Current progress of research on heat -resistant Mg alloys: A review

With the increasing attention received by lightweight metals,numerous essential fields have increased requirements for mag-nesium(Mg)alloys with good room-temperature and high-temperature mechanical properties.However,the high-temperature mechanic-al properties of commonly used commercial Mg alloys,such as AZ91D,deteriorate considerably with increasing temperatures.Over the past several decades,extensive efforts have been devoted to developing heat-resistant Mg alloys.These approaches either inhibit the gen-eration of thermally unstable phases or promote the formation of thermally stable precipitates/phases in matrices through solid solution or precipitation strengthening.In this review,numerous studies are systematically introduced and discussed.Different alloy systems,includ-ing those based on Mg–Al,Mg–Zn,and Mg–rare earth,are carefully classified and compared to reveal their mechanical properties and strengthening mechanisms.The emphasis,limitations,and future prospects of these heat-resistant Mg alloys are also pointed out and dis-cussed to develop heat-resistant Mg alloys and broaden their potential application areas in the future.

Effect of intermetallic compounds on heat resistance of hot roll bonded titanium alloy-stainless steel transition joint

The effect of intermetallic compounds on the heat resistance of transition joint was investigated. The experiment of post-weld heat treatment for the hot roll bonded titanium alloy-stainless steel joint using nickels interlayer was carried out, and the interface microstructure evolution due to heat treatment was presented. There was not found significant interdiffusion at stainless steel/nickel interface, when the specimens were heat treated in the temperature range of 600-800 °C for 10 and 30 min, while micro-cracks occurred at the stainless steel/nickel interface heat treated at 700 °C for 30 min. The thickness of intermetallic layers at nickel/titanium alloy interface increased at 600 °C, and micro-cracks occurred at 700 and 800 °C. The micro-cracks occurred between intermetallic layers or between intermetallic layer and nickel interlayer as well. The tensile strength of the transition joint decreased with the increase of heat treatment temperature or holding time.

Effects of composite scale on high temperature oxidation resistance of Fe-Cr-Ni heat resistant alloy

Fe-Cr-Ni heat resistant alloys with aluminum and silicon addition, alone and in combination, were melted using an intermediate frequency induction furnace with a non-oxidation method. By the oxidation weight gain method, the oxidation resistances of the test alloys were determined at 1,200 ℃ for 500 hours. According to the oxidation weight gains, the oxidation kinetic curves were plotted and the functions were regressed by the least squares method. The results show that the oxidation kinetic curves follow the power function of y = ax^b （a〉0, 0〈b〈1）. The effects of scale compositions on oxidation resistance were studied further by analyses using X-ray diffraction （XRD） and scanning electron microscope （SEM）. It is found that the composite scale compounds of Cr203, a-Al2O3, SiO2 and FeCr2O4, with compact structure and tiny grains, shows complete oxidation resistance at 1,200℃. When the composite scale lacks a-Al2O3 or SiO2, it becomes weak in oxidation resistance with a loose structure. By the criterion of standard Gibbs formation free energy, the model of the nucleation and growth of the composite scale is established. The forming of the composite scale is the result of the competition of being oxidized and reduced between aluminum, silicon and the matrix metal elements of iron, chromium and nickel. The protection of the composite scale is analyzed essentially by electrical conductivity and strength properties.

Enhanced elevated temperature wear resistance of Al-17Si-5Cu alloy after a novel short duration heat treatment

The goal of the present study is to improve the elevated temperature wear resistance of an Al-17 wt%Si-5 wt%Cu alloy(AR alloy) by a novel short duration heat treatment process. The elevated temperature(100°C) dry sliding wear behavior of an AR alloy was studied after microstructural modification using the proposed heat treatment. The study revealed considerable microstructural modifications after the heat treatment and the heat treated alloy was designated as HT(heat treatment) alloy. A higher hardness value was obtained for the HT alloy compared to the AR alloy. Accordingly, the wear rate for the HT alloy was found to be significantly lower compared to the as-cast AR alloy at all applied loads. Accelerated particle pull-out for the AR alloy at elevated temperatures resulted in poor wear behavior for it compared to the HT alloy. On the other hand, the Si particles remained intact on the worn surface of the HT alloy due to the good particle/matrix bonding that resulted from the isothermal heat treatment. Furthermore, the age hardening that occurred in the HT alloy during wear provided additional wear resistance. Thus, the HT alloy at 100°C exhibited a lower wear rate compared to the AR alloy even at room temperature for all applied loads. This improvement was attributed to microstructural modification upon isothermal heat treatment along with the age hardening effect.

