Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy

In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200℃, 250℃ and 300℃ are β’’(DO19) → β’(BCO) → β(FCC), β’’(DO19) → β’(BCO) → β_(1)(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200℃, 250℃ and 300℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200℃ and 250℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β’’ to β’ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β’ precipitate, the hardness of the alloy aged at 200℃ is higher than that aged at 250℃. For the alloy aged at 300℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.

Tribological behavior of ZK60Gd alloy reinforced by SiC particles after precipitation hardening

In this research, the effect of precipitation hardening on the tribological behavior of the ZK60Gd/SiC composite was studied. For this purpose, ZK60Gd alloy containing with 5 and 10 wt% SiC were produced with stir casting method. The microstructure characterization of the samples showed the wide distributions of Mg_(7)Zn_(3) and Gd(Mg_(0.5)Zn_(0.5)) precipitates were formed during casting. The results of hardness measurement after precipitation hardening at different temperatures showed that the hardness peck was obtained at 175 ℃. The wear tests with different loads(10, 40, 60, 90, and 120 N) and velocities(0.1, 0.3, 0.6, and 0.9 m/s) were performed on the as-cast and heat treated sample at 125, 175, and 225 for 12 h. Between the different precipitation hardening conditions, the precipitation hardened samples at 175 ℃ had the highest hardness values and least wear rate. The sample containing 10% reinforcement had the least wear rate between the unreinforced alloy and the composites. The results showed that abrasive, adhesive, delamination, MML, and fatigue wear mechanisms were the dominant wear mechanisms for the composite samples. In contrast, the dominant wear mechanism for the unreinforced samples was abrasive, adhesive,delamination, MML, and plastic deformation.

Hot Compression Behavior of As-Cast Precipitation-Hardening Stainless Steel

High temperature deformation characteristics of a semiaustenitic grade of precipitation-hardening stain- less steels were investigated by conducting hot compression tests at temperatures of 900--1 100 ℃ and strain rates of 0. 001--1 s^-1. Flow behavior of this alloy was investigated and it was realized that dynamic recrystallization （DRX） was responsible for flow softening. The correlation between critical strain for initiation of DRX and de- formation parameters including temperature and strain rate, and therefore, Zener-Hollomon parameter （Z） was studied. Metallographic observation was performed to determine the as-deformed microstructure. Microstructural observation shows that recrystallized grain size increases with increasing the temperature and decreasing the strain rate. The activation energy required for DRX of the investigated steel was determined using correlations of flow stress versus temperature and strain rate. The calculated value of activation energy, 460 kJ/mol, is in accordance with other studies on stainless steels. The relationship between peak strain and Z parameter is proposed.

PRECIPITATION HARDENING IN B2-ORDERED NiAl BY Ni_(2)AlTi COMPOUND

Microstructural variations and correlated hardness changes in B2-ordered NiAl containing fine precipitation of Ni2AlTi have been investigated by means of transmission electron microscopy (TEM) and hardness tests. The amount of age hardening is not large as compared to the large microstructural variations during aging. TEM observations have revealed that the L21-type Ni2AlTi precipitates keep a lattice coherency with the NiAl matrix at the beginning of aging. By longer periods of aging Ni2AlTi precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. The temperature dependence of the yield strength of precipitate-containing B2-ordered NiAl was investigated by compression tests over the temperature range of 873-1273 K. The fine precipitation of Ni2AlTi was found to enhance greatly the yield strength and the high-temperature strength is comparable with that of superalloy Mar-M200.

