AZ31镁合金高温热压缩流变应力行为的研究

在Gleeble 1500D型热模拟试验机上,在应变速率为0.01～1s-1、变形温度为573～723K条件下,对AZ31合金的流变应力行为进行了研究.结果表明:AZ31镁合金在热压缩变形时,当应变速率一定时,流变应力随着变形温度的升高而减小;而当变形温度一定时,流变应力随着应变速率的增大而增大;该合金的热压缩流变应力行为可用双曲正弦形式的本构方程来描述,在本实验条件下AZ31镁合金热变形应力指数n=8.34,其热变形激活能Q=196kJ/mol.     

铸态Ti–44Al–4Nb–(Mo, Cr, B)合金的热变形行为及热加工图

利用Gleeble热模拟机研究了铸态Ti-44Al-4Nb-(Mo,Cr,B)合金在1 050～1 200℃、0.005～0.5s-1下的热变形行为,并基于所得的真应力-真应变曲线绘制了热加工图。另外,通过透射电子显微镜(TEM)研究了片层和γ相的变形机制。结果表明,该合金是典型的应变速率和温度敏感材料,它的热加工性能较好,在1 100、1 150℃温度下的低应变速率区域以及1 200℃温度下高应变速率区域比较适合热加工。再结晶是流变软化的主要原因,较高的变形温度和较低应变速率有利于再结晶晶粒的进行。片层结构的变形机制为片层扭折,而γ相的主要变形机制为位错滑移和变形孪晶。     

7075铝合金高温等温变形的流变应力特征

在Gleeble-1500热模拟试验机上,采用高温等温压缩法,研究了7075铝合金在250～450℃温度范围及1.0～0.001 s-1应变速率范围内压缩变形时流变应力的变化规律.结果表明,应变速率和变形温度对合金流变应力的影响很大,流变应力随应变速率的提高而增大,随变形温度的提高而降低;其流变应力值可用Zener-Hollomon参数来描述.从流变应力、应变速率和温度的相关性,得出了该合金高温变形的应力指数n,应力水平参数α,结构因子A和变形激活能Q.     

5A30铝合金热变形的流变应力及材料常数

在Gleeble-1500热模拟试验机上,采用高温等温压缩试验,研究了5A30铝合金在300～500℃温度范围及应变速率在0.001～1s-1内压缩变形的流变应力变化规律,采用数学回归及最小偏差法求出了该合金的材料常数,建立了该合金流变应力与Zener-Hollomon参数的线性关系式.结果表明,该合金为正应变速率敏感材料,流变应力随变形温度升高而降低,随应变速率升高而增大;该合金的材料常数包括变形激活能Q为160.94kJ/mol,应力水平参数α为0.0184mm2/N,应力指数n为3.314,结构因子A为3.058×109s-1;合金流变应力模型可表达为σ=54.31ln{(Z/3.058×109)1/3.314+[(Z/3.058×109)2/3.314+1]1/2}.     

7075铝合金高温等温变形的流变应力特征

在Gleeble—1500热模拟试验机上．采用高温等温压缩法，研究了7075铝合金在250-450℃温度范围及1．0～0．001s^-1应变速率范围内压缩变形时流变应力的变化规律．结果表明。应变速率和变形温度对合金流变应力的影响很大，流变应力随应变速率的提高而增大，随变形温度的提高而降低；其流变应力值可用Zener-Hollomon参数来描述．从流变应力、应变速率和温度的相关性，得出了该合金高温变形的应力指数n，应力水平参数α，结构因子A和变形激活能Q。     

Cu-0.58Sn合金热变形行为和热加工图研究

利用Gleeble-3180热模拟试验机,对Cu-0.58Sn合金在400～700 ℃,应变速率0.01～10 s?1条件下的热变形行为进行了研究。结果表明：该合金具有正的应变速率敏感性和热敏性,其流动应力随应变速率的增大而增大,随变形温度的升高而减小。建立了本构方程及热加工图,计算得出CuSn合金的热变形激活能为239.928 kJ/mol,同时优化出合金的最佳热加工参数为：变形温度500～700 ℃,应变速率3.16～10 s-1。     

快速凝固/粉末冶金技术制备挤压态AZ91合金热加工性能研究

对快速凝固粉末冶金制备的挤压AZ91镁合金，在温度为250—400 ℃、应变速率范围为0.01—1 s^-1下进行了热压缩变形试验。同时，利用热加工图分析评价了合金的热加工特性。结果表明：合金的流变应力，随变形温度的升高或应变速率的减小而减小；同时，基于双曲正弦关系建立了描述流变应力行为的本构方程，RS/PM制备的AZ91挤压合金的热变形机制为晶格扩散控制的位错蠕变；通过微观组织验证了动态再结晶和流变失稳行为，当温度高于350 ℃、应变速率在0.01—0.1 s^-1时，合金的可加工性最佳。     