Effects of heat treatment process on thin film alloy resistance and its stability

Alloy thin film for advanced pressure sensors was manufactured by means of ion-beam sputtering SiO2 insulation film and NiCr thin film on the 17-4PH stainless steel elastic substrate. The thin film resistance was respectively heat-treated by four processes. The effects on stability of thin film alloy resistance were investigated, and paramaters of heat treatment that make thin film resistance stable were obtained. The experimental result indicates that the most stable thin film resistance can be obtained when it is heat-treated under protection of SiO2 and N2 at 673 K for 1 h, and then kept at 473 K for 24 h. Pressure sensor chips of high precision for harsh environments can be manufactured by this process.

Formation Mechanism and Existing Form of Sb in Heat Resistance Mg-Gd-Y-Sb Alloy

The existing form and reaction mechanism of Sb in heat resistane Mg-Gd-Y-Sb rare earth magnesium alloy were investigated by inductive coupled plasma emission spectroscopy(ICP),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD).It is found that Sb tends to form high melting point intermetallics with rare earth elements of Gd and Y.The existing form of Sb is determined to be GdSb and SbY,respectively,which has high melting point(GdSb:2142℃/SbY:1782℃).Meanwhile,the first principle calculation and electronegativity difference calculation were performed to further understand the reaction mechanism.Therefore,the forming heat and binding energy were calculated.The experimental results show that the binding tendency of Sb element to Gd and Y is much stronger than that of it with other elements in this alloy,which results in the formation of high melting point of Gd-Sb and Y-Sb intermetallics,and finally leads to the high temperature resistant further improvement of the Mg-Gd-Y magnesium alloy.

Microstructure and Corrosion Resistance of CrN and CrN/TiN Coated Heat-Resistant Steels in Molten Aluminum Alloy

The components of the equipment for processing the Al melts into the molded parts can be markedly corroded by the molten Al. In this study, a 4 μm CrN coating or CrN/TiN multilayer coating for providing the physical and chemical barriers between the molten reactive Al and the steel substrate were deposited by Cathodic Arc Evaporation onto 10 mm-thick heat-resistant steel plates. The dipping tests were conducted in a 700℃ A356 melt for 1 to 21 h at intervals of 3 h. The damage of the coated steel was eva...

Effect of heat treatment on corrosion behaviors of Mg-6Gd-3Y-0.5Zr alloy

The microstructures and corrosion behaviors of as-cast,T4-treated,and T6-treated Mg-6Gd-3Y-0.5Zr alloys were systematically investigated by SEM,TEM,immersion test,and electrochemical corrosion test.The results show that the microstructure of the as-cast alloy is composed ofα-Mg and Mg_(24)(Gd,Y)_(5) eutectic phase,and in T4-treated alloy,Mg_(24)(Gd,Y)_(5) phase dissolves into theα-Mg matrix,leading to an increase in the(Y,Gd)H_(2) phase.After T6 treatment,nanoscale Mg_(24)(Gd,Y)_(5) phase dispersedly precipitates from theα-Mg matrix,and exhibits a specific orientation relationship with the α-Mg:(332)Mg_((24)(Gd,Y)_(5))//(1011)_(α-Mg),[136]Mg_((24)(Gd,Y)_(5))//[1210]_(α-Mg).The corrosion resistance of the Mg-6Gd-3Y-0.5Zr alloys can be ranked in the following order:T6-treated alloy exhibits the highest corrosion resistance,followed by the T4-treated alloy,and finally,the as-cast alloy.The corrosion products of the alloys are all composed of MgO,Mg(OH)_(2),Gd_(2)O_(3),Y_(2)O_(3),and MgCl_(2).The corrosion behavior of Mg-6Gd-3Y-0.5Zr alloy is closely related to the precipitated phase.By establishing the relationship between corrosion rate,hydrogen evolution rate,and corrosion potential,it is further demonstrated that during the micro galvanic corrosion process,the coarse Mg_(24)(Gd,Y)_(5)phase in the as-cast alloy undergoes extensive dissolution,and(Y,Gd)H_(2) phase promotes the dissolution of theα-Mg matrix in the T4-treated alloy,intensifying the hydrogen evolution reaction.The T6-treated alloy,with dispersive precipitation of nanoscale Mg_(24)(Gd,Y)_(5) phase,exhibits better corrosion resistance performance.

