Non-isothermal aging behavior of in-situ AA2024−Al3NiCu composite

The effect of non-isothermal aging treatment on microstructure and mechanical properties of in-situ AA2024?Al₃NiCu composite fabricated by the stir casting process was examined. The Al₃NiCu intermetallic was created by adding 3 wt.% nickel powder during stir casting and homogenization treatment at 500 °C for 24 h after casting. The microstructural results obtained using optical and scanning electron microscope indicate that, after non-isothermal aging treatment, the S-Al₂CuMg precipitates become finer, forming a poor zone of this precipitate in the area between the dendrites. Also, adding nickel during stir casting reduces the precipitation rate and the contribution of S-Al₂CuMg precipitates in strengthening composite during non-isothermal aging. The maximum hardness, ultimate tensile strength, and toughness achieved in the 3 wt.% nickel-containing sample after non-isothermal aging at 250 °C are (121.30±4.21) HV, (221.67±8.31) MPa, and (1.67±0.08) MJ/m³, respectively. The maximum hardness and ultimate tensile strength of AA2024?Al₃NiCu composite are decreased by 6% and 4%, respectively, compared to those of nickel-free AA2024 aluminum alloy.     

固溶处理和人工时效对Al-Cu合金显微组织和力学性能的影响(英文)

对Al-Cu合金进行析出强化和人工时效处理以获得优异的力学性能,如高的强度、好的韧性。其热处理工艺条件为:510～530℃固溶处理2h;60℃水淬;160～190℃人工时效2～8h。采用光学显微镜、扫描电镜、能谱分析、透射电镜和拉伸实验对经固溶和人工时效处理的Al-Cu合金的组织和力学性能进行表征。固溶处理实验结果表明,Al-Cu合金的力学性能随着固溶处理温度的升高先增加,然后降低。这是由于Al-Cu合金的残余相逐渐溶解进入基体中,从而导致析出相的数量和再结晶晶粒尺寸不断增加。相较于固溶处理温度,固溶处理时间对Al-Cu合金的影响较小。人工时效处理实验结果表明,合金经180℃时效8h,可以获得最大的拉伸强度。合金的最大拉伸强度和屈服强度随着时效时间的延长和温度的升高而升高。     

半固态加工Ti-Cu合金的显微组织和力学行为（英文）

研究触变成形Ti-Cu合金的力学性能。Ti-Cu (25%,27%,29%Cu,质量分数)锭的制备流程为：先进行电弧熔炼,再在950℃均匀化处理24h,然后在900℃热锻,最后在1035℃热处理300 s后以8 mm/s的速度触变成形。结果显示,触变成形合金表现出良好的力学强度,但其在拉伸载荷下的塑性一般,在压缩载荷下的塑性尚可。随着Cu含量的增加,包晶Ti₂Cu相(转变液相区)的体积分数增加,与α+Ti₂Cu相区(转变固相区)相比,其力学强度和塑性更低,导致合金的力学强度和塑性降低。这些结果表明,Ti-Cu合金的力学性能和半固态加工性之间的平衡主要取决于Cu含量。     

The Mechanical and Microstructural Changes of Sn-Ag-Bi Solders with Cooling Rate and Bi Content Variations

The purpose of this study is to investigate the influence of cooling rate and Bi addition on the microstructure evolution and mechanical properties of Sn-3.5Ag alloy. A series of Sn-3.5Ag-xBi solders has been fabricated with Bi content in the range of 0.5-3.5 wt.%. After solution heat treatment at 170 °C for 24 h and subsequent aging heat treatment at 100 °C for 2 h, samples were divided into two groups. One group was rapidly quenched into iced water (water quenching) for the fast cooling rate (20 °C/s), while the second group was slowly cooled (furnace cooling) in the furnace for the slow cooling rate (0.2 °C/s) after the furnace reflow. The microstructural evolutions of the present solders have been investigated using x-ray diffraction and scanning electron microscopy. The microhardness was measured to correlate the mechanical properties to alloy compositions and cooling rate. It was found that the microhardness of Sn-3.5Ag-xBi solders increased with increasing cooling rate. The indentation creep curves have been evaluated from the obtained microhardness values. Results revealed the steady-state creep rate decreased with increasing Bi content exhibiting an anomalous behavior at 2.5Bi. The reason for improved creep resistance of Sn-3.5Ag-xBi solders is the result of the combination of the solid solution strengthening and precipitation strengthening of Bi. The mean values of stress exponent indicated that the operative creep mechanism is dislocation climb.     

