Activation Energy For Creep Research Articles

High temperature tensile creep behavior and microstructure evolution of Ti60 alloy rolled sheet

In this work, the high temperature tensile creep test of 2mm Ti60 alloy rolled sheet was carried out, and the creep behavior and microstructure evolution were studied. The initial microstructure of the 2mm Ti60 alloy rolled sheet is mainly composed of primary α phase and secondary α phase, and there are many dislocations and dynamic recrystallization (DRX) inside the rolled sheet. According to Norton-Bailey law, the creep stress exponent at 600 °C is calculated to be 3.6, and creep activation energy at 200MPa is calculated to be 286.4kJ/mol. It implies that the creep mechanism of the rolled sheet includes dislocation slip and dislocation climb. The initial Ti60 alloy rolled sheet has the {0001}<10-10>α texture. Stretching along the RD makes the texture direction gradually change from B-type (<0001>//ND) to T-type (<0001>//TD). Whether it is the B-type texture or T-type texture, the (0001) plane of the α phase always exhibits a basal texture state parallel to the tensile stress direction, contributing a good tensile creep property of this alloy sheet.

Simulation and Prediction of Creep Rate, Activation Energy, and Rupture Time of Sn94Sb5Ag1 Lead-Free Solder Alloy Using Artificial Neural Network Modeling

Simulation and Prediction of Creep Rate, Activation Energy, and Rupture Time of Sn94Sb5Ag1 Lead-Free Solder Alloy Using Artificial Neural Network Modeling

Experimental Observation of a New Attenuation Mechanism in hcp‐Metals That May Operate in the Earth's Inner Core

AbstractSeismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco‐ or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp‐iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation‐DIA combined with X‐radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q−1(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol−1). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp‐iron and the Earth's inner‐core it will be a contributor to inner‐core observed seismic attenuation and constrain the maximum inner‐core grain‐size to ≲10 km.

Role of threshold stress in creep of IN740H, a γ′-lean Ni-based superalloy

IN740H, a Ni-based superalloy comprising a low-volume fraction of γ′, is a candidate material to be used over long periods at high temperatures and stresses, as conditions prevalent in advanced ultra-supercritical (AUSC) power plants. In this study, IN740H has been tested at temperatures above 700 °C to gain mechanistic insights into its high-temperature creep behavior. The material showed classic signatures of the presence of threshold stress, marked by observation of a high apparent creep stress exponent, n, (e.g., 10 at 750 °C) and a high apparent activation energy for creep, Qc (e.g., ∼545 kJ/mol), along with a rapid increase in n at lower stresses. Accounting for the threshold stress led to a decrease in the values of n and Qc to 4 and 280 kJ/mol, respectively. Furthermore, the transmission electron microscopy and atomic scale compositional analysis reveal the pinning of dislocations at γ-γ′ interface and segregation of Co, Cr and Mo atoms in the regions of γ-γ′ interface rich in dislocations. The above combination of n and Qc and the observation of dislocation pinning at the γ-γ′ interface indicate the dislocation climb over γ′ as the dominant creep mechanism in this γ′-lean Ni-based superalloy, with the detachment of dislocations from the γ-γ′ interface, augmented by the segregation of Co, Cr and Mo, as the mechanism responsible for the realization of the threshold stress. This work, thus, provides a new impetus to research on the long-term structural integrity of γ′-lean Ni-based superalloys exposed to extreme service conditions.