Effect of Heat Treatment on the Microstructure and Corrosion Resistance of Cu-Zn Alloy

The microstructure of Cu-Zn alloy with different heat treatment conditions in 3.5%NaCl+NH3 solution were observed, and the average corrosion rates and electrochemical data of Cu-Zn alloy were measured, as well as the effect of heat treatment on microstructure and corrosion resistance of Cu-Zn alloy was analyzed. The results show that the microstructure of Cu-Zn alloy has been changed due to the heat treatment. As a results, the better corrosion resistance can be obtained for the Cu-Zn alloy quenched from 900 ℃ for 0.5 h followed by tempered at 100 ℃ for 2 h.

LONG-TERM CREEP AND RUPTURE PROPERTIES OF HEATRESISTING STEELS AND ALLOYS

The present status of NRIM Creep Data Sheet Project and the recent activities of long-term creep and rupture studies on heat resisting steels are described. The project has been continued to produce long-term data such as 100 000h-creep rupture strength for 47 kinds of principal heat resisting steels and alloys, including welded joints. The long-term creep deformation behavior and microstructural evolution during creep have been shown to be complicated.

Corrosion resistance of 2195 aluminum alloy treated by multi-step-heating-rate controlled process

2195 aluminum-lithium alloy was widely applied in the aviation and aerospace industry, but it is highly susceptible to pitting and intergranular corrosion undergoing sever corrosive circumstance and moisture atmosphere. To solve this problem and consequently to prolong its service life, a multi-step-heating-rate(MSRC) process was carried out. Investigations were carried out to find the effect of the MSRC process on the alloys corrosion resistance. It is found that the MSRC process is more favorable for the uniform phase precipitation by comparing the corrosion resistance of samples treated by traditional heat treatments. The potential difference between phases can be reduced and intergranular corrosion is able to be prohibited efficiently. Besides, the rare earth infiltration is beneficial to improving the corrosion resistance. As heating time increases, the corrosion resistance declines gradually, samples treated by artificial aging and solid solution also exhibit a better corrosion resistance.

Role of Heat Treatment Temperatures on Mechanical Properties and Corrosion Resistance Properties of Mg-10.16Li-8.14Al-1.46Er alloy

The microstructure and phase evolution of Mg-10.16Li-8.14Al-1.46Er alloy of as-cast,250℃+12 h,300℃+12 h,and 400℃+12 h were studied by optical microscopy,scanning electron microscope,and X-ray diffraction.The mechanical properties of Mg-10.16Li-8.14Al-1.46Er alloy in different states were tested by microhardness tester and tension tester.The corrosion resistance of Mg-10.16Li-8.14Al-1.46Er alloy in different states was measured by electrochemical workstation combined with hydrogen evolution and mass loss tests.The results show that the microstructure of as-cast Mg-10.16Li-8.14Al-1.46Er alloy consists ofα,β,AlLi,Al3Er and MgAlLi_(2)phases.Changes of microstructure are morphology and quantity ofαphase,and second phases of MgAlLi_(2)and AlLi by heat treatments at different temperatures.The best comprehensive tensile properties of Mg-10.16Li-8.14Al-1.46Er at 400℃are attributed to theαphase structure,solution strengthening and second phase strengthening.After heat treatments at different temperatures,the corrosion resistance of Mg-10.16Li-8.14Al-1.46Er was improved compared with as-cast samples.The Mg-10.16Li-8.14Al-1.46Er alloy has the best corrosion resistance at 250℃due to the best homogenization at this temperature.

