Multi-stage spark plasma sintering to study the densification mechanisms of boron carbide

The densification kinetics of boron carbide (B₄C) during multi-stage spark plasma sintering was studied. The densification mechanisms were analyzed according to the stress exponent n and the apparent activation energy Q_d using a creep deformation model. The results showed that the densification mechanisms were controlled by viscous flow and grain boundary diffusion at the low effective stress with initial temperature range of 1600–2000 °C, while the dominant mechanism is the dislocation climb at the effective stress regime with final temperature of 2100 °C and the multi-stage sintering can reduce the apparent activation energy. Meanwhile, the scheme of multi-stage sintering can obtain nearly theoretical dense B₄C and avoid grain growth. Therefore, the basic mechanical properties suggesting a good combination of high hardness (37.63GPa) and bending strength (539.86 MPa) was obtained by the multi-stage sintering.     

Spark plasma sintering of W-10Ti high-purity sputtering target: Densification mechanism and microstructure evolution

The densification mechanism and microstructure evolution of W-10Ti sputtering target prepared by spark plasma sintering (SPS) method at a temperature ranges from 900 to 1600 °C, with dwelling time of 6 min and fixed pressure of 30 MPa were investigated. Densification occurs mainly at low temperatures (900 to 1300 °C), while grain growth occurs at high temperatures (1400 to 1600 °C). The creep model has been used to reveal the densification process. The effective stress exponent n is calculated systematically, which indicates that the densification process is mainly due to the particle rearrangement (n < 1), grain boundary diffusion (n = 1–2), and dislocation climbing (n = 3.77 or 4.14). In addition, the apparent activation energy Q_d is calculated to be 119.30 and 271.79 kJ/mol when the effective stress exponent n is equal to 1 and 2, respectively. It is also found that the microstructure of W-10Ti alloys is greatly affected by the sintering temperatures. The solution between W and Ti significantly improves with the increase of the sintering temperature. The solubility of W in βTi(W) exceeded the eutectoid point (28.97 wt% W) and the eutectoid structure (βW(Ti) + αTi) forms in cooling process when the temperature is up to 1300 °C. With the temperature increasing to 1500 °C, the composition of the βTi(W) phase is located in the miscibility gap of the (βTi(W), βW(Ti)) system, which tends to decompose in to βTi(W) and βW(Ti) phases.     

A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

Zirconium oxycarbide powders with controlled composition ZrC_0.94O_0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (E_a) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n ? 2) governs the densification process, while a dislocation motion mechanism (n ? 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687–774 kJ mol^?1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification.     

Hot-pressing of magnesium aluminate spinel—I. Kinetics and densification mechanism

Deformation mechanisms in steady-state creep are adopted to interpret the rate-determining mechanism in the hot-pressing of the MgO-excess, near-stoichiometric, and Al₂O₃-excess compositions of magnesium aluminate (MgAl₂O₄) spinel. Effective stress has been modified to account for stress multiplication arising from the sintering of porous powder compacts. Stress exponents of n=1.5 to n=3–4.3, and activation enthalpies of ΔH≈495 kJ/mol for a high-stress regime and 473 kJ/mol for a low-stress regime suggest that the rate-controlling mechanism transits from a climb-controlled dislocation mechanism in the beginning to diffusion controlled by oxygen diffusion in the later stage. The transition stress decreases with the content of Al₂O₃. Calculated creep rates and grain-size exponent of p≈2 further suggest that the Nabarro–Herring creep dominates the densification in low-stress regimes. Sigmoidal hot-pressing curves observed in Al₂O₃-excess compositions are characterized by the zero-densification-rate period, which represents the incubation time required for the recovery of hardened microstructure. Similar to the stress-dip test, the zero-densification-rate period is originated from the decreasing effective stress during hot-pressing. The internal stress accumulated upon hot-pressing exceeds the decreasing effective stress and results in the incubation time.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.     