退火态TC4合金的热变形行为

采用Gleeble-3 5 0 0热模拟机系统研究退火态TC4(Ti 6Al 4V)合金在75 0～95 0℃,应变速率0 0 0 1～1s- 1 条件下的热变形行为。TC4合金的热变形激活能约为482kJ/mol,热变形方程为ε′=2 95×10 1 9[sinh(α·σp) ] 2 4 9exp(-4 82 0 0 0 /RT)。不同真应变下的热加工图相似,随变形温度升高及应变速率降低,能量消耗效率η逐渐升高。在变形温度90 0℃左右、应变速率为0 0 0 1s- 1 时,能量消耗效率η达到峰值,约为5 8%。     

Ti—6Al—2Zr—1Mo—1V合金的热压变形特性及塑性流动方程

在 Gleeble- 15 0 0热模拟机上对 Ti- 6 Al- 2 Zr- 1Mo- 1V钛合金铸态材料进行了恒温和恒应变速率下的热压缩变形试验 .在试验温度 70 0～ 10 0 0℃、应变速率 5× 10 -3～ 5 0 s-1条件下 ,测试了材料的稳态变形抗力 ,并绘制成 lnσ- lnε和 lnσ- 1/ T关系曲线 ,从而确定合金的变形激活能 Q和应力指数 n.观察热变形后的组织表明 :合金在 80 0℃热变形为不完全动态再结晶组织 ,变形机制受动态回复与动态再结晶共同影响 ;90 0℃为完全动态再结晶组织 ,变形机制完全受动态再结晶影响 .合金在 90 0℃以上具有较好的工艺塑性 ,并且应力指数 n随变形温度的升高而减小 .     

MgAlLiZnTi轻质高熵合金的热变形行为与热加工图研究

采用Gleeble-3500 热/力模拟试验机，在变形温度为250—350 ℃、应变速率分别为0.001、0.01、0.1和1 s^-1、真应变量分别为0.3、0.45、0.6的条件下，对Mg₅₄Al₂₂Li₁₁Zn₁₁Ti₂轻质高熵合金进行热压缩实验。基于Arrhennius模型对热压缩实验数据进行拟合，建立合金的本构方程，并绘制该合金在不同真应变下的热加工图。结果表明：在实验条件下，合金的热变形过程为加工硬化和动态再结晶为主的动态软化，且该合金的流变应力值与应变速率呈正相关，与变形温度呈负相关；该合金的热加工图表明，最佳的热加工工艺参数为变形温度335—350 ℃、应变速率1×10^-3—1 s^-1。     

变形条件对Mg-Gd-Y-Nd-Zr合金动态力学行为的影响

采用金相观察、扫描电子显微镜、能谱分析、显微硬度计及分离式霍普金森压杆等手段,研究了峰值时效挤压态Mg-8Gd-3Y-0.5Nd-0.5Zr合金在变形温度及应变速率下的动态冲击力学变形行为。结果表明：合金在220℃/14h到达峰值时效,其硬度约为143.2Hv,提升了55%左右。在不同的变形温度下均表现出优异的抗冲击性能,在室温及应变速率为3000 s_-1^{条件下合金抗压强度可}高达682MPa;在100℃及应变速率为1500 s_-1^{条件下抗压强度为}635MPa;在400℃及应变速率为3000 s_-1^{条件下抗压强度为}583MPa。合金在不同温度下优异的抗冲击性能主要得益于时效强化相、稳定存在的块状富稀土粒子以及冲击过程中在晶界形成的动态析出相协同强化机制。随着应变速率和变形温度的增大,合金热软化效应增强,合金力学性能有所降低。     

Review of rate constants for thiosulphate leaching of gold from ores,concentrates and flat surfaces: Effect of host minerals and pH

A review of literature data for different types of sulphide concentrates and gold ores has been carried out to examine the impact of host minerals and pH upon gold leaching. Analysis of initial rate data over the first 30–60 min of gold leaching from sulphide concentrates or silicate ores over a range of ammonia, thiosulphate, and copper(II) concentrations, pH (9–10.5) and temperatures up to 70 °C shows the applicability of a shrinking sphere kinetic model with an apparent rate constant of the order k_ss = 10⁻⁶–10⁻³ s⁻¹. The dependence of apparent rate constant on pH and initial concentrations of copper(II) and thiosulphate is used to determine a rate constant k_Au(ρr)⁻¹ of the order 1.0 × 10⁻⁴–7.4 × 10⁻⁴ s⁻¹ for the leaching of gold over the temperature range 25–50 °C (ρ = molar density of gold, r = particle radius). These values are in reasonable agreement with rate constants based on electrochemical and chemical dissolution of flat gold surfaces: k_Au = 1.7 × 10⁻⁴–4.2 × 10⁻⁴ mol m⁻² s⁻¹ over the temperature range 25–30 °C. The discrepancies reflect differences in surface roughness, particle size and the effect of host minerals.     