High Strength and Heat Resistance of Low-RE-Containing Mg Alloy Achieved via Substantial Dynamic Precipitates

Conventional high-strength Mg-RE-based wrought alloys usually contain a high amount of RE solutes,which largely increases the alloy cost and thus restricts their adoptions.In this work,we developed a low-RE-containing Mg-3Sm-1Nd-0.6Zn-0.4Zr alloy by hot extrusion with low extrusion ratio,which shows a high tensile yield strength(TYS)of 435 MPa and a satisfactory elongation of 5.6%at room temperature,outperforming most Mg-Gd-Y-based extrusion alloys with RE contents of 12 wt%at least.Outstanding high-temperature strength,such as the TYS of 280 MPa at 200℃ and 251 MPa at 250℃,is also obtained in this alloy.The alloy presented a typical bimodal grain structure including coarse hot-worked grains with a strong texture and fine recrystallized grains with random orientations.Also,abundant Mg3RE particles were mostly introduced in hot-worked grains and at recrystallized grain boundaries by dynamic precipitation during extrusion.Consequently,the high strength of this alloy is principally attributed to the combined hardening effect of numerous Mg_(3)RE particles,fine recrystallized grains,and strongly textural hot-worked grains,rather than the ultra-strong age-hardening effect in traditional high RE-alloyed Mg alloys.

Preparation of heat-resistant aluminum alloy pipe blanks by multi-layer spray deposition

A heat resistant aluminum alloy pipe blank with dimensions of d 700/300 mm×1 200 mm was prepared by the multi layer spray deposition technology. Optical microscopy, X ray diffractometry and transmission electron microscopy were used to analyze its morphologies and microstructures. The results show that the microstructures of the pipe blank are homogeneous and the precipitates are uniformly distributed d 25～70 nm spherical or sphere like Al 12 (Fe,V) 3Si particles, its mechanical properties at room temperature and 350 ℃ after densification by extrusion are σ b=412 MPa, δ =7.6% and σ b=187 MPa, δ =7.6%, respectively. The analyses indicate that the proper match of the motion rates of atomizer and substrate can produce deposited blanks with uniform thickness and relatively high cooling rate.

Influence of heat treatment on corrosion behavior of hot rolled Mg5Gd alloys

The influence of heat treatment on the corrosion behavior of rolled Mg5 Gd alloys in 3.5 wt.% Na Cl solution saturated with Mg(OH)2 was characterized by immersion test, electrochemical test, scanning electrochemical microscopy(SECM) and corrosion morphology analysis in order to improve the corrosion resistance of Mg alloys. The results showed that solution treatment reduced the corrosion rate of the Mg5 Gd significantly, resulting in relatively uniform corrosion and shallow corrosion cavities due to the dissolution of Cd-containing particles. The following aging process could further decrease the corrosion rate. Precipitation of nano-sized Cd-containing particles did not cause apparent micro-galvanic corrosion, which could be attributed to the formation of a protective corrosion product film fully covering the particles.

Characterizations of precipitation behavior of Al-Mg-Si alloys under different heat treatments

The solidification-precipitation behavior of Al-Mg-Si multicomponent alloys has long been an absorbing topic. Experiments were carried out to analyze the precipitation behaviors of Al-Mg-Si alloys under different heat treatments. All specimens were homogenized at 570 ℃ for 8 h, and then solution treated at 540 ℃ for 55 min. Subsequently, the specimens were age treated for different times at temperatures of 100 ℃, 150 ℃ and 180 ℃, respectively. The experimental results show that the occurrence of dispersed free zones （DFZ） is caused by the uneven distribution of dispersed phase. During the aging process, pre-β＂ phases form at the initial stage and an aging temperature of 100 ℃is too low to complete the transformation of pre-β＂ to β＂. At 150℃, the precipitation sequence is concluded as SSSS-pre-β＂-pre-β＂＋β＂-β＂-β＇-β. Moreover, changes in sizes and densities of the pre-β＂, β＂and β＇ phases during the aging process has an important influence on the evolution of microhardness and electrical resistivity. The microhardness peak value of 150 ℃ is similar to that of 180 ℃, which is -141 HV. While, at 100℃, the microhardness increases slowly, and the attainable value is 127 HV up to 19 days. When the aging temperature is 100 ℃, the electrical resistivity has the highest average value. When the aging temperature exceeds 100 ℃, with the occurrence and growth of β＂and β＇, the resistivity has a distinct decrease with prolonged aging time.