Effects of different tempers on precipitation hardening of 6000 series aluminium alloys

By means of Vickers hardness,mechanical property and formability tests,the effects of different tempers on precipitation hardening of 6000 series aluminium alloys for automotive body sheets were investigated.The results indicate that the short-time pre-aging at 170℃makes for subsequent artificial aging precipitation hardening.With the increase of pre-aging time,the artificial aging hardenability increases.The aging hardening rate reaches the maximum when pre-aging time is up to 10 min,and then it decreases.The short-time pre-aging at 170℃benefits sheets to obtain lower strength under delivery condition and consequently to improve stamping formability of automotive body sheets.The effects of different tempers on precipitation hardening are much more obvious than those of the alloying elements.It is a good treatment schedule to perform pre-aging for 5 min at 170℃right after solution treatment.     

Effect of non-isothermal aging on microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys

The evolution of microstructure and properties of Al–5.87Zn–2.07Mg–2.42Cu alloys during non-isothermal aging was studied. The mechanical properties of the alloy were tested by stretching at room temperature. The results show that in the non-isothermal aging process, when the alloy is cooled to 140 °C, the ultimate tensile strength of the alloy reaches a maximum value of 582 MPa and the elongation is 11.9%. The microstructure was tested through a transmission electron microscope, and the experimental results show that the GP zones and η’ phases are the main strengthening precipitates. At the cooling stage, when the temperature dropped to 180 °C, the GP zones were precipitated again. Besides, the experimental results show that the main strengthening phase during non-isothermal aging is η’ phases.     

AZ91镁合金的挤压和析出硬化行为(英文)

挤压比为4:1,将铸态AZ91镁合金分别在250,300和350℃下进行挤压,随后进行析出硬化处理(T6)。经过热挤压和析出硬化处理后,铸态AZ91镁合金中粗大的和偏析Mg17Al12析出相被细化并均匀分布在α-镁基体中。在不同的挤压温度下合金中发生了部分或全部动态再结晶。经挤压后,该合金的极限抗拉强度从铸态的190MPa增加到570MPa。AZ91镁合金的时效硬化特征与晶粒尺寸有关。在250、300和350℃下以4:1的挤压比挤压该合金后,获得峰值硬度的时效时间分别为35、30和20h。SEM观察到在AZ91基体中存在均匀细小的Mg17Al12析出相。     

211Z高强韧耐热铝合金的淬透性研究

采用类端淬设备、电导率计、差示热扫描分析仪和透射电镜等并结合数字模拟研究了211Z型铝合金的淬透性能。结果表明:试样在≤40℃水温淬火的过饱和固溶程度、时效后的硬度均高于水温较高(50~60℃)时淬火的相应值,但淬透深度相反;试样时效处理后的硬度值受淬火敏感温度区间(140~380℃)的平均冷却速度影响,超过临界值后,硬度值基本保持不变,小于临界值,则随平均冷却速度的增加而增大。     

Effects of heat treatment on microstructure and mechanical properties of Mg-6Zn-4Sm-0.4Zr alloy