Critical assessment of high-temperature deformation and deformed microstructure in high-purity tetragonal zirconia containing 3 mol.% yttria

Tensile creep has been investigated in a fine-grained yttria-stabilized tetragonal zirconia (3Y-TZP). The creep data corrected for grain growth reveal a high stress region with a stress exponent of n∼2.7 and a grain size exponent of p∼2.0–3.0, an intermediate stress region with n∼5.0, and a low stress region with n∼1.3 and p∼1.8. In the whole region, the apparent activation energy takes a value for the lattice diffusion of cations (580 kJ/mol). Microstructural observation reveals that intragranular dislocation motion is noticeably activated in the high stress region, limited in the intermediate stress region and absent in the low stress region. The results suggest that the rate of deformation is controlled by the recovery of the intragranular dislocations in the high stress region and by Nabarro–Herring creep in the low stress region. The intermediate stress region is related to a threshold stress for the dislocation motion.     

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.     

High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.     

Creep mechanisms in Ti–3Al–2.5V alloy tubing deformed under closed-end internal gas pressurization

Creep tests were carried out on Ti–3Al–2.5V alloy tubing in the temperature range of 723–873 K under closed-end internal pressurization. The data thus obtained were analyzed to obtain the mechanistic creep parameters (stress exponent and activation energy). Transitions in creep mechanisms were noted as the stress exponent varied from a lower value of 1 through 2 to a higher value of 5 with increasing stress where the activation energy assumed values of 232 and 325 kJ mol⁻¹, respectively. The creep mechanisms were elucidated in the light of standard creep models supported by the substructures studied by transmission electron microscopy. Newtonian viscous creep (n = 1) at lower stresses was identified to be in accordance with a slip band model named after Spingarn and Nix. Grain boundary sliding with n = 2 was noted in an intermediate stress region while climb of edge dislocations was observed to control creep at higher stresses. Microstructural observations along with parametric variations of creep rates were useful in identifying the underlying deformation mechanisms.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

Creep of cast Fe–36Al–2Ti alloy

Creep of an alloy based on the intermetallic compound FeAl with Ti addition was studied by compressive tests at constant stress in the temperature range from 873 to 973 K. The stress exponent n and the activation energy of creep Q were determined for the minimum creep rate. The stress exponent is slightly decreasing with increasing temperature. Its value is in agreement with stress exponents reported for Fe–Al alloys with similar additions of titanium. The values of n together with the observed shapes of the creep curves are consistent with the expected behaviour for solid solution hardened alloys where the dislocation motion is controlled by viscous glide of dislocations. Annealing of the alloy for 2 h at 1423 K with subsequent oil-quenching had no influence on the minimum creep rate in comparison with the as-cast material. On the other hand, this annealing accelerated “the inverse primary behaviour”.     

Separate contributions of texture and grain size on the creep mechanisms in a fine-grained magnesium alloy

The influence of texture and grain size on the creep behavior of a fine-grained magnesium alloy, over the temperature range 423–723 K was investigated. Equal channel angular pressing and rolling were used to produce samples with different textures. Two deformation regimes could be distinguished by their stress exponents. A stress exponent close to 2 and activation energy of 91 kJ mol⁻¹, close to that for grain boundary diffusion, were found at the lower strain rates. In this range, there is no detectable effect of texture. In the high stress exponent regime, within the range 3 < n < 12, a noticeable effect of texture and grain size has been found. The texture effect is related to the orientation of the basal planes. The influence of grain size distribution on flow stress is satisfactorily explained by modeling the deformation as a combination of grain boundary sliding and slip creep.     

等通道侧向挤压和板材挤压法制备细晶ZK60镁合金板材的热剪切变形本构分析（英文）

采用不同道次等通道侧向挤压(DECLE)和板材挤压成形发制备细晶ZK60合金板材。经过3和5道次DECLE和后续挤压后,退火样品中的粗大晶粒(68μm)分别变为6.0μm和5.2μm的细小晶粒。基于冲孔剪切实验(SPT)结果建立本构方程,研究合金的热剪切变形行为。SPT的温度范围为200～300℃,应变率范围为0.003～0.33 s^-1。计算结果表明,所有条件下制备的样品的活化能为125～139 k J/mol,应力指数为3.5～4.2,表明主要的热变形机制是位错蠕变,由位错攀移和溶质拖曳机制控制。材料常数n和Q取决于晶粒尺寸,第二相颗粒比例等微观结构因素,通过三维曲面曲线预测了二者的关系。此外,挤压后的ZK60板材具有相似的强基面织构,因此,合金在SPT过程中具有相同的变形机制与相近的n和Q值。     