The influence of microstructural heterogeneities on high-temperature mechanical properties of additively manufactured γ'-forming Ni-based alloys

AbstractAdditive manufacturing (AM) of metallic materials yields distinctive hierarchical and heterogeneous microstructures owing to the complex thermal conditions during the build-up process. Consequently, the knowledge gained from creep properties of conventionally manufactured (CM) Ni-based alloys cannot be directly applied to AM-processed alloys. Furthermore, insufficient creep life has posed a significant challenge in the development of Ni-based superalloys fabricated by laser powder bed fusion (LPBF), one of the most important AM techniques. Nevertheless, limited research has been conducted to understand their creep behavior due to the time-consuming nature of creep testing and extended research cycles. This study delves into investigating the creep behavior of an additively manufactured, precipitation-strengthened Ni-based alloy (NiCrAl) in comparison to its CM counterpart, focusing on the structure-property relationships. Constant-load creep tests were conducted at temperatures of 750 °C and 950 °C up to a maximum duration of nearly 1500 h. Although both the AM and CM states demonstrated high creep activation energy and creep exponents, indicative of a dislocation climb mechanism, the AM state demonstrated inferior creep life and ductility compared to the CM state for creep times below 500 h. To gain deeper insights into the underlying mechanisms, multi-scale microstructural characterization was performed to understand the effect of the AM-inherent microstructure. Overall, this study provides a comprehensive understanding of the creep behavior of Alloy 699XA after AM and CM processes, emphasizing the significance of AM-specific microstructural heterogeneities.

Influence of thermomechanical treatment on mechanical properties of CLAM steel

To enhance the creep resistance of CLAM steel and thus elevate its maximum service temperature, in our prior research, a superior thermomechanical treatment process was developed based on thermal simulation experiments conducted. This study employed forging to achieve the desired thermal deformation in accordance with the superior thermomechanical treatment process, followed by mechanical property testing post tempering. Comparative analysis with normalized and tempered CLAM steel (N + T CLAM steel) revealed notable improvements in yield strengths ranging from 31–55 %, ultimate tensile strengths from 30–46 %, and a reduction in total elongations by 12–21 % at both room temperature and elevated temperatures. Additionally, the ductile-to-brittle transition temperature (DBTT) increased from -65 °C to -27 °C. Creep behavior of N + T CLAM steel exhibited similarities to other RAFM steels, with stress exponents measured at 500 °C, 550 °C, 600 °C, and 650 °C recorded as 16.5, 15.0, 10.2, and 7.8, respectively. The creep activation energy was determined to be 556 kJ/mol, closely aligning with results from Eurofer 97 steel and F82H steel, and approximately twice the self-diffusion activation energy of iron. The Larson-Miller parameter method was employed to forecast stress-rupture limits under various conditions, with a predicted result of 120 MPa under typical design life conditions (550 °C / 100,000 h) for the cladding module, comparable to Eurofer 97 steel and F82H steel. TMT CLAM steel exhibited a stress exponent of 16.7 at 550 °C, with a creep activation energy of 578 kJ/mol. Under typical design life conditions (550 °C / 100,000 h) for the cladding module, the predicted stress-rupture limit increased to 176 MPa, representing a 47 % improvement over N+T CLAM steel. Moreover, under the same conditions (550 °C / 230 MPa), the rupture time of TMT CLAM steel was 54 times longer than that of N+T CLAM steel.

Effect of Si addition on the microstructure and creep properties of the forged titanium alloy

Creep experiments were carried out on pure Ti after annealed (720 °C/2 h, AC), Ti-0.4Si alloy after annealed (720 °C/2 h, AC) and thermal exposed (720 °C/2 h, AC+600 °C/500 h, AC) respectively. The presence mode of Si in Ti-0.4Si alloy after two heat treatments is partial precipitation and complete precipitation respectively. Combined with SEM and TEM characterization, the effects of Si addition on the creep properties and creep mechanism and the dominant roles of solid solution Si and silicide in improving the creep resistance of titanium alloys were investigated. The results show that the annealed Ti-0.4Si alloy has the best creep resistance, indicating that the addition of Si significantly improves the creep resistance of titanium alloys, and the Si has a creep strengthening effect whether it exists in solid solution state or in precipitated state. The high creep activation energy (Q = 253 kJ/mol) of the annealed Ti-0.4Si alloy is due to the strong inhibition of solid solution Si and dynamically precipitated silicide on matrix diffusion, indicating that the precipitation strengthening effect of dynamically precipitated silicide during creep is significantly stronger than that of precipitated silicide before creep, and obviously compensates the diffusion effect of dynamic precipitates during the precipitation process.

