A thermomechanical process for grain refinement in precipitation hardening AA6xxx aluminum alloys

A new thermomechanical processing method, consisting of conventional rolling and a continuous non-isothermal annealing process, has been designed to achieve substantial grain refinement through cost-efficient routes in heat treatable aluminum alloys. The method has been implemented on an alloy of interest for automotive applications and a highly-stable fine-grained microstructure with a very weak texture has been achieved.     

Case hardening nickel alloys

Boriding, or boronizing, can be used to create a hard, tough surface ‘case’ on nickel alloys, extending the application of these materials into more severe environments. The case layer is a complex nickel boride layer.     

Laser-arc hardening of aluminium alloys

The characteristics of a joint laser-arc hardening of the aluminium alloys V124 and Al25 are investigated. Optimum parameters for the process are established. The laser-arc source gives a greater hardening effect in comparison with that of the arc source at treatment rates over 17 mm s. A microstructural analysis of the solidified melt zone showed that it represents α-phase dendrites surrounded by the eutectic Al-Si in which silicon has a fibre structure. With the aid of quantitative metallography, the parameters of the melt crystallization are determined for the case of fusion by the laser-arc source.     

长周期结构增强镁合金的研究进展

长周期结构增强镁合金是近年来新发展的一种具有应用前景的镁合金新型结构材料.长周期结构作为镁合金中一种新发现的增强相,极大地提高了合金的力学性能.主要介绍了长周期结构增强镁合金的各种类型、特点、力学性能以及目前国内外的研究现状.     

Influence of cold rolling degree and ageing treatments on the precipitation hardening of 2024 and 7075 alloys

In the present work, the precipitation hardening of 2024 and 7075 aluminum alloys is investigated as a function of cold rolling degree, ageing time and temperature using Vickers microhardness measurements and differential scanning calorimetry (DSC). It is found that a variation in such parameters can improve the hardness and plays an important role in the precipitation hardening process. At specific ageing temperature, the large cold rolled 7075 alloy exhibits two peaks of hardness. Moreover, for both alloys, the increment of hardness during ageing decreases with increasing the cold rolling degree. While in some cases microhardness measurements give impression that the precipitation reaction is slowed down by deformation, DSC analysis indicates that the precipitation is much accelerated since only a slight deformation decreases strongly the temperatures of reactions. However, the degree of cold rolling does not play a crucial role.     

A micromechanical method for predicting the precipitation hardening response of particle strengthened alloys hardened by ordered precipitates

Summary A micromechanical method was developed for predicting the precipitation hardening response of particle strengthened alloys hardened by ordered precipitates based on the microstructure, composition, and heat treatment, and utilizing a minimum number of experimental tests to evaluate the microstructural constants of the overall model. The overall approach was based on incorporating the dislocation particle interaction mechanics, particle growth and coarsening theory, thermodynamics, and particle strengthening mechanisms applicable to precipitation hardened alloys as part of the overall micromechanical method. The method/model evaluates, from a minimum number of experimental tensile tests, microstructural constants necessary in determining the precipitation srengthening response of a particle strengthened alloy. The materials that were used as vehicles to demonstrate and evaluate the model were precipitation hardenable aluminium-lithium-zirconium and nickel-aluminum alloys. Utilizing these demonstration alloys, the method used a total of four tensile tests to evaluate the necessary microstructural constants and thus predict the variation in strength as a function of aging time, aging temperature, and composition, for the underaged, the peak-aged, and the overaged conditions. Predictions of the precipitation strengthening response were made incorporating the Wagner particle distribution model to evaluate the size distributions of particles in the microstructures. The predicted variation of strength with aging practice and composition using the Wagner distribution model compared well with the corresponding experimental yield strength results.Notation b Burgers vector - average particle size diameter for a particle distribution - d _loop particle looping diameter for dislocation bypassing by Orowan looping - f _v volume fraction of precipitates - h() Wagner particle size distribution function - n _total total number of precipitate particles per unit area on a given microstructural plane - average particle size radius for a particle distribution - average planar particle size radius on a given microstructural plane - t aging time, in hours - average planar particle cross sectional area - G _t total shear modulus of the material - K _c particle growth rate constant - texture or Taylor grain orientation factor - N _v total number of precipitate particles per unit volume - Q _A activation energy for diffusion - R universal gas constant - T aging temperature - the interparticle separation or spacing - _y yield strength - _q as-quenched strength - _i intrinsic lattice strength - _c critical resolved shear strength - _loop critical resolved shear strength for dislocation particle bypassing via. Orowan looping - _particle total critical resolved shear strength for particle strengthening - _shear critical resolved shear strength for dislocation particle shearing, in underaged state     

Order hardening in nickel-molybdenum and nickel-tungsten alloys

Order hardening characteristics were studied in Ni-20% Mo, Ni-25% Mo and Ni-20% W. They were processed through quench and reheat cycles and the progress of order hardening was monitored through microhardness measurements. A correlation with the microstructure as observed with polarized light microscopy and transmission electron microscopy was established. Among the various factors, such as degree of order, domain size, coherency strains and dislocations, which can contribute to strengthening, domain size seems to play a major role.     

