Constitution and age hardening of Al-Sc alloys

Aluminium-rich alloys from the Al-Sc system were examined to determine the form of the equilibrium phase diagram and to obtain information on age hardening of chill cast alloys. Samples containing up to 8.75wt% Sc were examined using thermal analysis and optical microscopy. This work indicated a eutectic type of phase diagram with a eutectic temperature of about 665° C and a eutectic composition of about 0.6wt% Sc. The scandium-rich primary phase was found to be ScAl₃ which is f c c with a lattice parameter of 0.4105nm. Chill cast samples of a 1 wt% Sc alloy were examined for their age hardening behaviour over the temperature range of 225 to 360° C. A maximum hardness of 77 VHN was obtained after ageing at 250° C for 3 days. This hardness was retained after ageing for a total of at least 12 days. The hardening precipitates were ScAl₃ which were observed to form via a discontinuous precipitation mechanism. The ScAl₃ precipitates were observed to have a parallel orientation relationship with the matrix.     

时效硬化铜钛合金的相变和应用

铜钛合金是典型的时效硬化高强高导电合金。本文详细阐述了铜钛合金的相变过程;综述了铜钛合金的研究现状,即通过氢气氛时效、预冷变形与时效处理结合的方法可以在不同程度上提高铜钛合金的导电性、强度等性能。指出,晶界对合金性能的影响以及塑性变形对相变的影响有待进一步研究,预冷变形与时效处理结合将是今后研究的热点。     

Influence of cold rolling degree and ageing treatments on the precipitation hardening of 2024 and 7075 alloys

In the present work, the precipitation hardening of 2024 and 7075 aluminum alloys is investigated as a function of cold rolling degree, ageing time and temperature using Vickers microhardness measurements and differential scanning calorimetry (DSC). It is found that a variation in such parameters can improve the hardness and plays an important role in the precipitation hardening process. At specific ageing temperature, the large cold rolled 7075 alloy exhibits two peaks of hardness. Moreover, for both alloys, the increment of hardness during ageing decreases with increasing the cold rolling degree. While in some cases microhardness measurements give impression that the precipitation reaction is slowed down by deformation, DSC analysis indicates that the precipitation is much accelerated since only a slight deformation decreases strongly the temperatures of reactions. However, the degree of cold rolling does not play a crucial role.     

An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Effect of reinforcement size on age hardening of PM 2009 Al-SiC 20 vol% participate composites

The influence of SiC particulate size on the age-hardening response of 2009 aluminium has been monitored utilizing hardness, electrical conductivity and differential scanning calorimetry. Ageing involved either two or three stages depending on the reinforcement size. For 2009 Al reinforced with 29 μm SiC_p, initial ageing consisted of GPB (Guinier Preston Baqaryatsky) I zone formation. Decreasing the particulate size to 4 μm eliminated GPB I formation. This suppression of GPB I formation during ageing suggests that decreasing SiC size decreases the vacancy supersaturation following quenching by providing additional vacancy sinks at SiC/matrix interfaces. Subsequent ageing involved, at the larger reinforcement sizes, a transition with increasing time from GPB I zones to GPB II-dislocation complexes. At smaller (4 μm) reinforcement sizes, the formation of GPB II-dislocation complexes occurs directly. Finally, the last stages of age hardening in all SiC_p-reinforced composites examined consisted of heterogeneous nucleation of S′/S and GPB II→S′/S transformation.     

An investigation of the hardening process for several phosphate glass-containing cements

The hardening process for several phosphate glass-containing cements that have potential dental applications was investigated using both powder X-ray diffraction and Fourier-transform infrared spectral techniques. The investigated phosphate glasses contained various amounts of calcium, strontium and sodium ions. Crystalline brushite and amorphous phases whose concentrations and compositions depended on the compositions of the starting phases were formed during the hardening process for the cement. The study indicates that the use of phosphate glasses containing calcium and strontium ions in cement mixtures, rather than solid mixtures of the related metal oxides, sufficiently slows their hardening process so that commercially useful cements might be formed.     

An investigation on the application of process control agents in the preparation and consolidation behavior of nanocrystalline silver by mechanochemical method

In this paper, the effect of various amounts and types of process control agent (PCA), i.e., stearic acid (SA) and ethylene bis-stearamide (EBS), in the production and consolidation behavior of nanocrystalline silver prepared by mechanochemical reduction of Ag₂O by graphite was studied. The structural evolution and morphology of powders were investigated using XRD, HRSEM and particle size analyzer techniques. The results showed the nanocrystalline Ag formed after 25 h of milling and the addition of PCA prolonged the synthesis process time. Also, the effect of EBS on prevention of the excessive cold welding of ultra-fine Ag particles in the final stages of milling was more serious than SA. In fact, the presence of PCA effectively inhibited the creation of coarse Ag particles and finally decreased the crystallite size to 14 nm. Moreover, with the addition of PCAs, the Brinell hardness of sintered Ag samples was considerably increased.     