Relation between rheological properties and the stress state in subducting slabs

The distribution of different stress states in the subducting slab indicated by centroid moment tensor solutions for intra-slab earthquakes can help constrain the rheological properties of the slabs. A comparison of slabs in the western and eastern Pacific realms shows contrasting patterns in the stress states down to depths of ~ 350 km. The majority of slabs in the western Pacific show a pair of down-dip compression (DC) and down-dip tension (DT) domains in the upper and lower parts of the slab reflecting the effects of the slab unbending during progressive subduction. In contrast, slabs in the eastern Pacific show predominantly in-plane DT stress irrespective of slab geometry. Two-dimensional numerical simulations assuming constant slab thickness and viscosity indicate that the development of slabs with in-plane DT stress at depths of 100–300 km requires the slabs to be thin and have a low viscosity (1023 Pa s). Weak slabs bend easily and tend to fold when they encounter increased resistance to downward movement at the 660-km boundary. The associated DC stresses are not transmitted up the slab so negative buoyancy of the slab and DT stress dominates at intermediate depths for this type of slab. Most experimentally derived rheological parameters predict a high viscosity (> 1024 Pa s) for such slabs. However, two-dimensional numerical simulations using temperature- and pressure-dependent viscosity show that a relatively low activation energy (~ 110 kJ/mol) for diffusion creep is a possible explanation for the observed distribution of stresses in the slabs. Such low activation energies are compatible with recent experimental work on diffusion creep of polyphase mantle materials in which a low effective activation energy for creep results from a slow grain growth due to pinning effect of the secondary phase. The simulations provide a mechanical explanation for the observed dominantly DT stress state at 100–300 km depths for young slabs and paired DT and DC stress states at the same depth range for old slabs.Graphical

A new tensile creep model for predicting long-term creep strengths with short-term test data for creep resistant alloys

The difficulties in using the conventional Norton creep model to rationalise short-term creep data and to subsequently predict long-term creep rupture strengths for creep resistant alloys are presented and analysed. The results of this study show that these difficulties can be resolved if a new tensile creep model that integrates the tensile strength at creep temperature is applied to rationalise the short-term creep data. This is illustrated with the creep and tensile strength data measured for a grade of Ni-based superalloy. Based on this new tensile creep model, the activation energy of creep determined is independent of stress and the stress exponent is not influenced by temperature. Consequently, the model constants obtained from the short-term creep data can be applied together with the Monkman-Grant relationship to make the long-term creep rupture strength predictions at different temperatures. The factors affecting the reliability of the predictions made by this method are also analysed.

Tensile creep mechanisms of Al-Mn-Sc alloy fabricated by additive manufacturing

The incorporation of Sc/Zr elements into aluminum (Al) alloys provides great opportunities for high temperature applications, due to the high coherency with the Al matrix, the high thermal stability and the high strengthening potentials of the generated Al3(Sc,Zr) precipitates. In this study, the microstructure, thermal stability, high temperature tensile and creep behaviors of additively manufactured Al-Mn-Sc alloy were systematically evaluated. The Al-Mn-Sc alloy demonstrated sound high temperature yield strength as compared with other reported Al alloys made by additive manufacturing. However, the formed large area fraction (approximately 60%) of fine grain boundaries significantly promoted the dislocation annihilation and reduced the creep threshold stress, though the inherent thermally stable solutes and/or precipitates contributed to its high creep activation energy. On the other hand, the primary Alx(Mn,Fe) and Aly(Sc,Zr) particles and the fine grain boundaries severed as the potential cavity nucleation sites, enhancing the cavity kinetics during creep loading. The cavity nucleation and coalescence process, which basically dominated the tertiary stage of the creep lifetime and led to the final fracture, were further revealed through a cavity evolution model. Future creep-resistant Al alloy design outlook, especially for the inoculates modified Al alloys with fine grain structures, were proposed based on the present findings.

