Effect of Cu on precipitation hardening and clustering behavior of Al-Zn-Mg alloys in the early stage of aging
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This work demonstrates that Cd microadditions can enhance the precipitation of age-hardening precipitates in 2xxx and 6xxx alloys as well as the dispersoid precipitation in 3xxx alloys, but have a minimal effect on the precipitation in 7xxx alloys. Experimental and theoretical methods are utilised to investigate the nucleation and precipitation of precipitates/dispersoids in Cd-containing Al alloys. The enhanced precipitation of θ′ and α-dispersoids in 2xxx and 3xxx alloys, respectively, are proposed to result from the heterogeneous nucleation on earlier-formed Cd-rich nanoparticles, while the enhanced precipitation of β′′ in 6xxx alloys is attributed to the increased number density of effective Mg-Si co-clusters due to the Cd incorporation. This investigation provides theoretical basis for the design of precipitation-strengthened Al alloys via microalloying.
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In the previous paper, the phase changes in 17-7 PH steel were determined. In the present paper, the effect of Al upon its age-hardening characteristics was studied, and its nature was clarified.The age-hardening of alloys free from δ(0∼1.2%Al) is enhanced markedly with Al content owing to an increase of α′ precipitates. In alloys with δ alone (>4%Al), however, α′ precipitates so rapidly that it cannot be suppressed even by water-quenching after solution treatment, and hence no appreciable age-hardening is observed on subsequent aging. Thus, the maximum hardening is obtained in alloys which consist of αM and a small amount of δ with about 1.5%Al corresponding to the Al content of the standard 17-7 PH steel. In commercial 17-7 PH steel with about 0.07%C, M23C6 carbide also precipitates from the matrix during aging, which, however, seems unlikely to be responsible for the age-hardening of this steel, because its amount is negligibly small.Considering the results of the present and previous experiments, the remarkable age-hardening of 17-7 PH steel was attributed to the precipitation of α′ from αM.
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The influences of added C, Mo, and N contents on precipitation hardening were investi-gated. There were 23 kinds of samples with varying chemical composition of C, Mo, and N. For the purpose of studying the influence of varied contents of added elements on precipitation hardening, 13 group were made by various combinations of all samples. After hot forging to bars, all samples were solution treated at 1200°C for 1 hour. Then, they were aged at 800°C up to 150 hours. Hardening was measured by hardness at 1, 3, 15, 30, 50, 75, 100, and 150 hours. And, the microstructure of such aged samples was also observed.It was concluded from this experiments as follows;(1) Carbon contents had more remarkable influences on precipitation hardening than Mo or N contents. If the carbon contents increased, hardness was raised and phenomena of double peaks on hardening process became more evident.(2) Although influence of Mo contents was not found immediately after solution treatment, more Mo meant more hardening when it was aged at 800°C. If Mo content was much less than 6%, softening was indicated after 150 hours aging at 800°C.(3) In the experiment of Reports (IX), the influence of N content on precipitation hardening was not apparent. This was already investigated in Reports (V) in much more details.
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KeyLos? 2001 is a new precipitation-hardening steel especially developed for plastic moulds. In this study the precipitation stage of KeyLos? 2001 steel has been investigated and compared to the results obtained with 17-4 PH steel. Precipitation-hardening has been carried out at three different temperatures and the stages of hardening and overageing have been studied in order to clarify the hardening mechanisms. It has been found that hardening and softening mechanism during the precipitation-hardening treatment occur at higher temperatures and in correspondence with more prolonged treatment times than those typical for the best known 17-4 PH steel; hardness is then expected to remain stable also for very extended mould lives. Microstructural investigations by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) have also been carried out and the microstructural parameters responsible for the hardening and overageing have been pointed out.
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The age-hardening of the Cu-7%Al-1.5%Co alloy containing 0.01%P has been experimented by a variety of methods. The main results are as follows:(1) The age-hardening of the alloy, solution-treated by heating at 970°C for 2 hr and quenching in water, takes place when it reheated above 350°C. The behavior of hardness in the course of the age-hardening is such that is observed with usual precipitation hardening alloys.(2) The activation energy for the the hardening is 37.3 kcal/mol, being almost equivalent to that for the diffusion of Al in Cu.(3) The process of the age-hardening was examined by electron micrography, X-ray diffraction, electron probe micro-analysis. The results imply that the hardening is ascribed to the formation of G·P zones, where Al and Co are concentrated. At later stages of the heat treatment the zones grow into precipitates, whose electron microdiffraction indicates that they have a cubic structure.(4) Between 500°C and 600°C a heat evolution due to the formation of G·P zones is observed by the thermal analysis of the solution-treated specimens, and the evolved heat is determined to be 5.5 cal/g. Below 300°C another heat evolution is observed in the water quenched specimens, but the latter heat evolution seems to do nothing with the age-hardening of this alloy.
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The present investigation was carried out to acertain the effect of heat treatment on the natural age hardening and temper hardening of Al alloys containing Cu and Sn which had been used for bearing materials.As these test pieces were prepared by addition of the mother alloys (Cu6Sn5Cu3Sn) and Mg to Al, the results obtained from the study of Al-Cu6Sn5, Al-Cu3 Sn, and Al-Cu6 Sn5-Mg systems were as follows:(1) All the alloys of these systems did not show a natural age hardening at room temperature, and the temper hardening was remarkable except the two alloys containing 2%Cu6 Sn5 or Cu3 Sn. But the addition of 1.5%Mg to these two alloys gave an influence upon the temper age hardening. From these results, it was cartain that Cu and Sn were soluble in Al.(2) By heating these natural aged alloys at 220° for a short time, the hardness increased at first and decreased, and then increased. This first hardening seemed to be a temper hardening without a certain precipitate and the second by it.
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In order to clarify the precipitation and its hardening effect, and the relation between aging temperature and composition of precipitates in Ni-rich NiTi, the changes in hardness and microstructure with aging temperature and time have been examined. The composition of precipitates has also been determined by an electron probe microanalyser.In the solution treatment of Ni-rich NiTi, the higher the cooling rate the higher becomes the hardness. A significant hardening effect is obtained even by air cooling. The hardness of water-quenched specimens decreases with aging time in the temperature range from 400 to 700°C. No hardness peak is detected at these temperatures. Optical microstructure experiment shows that no precipitates exist in the water-quenched specimens, but the precipitates appear after aging for 1 to 10 hr at 600°C or 0.1 to 1 hr at 700°C. The compositions of the precipitates determined in the specimens aged at 600 and 700°C are close to Ni3Ti2 and Ni3Ti, respectively.The above results indicate that the quench-hardening is caused by quenched-in substitutional defects and that although age-hardening occurs the hardness peak does not appear because the hardening effect is cancelled by lowering of quench-hardening as the number of substitutional defects decreases.
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The effect of ageing on the hardening and softening of Mg-Y-Nd alloys has been studied. An obvious age hardening occurs in alloys aged at 473 K for about 2 h, and then the hardness decreases sharply. A second hardening occurs during the following ageing. But alloys started softening after being aged at 523 K for 600 h. The microstructure investigations by TEM reveal that the first hardening owes to the occurrence of the dispersed precipitate, and the sharp fall of the hardness results from the dissolution of MgY phase. The occurrence and increasing of the precipitate phase and the forming of lathy structure in alloys result in the second hardening. The enlargement of j3 precipitate phase and the formation of nano-size MgO grains at the interface between matrix and (3 phase result in the softening of the Mg-Y-Nd alloy.
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