PRECIPITATION BEHAVIOUR AND HARDENING OFB2-ORDERED Ni(Al, Ta)

Precipitation of Ni2Al in supersaturated NiAl containing Ta and corresponding hardening have been investigated by hardness measurements and by transmission electron microscope observations. Selected area electron diffraction, high resolution electron microscopy and energy dispersive X-ray spectrometer analysis have shown that the Ni2Al with a hexagonal structure precipitates coherently in the B2-NiAl matrix. Ni2Al is an ordered phase with a. unit cell parameter, aNi2All=aNiAl and CNi2Al= aNiAl. The orientation relationship between the Ni2Al precipitate and the B2-NiAl matrix is (0001)Ni2Al (111)NiAl and [1120]Ni2Al[110]NiAl. NiAlhardens appreciably by the precipitation of Ni2Al phase.

The Effect of Time, Percent of Copper and Nickel on the Natural Precipitation Hardness of Al – Cu – Ni Powder Metallurgy Alloys Using Design of Experiments

In this investigation, the effect of time, percentage of copper and nickel on the hardness property of aluminum based powder metallurgy alloys were studied. A full factorial analysis with four levels for each factor was used. The samples were produced using powder metallurgy process, and then subjected to natural aging where heat treatment was conducted for all samples together at 550°C for 3 hours before quenching in water. Then, the samples were left at room temperature for 936 hours (39 days) to allow traces atoms to diffuse and form coherent phases which increase the hardness. It was found that the hardness was firstly increased with time for about 300 hours after the quenching time, and then it tends to remain constant after that. However, the hardness drop at overage stage was not observed until the end of 936 hours. To get a full analysis of the natural aging we used design of experiment tool to study the effect of %Cu, %Ni and aging time on the hardness. The results showed that the hardness was influenced significantly by all considered factors and interactions between them.

Numerical Calculation and Experimental Research on Residual Stresses in Precipitation-hardening Layer of NAK80 Steel for Shot Peening

Shot peening can improve fatigue strength of materials by creating compressive residual stress field in their surface layers,and offers a protection against crack initiation and propagation,corrosion,etc.And fatigue fracture and stress corrosion cracking of NAK80 steel parts are improved effectively.Currently there lacks in-depth research in which the beneficial effect of the residual stress may be offset by the surface damage associated with shot peening,especially in terms of the research on the effective control of shot peening intensity.In order to obtain the surface residual stress field of NAK80 steel after shot peening,the samples are shot peened by pneumatic shot peening machine with different rules.The residual stress in the precipitation-hardening layer of NAK80 steel is measured before and after a shot peening treatment by X-ray diffraction method.In order to obtain true residual stress field,integral compensation method is used to correct results.By setting up analytical model of the residual stress in the process of shot peening,the surface residual stress is calculated after shot peening,and mentioning the reason of errors occurred between calculated and experimental residual stresses,which is mainly caused by the measurement error of the shoot arc height.At the same time,micro hardness,microstructure and roughness in the precipitation-hardening layer of NAK80 steel before and after shot peening were measured and surveyed in order to obtain the relation between shot peening strength and surface quality in the precipitation-hardening layer.The results show that the surface quality of NAK80 steel is significantly improved by shot peening process.The over peening effect is produced when the shot peening intensity is too high,it is disadvantageous to improve sample＇s surface integrity,and leading to reduce the fatigue life.When arc high value of optimal shot peening is 0.40 mm,the surface quality is the best,and the depth of residual stress in the precipitation-hardening layer reaches to about 450 μm.Numerical calculation is very useful to define the process parameters when a specific residual stress profile is intended,either to quantify the benefits on a specific property like fatigue life or to help on modeling a forming process like shot peen forming.In particular,the proposed parameter optimization in the progress of shot peening and effective control of the surface texture provide new rules for the quantitative evaluations of shot peening surface modification of NAK80 steel.