Transient Creep of Quartz and Granulite at High Temperature Under Wet Conditions

AbstractTransient creep of crustal rocks is important to explain time‐dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady‐state creep flow law parameters of quartz and feldspar, major minerals in the upper and lower crust, are well known, the physical mechanism behind transient creep and the corresponding flow law parameters are poorly understood. We quantify the flow law parameters for both quartz and granulite (mixture of plagioclase and pyroxene) under wet conditions with a nonlinear Burgers model using a Markov chain Monte Carlo (MCMC) method. Modeling results yield an activation energy of 70 ± 20 kJ/mol and a stress exponent of 2.0 ± 0.1 for transient creep of quartz aggregates. For granulite/feldspar, we find activation energies of 280 ± 30 and 220 ± 20 kJ/mol and stress exponents of 1.0 ± 0.2 and 0.9 ± 0.1 under mid (1050–1100°C) and high (1125–1150°C), temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady‐state creep for both quartz and granulite/feldspar. Combined with results for transient creep of olivine that were previously obtained (Masuti &amp; Barbot, 2021, https://doi.org/10.1186/s40623-021-01543-9), we suggest that the activation energies and stress exponents of transient creep are smaller than those of steady‐state creep for volumetrically important silicate minerals of the crust and upper mantle. Extrapolation of the estimated flow law parameters of granulite/feldspar to natural conditions suggests that transient creep may dominate during the postseismic period and lasts longer than previously thought.

Transition of superplastic and dislocation creep of duplex Mg–Li–Al–Zn alloy

In this study, temperature dependence of creep behavior in a duplex Mg–Li–Al–Zn alloy was investigated. Below 473 K, creep deformation in this alloy was primarily governed by the viscous motion of dislocations. In contrast, at temperatures above 493 K, dynamic recrystallization (DRX) occurred during creep deformation, leading to a remarkable elongation exceeding 500%. Consequently, the creep behavior of the duplex Mg–Li–Al–Zn alloy exhibits a transition from viscous glide-control creep at relatively low temperatures to DRX-control superplastic creep at relatively high temperatures. These creep deformation mechanisms were elucidated through analyses based on stress exponent, activation energy of creep, and dynamic microstructural evolution.

Study on creep behaviors of nickel-based single-crystal alloys considering microstructure evolution

Purpose The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution.Design/methodology/approachThe mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM).FindingsWith the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress.Originality/valueA creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band.

Enhancing the creep resistance of a cast AZ91 alloy via minor Sc addition

The influence of 0.2 wt% Sc addition on the microstructure and creep characteristics of a cast AZ91 magnesium alloy was studied by impression creep tests under the constant punch stresses ranging from 175 to 700 MPa in the temperature range 425–525 K. The findings indicated that creep rates decreased at all stress levels and temperatures after Sc addition. This was ascribed to the formation of the thermally stable Al3Sc and Mg5Al4Sc particles, reduction in the quantity of the unstable β-Mg17Al12 phase, and solid solution strengthening of Al in the Mg matrix. The creep stress exponent and activation energy values obtained from the studied alloys ranged from 5–6 and 95–117 kJ/mol, respectively. The detected drop in creep activation energy with increasing stress implies that two concurrent mechanisms of dislocation climb controlled by lattice and pipe-diffusion are competing with each other. The former one was the governing mechanism at low stress levels, while the latter one was the prevailing mechanism in the high stress regimes.