A new rare earth magnesium alloy(Mg-6 Zn-4 Sm-0.4 Zr, wt.%) was prepared by permanent mould casting. The microstructure and mechanical properties of the alloy sample in as-cast and various heat treatment situations were characterized with an optical microscope(OM), X-ray diffractometer(XRD), scanning electron microscope(SEM) equipped with energy dispersive spectroscope(EDS), transmission electron microscope(TEM) and mechanical tests at room temperature, respectively. The experimental results show that the as-cast alloy mainly consists of α-Mg, eutectic Mg_2Zn_3, MgZnSm and Mg_(41)Sm_5. These eutectic phases with continuous or semicontinuous morphology principally distribute along grain boundaries. Almost all the eutectic compounds dissolve in α-Mg and the grains have no obvious growth trend after optimum solution treatment at 490 °C for 18 h. Meanwhile, the ultimate tensile strength(UTS) of 229 MPa and elongation(EL) to rupture of 9.78% can be achieved through the optimal solution treatment, which increase by 37 MPa and 57.74%, respectively, compared with that of the as-cast alloy. Further aging treatments at 200 °C for different durations lead to the conspicuous increment of mechanical properties and prominent age-hardening response. Peak-aged alloy(treated at 200 °C for 12 h) reveals better mechanical properties(UTS 258 MPa, EL 9.42%, hardness 73.4 HV) compared with the same alloy treated in other aging conditions, which is mainly ascribed to precipitated Mg_2Zn_3 and MgZn_2 phases. Fracture analysis demonstrates that the as-cast alloy belongs to inter-granular and cleavage fracture patterns, while the solutionized alloy(treated at 490 °C for 18 h) reveals trans-granular and quasi-cleavage fracture modes. For the peak-aged alloy, the fracture pattern obeys the mixture of trans-granular and cleavage modes.     

Effect of three-step homogenization on microstructure and properties of 7N01 aluminum alloys

The effect of different homogenization treatments on the microstructure and properties of the 7N01 aluminum alloy was investigated using hardness measurements, electrical conductivity measurements, tensile and slow strain rate tests, electron probe microanalysis, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results revealed that three-step homogenization improved the uniformity of Zr distribution by eliminating segregation of the main alloying elements. During the second homogenization step at 350 °C for 10 h, coarse and strip-like equilibrium η phases formed which assisted the nucleation of Al₃Zr dispersoids and reduced the width of the precipitate-free zone of A1₃Zr dispersoids. As a result, coarse recrystallization was greatly reduced after homogenization at 200 °C, 2 h + 350 °C, 10 h + 470 °C, 12 h, which contributed to improving the overall properties of the 7N01 aluminum alloys.     

Microstructure and Precipitate's Characterization of the Cu-Ni-Si-P Alloy

Microstructure of the Cu-Ni-Si-P alloy was investigated by transmission electron microscopy (TEM). The alloy had 551 MPa tensile strength, 226 HV hardness, and 36% IACS electrical conductivity after 80% cold rolling and aging at 450 °C for 2 h. Under the same aging conditions, but without the cold rolling, the strength, hardness, and electrical conductivity were 379 MPa, 216 HV, and 32% IACS, respectively. The precipitates identified by TEM characterization were δ-Ni₂Si. Some semi-coherent spherical precipitates with a typical coffee bean contrast were found after aging for 48 h at 450 °C. The average diameter of the observed semi-coherent precipitates is about 5 nm. The morphology of the fracture surface was observed by scanning electron microscopy. All samples showed typical ductile fracture. The addition of P refined the grain size and increased the nucleation rate of the precipitates. The precipitated phase coarsening was inhibited by the small additions of P. After aging, the Cu-Ni-Si-P alloy can gain excellent mechanical properties with 804 MPa strength and 49% IACS conductivity. This study aimed to optimize processing conditions of the Cu-Ni-Si-P alloys.     

Effect of T6-treatments on microstructure and mechanical properties of forged Al-4.4Cu-0.7Mg-0.6Si alloy

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), hardness tests and tensile tests were performed to investigate the effect of aging on microstructure and mechanical properties of forged Al-4.4Cu-0.7Mg-0.6Si alloy. The results show that the alloy exhibits splendid mechanical properties with an ultimate tensile strength of 504 MPa and an elongation of 10.1% after aging at 170 °C for 16 h. With tensile testing temperature increasing to 150 °C, the strength of the alloy declines slightly to 483 MPa. Then, the strength drops quickly when temperature reaches over 200 °C. The high strength of the alloy in peak-aged condition is caused by a considerable amount of θ′ and AlMgSiCu (Q) precipitates. The relatively stable mechanical properties tested below 150 °C are mainly ascribed to the stability of θ′ precipitates. The growth of θ′ and Q precipitates and the generation of θ phase lead to a rapid drop of the strength when temperature is over 150 °C.     

