Showing papers in "Scripta Metallurgica Et Materialia in 1990"

On the contribution of triple junctions to the structure and properties of nanocrystalline materials

Abstract: Nanocrystalline materials, having a crystal size less than ~10 rim, have been shown to possess unusual properties [1-3]. These properties are primarily the result of a substantial fraction of the atoms (20 50%) lying in intercrystalline regions [3]. On the basis of x-ray scattering [4], EXAFS [5], hydrogen solubility [6], small angle neutron scattering [7], and self diffusivity measurements [8] conducted on bulk nanocrystalline materials, it has been suggested [3] that the grain boundaries in these materials are more disordered than those in conventional polycrystals. Recent studies[9,10] involving direct observation of nanocrystalline interfaces by HREM, provide contradictory results. Wunderlich and co-workers[9] have shown that interfaces in nanocrystalline Pd show an 'extended' structure not typically observed in conventional systems. However, Thomas et al [10] observed that the interfacial structure of nanocrystalline Pd is consistent with that typically observed in coarse-grained materials.

Microhardness and fracture properties of nanocrystalline NiP alloy

Abstract: lu, k (reprint author), acad sinica,inst met res,natl lab rsa,shenyang 110015,peoples r china

Grain growth in nanocrystalline TiO2 and its relation to vickers hardness and fracture toughness

Abstract: This paper reports on scientific interest in ultra-fine grained powders for processing of ceramic components motivated by the possibilities for the enhancement of sintering rates, reduction in flaw sizes and low-temperature superplastic deformation. Previous works have developed a technique, which combines the methods established of inert gas condensation of small particles and in situ powder compaction, for synthesizing materials with grain sizes {lt}10 nm. It has been shown that this method can be adapted for the production of ceramic nanocrystalline particles. Subsequent work has demonstrated that enhanced sintering and superplastic deformation is possible in nanocrystalline ceramics (TiO{sub 2}), but not without significant grain growth. Control of grain growth, however, is necessary if the capability for synthesizing nanoscale powders is to have benefit for structural applications. It is well known that the initial particle size in green body compacts is not always indicative of the final grain sizes in fully sintered ceramic bodies. This study was initiated to examine grain growth kinetics in nanocrystalline materials. TiO{sub 2} was selected for this initial study since sintering, deformation and diffusion data are available.

New cubic phases formed by alloying Al3Ti with Mn and Cr

Abstract: It has been found that Mn or Cr can substitute for Al in Al 3 Ti and change the crystal structure from tetragonal DO 22 to cubic L1 2 . The new phases have been produced with negligible porosity and second phases, and have compositions near Al 67 Mn 8 Ti 25 and Al 67 Cr 8 Ti 25 . As with other cubic forms of Al 3 Ti, the ternary addition can range from bout 5 to 15 at. pct. while still maintaining the cubic phase. These new cubic phases have significantly reduced hardnesses and greatly improved crack resistance compared to binary Al 3 Ti, as well as other cubic forms based on alloying with Fe, Ni, or Cu. In addition to appreciable ductility in compression, the new phases show some tensile ductility

Chromium addition and environmental embrittlement in Fe3Al

Abstract: Iron aluminides based on Fe[sub 3]Al afford excellent oxidation properties at relatively low cost, making them candidates for use as structural material in corrosive environments. Recently, efforts have been devoted to understanding and improving their ductility through control of grain structure, alloy additions and material processing. Studies at this laboratory have shown that the ambient temperature ductility can be increased significantly by additions of up to 6% Cr. This increase in ductility was earlier attributed to increased cleavage strength, easier cross slip due to lower antiphase boundary (APB) energy, and solid softening. Very recent studies of FeAl and Fe[sub 3]Al in various tensile testing environments have indicated that both alloy systems are relatively more ductile at room temperature when tested in vacuum or dry oxygen. Ductilities of 12--18% were attained in both iron aluminide systems in an oxygen pressure of 6.7 [times] 10[sup 4] Pa, while only 2--4% ductility was achieved in normal laboratory air. It seems appropriate to reexamine the mechanism by which chromium produces improved ductility at room temperature in laboratory air and to correlate it with the environmental effects on mechanical properties. In the current investigation, the authors have evaluated room temperature tensile properties of the binary more » alloy (Fe-28Al, at.%) and ternary alloy containing chromium (Fe-28Al-4Cr) as a function of surface condition and heat treatment. The results indicate that, although chromium may affect cleavage strength and APB energies, its most significant effect on room temperature ductility is to modify the protective surface oxide, resulting in a minimization of environmental embrittlement. « less

Structure and properties during aging of an ultra-high strength Al-Cu-Li-Ag-Mg alloy

Abstract: The structure and properties of the strengthening phases formed during aging in an Al-Cu-Li-Ag-Mg alloy (Weldalite 049) were elulcidated, by following the development of the microstructure by means of TEM. The results of observations showed that the Weldalite 049 alloy has a series of unusual and technologically useful combinations of mechanical properties in different aging conditions, such as natural aging without prior cold work to produce high strengths, a reversion temper of lower yield strength and unusually high ductility, a room temperature reaging of the reversion temper eventually leading to the original T4 hardness, and ultrahigh-strength T6 properties.

Influence of testing environment on the room temperature ductility of FeAl alloys

Abstract: The effects of testing atmospheres (air, O2, N2, and vacuum) on the room-temperature ductility of Fe-40Al, Fe-40Al-0.5B, and Fe-50Al alloys were investigated. The results confirmed the decrease in room-temperature ductility of Fe-rich FeAl alloys by the interaction of the aluminide with water vapor, reported previously by Liu et al. (1989). The highest ductilities were measured in the atmosphere with the lowest moisture levels, i.e., in vacuum. It was found that significant ductility is still restricted to Fe-rich alloys (Fe-40Al), as the Fe-50Al alloy remained brittle under all testing conditions. It was also found that slow cooling after annealing was beneficial, and the effect was additive to the environmental effect. The highest ductility measurements in this study were 9 percent elongation in furnace-cooled Fe-40Al and in Fe-40Al-0.5B, when tested in vacuum.

Effects of particle size on inhibited grain growth

Abstract: The purpose of this communication is to re-examine pinned grain growth simulation results for a range of particle sizes