The failure of large second phase particles in a cracking aluminum alloy

The fracture of large second phase particles in advance of a crack tip was investigated. The experiments allow establishment of a fracture criterion for particles. Fracture toughness is shown to increase almost linearly with particle distance.     

Debonding of circular second phase particles

The plastic relaxation of a shear crack situated normal to the interface of a second phase particle of circular cross section is quantitatively analyzed. The ratio of applied stress to yield stress and the relative displacement of crack faces at the tips in the matrix and at the interface of the second phase particle are related to the crack parameters—namely the length of the crack, the width of the plastic zone along the interface and the width of the plastic zone in the matrix. The effect of the shear modulus and size of the second phase particle on the behavior of the plastic zones is determined. A critical value of the relative displacement of the crack faces at the tip is used as the criterion to determine the tendency to brittle extension of the crack into the matrix or along the interface. Conclusions are made on the debonding of the second phase particle from the matrix.     

The second phase particles and mechanical properties of 2124 aluminum alloy processed by accumulative back extrusion

An age-hardenable 2124 aluminum alloy was severely deformed by accumulative back extrusion (ABE) method up to three passes at 100 and 200 °C. The characteristics of the second phase particles were studied using scanning electron microscopy. The results indicated that the size of primary particles had been reduced after the first ABE pass where even much finer particles was obtained as the successive passes were applied. In addition, the secondary particles were fragmented into finer pieces after ABE at 100 °C, whereas a particle coarsening was realized as the deformation temperature rose to 200 °C. The latter was attributed to the Ostwald ripening mechanism. However, the volume fraction of secondary particles was significantly decreased after three ABE passes at 200 °C due to the occurrence of deformation induced dissolution. Additionally, the tensile properties of the processed materials were measured utilizing a miniaturized tensile testing method. The results were justified considering the evolution of the second phase particles.     

Dissolving behaviour of second phase particles in Nb-Ti microalloyed steel

The dissolving behaviours of second phase particles in low carbon Nb-Ti microalloyed steel during isothermal holding at 1300 °C were investigated by carbon extraction replicas, TEM and EDX analysis. The experimental results indicate that there are two types of precipitates in as-forged steel: the coarsened Ti-rich precipitates originate from solidification, and the other finer Nb-rich particles attribute to strain-induced process. The strain-induced precipitates disappear after holding for 2 h at 1300 °C, while the coarsened particles from the solidification containing Nb-bearing (Nb,Ti)(C,N) still remain even holding for 48 h. With addition of Ti to the Nb-bearing microalloyed steel, the thermal stability of the carbonitrides would enhance remarkably. These results rationalize that formation and thermostability of precipitates are considerably influenced by interaction between Nb and Ti atoms.     

Analysis of second phase particles in a powder metallurgy HIP nickel-base superalloy

The nature of the second phase particles associated with LC Astroloy prepared using powder metallurgy techniques has been examined. The individual particles have been identified using energy dispersive X-ray analysis and convergent beam electron diffraction. Four distinct types of particles have been observed: a cubic MC carbide in which M is either titanium or titanium plus molybdenum, a monoclinic phase ZrO₂, a trigonalα-Al₂O₃ and a tetragonal M₃B₂ phase in which M is molybdenum or molybdenum and chromium. The observations indicate that, although some MC carbides are associated with the ZrO₂ phase, the majority of the prior particle boundary precipitates are separate entities. Hot isostatic pressing or subsequent heat treatments above or below the γ′ solvus were observed to have little effect on the incidence or distribution of the precipitation associated with the prior particle boundaries. In contrast, heat treatments above the γ′ solvus resulted in the dissolution of the M₂B₃ phase and its preferential precipitation on the grain boundaries.     

不同退火工艺下Ti—IF钢二相料演变的比较

对罩式退火和连续退火过程中Ｔｉ－ＩＦ钢二相粒子的演变规律进行了研究。结果表明：在退火以前形成的二相凿子，ＴｉＮ，ＴｉＳ和Ｔｉ４（ＣＳ）２经过罩式退火和连续退火后，春尺寸，形状和分布无较大的差别，而ＴｉＣ的变化较大；Ｔｉ－ＩＦ钢在连续退火中不形成新的相粒子，而在罩式退火中形成两类新的二相粒子－ＦｅＴｉＰ和（Ｔｉ，Ｍｎ）Ｓ。     

Properties of conductive films made from fine spherical silver-palladium alloy particles

Silver-palladium alloy films were made by a thick-film technique using several metal powders prepared by the spray-pyrolysis method. Two types of powders were used: silver-palladium alloy powders (15 mol % Pd and 30 mol % Pd) and mixtures of silver and palladium powders. The fired films were about 2–3 m in thickness. The alloy particles sintered uniformly in firing, whereas, in the case of the powder mixtures, the alloying of silver and palladium particles caused uneven particle growth and large voids were formed in the fired films. For this reason the films made from the alloy powders had better conductive properties than those made from the powder mixtures. The resistivities of the films made from the alloy powders were close to the intrinsic value for silver-palladium alloys. Further, the oxidation of palladium during heating in air was significantly suppressed in the alloy powders when the palladium content was lower than 30 mol % in accordance with thermodynamic considerations.     

