 1 × 10 −3 ).

Metal-contact-induced crystallization of semiconductors

Abstract: The crystallization of amorphous semiconductors in amorphous Si (a-Si)/Al, a-Si/Ag, and amorphous Ge (a-Ge)/Ag layered thin films has been investigated by cross-section in situ transmission electron microscopy and differential scanning calorimetry. In all cases, the amorphous semiconductors were found to crystallize at substantially lower temperatures than those without the metal contact. During the reaction, the metal phase always exists between the amorphous and crystalline semiconductor phases, migrating into the amorphous matrix, leaving the crystalline phase behind. This observation strongly indicates that the semiconductor atoms diffuse through the metal grain from amorphous to crystalline phases, which was confirmed by in situ high resolution recordings. We suggest that this mechanism provides the fastest reaction path for the system to reduce its excess free energy, as the direct rearrangement of the amorphous tetrahedral networks cannot occur at such low temperatures.

Shape memory TiNi synthesis from elemental powders

Abstract: Commercially, the shape memory alloy TiNi is produced either by vacuum induction melting or by vacuum arc remelting of the pure metal ingots. Powder metallurgy techniques provide an alternative fabrication route but problems arise in achieving chemical homogeneity. In this study TiNi compacts were cold-pressed from the blended elemental powders and sintered in vacuum for varying times at temperatures from 800 °C to 1000 °C. Two heating rates were used, 5 K min−1 and 10 K min−1. A TiNi microstructure could be produced after annealing at 1000 °c for 6 h, although some TiNi3 was still observed. This is likely to be difficult to completely remove as TiNi3 is thermodynamically more stable than TiNi. Thus, homogenization is unlikely to be completed by solid-state diffusion processes. The martensitic B19′ structure was observed to be highly oriented after processing.

Application of grain boundary engineering concepts to alleviate intergranular cracking in Alloys 600 and 690

Abstract: Nickel-based alloys used in nuclear steam generator tubing have been found to be susceptible to intergranular stress corrosion cracking while in service. Following a recently developed model, grain boundary engineering concepts may be used to alleviate these concerns. This paper presents a report on the grain refinement approach in the proposed model which results in a higher probability of arresting stress corrosion cracks before reaching a critical length at which failure occurs. For this purpose, an electroplating system was developed to produce ternary Ni-Cr-Fe alloy coatings. Electroplating conditions are given for the production of Alloys 600 and 690 having an average grain size in the range 100-250 nm.

Cyclic response and dislocation structures of AISI 316L stainless steel. Part 1: single crystals fatigued at intermediate strain amplitude

Abstract: Polycrystalline AISI 316L stainless steel has been cycled at intermediate strain amplitudes. Cyclic response was analyzed in terms of the cyclic stress-strain curve (CSSC), softening behavior, the friction stress and the back stress. Dislocation structures were determined by transmission electron microscopy (TEM). It was found that cyclic softening behavior occurred and this has a significant influence on the dislocation structures by stimulating the activity of cross slip. The cross slip system becomes the second most important slip system, and causes the development of dislocation wall structure. Compared with the cyclic response of 316L single crystals, which are planar in slip character, polycrystals show a tendency towards wavy slip, despite the low stacking fault energy and high friction stress. It is concluded that grain boundaries, together with dislocation “starvation”, have a major influence on the fatigue behavior of polycrystals by increasing the tendency to multi-slip, decreasing the capacity of strain transfer from grain to grain and from band to band, and changing the limit of stress or strain sustainable by a given dislocation structure in the sequence that occurs.

Dynamic recrystallization—scientific curiosity or industrial tool?

Abstract: Rolling processes are categorized by the lengths of their interpass times: those that involve interpass intervals significantly longer than 1 s ( e.g. reversing mills) are particularly suitable for conventional controlled rolling or recrystallization-controlled rolling; by contrast, those involving interpass times of 15–100 ms or more ( e.g. the finishing trains of tandem mills) are shown to be suitable for dynamic or metadynamic recrystallization-controlled rolling. This distinction relies on the minimum time required to permit strain-induced precipitation, the occurrence of which inhibits both static and dynamic recrystallization. The effect of metadynamic recrystallization can play an important strain is described. Examples are given of industrial processes in which dynamic recrystallization can play an important role. These include the stretch-reducing mill for the finishing of seamless tubes, the finishing stages of rod rolling, and the finishing of microalloyed steels in hot-strip mills. The practical consequences of dynamic recrystallization on the microstructures produced by these operations and on the modelling of rolling load are also described.

Crystallization of highly undercooled metallic melts and metallic glasses around the glass transition temperature

Abstract: Crystallization in highly undercooled melts can be studied either after severe undercooling of the melt or after heating up metallic glasses above their glass transition temperature. Whereas the crystallization of silicate glasses proceeds only above the glass transition temperature, the crystallization of metallic glasses can occur in both temperature ranges. Below the glass transition temperature, nucleation and crystal growth are controlled by diffusivity with an Arrhenius-type temperature dependence; above the glass transition, crystallization kinetics can be better described by the Vogel-Fulcher-Tammann equation which is usually used to describe the temperature dependence of shear viscosity. The different behaviour in comparison with silicate glasses is assumed to be due to the metallic bonding which allows atomic exchange of the glass-forming elements by diffusion even at temperatures below the glass transition temperature. Usually, metallic glasses are found to crystallize very rapidly at temperatures close to the glass transition, thus hiding the glass transition itself. For example, metal-metalloid glasses ( e.g. Fe 75 B 25 ) and zirconium based transition metal glasses ( e.g. Co 33 Zr 67 or Co 50 Zr 50 ) are known to crystallize within a few seconds in this temperature range. Zr 60 Ni 25 Al 15 glasses, however, can be held without crystallization for relatively long times in the highly undercooled state, i.e. in the temperature range above the glass transition temperature. During primary crystallization of metallic glasses, e.g. FINEMET (Fe 73.4 Cu 1 Nb 3.1 Si 13.4 B 9.1 , with size-dependent growth rates, the microstructure can be controlled by the addition of slow diffusing elements such as Nb and/or elements such as Cu or Au which enhance the nucleation rate.