Characterization of creep properties and creep textures in pure aluminum processed by equal-channel angular pressing

High-purity aluminum was processed by equal-channel angular pressing (ECAP) and then tested under creep conditions at 473 K. The results show conventional power-law creep with a stress exponent of n = 5 which is consistent with an intragranular dislocation process involving the glide and climb of dislocations. It is demonstrated that diffusion creep is not important in these tests because the ultrafine grains produced by ECAP are not stable at this temperature. Texture measurements were undertaken using the high-pressure preferred orientation neutron time-of-flight diffractometer and they reveal significant differences in the evolution of texture during creep in pressed and unpressed specimens. These experimental measurements of texture are in excellent agreement with theoretical textures predicted using a visco-plastic self-consistent model that limits deformation to plastic slip. The calculations provide additional confirmation that creep occurs through an intragranular dislocation process.     

高密度纯钨的低温活化烧结工艺及其致密化行为

采用溶胶喷雾干燥-煅烧-氢热还原法制备了BET粒径为0.21μm的超细纯钨粉末,并利用球磨处理进一步活化粉末。研究了超细纯钨粉末形貌及其性能随球磨时间的变化特征,探索了未球磨、球磨5h及球磨10h3种超细纯钨粉末烧结致密工艺,此外还详细研究了纯钨烧结体组织形貌、晶粒尺寸及显微硬度等性能随烧结温度及球磨时间的变化规律。结果表明,球磨处理对超细纯钨粉末的烧结起到了极大的活化作用,由球磨10h粉末制成的压块在1900℃下烧结2h其致密度即可达97.3%,比传统微米级纯钨粉末制成的压块达到相同烧结致密度的温度降低了600℃以上。同时,球磨处理可以大幅降低钨粉的起始烧结温度和再结晶温度,获得组织更加均匀细小、力学性能(硬度)更加优良的钨烧结体。     

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.     

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.     

Sintering of a nano-crystalline tungsten heavy alloy powder

A nano-crystalline Tungsten heavy alloy powder was obtained by mechanical alloying of elemental powders in a jar mill with a high ball to powder ratio. The chemical composition of the primary powder was 93 W-4.9Ni-2.1Fe (wt%). The mechanically alloyed powder had 22 nm sized tungsten crystallites distributed in an amorphous nickel base phase. Mechanical alloying reduced particle size of powders and also yielded to more uniform particles size distribution. Sintering behavior and microstructural development of that powder were studied and compared with a conventionally mixed powder. Mechanically stored energy and better distribution of primary elements in Nano-crystalline powder had decreased motivation energy of sintering and that powders showed more densification at relatively lower sintering temperatures. Sintering at low temperatures can depress grain growth during sintering and provide desirable properties. A transient intermetallic phase was formed in the nano-crystalline powder during sintering that has not been seen in conventionally mixed powders.     

The effect of carbon and silicon additions on the creep properties of Fe-40 at. % Al type alloys at elevated temperatures

The mechanical properties of Fe–Al alloys with 39–43 at.% Al, C contents up to 4.9 at.% and Si contents up to 1.2 at.% were studied using uniaxial compressive creep at temperatures from 600 to 800 °C. The stress and temperature dependence of the creep rate were determined by stepwise loading and evaluated in terms of the stress exponent n and the activation energy Q, respectively. These quantities can be interpreted by means of dislocation motion controlled by climb and by the presence of second-phase particles. The dislocation motion is obstructed by precipitates of carbide κ in alloys E and F and by particles of Al₄C₃ in the alloys with either higher content of C or of C and Si. Both carbon and silicon improved the creep resistance, but the effect of silicon was more significant.     

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.