Phase transition of Al-Fe-V-Si heat-resistant alloy by spray deposition

Solidification curve of Al 8.5Fe 1.7Si was calculated by thermodynamic models. Phase constitutions of Al 8.5Fe, Al 8.5Fe 1.7Si and Al 8.5Fe 1.7Si 1.3V alloys were analysed at different cooling rates by means of optical microscopy, X ray diffraction, transmission electron microscopy and so on. Microstructures of spraying deposited ingots and their hot processed samples were detected, and the phase transitions during spraying deposition, heating process and hot processing were also studied. The results show that, by spraying deposition, the ideal structures of α (Al) and Al 12 (Fe, V) 3Si can be obtained when cooling rate is above 10 5 K/s; and restraining the formation of Al 8(Fe,V) 2Si/Al 13 Fe 4 can improve the properties of the alloy.

Mechanical properties and tribological behavior of a cast heat-resisting copper based alloy

Mechanical properties and tribological behavior of a novel cast heat resisting copper based alloy are investigated. The corresponding properties of a commercial aluminum bronze C95500 (ASTM B30) are compared with the alloy. The results show that the alloy possesses better mechanical properties and tribological behaviors than that of C95500 at elevated temperature. The tensile strength, elongation and hardness at 500℃ are 470MPa, 2.5% and HB220, respectively. The wear rate of the developed alloy at ambient and elevated temperature is about one sixth and one fortieth of that of C95500, respectively. The alloy is very suitable for ma nufacturing heat resisting and wear resisting parts. Major strengthening mechanisms for the alloy are solution strengthening and the second phase strengthening.

Experimental Study on Machining Shape Hole of Ni-based Super-heat-resistant Alloy

Inconel 718 and Waspaloy, Nickel-based super-heat-resistant alloy, are high-strength, thermal-resistant and corrosion-resistant alloy that are widely used in parts of gas turbines and airplane engines. Due to their extremely tough and thermal-resistant nature, they are well known as materials that are difficult to cut. Shape holes on a disc of an aircraft engine, made of Ni-based super-heat-resistant alloy, are required with good surface integrity and geometric accuracy. This kind of shape hole is produced by EDM (Electro-discharge machining) currently. It is necessary to investigate an alternative machining process to reduce the process time and improve the surface quality. This paper presents an experimental study on the machining methods for the shape hole of Ni-base super-heat-resistant alloy Inconel 718 and Waspaloy. The feasibility using milling or/and grinding as an alternative for currently EDM process to machine shape holes is assessed by observing the wear and breakage of the cutting tools and grinder and analyzing the hole surface integrity and geometric accuracy. The results show that the milling process of Inconel 718 and Waspaloy can produce shape holes with acceptable surface roughness and geometric accuracy efficiently. The machined Waspaloy shape holes reveal a slight decrease tendency in section dimension from the top of the its bottom. There is a larger deviation from the nominal profile at the segment with smaller radius. A thin softened surface layer with thickness smaller than 60 μm occurs on the machined Waspaloy shape holes. The softening and work hardening become remarkable with the progress of tool wear.

THERMAL ANALYSIS STUDY OF Fe-Cr-Mn HEAT RESISTANT ALLOY

Thermal analysis has been adopted to determine the liquidus and eutectic temperature of Fe-20%Cr-Mn alloy over range of Mn and C contents.Using regression analysis,the par- tial quasi-binary phase diagrams of Fe-Mn alloy(solidification range)have been con- structed.And the effect of Mn content on liquidus and eutectic temperature was discussed.