置氢Ti6Al4V合金的微观组织演变规律

为研究置氢Ti6Al4V合金的高温加工改性机理,从微观组织的角度对合金进行了对比分析.利用OM、SEM、XRD等研究了置氢对Ti6Al4V合金变形前后微观组织演变的影响.研究结果表明:氢的加入不仅使置氢Ti6Al4V合金中β相比例明显增大,而且改变了α相与β相之间的电势差,在氢含量为0.3%~0.5%两相颜色将发生互换,氢含量增加到0.50%以上时,合金中将出现面心立方结构的δ氢化物;随氢含量的增加,合金超塑拉伸变形后的组织由α+β两相等轴晶粒变为粗大的β晶粒,造成α与β界面的协调能力下降,并改变了合金的变形机制.     

淬火对V40Ti26Cr26Fe8贮氢合金的影响

淬火工艺被广泛应用于钒基固溶体贮氢合金的热处理过程中,并被认为能够有效改善合金成份均匀性,提高合金的有效放氢量.考察了淬火对V40Ti26Cr26Fe8贮氢合金放氢PCT性能的影响.合金微观结构变化的分析表明,合金在退火态时存在C14 Laves相,淬火后Laves相固溶于BCC主相中,含量下降.成分分析表明,合金BCC主相淬火后Ti含量上升.与退火处理相比,合金放氢量和平台压在淬火后均出现下降.而合金在淬火过程中产生的缺陷及应力,导致了放氢平台倾斜,滞后增大且放氢量下降.     

氢对Ti-6Al-4V合金室温压缩性能的影响

为了研究氢对Ti-6Al-4V合金室温压缩性能的影响,采用Zwick/Z100型材料试验机对置氢Ti-6Al-4V合金进行了压缩试验,并利用OM、XRD和TEM等材料分析方法对合金的微观组织进行了观察.研究表明:置氢前,Ti-6Al-4V合金由等轴的α相和β相组成,置氢后,出现马氏体组织和氢化物;随氢含量增加,马氏体和剩余β相数量增多;氢提高了Ti-6Al-4V合金的抗压强度和塑性等室温压缩性能,最大增幅分别为33.9%和56.3%;置氢Ti-6Al-4V合金抗压强度的提高主要归因于氢的固溶强化、马氏体相变强化和氢化物强化;塑性指标的提高主要是置氢合金中塑性β相数量的增多所致.     

脉冲电流对0.27%置氢Ti-6Al-4V合金变形行为的影响

对0.27%(质量分数)置氢Ti-6Al-4V合金进行压缩试验,对断口进行观察与分析,研究了脉冲电流对其变形行为的影响。结果表明,脉冲电流使0.27%置氢和未置氢Ti-6Al-4V合金的变形抗力降低和压缩延伸率提高。脉冲电流产生的焦耳热效应,是影响0.27%置氢Ti-6Al-4V合金力学性能变化的主要原因。由于电致塑性效应的影响,Ti-6Al-4V合金的断裂形式由延性脆性沿晶断裂向准解理穿晶断裂转变,但是对于0.27%置氢Ti-6Al-4V合金,由于在低温条件下氢脆的影响其准解理断裂特征不如未置氢Ti-6Al-4V合金的明显。     

0.2%H对Ti_2AlNb基合金板材高温拉伸变形行为的影响

为研究热氢处理技术对Ti2AlNb基合金板材高温拉伸变形行为的影响,在实验温度为870、900、930和990℃,应变速率为2.5×10-2s-1条件下,对H含量为0%和0.2%(质量分数)的Ti-22Al-25Nb合金板材进行了高温拉伸实验,并对其微观组织进行了分析。结果表明:0.2%H可以降低合金板材的高温流动应力,提高延伸率。在930℃变形时,置氢合金的峰值应力比未置氢合金下降了约36%,延伸率比未置氢合金提高了约53%。氢致合金软化和增塑的主要机制是H促进了α2相的动态再结晶,促进了b/B2相的位错运动和动态回复,提高了b/B2相的含量。     

