Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases     

SnCu钎焊接头稳态蠕变本构方程建立

以搭接面积为1 mm2微型单搭接钎焊接头为研究对象,采用新型高温蠕变测试装置,测定了Sncu钎焊接头应力指数n和蠕变激活能Q,构建了稳态蠕变本构方程,探讨了蠕变变形机制.结果表明,在低温高应力下,SnCu共晶钎料钎焊接头应力指数为8.73,激活能在59.1～63.2 kJ/mol,位错攀移运动主要受位错管道扩散机制控制;在高温低应力区,Sncu共晶钎料钎焊接头应力指数为6.45,激活能在88.4～97.5 kJ/mol,位错攀移运动主要由晶格自扩散机制控制.     

Relationship between tensile and primary creep properties of near γ-TiAl intermetallics

This paper presents the tensile and primary creep behaviour of fully lamellar microstructures for three near γ-TiAl compositions: binary TiAl, ternary TiAl+W and TiAl+NbMnWMoSi. Reducing the lamellar interface spacing or aging to precipitate β along interfaces increases the 760 °C tensile strength. Primary creep at stresses from 140 to 276 MPa and 760 °C is characterized in terms of the instantaneous strain occurring during creep loading and a subsequent primary transient. The presence of interface β precipitates reduces the instantaneous strain and increases the time to 0.5% creep strain. Reducing lamellar interface spacing does not influence the instantaneous strain, but decreases the strain rate during the primary transient. Correlating the creep response with deformation structures after primary creep suggests that the instantaneous strain depends on the mobility of interface dislocations and the primary transient depends on dislocation mobility in both the lamellar interfaces and γ lamellae.     

Ti—47Al—2Mn—2Nb—0.8TiB2合金韧脆转变温度及其应变速率敏感性

采用拉伸试验,研究了不同应变速率（１０＾－５－１０＾－１ｓ＾－１）下温度对含体积分数为０．８％的ＴｉＢ、具体近全片层组织的Ｔｉ－４７Ａｌ－２Ｍｎ－２Ｎｂ金属间化合物的屈服强度和延伸率的影响,得到合金韧脆转变温度随应变速率升高而升高的变化关系,确定这种ＴｉＢ２的ＴｉＡｌ合金的韧脆转变激活能为２５６ｋＪ／ｍｏｌ。这一数值低于无ＴｉＢ２的ＴｉＡｌ合金韧脆转变过程可能受位错攀移机制控制。     

T45Al10Nb附带 金全片层组织韧脆转变机制的研究

研究了应变速率和温度对Ｔｉ４５Ａｌ１０Ｎｂ合金的屈服强度和延伸率的影响。结果表明：随着应经升高,合金的屈服强度升高而延伸率下降,由此得到韧脆温度ＴＢＤＴ随应变束率升高而升工计算出Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变的激活能为３３０ＫＪ／ｍｏｌ。这一数值与ｒ－ＴｉＡｌ合金中原子的自扩散激活能（２９０ＫＪ／ｍｏｌ）相当,说明Ｔｉ４５Ａｌ１０Ｎｂ合金韧脆转变过程受扩散控制的形迹机制,即位错攀移控制,ＴＥＭ形     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

An internal variable approach for anomalous yield phenomena of β-CuZn alloy

A new interpretation for the anomalous yield phenomena of β-CuZn alloy (B2 ordered structure) has been made using an internal variable approach. The high temperature deformation behavior of β-CuZn alloy has been examined through a series of load relaxation tests (28–430 °C) and analyzed. The deformation behavior has been well described by the internal variable theory of inelastic deformation, consisting of three deformation modes; plastic, inelastic, and dislocation creep deformation modes. Among these, the inelastic deformation mode has proved to be most important in affecting the anomalous phenomena. Microstructural factors related to the inelastic deformation mode characteristics in B2 ordered alloys are discussed. It has also been proposed that the anomalous phenomena can occur only when the reference strain rate ($\text{ε}\text{̇}{}_{\mathrm{<mtext>o</mtext>}}$) falls into the region of lower strain rate and lower stress with increasing temperature. The decrease of yield stress above anomalous peak temperature (T_p) in tensile results can be attributed to the dislocation creep process. The activation energy of the process is obtained as 159 kJ/mol, similar to that for the self-diffusion of Cu or Zn.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Microstructure and high-temperature mechanical behavior of the NiAl–27 at.% Cr intermetallic composite