Improved creep resistance of AZ91 magnesium alloy after the micro arc oxidation process

AZ91 Mg alloy has a wide range of applications in the automotive industry, although its use is restricted to powertrain applications due to its low creep resistance. In this study, the effect of the micro arc oxidation (MAO) coating on the creep resistance of an AZ91 Mg alloy was investigated to take advantage of the coating layer with high thermal insulation properties. In this context, the MAO process was applied to AZ91 Mg alloy using a bipolar pulsed DC power supply. The creep tests were conducted at different temperatures (150–200 °C) and stresses (25–90 MPa) for the bare and coated samples, and the minimum creep rates were determined. It has been shown that the MAO coating reduces the creep rate of the bare alloy by 35%–84% depending on the temperature and the stress due to the stress-reducing effect and thermal barrier properties of the MAO coating. Based on the calculated creep activation energy and stress exponents, creep mechanisms were proposed for the bare and coated alloys. Effective activation energy was also calculated and lattice diffusion-controlled dislocation climb was determined to be the effective creep mechanism for both samples at lower stresses, while pipe diffusion-controlled dislocation climb was effective at higher stresses.

High temperature creep study on thermally grown oxide by tensile combined with a digital image correlation method

A thermally grown oxide (TGO) layer usually generates at the interface between the ceramic coating and the bond coating during the long-term high temperature service. TGO creep can affect the service life of thermal barrier coatings. In this paper, a dense TGO layer with a thickness of 4 μm is formed on the surface of the PtAl bond coating. The high-temperature creep behavior of the TGO layer is investigated by tensile combined with a digital image correlation method. The creep mechanism of TGO at 750 °C–950 °C is dislocation creep (stress exponent ∼4, and creep activation energy of 706–815 kJ/mol). The creep strain rate of TGO at 750 °C–950 °C is in the range of 4.7 × 10−6 s−1–4.7 × 10−4 s−1. The failure behavior of the TGO layer is significantly affected by temperature and applied stress. As the temperature and stress increase, the surface vertical cracks initiate at the TGO surface and propagate to the interface of TGO/PtAl bond coating, leading to the delamination and cracking of TGO layer.

Development of creep resistant high yttria 18Cr ferritic ODS steel through hot powder forging route

The present work reports the creep behaviour of a high yttria Fe-18Cr-2W-0.57Ti-1Y2O3 (all compositions in wt.%) ferritic oxide dispersion strengthened (ODS) steel produced by powder forging. The creep tests were carried out in a temperature range of 873 K to1023K and a stress range of 150 MPa to 425 MPa. Creep data is analysed using the power law creep equation. A large value of stress exponents (n ∼ 10 at 873 K, ∼ 18 at 923 K and 8.5 at 973 K) and a high activation energy (Q ∼581 kJ/mol) are obtained. These high values are attributed to threshold stress (σth) generated due to attractive interaction between dislocation and oxide particles. Threshold stress for different creep conditions was determined. True stress exponent values of 4 to 5 were obtained after accounting for the threshold stress. Electron back scattered diffraction (EBSD) revealed an equiaxed strain free grain structure in the powder forged ODS steel. Transmission electron microscopy (TEM) of creep tested samples indicates that microstructure remains stable after creep with no significant coarsening of nano size oxide particles. Dislocation pinning by the oxide particles was observed in creep tested samples. The observed values of true stress exponent suggest dislocation climb over the nano oxide particles to be the creep mechanism operating in the ODS steel. Samples sections close to creep fracture surfaces were investigated by EBSD. Void formation at triple junctions followed by crack propagation normal to the loading direction was observed. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. Conventional ODS steel compositions contain up to 0.35% yttria. A significant improvement in creep life as well as an increase in activation energy for creep was observed because of high (1%) yttria content used in the ODS steel studied.