Interfacial reactions and age hardening in AlMgSi metal matrix composites reinforced with SiC particles

Abstract: AlSiMg matrix composites have been fabricated by the compocasting technique using as-received and oxidized particles. Interfacial reactions occurring during fabrication and during subsequent remelting of the composites were characterized by detailed analytical transmission electron microscopy. The effects of these reactions on the age hardening response of the materials are also characterized and discussed in detail. The results show that it is possible to control the interfacial reactions either to totally suppress age hardening or to give rise to hardening in the case of a normally nonhardenable matrix alloy.

Grain boundary structure, energy and strength in molybdenum

Abstract: Molybdenum has many attractive properties for high-temperature structural applications. However, its usefulness as a structural material is impaired by brittleness at and below ambient temperature, resulting from its intrinsically weak grain boundary strength. According to our previous study, the fracture strength in molybdenum depends markedly on the grain boundary character and, in particular, on the orientation relationship between the two adjacent crystals and the orientation of the grain boundary plane. However, our present knowledge is still far from a complete understanding. In this study, the relationship between the strength, grain boundary structure and grain boundary energy has been investigated for purified biccrystals of molybdenum with various symmetric tilt boundaries, by means of transmission electron microscopy and optical interferometry. The main results obtained are as follows. (1) There is a relatively good correlation between the fracture strength and the grain boundary energy. The energy cusps, for instance, are observed for (112) and (111) Σ3 coincidence boundaries, which are high in fracture strength, while the energy is higher for near (114) and (122) Σ9 boundaries, which are low in strength. (2) Σ1 small angle and near (112) Σ3 coincidence boundaries have a good coherence, which agrees well with the result of the boundary energy measurement. Grain boundary dislocations are observed and they can be described by the boundary dislocation model. (3) On the (114) Σ9 coincidence boundary with low fracture strength, the structure is poor in coherence compared with Σ1 and Σ3 boundaries. However, the grain boundary structure can also be described by the grain boundary dislocation (displacement shift complete (DSC) dislocation) model. (4) It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.

Effects of strain rate and temperature on deformation behaviour of IN 718 during high temperature deformation

Abstract: The hot deformation characteristics of a wrought IN 718 alloy were investigated by compression testing at constant strain rates in the range of 0.1 to 5 × 10 −3 s −1 , and testing temperatures in the range of 950 to 1100 °C using a 200 ton capacity microprocessor controlled Fielding hydraulic press. Examination of the microstructures was carried out by optical microscopy and TEM. The flow stress of the compression tests showed a single peak in the flow stress-strain curves, and indicated that a dynamic recrystallization transition took place during the hot compression. The relationship between the peak stresses (σ p ) and the Zener-Hollomon parameter ( z ) can be expressed by σ p = 0.5Z 0.17 . “Necklace” microstructures were observed at testing temperatures below 1050 °C, for strain of 0.7. The fraction of recrystallized grains increased with the increasing temperature and strain, and decreasing strain rate. Fully recrystallized microstructures were observed at temperatures 1050 °C or greater, with a strain of 0.7.

Fabrication and novel properties of nanostructured Al base alloys

Abstract: This paper deals with the recent drastic improvements in the mechanical strength and ductility obtained by the homogenous dispersion of nanoscale Al particles in an amorphous phase and by the formation of a finely mixed structure consisting mainly of nanoscale icosahedral particles surrounded by the Al phase. The generation of ferromagnetism with high magnetization by subnanoscale control of the icosahedral atomic configuration in Al base alloys is also discussed.

Use of position-dependent stress-free standards for diffraction stress measurements

Abstract: Residual stress measurements using neutron diffraction are inherently triaxial in nature due to the low adsorption, and thus high penetration, of neutrons in samples of interest. This means that stress-free reference standards are required to convert measured changes in peak position to strain and stress tensors. Althogh a number of empirical and analytical approaches have been utilized to obtain accurate stress-free reference cell parameters, they all presume the presence of a single reference value for the material being measured. However, important cases can arise for which the local stress-free cell parameter varies from point to point. One such case, a circumferentially welded cylinder, is presented here. A method of point-to-point correction is employed, involving the sectioning of a companion piece into small cubes corresponding to the positions of stress measurement. The variations in peak position and peak breadth are presented. It is shown that accurate stress tensors can be obtained and that use of a constant value of stress-free reference cell parameter from the unaffected base metal leads to errors of up to 700 MPa in some stress tensor components. Effects on the principal stresses are also presented.

Towards applications of quasicrystals

Abstract: Quasicrystals are hard, low-friction materials which show specific electronic properties and low thermal conductivity. They may be produced as coatings by thermal spraying. This technology may enable these materials to be applied in different areas.