Evaluation of precipitation hardening in TiC-reinforced Ti_2AlNb-based alloys

Ti2AlNb-based alloys with 0.0 wt%, 0.6 wt%, and 2.0 wt% carbon nanotube（CNT） addition were fabricated from spherical Ti–22 Al–25 Nb powder by sintering in the B2 single-phase region. Phase identification and microstructural examination were performed to evaluate the effect of carbon addition on the hardness of the alloys. Carbon was either in a soluble state or in carbide form depending on its concentration. The acicular carbides formed around 1050℃ were identified as TiC and facilitated the transformation of α2 ＋ B2 → O. The TiC was located within the acicular O phase. The surrounding O phase was distributed in certain orientations with angles of 65° or 90° O phase particles. The obtained alloy was composed of acicular O, Widmanstatten B2 ＋O, and acicular TiC. As a result of the precipitation of carbides as well as the O phase, the hardness of the alloy with 2.0 wt% CNT addition increased to HV 429 ± 9.

Effect of MWCNT reinforcement on the precipitation-hardening behavior of AA2219

Aluminum alloy matrix composites have found a predominant place in research, and their applications are explored in almost all industries. The aerospace industry has been using precipitation-hardenable alloys in structural applications. However, insufficient literature is available on the influence of multiwalled carbon nanotubes （MWCNTs） on precipitation-hardenable alloy composite materials; thus, this work was designed to elucidate the effect on MWCNT reinforcement on AA2219 with and without precipitation hardening. Reinforcement with MWCNTs has been reported to accelerate precipitation and to achieve greater hardness within a much shorter time. The addition of 0.75wt% MWCNTs resulted in maximal hardness at 90 min, which is approximately 27% of improvement over the maximum hardness achieved by the corresponding monolithic alloy after 10 h of aging. The sample reinforced with 0.75wt% MWCNTs showed an improve- ment of 82% in hardness by solutionizing and aging compared to that achieved by sintering.

PRECIPITATION HARDENING OF ELINVAR ALLOY

The microstructural evolution and precipitation hardening of an Elinvar alloy doped with Ti and Al during isothermal aging at 700℃ have been investigated by atom probe field ion microscopy and microhardness measurements.The γ′ precipiates are spherical and coherent with the matrix.The chemical composition of the precipitates are(Ni_(0.53)Fe_(0.47)_3 (Ti_(0.(?))Al_(0.4)). During aging,a Lifshitz-Wagner type dissolution and coarsening reaction of the precipitates has been observed,The hardness of the material varies with the aging time and reaches maxi- mum when the average diameter of the precipitates was about 11 nm.

Investigation on microstructure and properties of a combined precipitation hardening ultrahigh strength steel

The microstructure and properties of a combined precipitation hardening ultrahigh strength steel with nano-sized carbides and intermetallics were studied systematically.The results show that after tempering at 300℃lots ofε-carbides are precipitated in the martensite,the strength rises and the toughness falls slightly.After tempering at 430℃,much coarser cementite lamina are precipitated in martensitic laths,which causes the impact toughness falls to the minimum value.With temperature further increasing the cementites are dissolved and M_2C carbides,β-NiAl intermetallics and reverse austenite begin to precipitate.The tensile strength and yield strength achieve the peak value at 470℃,490℃respectively.The tested steel achieve a tensile strength of 2 120 MPa,a yield strength of 1 950 MPa and impact energy of 54 J/cm^2 after optimum tempering at 510℃.When tempering temperature is above 530℃the M_2C carbides and reverse austenite is coarsening.After tempering at 560℃the reverse austenite reaches the maximum volume fraction in present work.

Removal of Aqueous Lead and Copper Ions by Using Natural Hydroxyapatite Powder and Sulphide Precipitation in Cyanidation Process

Mining, ore processing, and smelting activities have contaminated soil and water resources with cyanide and heavy metals throughout the world. In-situ chemical immobilization is a remediation technology that decreases the concentrations of dissolved ions of copper cyanide and lead hydroxides by adsorption or precipitation. This study deals, with a process to recover lead hydroxides with natural hydroxyapatite powder and copper cyanide ions with sulphide precipitation from solutions of the cyanidation process. This technology is based on inducing nucleated precipitation of copper and silver in a serpentine reactor, using sodium sulfide as the precipitator, and sulfuric acid for pH control. Results show that pH value has a significant effect on copper cyanide removal efficiency, and it was determined the optimal pH range to be 2.5 - 3. At this pH value, the copper cyanide removal efficiency achieved was up to 99% with 60 percent of copper purity and the cyanide ions associated with this complex the recovery was 90%. For the elimination of lead hydroxides ions from the aqueous cyanidation process with natural hydroxiapatite powder the recovery was 99%. The abundance of cow bones, its low price and non-aggressive nature towards the environment are advantages for its utilization in point of view of wastewater cleanup and Dore with higher quality.