Variations in Creep Performance of P92 Steel Within Its Composition Range: Influence of N Solubility

Underspecified composition ranges often lead to alloys with unpredictable mechanical performance. To better understand the changes in microstructure and mechanical performance associated with variations of key elements, three versions of P92 are formulated within, or close to, the specified allowable for N, B, and C ranges. Chromium and Si are also varied to influence N solubility. Different service conditions (i.e., temperature and stress) are explored. It is observed that &gt;80% decrease in creep life occurs at 625 °C and 155 MPa for the highest B and N containing alloy. Multiscale characterization reveals key changes due to the trace element variation. The high B and N containing alloy forms deleterious BN precipitates with morphology that promotes crack nucleation and damage accumulation, but this alloy additionally forms higher fractions of beneficial MX precipitates. The alloy with the lowest B and N concentrations but greater C content shows the best creep performance—a consequence of the refined M23C6 carbide precipitate population and the absence of large‐scale inclusions or BN precipitates. Calculations of creep activation energy reveal that the high B and N containing alloy is more prone to damage accumulation which causes an early onset of accelerating creep and greater minimum creep rate.

Intermediate-temperature creep behaviors of an equiatomic VNbMoTaW refractory high-entropy alloy

As one of refractory high-entropy alloys (RHEAs), VNbMoTaW is considered to be a promising candidate for elevated-temperature application. However, its creep behavior is rarely reported. In the present work, the compressive creep behaviors of an equiatomic VNbMoTaW RHEA with a grain size of 138 ± 36 μm were well studied over an intermediate temperature range (973–1173 K) and under high applied stress (130–520 MPa). The stress exponent of the alloy is found to remain stable (∼1) at relatively low temperatures (973–1073 K), whereas a stress-dependent transition behavior occurs at high temperatures (1123–1173 K), i.e., the stress exponent of the alloy changes from ∼1 in the low stress region (130–390 MPa) to ∼ 4 in the high stress region (390–520 MPa). Meanwhile, the creep activation energy increases from 139 to 156 kJ mol−1 at low temperatures to 307–373 kJ mol−1 at high temperatures. The low stress exponent and low activation energy at low temperatures suggest that the creep is controlled by dislocation pipe diffusion. The low stress exponent and relatively high activation energy in the high-temperature low-stress region suggest that the creep is controlled by lattice diffusion. In the high-temperature high-stress region, the prevalent dislocations detected by the post-mortem microstructural observation, the high stress exponent, and high activation energy suggest that the creep deformation is controlled by a lattice diffusion mediated dislocation climb process. These findings provide a fundamental understanding of the creep behavior and deformation mechanism of VNbMoTaW, which can be applied to design advanced creep-resistant RHEAs.

The synergistic effects of nano-sized α-Al(Mn,Cr,Fe)Si and Al–Si–Zr dispersoids on the creep behavior Al–Si–Mg–Cu (Mn, Cr, Zr) diesel engine alloy

The influence of Mn, Cr and Zr on the creep behavior of an Al–Si–Mg–Cu alloy intended for Diesel engine applications was studied. Two alloys, base alloy Al–7Si-0.3Mg-0.5Cu (MG1) and Al–7Si-0.3Mg-0.5Cu-0.15Mn-0.25Cr-0.15Zr (MG2-1) were creep tested at 300–350 °C and 28 MPa in the T7 heat-treated and soaked (100 h at 300 °C) condition. The apparent activation energy for creep for MG1 was 184 kJ mol−1 and for the MG2-1 was 234 kJ mol−1. The two alloys exhibited differing rate controlling creep mechanisms. The minimum creep rates of the MG2-1 alloy were 3–5 times lower than the MG1 alloy at 300 °C and 325 °C, respectively and only 1.3 times lower at 350 °C. The TEM investigation indicated creep deformation in the MG2-1 alloy was due to thermally activated restoration processes where grain boundary migration experienced Zener pinning by coherent and ∼5 nm αMG2-1-Al(Mn,Cr,Fe)Si. In the MG1 alloy, the creep mechanism was identified as climb over nano-sized (∼50 nm) non-coherent α-Al(Mn,Fe) dispersoids. EDS analysis showed that semi coherent Q’ phase (near equilibrium version of Q phase) existed in the MG1 alloy that had a nucleating role for nano-sized non-coherent α-Al(Mn,Fe) dispersoids.