A NiAl–27 at.% Cr composite material was prepared by a powder metallurgical route, involving argon atomization and consolidation by hot isostatic pressing at 1350°C for 4 h at 400 MPa. The consolidated material exhibited a fine-grained microstructure consisting of a fine dispersion of Cr particles of about 1.7 μm in a NiAl matrix. The mechanical behavior at temperatures ranging from 650 to 1100°C was investigated by tensile-strain-rate-change tests. Analysis of the strain–stress data with both power law creep and Garofalo’s hyperbolic sine relation shows the transition to a low stress exponent creep regime with decreasing stress and/or increasing testing temperature. The measured activation energy for deformation of 300 kJ/mol is consistent with the activation energy for Ni self-diffusion in Ni–50Al. Experiments with coarse grain sizes established that the creep rate is independent of grain size which suggests that the deformation mechanisms must be associated with the motion of lattice dislocations.     

C含量对铸造TiAl合金组织和力学性能的影响

在Ti-47.5Al-3.7(Cr,V,Zr)合金中添加0.05%~0.2%C(原子分数,下同),采用冷坩埚悬浮熔炼方法制备出了层片组织TiAl合金铸棒,通过组织观察、室温拉伸和蠕变性能测试研究了C含量对TiAl合金组织和力学性能的影响。结果表明,添加0.05%~0.2%C后,合金仍可获得择优取向层片组织。随C含量增加α2层片体积分数略有增加,层片间距呈细化趋势。当C含量超过0.1%时,在α2和γ层片内和层片界面上有细小的Ti2AlC型碳化物析出,碳化物析出相的尺寸和数量随C含量增加有所增加。添加0.05%~0.2%C后提高了合金室温的抗拉强度和屈服强度,且随C含量增加提升幅度逐渐增大,当C含量为0.2%时,分别将抗拉强度和屈服强度提升了101和123 MPa。添加C元素后显著改善了合金的蠕变性能,当C含量为0.1%时蠕变性能最佳,与不含C的合金相比,其塑性蠕变应变降低了一半、相同应变时的蠕变速率降低了1个数量级以上。添加0.1%C提升合金蠕变抗力的机制主要是通过抑制合金在蠕变初期的位错萌生和增殖过程;在γ层片中形成割阶和位错碎片阻碍位错继续运动,使得合金在蠕变第一阶段的应变硬化程度迅速增加;此外,析出的Ti2AlC型碳化物进一步强化层片界面和基体,与层片间距细化共同提高了穿层片滑移位错的运动阻力。     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.     

片层厚度对TA15合金β相区变形行为的影响

将具有不同α片层厚度的两种转变组织的TA15合金分别在Ther mecmaster-Z型热模拟试验机上进行等温恒应变速率压缩试验,研究了温度为1000～1100℃、应变速率为0.001～10s-1时合金的热变形行为。结果表明,两种不同组织的TA15合金在β相区相同热力参数变形时,真应力-真应变曲线的形貌和真应力值基本相同,变形激活能为(170±2)kJ/mol,且微观组织特征基本相似,以β相的动态再结晶为主。但在1050℃、0.01～1s-1变形时,细片层组织的合金发生β相动态再结晶的体积分数总是略高于粗片层组织的合金,这可能与细片层组织的合金较早发生α→β相转变、且β相也较早开始再结晶有关。     

Al-5Zn-2Mg合金在动态回复过程中的流变软化与微观组织演变(英文)

将7005铝合金在变形温度为300~500°C、应变速率为0.05~50 s-1的条件下进行等温压缩实验,研究材料的流变应力行为及微观组织演变规律,使用金相显微镜(OM)、透射电子显微镜(TEM)、电子背散射花样(EBSD)等方法观察、分析热压缩试样。通过计算得到7005铝合金的激活能为147 kJ/mol,与纯铝的晶格自扩散能(142kJ/mol)相近。7005铝合金热变形过程中主要的恢复机制为动态回复。在高应变速率(50 s-1)条件下,试样由于变形温升的影响会发生流变软化。经过温升修正后,在较高变形温度下材料依然存在软化现象。通过微观组织分析可知,该现象主要与材料动态回复过程中晶界迁移引起的晶粒粗化有关。     

Creep behavior of Ti–6Al–2Sn–4Zr–2Mo: I. The effect of nickel on creep deformation and microstructure

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal 1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in -titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the -phase. We suggest that trace levels of Ni in the -phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.     