HARDENING OF L1_2-ORDERED Co_3Ti BY PRECIPITATION OF FCC-Co PHASE

The Co3 Ti phase hardens appreciably by the fine precipitation of disordered fcc Co-rich phase upon aging after quenching from solution annealing temperature. Transmission electron microscope (TEM)observations revealed that the precipitates are platelet in shape, lying nearly parallel to the {100} planes of the L12-ordered matrix, and perfectly coherent with the matrix lattice at the beginning of aging. The high temperature strength increases appreciably with the fine precipitation of disondered Co-rich phase over the whole temperature range investigated. TEM observations of the deformed alloys after underaging revealed that saperdislocations are pinned by precipitates indicating an attractive interaction between dislocations and precipitates.

Decomposition processing and precipitation hardening of rapidly solidified Al-Cr-Y-Zr alloy

Foil powders of Al 5.0 Cr 4.0 Y 1.5 Zr (%) were prepared by using a multi stage atomization rapid solidification powder making device. The obtained powders were exposed thermally at various temperatures. Variation of microstructures and properties of the alloy powders was investigated by micro hardness measurement, X ray diffraction, differential thermal analysis, and transmission electron microscopy with energy disperse X ray analyses. The results show that cubic Al 20 Cr 2Y ( a = 1.437?nm) and metastable Ll 2 Al 3Zr (FCC, a = 0.405?1?nm) or equilibrium DO 23 Al 3Zr (tetragonal structure, a = 0.409?1?nm, b = 1.73?0?nm) are main second phases precipitated from supersaturated solid solution of the rapidly solidified foil powders during thermal exposure. The cubic dispersion precipitates prior to the two other Al 3Zr type intermetallic phases in the course of the decomposition. Precipitation of incoherent Al 20 Cr 2Y results in softening of foil powder, and coherent Ll 2 Al 3Zr has intensive precipitation strengthening effect. The Al 20 Cr 2Y phase is structurally stable, but it is prone to coarsen and polygonize above 450?℃. Both Al 3Zr type intermetallic phases have much smaller coarsening rate than Al 20 Cr 2Y at temperature higher than 450?℃. These two phases are able to keep their fine spherical morphologies up to 550?℃, but Al 3Zr transforms into DO 23 structure from Ll 2 structure during thermal exposure above 550?℃.

Hardening effects of sheared precipitates on {1121} twinning in magnesium alloys

The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the(110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length(in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning(non-sheared precipitate), {1121} twinning(sheared precipitate) and basal dislocations(non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.

预变形Mg-Nd合金时效析出行为的原子尺度

使用球差校正的高角环形暗场扫描透射电镜(HAADF-STEM)系统研究了预变形Mg-20%Nd(质量分数)合金250℃等温时效2 h后的时效析出行为结果表明,预变形产生的位错促进了第二相的形核,导致高密度且有别于传统沿{101-0}α惯习面均匀分布的亚稳析出相的形成原子尺度的表征结果表明,合金内部形成的析出链主要由βS′相和β1相组成此外,析出链中还含有少量的β″和βH′相,以及βL′相这些片状或透镜状的βS′和β1相与α-Mg基体完全共格,且均具有一个{101-0}α的惯习面,但沿[112-0]α,[12-10]α和[2-110]α3个方向非均匀分布这些结果有望为形变调控合金的析出行为,提升合金力学性能提供理论指导.