Experimental and theoretical investigation of the effect of kinematic hardening on spherical indentation

When investigating material properties, the indentation test is often used in spite of the fact that very complicated nonhomogeneous processes are involved. In order to understand this test in more detail, an analysis was given by Huber and Tsakmakis using Finite Element calculations. It was shown that there exists an analogy between uniaxial homogeneous tensile experiments and spherical indentation for cyclic loading conditions. In fact, in both cases, existence, e.g., of kinematic hardening can be identified by the existence of hysteresis loops in the strain–stress diagram and the depth–load plot, respectively. The present paper deals with an experimental verification of the existence of such hysteresis loops for the case of depth-sensing indentation tests. Further, two measures are considered in order to quantify the size of hysteresis loops. The first one is the area enclosed by the hysteresis loop while the second one is a suitable defined middle opening of the hysteresis loop. Using various Finite Element calculations, it is shown that both measures can be regarded to be correlated. These theoretical relationships are proved to be in agreement with experimental results as well. Finally, the effect of kinematic hardening on the hysteresis loops is discussed experimentally by studying the opening of the hysteresis loop as a function of the depth.     

Experimental and theoretical investigation on the compressive behavior of aluminum borate whisker reinforced 2024Al composites

The compressive behavior of Al₁₈B₄O₃₃w/2024Al composites fabricated by squeeze casting was investigated under low and elevated temperature. Microstructure shows that the compression exerts a significant effect on whisker fracture and rotation. The theory of synergistic effects caused by different strengthening mechanisms is used to predict the yield strength. Experiments show that compressive yield strength of composites improves by 47% compared with those of 2024Al at 623 K and agrees relatively well theoretical value. The compressive deformation depends on matrix mainly at lower temperature and the main failure mode is shear fracture. Additionally, fracture mechanisms are investigated further through fracture surface analysis. During hot compression, the predominated softening mechanisms also include dynamic recrystallization and strain softening except for dynamic recovery, which corresponds well with the shape of flow curves, microstructural observation and change of activation energy. Lastly, the optimum process parameters are determined to be about 0.1 s^− 1 and 723 K based on Dynamic Material Model and validated by microstructure evolution. Experiments show that the strain rate has a mixed effect on whisker fracture.     

The effect of thermal exposure on the electrical conductivity and static mechanical behavior of several age hardenable aluminum alloys

Aluminum alloys 2014-T6, 2024-T3, 6061-T6, 7050-T7451, and 7075-T6 were thermally exposed at different times (1 min to 20 days) and temperatures 177–482 °C (350–900 F). This study was conducted to simulate the effects of heat damage on aluminum alloys and to determine the correlations existing between the static mechanical and electrical conductivity properties. Results indicate that at the temperatures below 260 °C (500 F) all five alloys showed clear correlations between the mechanical and physical properties.     

Influence of alloying and secondary annealing on anneal hardening effect at sintered copper alloys

This paper reports results of investigation carried out on sintered copper alloys (Cu, 8 at%; Zn, Ni, Al and Cu-Au with 4 at%Au). The alloys were subjected to cold rolling (30, 50 and 70%) and annealed isochronally up to recrystallization temperature. Changes in hardness and electrical conductivity were followed in order to investigate the anneal hardening effect. This effect was observed after secondary annealing also. Au and Al have been found to be more effective in inducing anneal hardening effect.     

An experimental investigation on crack effect on the mechanical behavior and energy absorption of thin-walled tubes

Energy absorption capacity and collapse of cylindrical and square thin-walled aluminum tubes with a crack shaped trigger under axial compression are studied in this paper. Furthermore, the effects of length, angle, location and situation of cracks on the mechanical behavior of tubes are investigated. The results of this research show that the cracks change the collapse processes and folding modes; this effects are greater for the cylindrical tubes; the maximum load is reduced between 4.92% and 31.33% for cylindrical and between 2.55% and 18.52% for square tubes; the cracks increase the crush force efficiency up to 67.03% and 31.06%, and absorbed energy up to 30.45% and 30.16% for cylindrical and square tubes, respectively. The maximum load for all of the cracked tubes is less than that of intact tubes and increasing the crack angle from 0° to 45° decreases the maximum load and from 45° to 60° increases it. Finally, parallel cracks are more effective than perpendicular cracks.     

Reduced fracturing of intermetallic particles during crack propagation in age hardening Al-based alloys due to PFZs

Based on a shear lag model approach, a simple analytic model for the stress shielding of intermetallic particles due to the presence of a weakened precipitate free zone (PFZ) is derived. The PFZ desensitises the material to the breaking of particles. For the S and θ phase particles in the 2091 aluminium based alloy, particles with average sizes in the range 3–15 μm receive effective shielding from matrix stresses, which raise the matrix stress required to break the particle, and causes a propensity for larger particles to break at smaller applied macroscopic stress.