热机械处理中退火时间对Fe-Mn-Si基合金形状记忆效应的影响

热机械处理能显著改善Fe-Mn-Si基合金的形状记忆效应。本工作中，对铸态Fe-Mn-Si基合金进行热锻、20%冷轧变形及固溶处理，随后在1073 K对部分经10%室温拉伸变形的试样进行不同时间退火处理。借助背散射电子显微分析(EBSD)、RGM-4300型万能试验机、示差四端电阻法及弯曲法研究热机械处理中退火时间对Fe-Mn-Si基合金微观组织和形状记忆效应的影响规律。结果表明：固溶态Fe-Mn-Si基合金的可恢复应变为4.1%;当经过退火时间为10 min的热机械处理后，合金的可恢复应变显著提高至5.3%;当进一步延长热机械处理的退火时间，合金的可恢复应变随之下降；当热机械处理的退火时间为180 min时，合金的可恢复应变下降至4.9%。随着热机械处理的退火时间延长，退火孪晶分数增加，堆垛层错密度下降，从而导致Fe-Mn-Si基合金促进应力诱发ε马氏体相变和抑制塑性滑移的能力变弱。这是Fe-Mn-Si基合金的可恢复应变随着热机械处理的退火时间延长而降低的主要原因。     

TI-6Al-4V合金置氢-除氢组织与力学性能

为研究除氢处理对置氢钛合金组织与性能的影响,对Ti-6Al-4V合金在不同参数条件下进行了置氢与除氢处理,采用光学显微镜分析了置氢-除氢处理过程中Ti-6Al-4V合金微观组织的演化规律,通过室温拉伸试验研究了置氢-除氢处理后Ti-6Al-4V合金的力学性能,探讨了Ti-6Al-4V合金置氢-除氢组织与力学性能之间的相...     

变形方向对TiAl合金二次热变形行为的影响

通过等温热压缩实验研究变形方向对经一次锻造后的Ti-46.2Al-2.5V-1.0Cr-0.3Ni合金热变形行为的影响,所选定的二次热变形方向垂直或平行于一次锻造应力轴方向,试样初始组织状态分别为锻造态、去应力退火态和双态组织。结果表明:变向二次热变形均加快了三种热处理状态钛铝合金加工硬化率的衰减过程;对于其中具有双态组织的钛铝合金继续施加变向或不变向二次热变形均有利于减小流变软化程度,提高二次热变形后的组织均匀性。进一步的微观组织观察表明,变向二次热变形与不变向二次热变形相比,更有利于合金的残余层片分解和动态再结晶的进行。     

工艺参数对半固态Al-4Cu-Mg 合金力学行为的影响

在热模拟实验的基础上,研究了各工艺参数对半固态和供应态Al-4Cu-Mg合金变形力学行为的影响.研究结果表明:在液-固温度区间变形时,变形温度、应变速率对半固态和供应态Al-4Cu-Mg合金的流变应力峰值影响显著,对稳态流变应力影响较小.当变形温度超过某一临界值后,流变应力随变形温度的变化趋于稳定.在固相温度区间变形时,半固态与供应态材料流变应力的变化趋势基本一致,变形均为固相间的塑性变形.但供应态Al-4Cu-Mg合金的原始变形组织和半固态Al-4Cu-Mg合金的球状组织对变形的影响有所不同.     

氢对TC21合金高温变形行为的影响

采用连续升温金相法研究了氢对TC21合金相变温度的影响,通过热模拟压缩实验,研究了氢对TC21合金高温变形行为的影响.结果表明:置氢可以显著降低TC21合金的相变温度,置氢0.7%(质量分数,下同)的TC21合金相变温度为810℃,与未置氢合金相比,降幅达145℃;同时,置氢还可以有效降低TC21合金的流变应力,并且温...     