7075高强铝合金构件冷成形强化机制研究

目的针对7075高强铝合金构件在固溶-淬火-时效处理过程中成形精度低的问题,提出了7075高强铝合金预强化冷成形工艺,研究7075高强铝合金构件冷成形强化机制。方法基于高强铝合金短流程高性能成形技术,经过固溶-时效处理,获得预强化处理的7075铝合金板料,使用预强化处理的7075铝合金板料冷成形试制帽形梁。通过拉伸试验、杯突试验测试预强化处理的7075铝合金板料及帽形梁力学性能,并通过透射电子显微镜试验解释7075高强铝合金构件冷成形强化机制。结果预强化处理的7075铝合金板料抗拉强度为540MPa,延伸率为19.3%,强度接近7075铝合金T6态强度水平,塑性接近7075铝合金O态塑性水平。杯突值为16.6 mm,达到7075铝合金O态的87%。使用预强化处理的7075铝合金板料冷成形试制的帽形梁表面质量良好,无破裂等情况。经过烤漆工艺后,帽形梁抗拉强度为(560±5)MPa,屈服强度为(480±5)MPa,与7075高强铝合金T6态强度相当。结论预强化处理的7075铝合金板料基体内部存在大量GPⅡ区组织,这有助于提高7075高强铝合金的强度和塑性。使用预强化处理的7075铝合金板料冷成形试制的帽形梁在烤漆工艺处理时,基体中部分GPⅡ区会转变为η'相,析出相的转变和加工硬化的结合提高了成形构件的强度,使其强度可以达到7075高强铝合金T6态强度水平。

发泡剂和增硬树脂对轻质天然橡胶材料性能的影响

采用均匀设计法,考察了发泡剂和增硬树脂对轻质天然橡胶材料性能的影响。结果表明,发泡剂N,N′-二亚硝基五亚甲基四胺(H)和增硬树脂(P1)用量变化对NR密度和拉伸强度等有显著影响。增大发泡剂H和增硬树脂P用量,发泡压力增大,硫化起步提前,最高硫化扭矩先增大后减小,拉伸强度和密度均呈线性变化。与单一发泡剂H胶料相比,发泡剂偶氮二甲酰胺(AC)/发泡剂H并用胶料硬度、密度、撕裂强度和定伸应力均较高,发泡材料孔径较小;发泡剂4,4′-氧代双苯磺酰肼(OBSH)/发泡剂H并用胶料密度最小,拉伸强度最大。

影响15-5PH沉淀硬化不锈钢硬度的因素

炼制了两炉化学成分不同的15-5PH马氏体沉淀硬化不锈钢并制备了试样。对试样进行了1040℃保温90 min油冷的固溶处理及铣削、钻削、磨削试验和分别在400~620℃时效1.5、4、8和12 h。随后检测了试样的硬度和显微组织。结果表明:切削加工对15-5PH马氏体沉淀硬化不锈钢的硬度无明显影响;在450℃时效,随着时效时间延长至4 h,15-5PH钢的硬度提高,并达到了44~46 HRC的硬度要求;15-5PH钢的最佳时效工艺为450℃保温4 h。

7种植物精油对空气自然菌落抑制作用研究

本研究利用自然沉降法收集空气微生物,采用平板计数法统计植物精油添加前后的空气微生物菌落数,并计算其抑菌率。根据抑菌率筛选出2种精油在其最佳抑菌浓度下进行添加不同体积精油的实验以及进行干扰项测试实验。结果表明,7种植物精油中有5种精油的最佳抑菌率达到70%以上,抑菌作用由强至弱的顺序为:柠檬草、柠檬、野橘、薰衣草、艾叶、薄荷、依兰。根据抑菌作用和筛选出的2种较易获得的植物精油为柠檬草精油和艾叶精油。在干扰项测试中,市售祖马龙香水对2种精油无干扰作用。