Effect of WC addition and cooling rate on microstructure,magnetic and mechanical properties of Ti(C0.6,N0.4)-WC-Mo-Ni cermets

Ti(C_0.6,N_0.4)-8Mo-xWC-25Ni (x = 0, 3, 6 and 9 wt%) cermets were synthesized under different cooling rates by vacuum sintering. The influence of WC addition and cooling rate on microstructure, magnetic and mechanical properties of the as-prepared Ti(C,N)-based cermets was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and physical property measurement system (PPMS). The results revealed that the grain size of the Ti(C,N)-based cermets became finer with WC addition. Furthermore, room-temperature saturation magnetization (M_s), remanence (M_r) and Curie temperature (T_c) of the Ti(C,N)-based cermets initially decreased with increasing WC content, followed by a gradual increase. Cermets bacame paramagnetic at x = 6 under the cooling rate of 2 °C/min, x = 6 and 9 under the cooling rate 35 °C/min, respectively. The decrease in magnetic properties could be ascribed to the enhanced solid solubility of alloy elements in Ni-based binder phase. Moreover, the hardness and transverse rupture strength (TRS) of the Ti(C,N)-based cermets initially increased and followed by a gradual decrease, whereas the fracture toughness initially decreased followed by an increase with increasing WC content. At the same value of x, the Ti(C,N)-based cermets exhibited better magnetic and mechanical properties at the cooling rate of 35 °C/min than that at the cooling rate of 2 °C/min, which could be attributed to the grain refinement strengthening and solid-solution strengthening of the binder phase.     

Effect of precipitation from tweed matrix on the mechanical properties in a metastable beta alloy of Ti-15-3

The effects of aging treatment on the microstructures and mechanical properties of a metastable beta titanium alloy Ti-15-3 (Ti−15V−3Al−3Sn-3Cr) have been investigated with hardness measurements, tensile test, and optical and electron microscopy. Precipitate-free beta structure with average grain size of about 56 μm was obtained after solution treatment at 800°C for 15 min followed by air cooling. Solution treated specimens were aged up to 800 h in the temperature range between 350 and 600°C. The morphology of the precipitates was varied significantly, depending on the aging temperature. The fine aggregates of α precipitates were dominant above 450°C. Peak hardness values were maintained up to 800 h at 500°C, which showed the superior thermal stability of α precipitates. Tensile strength increased up to 1600 MPa along with the decrease of elongation after aging at 350 and 400°C.     

Microstructure Evolution and Mechanical Properties of 2219 Al Alloy During Aging Treatment

Hardness and tensile properties of 2219 Al alloys were tested at various temperature (150, 165, 175 °C) and subjected to T6 temper heat treatment to identify the peak aging time at various temperature. Microstructure evolution and precipitate behavior were analyzed with transmission electron microscope (TEM), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). It is found that the peak aging time is 24 h at 150 °C and does not vary down to 165 °C. When the aging temperature rise to 175 °C, the peak aging time down to 12 h. Considering the strength and elongation, the optimum aging treatment is at 165 °C for 24 h after the solution treatment at 535 °C for 1.5 h. Compared with that of only water-quenched sample, after aged at 165 °C for 24 h, the tensile strength of the 2219 Al alloy increases from 324.5 to 411.8 MPa, yield strength from 168 to 310.8 MPa, respectively. The improvement in the mechanical performance is mainly attributed to the precipitation strengthening of the GP zones, θ″ and θ’ phases.     