Dynamic strength and failure behavior of titanium alloy Ti-6Al-4V for a variation of heat treatments

In our study, samples of Ti-6Al-4V were subjected to modifications of an aging treatment where temperatures for solution annealing and final aging as well as the cooling rate were varied. The titanium alloy was annealed above and below the β-transus temperature followed by cooling in a vacuum furnace or by water quenching. Additionally, the final annealing temperature was varied. Compression tests under quasistatic and dynamic loading rates were performed to determine the flow stress and strain hardening behavior. Furthermore, instrumented Charpy impact tests on U-notch specimen were performed at room temperature to monitor the load-time response of deformation and fracturing. The obtained high rate mechanical properties are discussed and correlated with the present microstructure. Our results reveal a very strong effect of the microstructure on the material behavior and will assist to choose the appropriate heat treatment technology, especially if impact loaded or safety structures have to be considered.     

TC21合金在形变热处理工艺下的组织特征

对TC21钛合金在不同条件下超塑拉伸后,进行双重退火热处理,研究热加工工艺对TC21合金显微组织演变的影响。结果表明,当变形温度在890～960℃时,TC21合金的伸长率随变形温度的增加先增加后减少,最佳超塑性变形温度为910℃;TC21合金在α+β相区超塑变形,然后在α+β相区双重退火处理后得到双态组织;在β区进行超塑变形、α+β相区双重退火处理后得到网篮组织。     

6vol%纳米B_4C_P/2009Al复合材料的颗粒分布特征及性能

采用三维高效混料机混料、多级真空热压和热挤压制备了增强体含量为6vol%的纳米B_4C_P(n-B_4C_P,50nm)/2009Al复合材料,研究复合材料中n-B_4C_P分布、形成机制以及对n-B_4C_P/2009Al复合材料力学性能的影响。结果表明:当混料球料比为5∶1时,复合粉末经过30h混料后,纳米B_4C_P基本均匀分布于Al合金颗粒表面;热压态复合材料中n-B_4C_P偏聚在基体晶界处,经过热挤压后,复合材料中的n-B_4C_P呈弥散均匀分布。热挤压过程中,基体合金的塑性流动对分布于晶界处的纳米B_4C_P形成剪切作用力,断裂的纳米B_4C_P团聚体沿着剪切应力方向发生重新分布是实现n-B_4C_P均匀分布的主要机制。经过495℃保温1h后水淬,175℃人工时效16h后,增强体含量为6vol%n-B_4C_P/2009Al复合材料硬度比基体合金提高了36.4%,抗拉强度和屈服强度分别提高10.9%和26.2%。n-B_4C_P/2009Al复合材料的拉伸断口表现出韧性断裂和脆性断裂混合特征。     

TC4钛合金板不同加工条件下组织对比研究

通过对Ti-6Al-4V钛合金进行常规锻造及换向墩拔锻造热塑性变形加工,然后再进行多火次换向热轧变形,研究换向墩拔锻造热塑性变形加工及多火次换向热轧变形过程中Ti-6Al-4V钛合金组织的演变。试验结果表明,采用墩拔锻造与多火次轧制加工方式,板坯组织由原始双态组织(等轴α及转变(α+β))逐步演变成完全等轴α组织形态,组织更加细小、均匀,有效保证了板材组织及性能各向同性。     

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu-coated Ti – 6Al – 4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti – 6Al – 4V substrate. The formed intermetallic compounds at the interface between the Ti – 6Al – 4V substrate and Cu coating are CuTi₂, CuTi, Cu₄Ti₃, and β-Cu₄Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu-coated Ti – 6Al – 4V is 126.0 kJ mol⁻¹, and the pre-exponential factor is 0.1 m² s⁻¹. The tribological properties of the Cu-coated Ti – 6Al – 4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti – 6Al – 4V alloy. The wear resistance of the Ti – 6Al – 4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu – Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.     