The reduction of chromite in the presence of silica flux

The mechanism and kinetics of the carbothermic reduction of a natural chromite was studied at 1300–1500 °C in the presence of silica. Thermogravimetry, X-ray diffraction (XRD) analysis, energy dispersive X-ray analysis (EDAX) and metallography were the experimental techniques used. Silica affected the reduction at and above 1400 °C. A two stage reduction mechanism was established. The first stage, up to about 40% reduction, is primarily limited to iron metallization and zoning is observed in partially reduced chromites. In this stage silica does not interfere with the reduction. The second stage is mainly confined to chromium metallization and formation of a silicate slag alters the reduction mechanism. Ion-exchange reactions between the reducible cations (Cr³⁺ and Fe²⁺) in the spinel and the dissolved cations (A1³⁺ and Mg²⁺) in the slag allow further reduction. Due to the very high driving force for the diffusion, the overall process is shifted toward a more chemical reaction controlled mechanism. A generalized rate equation was then applied to the individual metallization curves of iron and chromium from which respective rate constants and diffusion coefficients were derived. The rate constants were in the range 6.74 × 10⁻⁴–9.01 × 10⁻⁴ s⁻¹ for iron and 7.20 × 10⁻⁴–8.50 × 10⁻⁴ s⁻¹ for chromium reduction at 1500 °C in the presence of silica. At 1500 °C, the corresponding diffusion coefficients were in the range 3.14 × 10⁻⁸–4.78 × 10⁻⁸ m²/s for Fe²⁺ diffusion in the spinel and in the range 1.70 × 10⁻⁸–2.03 × 10⁻⁸ m²/s for the respective diffusion of Cr³⁺. Finally using Arrhenius plots activation energies were derived.     

Study on structural and compositional transitions of coal ash by using NMR

Structural and compositional transitions of Datong coal ash and its CaCO₃ additional effects were carefully examined at a temperature range of 300 to 1 600 °C by using XRD and solid state NMR. The quantitative estimation of amorphous structures of ashes can be successfully obtained through the analyses of solid state NMR spectra. Viscosity of molten ash and its changes with CaCO₃ addition were also evaluated up to 1 700 °C by using a rotary type viscometer. Glasses with poor crystalline and amorphous phase were continuously formed through the eutectic reaction of silica above fusing temperature (FT>1 500 °C) that caused broadening and shift of ²⁹Si and ²⁷Al peaks in NMR results. With the additional amount of CaCO₃ increasing, the peaks shifted to downfield obviously; the fraction of Si(OAl)₀(OSi)₄ decreased, while that of Si(OAl)₁(OSi)₁ at −84.3×10⁻⁶ increased apparently. These transitions indicated the destruction of large alumina-silicate framework into small segments by the addition of Ca ion.     

Self-heating activation energy and specific heat capacity of sulphide mixtures at low temperature

Energy of activation (E_a) and specific heat capacity (C_p) for mixtures of sulphide minerals that on their own do not self-heat (SH), sphalerite/pyrite, pyrite/galena, chalcopyrite/galena and sphalerite/galena, were determined using a self-heating apparatus at temperatures below 100 °C in the presence of moisture. The mixtures all gave E_a ranging from 22.0 to 27.8 kJ mol⁻¹, similar to the range reported for Ni- and Cu-concentrates. The E_a is close to that for partial oxidation of H₂S which adds to the contention that the partial oxidation of H₂S contributes to SH of sulphides at low temperature. The C_p values ranged from 0.152 to 1.071 JK⁻¹ g⁻¹ as temperature rose from 50 °C to 80 °C, similar to the reported findings on Ni- and Cu-concentrates. The role of galvanic interaction in promoting SH is tested by examining correlations with the rest potential difference of the sulphides in the mixture.     

Time-dependent rheological behaviour of cemented backfill mixture

When cemented backfill mixture (CBM) is transported through pipelines from a backfill plant to stopes, it experiences shearing forces over the transport time. In this paper, the effects of solids concentration, binder content, shear rate and curing time on the time-dependent rheological behaviour of CBM were studied. It was found that over long periods of shearing at a constant rate greater than 5 s^?1, the shear stress decreased at first and then increased gradually with time. When the shear rate was less than 0.5 s^?1, shear stress increased slightly firstly with shearing time, then it started to behave similar to the test with a higher shear rate whose shear stress decreased firstly and then increased. The samples that were sheared at a higher shear rate exhibited a lower apparent viscosity and the higher yield stress CBM samples displayed more pronounced shear-thinning properties. It was also found that for transient flow, increasing the solids concentration and the curing time lead to both a higher initial shear stress (τ₀) and minimum shear stress (τ_min). When the cement to tailings ratio increased, the τ₀, τ_min and final shear stress after shearing for 3600 s (τ₃₆₀₀) increased at first, and then subsequently decreased. Moreover, changes in the solids concentration profile and the cement hydration property were displayed during the rheological tests.     