Steady-State Creep Behavior of Super304H Austenitic Steel at Elevated Temperatures

Creep behavior of Super304 H austenitic steel has been investigated at elevated temperatures of 923-973 K and at applied stress of 190-210 MPa.The results show that the apparent stress exponent and activation energy in the creep deformation range from 16.2 to 27.4 and from 602.1 to 769.3 kJ/mol at different temperatures,respectively.These high values imply the presence of a threshold stress due to an interaction between the dislocations and Cu-rich precipitates during creep deformation.The creep mechanism is associated with the dislocation climbing governed by the matrix lattice diffusion.The origin of the threshold stress is mainly attributed to the coherency strain induced in the matrix by Cu-rich precipitates.The theoretically estimated threshold stresses from Cu-rich precipitates agree reasonably with the experimental results.     

AgInCd合金压缩蠕变性能研究

用RDL-50型拉伸蠕变试验机进行改装后的实验装置研究了铸态AgInCd合金在温度300~400℃及应力范围12~24 MPa内的压缩蠕变行为,分析了稳态速率与温度和应力的关系,计算了应力指数(n)和蠕变激活能(Q_a),并结合蠕变后样品在透射电子显微镜下的微观形貌及位错组态,探讨了合金的压缩蠕变机制。结果表明:随温度和应力水平的升高,合金的稳态蠕变速率增加。相比较指数关系,蠕变速率与应力之间更符合幂函数关系。300、350和400℃条件下,合金的蠕变应力指数n分别为3.31、4.09和5.77;12、18和24 MPa条件下,合金的蠕变激活能Q_a分别为68.1、103.7和131.6 kJ/mol。微观形貌以层错为主,孪生为300℃的主要蠕变机制,位错攀移生成位错墙为400℃的主要蠕变机制。     

Creep behaviour of a new air-hardenable intermetallic Ti–46Al–8Ta alloy

Creep behaviour of a new cast air-hardenable intermetallic Ti–46Al–8Ta (at.%) alloy was investigated. Constant load tensile creep tests were performed at initial applied stresses ranging from 200 to 400 MPa in the temperature range from 973 to 1073 K. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of the minimum creep rate is n = 5.8 and the apparent activation energy for creep is calculated to be Q_a = (382.9 ± 14.5) kJ/mol. The kinetics of creep deformation of the specimens tested to a minimum creep rate (creep deformation about 2%) is suggested to be controlled by non-conservative motion of dislocations in the γ(TiAl) matrix. Besides dislocation mechanisms, deformation twinning contributes significantly to overall measured strains in the specimens tested to fracture. The initial γ(TiAl) + α₂(Ti₃Al) microstructure of the creep specimens is unstable and transforms to the γ + α₂ + τ type during creep. The particles of the τ phase are preferentially formed along the grain and lamellar colony boundaries.     

Primary creep deformation behaviors related with lamellar interface in TiAl alloy

Constant tensile stress creep tests under the condition of 760~816°C/172~276 MPa in an air environment are conducted, and the microstructural evolution during primary creep deformation at the creep condition of 816°C/172 MPa was observed by transmission electron microscopy (TEM) for the lamellar structured Ti-45. 5Al-2Cr-2.6Nb-0.17W-0.lB-0.2C-0.15Si (at.%) alloy. The amount of creep strain deformed during primary creep stage is considered to be the summation of the strains occurred by gliding of initial dislocations and of newly generated dislocations. Creep rate controlling process within the primary stage seems to be shifting from the initial dislocation climb controlled to the generation of the new dislocations by the phase transformation of 2 to as creep strain increases.     

Dislocation annihilation in plastic deformation: I. Multiscale irreversible thermodynamics

Irreversible thermodynamics is employed as a framework to describe plastic deformation in pure metals and alloys. Expressions to describe saturation stress in single crystals and nanocrystals are employed over wide ranges of temperature, strain rate and grain size. The importance of the roles played by vacancy self-diffusion in dislocation climb and in plasticity is shown. Equations to describe the stress–strain response of single crystals and ultrafine-grained metals are derived. The activation energy for dislocation annihilation plays a central role in the mechanical response of the systems. Succinct formulations for predicting hot deformation behaviour and relaxation of industrial alloys are presented; the influence of composition in the activation energy for dislocation annihilation is shown. All formulations describing stress–strain relationships can be reduced to Kocks–Mecking classical formulation, but incorporating grain size and compositional effects. The importance of the recovery term in such formulation is established, as well as the need to obtain it employing more fundamental approaches.