Annealing behaviour of dilute aluminium alloys following hot deformation

Abstract

The microstructure and texture of three dilute aluminium alloys after hot deformation and annealing was assessed; In particular, the influence of deformation temperature, strain rate, and strain on the annealed texture was examined, as well as the effect of alloy composition. The microstructures of the commercially pure materials studied (Al, Al+1%Mn and Al+1%Mg) varied in the volume fraction of coarse intermetallic particles, the type of dispersoid present, and the level and type of solute in solid solution. Furthermore, the initial stages of recovery and recrystallisation were studied in detail for one of the alloys (commercially pure Al). It was found that the main recrystallisation texture component was the cube and its strength, as well as the recrystallised grain size, depended strongly on the deformation strain. The deformation strain rate and temperature, and the alloy composition also strongly influenced the grain size and cube texture strength. These results are discussed in the context of current theories for cube nucleation within cube bands in the hot deformed microstructure. The present work was carried out as part of a wider research programme, partially supported by the European Union (Brite/Euram funded), to develop micromechanical models to describe the evolution of microstructure and texture during hot deformation and annealing of aluminium alloys.

MST/3376     

Effect of stress relief at 350 °C and 550 °C on the impact properties of duplex stainless steels

The production of massive duplex stainless steel castings weighting over 2 t, with thicknesses exceeding 5 in. represents a major challenge for the foundry industry. The difficulty in manufacturing such castings lies in the fact that thick sections experiment low cooling rates during the solidification process and during the solution annealing and water quenching heat treatment.As a result, intermetallic phases such as sigma phase (σ), Chi phase (χ), G phase, R phase, and complex carbides may precipitate, causing the material to be extremely brittle [Martins M, Casteletti LC. Effect of heat treatment on the mechanical properties of ASTM A890 grade 6A super duplex stainless steel. J ASTM Int 2005;2(1) [January]. [1]].After solution annealing and water quenching, the steel is, in principle, free of intermetallic precipitates, but will contain residual stresses resulting from rapid cooling on quenching. During and after machining, these stresses may produce dimensional distortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures, taking care to prevent the loss of mechanical properties, mainly impact toughness.The purpose of this study was to investigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact tests under the conditions of solution annealing and water quenching and stress relief at 350 °C for 4 h and at 550 °C for 2 h. Compared to CD4MCu the high nitrogen content of CD4MCuN stainless steel has a more balanced microstructure with similar ferrite and austenite contents, providing it with higher energy-absorbing capacity in impact tests. CD4MCuN fracture surfaces have predominantly fibrous structures typical of high toughness materials, while the CD4MCu steel's fracture surface shows cleavage facets typical of low toughness materials. The stress relief heat treatments reduced the impact toughness of the CD4MCu alloy but did not affect the CD4MCuN alloy.     

Electrically Assisted Ultrasonic Nanocrystal Surface Modification of Ti6Al4V Alloy

In this study, an innovative process, electrically assisted ultrasonic nanocrystal surface modification (EA‐UNSM), is used to process Ti6Al4V alloy. As compared with traditional UNSM, EA‐UNSM results in lower dislocation density and larger grains due to the thermal annealing effect caused by resistive heating. In addition, deeper plastic deformation layer is observed in the electrically assisted case. By supplying mechanical energy and thermal energy simultaneously, a strong dynamic precipitation effect is induced, which generates nanoscale precipitates in the EA‐UNSM‐treated Ti6Al4V alloy. These nanoscale precipitates can effectively pin dislocations during plastic deformation and thus significantly improve the surface hardness.     

Formation of Ultrafine‐Grained Microstructure of Ti–6Al–4V Alloy by Hot Deformation of α′ Martensite Starting Microstructure

We have presented a formation of ultrafine‐grained microstructure (d_α ≈ 0.2 µm) of industrial Ti–6Al–4V alloy produced by the hot compression of a sample with the acicular α′ martensite starting microstructure. The hot‐deformation behavior was different from the case of the conventional (α + β) starting microstructure, that is, the phase transformation of α′/(α + β) during hot working enhanced the microstructural conversion, especially under the conditions of a low temperature and a high‐strain rate.