Preconcentration of gold by rice husk ash

The goal of this research was to develop a new, efficient adsorbent of gold-thiourea complex, [Au(CS(NH₂)₂]⁺. In this experiment, rice husk was heated at three different temperature: 300°C, 400°C and 500°C. Rice husk ash heated at 300°C adsorbed gold thiourea complex, whereas rice husk ash heated at 400°C and 500°C did not. The structure of rice husk ash heated at 300°C had specific silanol groups and oxygen functional groups of carbon, while rice husk ash heated at 400°C and 500°C contained siloxane groups. Maximum gold adsorption of rice husk ash heated at 300°C and activated carbon, was 21.12 and 33.27 mg Au/g adsorbent, respectively. However, rice husk ash absorbed 0.072 mg thiourea/g adsorbent, which was less than activated carbon adsorbed (0.377 mg thiourea/g adsorbent). In addition, the adsorbed gold could be eluted from this rice husk ash by sodiumthiosulfate more easily than from activated carbon. The results revealed that rice husk ash heated at 300°C can be used as a new adsorbent for gold thiourea complex.     

Determination of specific heat capacity of sulphide materials at temperatures below 100 °C in presence of moisture

The specific heat capacity (C_p) of one copper and three nickel concentrates was determined using a self-heating apparatus and by drop calorimetry over the temperature range 50 to 80 °C in the presence of 6% moisture. The C_p values from both techniques were comparable and shown to be measuring the same property. The C_p values were similar for all four concentrates increasing from ca 0.4 to 1.4 J g⁻¹ K⁻¹ as temperature increased from 50 to 80 °C. Uses of C_p to identify self-heating risk and to modify the Rosenblum standard test are discussed.     

Study on the pyrolysis behavior of Shendong Shangwan coal and its macerals concentrate

The pyrolysis characteristics of Shendong Shangwan coal and its macerals concentrate were investigated using thermogravimetry (TG) coupled with mass spectrometry (MS). The evolved gases were analyzed online by MS spectroscopy. The results of TG/DTG (derivative thermogravimetry) show ths vitrinite concentrate has greater weight loss rate and higher volatile yield than the other two samples. More light hydrocarbons C₁-C₅ are released from the vitrinite concentrate than from the Shendong Shangwan coal and inertinite concentrated in the process of pyrolysis. Three samples have similar shape curves of evolved gases of C₂–C₅ with different intensities. When the pyrolysis temperature was lower than 418 °C, the amount of C₆H₆ evolved in the process of pyrolysis of inertinite concentrated was higher than that of raw coal and vitrinite concentrate. As the temperature rising, the production rate of C₆H₆ increased. Below 672 °C, C₆H₆ evolution rate of vitrinite concentrate was far greater than the other two samples; the main evolution temperature range of C₇H₈ was 400 °C to 700 °C for the three samples. The amount of H₂ and H₂O released first increased and then decreased with the temperature increase while more H₂ released for pyrolysis of inertinite concentrated and more H₂O released for the pyrolysis of vitrinite concentrate.     

TGA kinetic study on the hydrogen reduction of an iron nickel oxide

Reduction of a mixed iron nickel oxide by hydrogen to produce ferronickel alloy was investigated by thermo-gravimetric analysis (TGA). The iron nickel oxide, which contains 50 wt% Fe and 10 wt% Ni mainly in the form of hematite (Fe₂O₃) and nickel ferrite (NiFe₂O₄), is a residue produced from the water leaching of a selectively sulfation roasted nickel calcine. Continuous heating tests with a fixed heating rate as well as isothermal tests were performed. Results indicate that the reduction of the mixed oxide began above 350 °C. The reduction reactions occurred in a non-topochemical mode at low temperatures between 350 °C and 600 °C with its rate controlled by the solid–gas chemical reactions. The reduction rate increased with the increase in temperature from 350 °C up to 1100 °C. Between 600 °C and 1200 °C, the rate controlling step was the diffusion of reducing gas through the pores of the sample bed with an apparent activation energy of 34.1 kJ/mol. Due to the melting of the silicate material at 1200 °C which substantially reduced the sample porosity, the reduction rate decreased. Above 1200 °C, the reduction rate increased again due to the increased diffusion of the reducing gas through the molten sample at higher temperatures.