Discontinuous precipitation in Cu-Mg alloys

The relation between discontinuous precipitation and continuous precipitation and the cell growth kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 wt % Mg have been investigated, mainly by metallographic observation. The volume fraction of cells, the cell width and the interlamellar spacing have been determined by quantitative metallographic measurements. The cell growth rate decreases progressively with ageing time after the initial linear growth of the cells. This may be attributed to the influence of continuous precipitation on the cell growth. The volume fraction of the discontinuous precipitation cells,f, can be represented by the Johnson-Mehl equation: f=1–exp (–bt ⁿ). The value of the parameter n is about 2 and is independent of both the ageing temperature and alloy composition in the ageing range where the cell growth rates are constants. Mass transport of magnesium during the linear growth of cells occurs by grain boundary diffusion in a Cu-Mg solid solution.     

The study of work hardening in Fe-Mn-Al-C alloys

Three austenitic Fe-Mn-Al-C alloys with different aluminium content from 0, 5.1 and to 8.5wt% are chosen for the present work hardening study. Serrated stress-strain curves with pronounced work hardening were observed during tensile testing, also the serration of stress-strain curve is found to be decreasing as the increase of aluminium content. The serration can, however, still be observed even if the aluminium content is increased to as much as 8.5wt%. According to morphology studies and electron microscopic investigations, it is found that strain-induced deformation twins are closely related to the work hardening in the present alloys. Therefore, deformation twinning is strongly suggested as a major cause of work hardening in Fe-Mn-Al-C alloys, and also plays an important role on the serration of stress-strain curve.     

Iron precipitation in Cu-Au-Fe alloys

Interest is focused on the segregation and clustering behaviour of iron atoms in fcc Cu₁₀₀–x–yAu_xFe_y, alloys (x=6 to 50.7 at%, y=0.2 to 1.0 at%) of three different metallurgical states: as-rolled, fast-quenched and melt-spun. The gold concentration was varied to assess the effect of change in lattice parameter. Mössbauer spectroscopy has been used to examine iron clusters and phases in samples as functions of annealing temperature and time. The development of f c c -Fe, segregates with increasing annealing time at temperatures around 410° C has been monitored and the Néel temperature of the antiferromagnetic -Fe precipitates as well as their particle size determined, the latter by transmission electron microscopy. The increase in isomer shift with increasing gold concentration is accounted for mainly by changes in atomic volume.     

Mg2Ni系储氢合金的制备及其性能研究

采用感应熔炼法制备出两种镁基储氢合金,即Mg2Ni和Mg2Ni0.9Zr0.1;对两种合金进行扩散退火处理,并对退火处理前后的金相显微组织进行了比较研究;最后对这两种合金进行了X射线衍射实验和吸放氢性能测试实验.研究结果表明该方法制备出的镁基储氢合金活化性能优良;扩散退火能显著改善镁基储氢合金的显微组织和储氢性能.     

Dispersion hardening by niobium carbonitride precipitation in ferrite

Abstract

The formation offine carbonitride distributions in an experimental, low carbon, high niobium microalloyed steel transformed at various cooling rates was accompanied by TEM observations, tensile tests, and hardness measurements. Interphase precipitation or carbonitride formation on ferrite dislocations were observed to be the only precipitation modes which were responsible for the nucleation of new particles inferrite. An increase of only ～ 90 MPa in yield strength was found as the result of carbonitride precipitation on dislocations that occurred in an acicular ferrite microstructure with a dislocation density of (3–5) × 10¹⁰ cm^-2. A yield strength contribution of ～ 200 MPa was associated with interphase precipitation in polygonal ferrite formed during slow cooling of undeformed austenite. However, interphase precipitation was encountered only occasionally and did not contribute to the strength in the presence of carbonitride particles nucleated during austenite deformation.     

Age hardening of rapidly quenched Al-Zr-B alloys

Ternary alloys from the Al-Zr-B system were prepared as thin foils by a rapid quench technique. These foils were annealed isothermally as well as isochronally at various temperatures (150 to 550° C). The microstructures show that at high temperature, the grain growth is significantly retarded by the grain boundary pinning of boride precipitates. A strong age hardening is also a characteristic phenomenon in these alloys. It is found that microstructure and microhardness largely depend upon the zirconium/boron ratio of the alloy, indicating that the ratio determines the types of compounds occurring in this alloy system. It appears that in these alloys, high concentration of boron and the low ratio of zirconium/boron together yield stable precipitates at high temperatures.