Effect of Holding Pressure on Microstructure and Mechanical Properties of A356 Aluminum Alloy

In this study, the effect of holding pressure on microstructure and mechanical properties of low-pressure die cast A356 aluminum alloy was investigated. The results showed that the application of high holding pressure (300 kPa) generated castings with denser structure and superior mechanical properties. By increasing the holding pressure up to 300 kPa, the size of secondary dendrite arm spacing greatly reduced by 22.7% at the cooling rate of 1°C/s and decreased by 12.8% at 10°C/s. The Feret’s diameter and aspect ratio of eutectic silicon particles decreased by 8.4 and 5.1% at the cooling rate of 1°C/s and decreased by 9.3 and 6.4% at 10°C/s, respectively. Meanwhile, the density of A356 aluminum alloy increased to 2.678 g/cm³ and the area fraction of porosity decreased to 0.035%. Thus, tensile properties of A356 aluminum alloy obtained at high holding pressure were enhanced, especially the ductility. All these could be associated with the better filling capability and faster cooling rate caused by high holding pressure. In the analytical range of experimental conditions, the correlation of mechanical properties with process parameters was established by statistical models to predict the ultimate tensile strength and elongation of low-pressure die cast A356 aluminum alloy.     

Effect of chromium additions on the mechanical and physical properties and microstructure of Fe-Co-Ni-Cr-Mo-C ultra-high strength steel: Part I

The effect of chromium additions to an Fe-14Co-10Ni-0.1Mo-0.16C (AF1410 based) secondary hardening steel was evaluated by mechanical and physical properties and by microstructural examination. This unique behavior was extended to encompass a large range of aging temperatures and times that may be encountered during commercial thermal treatment and/or welding. In the aging range of 482 to 550 °C, an increase in chromium from 2 to 3% in the AF1410 based steel resulted in a substantial strength decrease concomitant with an increase in toughness. This behavior is related to a peak hardening shift, early M₂C carbide coarsening, and an increase in reverted austenite for the 1 wt% Cr increase. The increased aging kinetics resulting from the 3Cr steel caused a faster dissolution of Fe₃C and rapid changes in chromium partitioning in the (Mo,Cr)₂C carbide resulting in a coherency loss with a corresponding decrease in lattice parameter. The kinetics of the secondary hardening reaction, for the two steels, was determined by resistivity data for changes in aging parameters (time/temperature).     

Influence of cooling rate on microstructure and properties of high strength steel weld metal

Abstract

Microstructures, and hence mechanical properties, of high strength steel weld metals are affected by cooling rate. Weld metal microstructures for a nominal composition of Fe–0·05C–0·3Si–2Mn–3Ni–0·5Cr–0·6Mo (wt-%) were therefore characterised for a range of cooling rates using high resolution scanning electron microscopy, and transformation behaviour, assessed from cooling curves, is presented as a continuous cooling transformation diagram. As deposited last bead microstructure changes gradually from lower bainite and martensite interspersed with coalesced bainite, via a mixture of relatively fine upper and lower bainite, to coarse upper bainite as cooling rate decreases. The microstructure of reheated beads follows the as deposited structure closely and becomes coarse with slower cooling. Mechanical properties correlate with observed microstructure and transformation behaviour. Results suggest high strength and good toughness for cooling rates between 800 and 500°C of about 3–13 s. A fine microstructure will then form with varying proportions of martensite, lower bainite, coalesced bainite and fine upper bainite.     

Creep aging behavior of retrogression and re-aged 7150 aluminum alloy

Creep aging behavior of retrogression and re-aged (RRAed) 7150 aluminum alloy (AA7150) was systematically investigated using the creep aging experiments, mechanical properties tests, electrical conductivity tests and transmission electron microscope (TEM) observations. Creep aging results show that the steady-state creep mechanism of RRAed alloys is mainly dislocation climb (stress exponent≈5.8), which is insensitive to the grain interior and boundary precipitates. However, the total creep deformation increases over the re-aging time. In addition, the yield strength and tensile strength of the four RRAed samples are essentially the same after creep aging at 140 °C for 16 h, but the elongation decreases slightly with the re-aging time. What's more, the retrogression and re-aging treatment are beneficial to increase the hardness and electrical conductivity of the creep-aged 7150 aluminum alloy. It can be concluded that the retrogression and re-aging treatment before creep aging forming process can improve the microstructure within grain and at grain boundary, forming efficiency and comprehensive performance of mechanical properties and electrical conductivity of 7150 aluminum alloy.