Interaction effect of second particles on the stress concentration in ZK60 magnesium alloy

Second particles play a crucial role in influencing the performance of magnesium alloys. A previous work showed that during the fracture initiation process a competitive relationship exists between micron MgZn₂ particles and submicron Mg₄Zn₇ particles of ZK60 magnesium alloy. Especially, some of the MgZn₂ particles were found to be gathered in pairs, which resulted in the interaction effect. In this work, three-dimensional MgZn₂ particles were characterized in terms of size, morphology, and distribution using computed tomography technology. Based on the visualization and quantification results of MgZn₂ particles, the typical structural models were established. The relationship between stress concentration and shape ratio induced by a single particle was investigated using finite element computations. Furthermore, empirical equations were proposed to quantify the interaction effect induced by two adjacent particles. The validity of the equations was verified by models based on real particle. Comparing the stress concentration between micron MgZn₂ particles and submicron Mg₄Zn₇ particles, the fracture initiation was confirmed to be caused by Mg₄Zn₇ particles considering the interaction effect of micron particles. The obtained quantitative information would be useful for understanding the crack initiation of engineering materials.     

Microcrystalline to nanocrystalline silicon phase transition in hydrogenated silicon-carbon alloy films

Different classes of interesting materials (such as protocrystalline, microcrystalline and nanocrystalline) have been grown under conditions very near to those for the microcrystalline phase. In spite of the importance of these materials, a clear picture regarding their phase transitions is missing. A smooth transition from the microcrystalline to the nanocrystalline silicon phase, distinctly different from an abrupt order-disorder phase transition, has been demonstrated, for the first time, in hydrogenated silicon-carbon alloy films, prepared from a silane-methane gas mixture highly diluted in hydrogen, by varying the rf power in a plasma enhanced chemical vapour deposition system. The study has also provided the signature of medium range order in hydrogenated silicon-carbon alloy films.     

Liquid film simulation of Zener grain boundary pinning by second phase particles

Abstract

The geometry of the interaction of second phase particles with migrating grain boundaries has been studied using a liquid film simulation. The geometries observed show that the perimeter of intersection between boundary and particle can be complex, often not lying in a single plane through the particle. The curvature of the boundary when in contact with the particle is also complex, sometimes displaying double curvature and often of opposite sign. These observations have been used to modify previous calculations quantifying the grain boundary pinning effect. Measurements from the photographs of these interactions have been applied to a force equilibrium approach for calculating the pinning force and these results are found to be in good agreement with values obtained using an energy analysis based on grain boundary shape change.

MST/1259     

Dependence of fracture toughness on multiscale second phase particles in high strength Al alloys

Abstract

An improved model is described to predict variations in fracture toughness of high strength aluminium alloys with volume fraction, size, and characteristics of the contained multiscale second phases, i.e. ellipse shaped constituents, sphere shaped dispersoids, and disc shaped precipitates, in an integrated manner. Results show that predictions are in broad agreement with values measured experimentally for an aged Al-Cu-Mg alloy. Furthermore, the model was employed torelate the anisotropic fracture toughness ofalloy plate to its orientation. A diagramis presented to illustrate the relationship between yield strength and fracture toughness of the aged alloy.     

Characterization of phase transformation behavior in electrolytically produced indium-thallium shape memory alloy films

Indium-thallium alloys in the range 15–38 at. % Tl were electrodeposited from a sulfate electrolyte by using pulsed current. The relationship between composition and phase structure of the alloys deposited was investigated with regard to the shape memory effect. To date there have been no reported experimental studies describing the electrodeposition of In---Tl alloys capable of undergoing such transformations. Not only did the deposited alloys exhibit the shape memory effect, but the composition-phase relationship exhibited was shown to deviate from that reported for alloys in an equilibrium condition. The results show that the temperature and the temperature interval for the transformation of the electrodeposited alloys are substantially different from those of the thermally prepared alloys. The shape recovery for the electrodeposited alloys was also confirmed.     

Energy gap variations and structural phase changes in CdSTe alloy thin films

Thin films of CdS_x Te_1?x have been prepared by electron-beam evaporation throughout the composition range. The optical energy gaps have been measured, and the observed variation of energy gap with composition is compared with previous results and with theoretical prediction. A plot is given in which a sharp discontinuity of slope occurs at the composition at which the wurtzite to zinc blende phase change commences.     

Role of second phase particles on microstructure and texture evolution of ARB processed aluminium sheets

Abstract

The accumulative roll bonding (ARB) process was carried out on a high purity alloy (AA1100) and a particle containing aluminium alloy (AA3003) for up to eight cycles. The electron backscattered diffraction (EBSD) method was utilised to investigate the microstructural and microtextural evolution in ARB processed sheets. The results indicate that the lack of second phase particles in pure aluminium hinders grain refinement and leads to the formation of unrefined bands, which results in the increase of the overall texture intensity and the development of a strong texture. A submicrometre grain structure in this alloy develops at the final stages of the process. It was also found that the presence of second phase particles in AA3003 alloy prevents the development of such unrefined bands and improves grain refinement during the ARB process, which results in a more homogenous microstructure of ultrafine grains.     

The effect of HfO2 second phase in Fe films upon ion irradiation

Ferrum of BCC crystal structure is a typical kind of matrix in structural alloy steels which could be strengthened by introducing some second phase. In the present study, BCC Fe thin films with hafnium oxide (HfO₂) second phase have been synthesized in an electron beam evaporation system. Multi-layered and glancing angle deposition (GLAD) techniques were taken to form some HfO₂ second phase in Fe films. Ion irradiation was conducted to investigate the irradiation resistance of the obtained samples with and without HfO₂ second phase.