Iranian Journal of Materials Science and Engineering

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Abstract: In the current work, the strain hardening behavior of dual-phase steels with different silicon content (0.34- 2.26 Wt. %) was examined using the modified Crussard-Jaoul analysis. It was shown that these dual-phase steels deform in two stages over a uniform strain range. Each stage exhibited a different strain hardening exponent varying with silicon content. At the first stage, work hardening exponent remind significantly constant, while during the second stage, it decreased with increasing silicon content from 0.34% to 1.51% and then increased for the higher silicon contents (1.51% to 2.26%). It was found that the strain hardening behavior of these steels was predominantly affected by the volume fraction of martensite at low silicon contet and the ferrite strengthening induced by silicon at the higher silicon content. The effect of silicon content on the volume fraction of martensite and tensile properties were also considered.

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Microstructure and properties of the Al-6Si-0.3Mg alloys containing scandium (0.2 to 0.6wt %) were investigated. The microstructure was observed by optical microscopy, the hardness was determined by Vickers tester and phase transformation was investigated by differential scanning calorimetry (DSC) technique. The results showed that scandium can refine dendrites, enhance hardness in the aged alloys and suppress softening effect during prolonged ageing treatment.

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Precipitation behaviour of wrought Al-6Mg alloys with ternary scandium and quaternary zirconium and titanium has been studied. Hardness measurements and resistivity studies are employed to assess the precipitation behaviour of scandium doped Al-6Mg alloy without or with quaternary additions of zirconium and titanium. Further, the kinetics of precipitations are studied by differential scanning calorimetric technique. Scandium has been observed to form fine coherent Al3Sc precipitates during ageing and these are responsible for strengthening of the alloys. The precipitation kinetics of Al3Sc depends on the diffusion of scandium in aluminium. Presence of fine coherent precipitates of Al3Sc impedes the migration of dislocations and increase the recovery temperature. The kinetics of recrystallisation is also delayed.

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The presence of alloying elements, sometimes in a very small amount, affects mechanical properties one of these elements is Boron. In Aluminum industries, Boron master alloy is widely used as a grain refiner In this research, the production process of Aluminum –Boron master alloy was studied at first then, it was concurrently added to 2024 Aluminum alloy. After rolling and homogenizing the resulting alloy, the optimal temperature and time of aging were determined during the precipitation hardening heat treatment by controlled quenching (T6C). Then, in order to find the effect of controlled quenching, different cycles of heat treatment including precipitation heat treatment by controlled quenching (T6C) and conventional quenching (T6) were applied on the alloy at the aging temperature of 110°C. Mechanical properties of the resulting alloy were evaluated after aging at optimum temperature of 110°C by performing mechanical tests including hardness and tensile tests. The results of hardness test showed that applying the controlled quenching instead of conventional quenching in precipitation heat treatment caused reduction in the time of reaching the maximum hardness and also increase in hardness rate due to the generated thermo-elastic stresses rather than hydrostatic stresses and increased atomic diffusion coefficient as well. Tensile test results demonstrated that, due to the presence of boride particles in the microstructure of the present alloy, the ultimate tensile strength in the specimens containing Boron additive increased by 3.40% in comparison with the specimens without such an additive and elongation (percentage of relative length increase) which approximately increased by 38.80% due to the role of Boron in the increase of alloy ductility

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The effect of aging time and temperature on the microstructure and mechanical properties of Ti-13V-11Cr-3Al and Ti-13V-11Cr-3Al-0.2C was studied. The carbon addition increases the rate of age hardening as well as the peak hardness of aged samples. The presence of titanium carbides in Ti-13V-11Cr-3Al-0.2C limits grain growth during the process. The observations in this work are discussed in terms of the effect of the microstructural changes in quenched and aged samples associated with the presence of carbide precipitates

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Precipitation hardening is the most common method in the strengthening of aluminium alloys. This method relies on the decrease of solid solubility with temperature reduction to produce fine precipitations which impede the movement of dislocations. The quality control of aluminium alloy specimens is an important concern of engineers. Among different methods, non-destructive techniques are the fastest, cheapest and able to be used for all of parts in a production line. To assess the ability of eddy current as a non-destructive method in the evaluation of precipitation hardening of aluminium alloys, 7075 aluminium alloy specimens were solution treated at 480°C for 1 hr. and followed by water quenching. Afterwards, the specimens were aged at different temperatures of 200, 170, 140, 110 and 80°C for 8 hr. Eddy current measurements was conducted on the aged specimens. Hardness measurement and tensile test were employed to investigate the mechanical properties. It was demonstrated that eddy current is effectively able to separate the specimens with different aging degree due to the change of electrical conductivity during aging process

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Creep age forming (CAF) is one of the novel methods in aerospace industry that has been used to manufacture components of panels with improved mechanical properties and reduced fabrication cost. CAF is a combined age-hardening and stress-relaxation that are responsible for strengthening and forming, respectively. This paper deals with the experimental investigations of mechanical and springback properties of Al-Zn-Mg Al alloy in creep forming process. Creep forming experiments have been performed at temperatures of 120◦C and 180◦C for 6–72 h. Results indicated that yield stress and hardness of creep age formed specimens increased with increasing forming time and temperature,  simultaneously induced deflection by stress-relaxation increased. Incorporating spring back and mechanical properties, it can be found that the appropriate forming cycle was 180 ◦C/24 h among all forming conditions. CAF Time increase to a certain extent increased mechanical properties. This can be attributed to presence of stress in CAF that causes the precipitates be finer because of creation more nucleation sites. Therefor the growth of precipitates, takes place at long time and postpones the decreasing of the yield stress

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In this paper, the influence of heat treatment on PH17-7 stainless steel spring was evaluated. Precipitation hardening phenomenon of  PH 17-7 steel was evaluated in three stages. First, the spring constant changes with time and temperature was evaluated. Second, the spring constant changes with respect to its original length at constant temperature and time with blocking (spring length after compression, 18 and 21 mm) were investigated.  And finally, the spring heat treatment at 480 °C for 80 min and then holding at 230 °C in oil bath for 60 min without blocking were investigated. The results showed that the use of 18 mm block have large spring constant than 21 mm block. The optimal conditions (480°C for 80 min) for this spring to reaching maximum spring constant were determined.

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The boronizing of a tungsten heavy alloy containing Ni and Fe as the major alloying elements were performed in the present study to increase its surface hardening. Pack cementation method was employed as a well-known, successful solid-state process for boronizing. The coating treatment was accomplished at different temperatures of 1000, 1050 and 1100°C for 6 and 9 hours. The formation of tungsten boride phase was confirmed, although a silicide layer covered the surface of the specimen as the outer layer. The mechanism of the formation of a multilayered surface was explained. The maximum thickness of reaction zone and surface hardness achieved in the current work were 300 µm and 2470 HV, respectively.

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The objective of the present paper is to investigate the effects of rapid heating and cryogenic cooling on on the microstructure and tensile properties of Al-Cu-Mg. The specimens were subjected to three heat treatment cycles in which the Infrared heating (IR) were used as the heating medium at the ageing stage, and the liquid nitrogen and water were used as the quenching mediums. The ageing temperature and time were 190⁰C and from 2 hours to 10 hours, respectively.The results indicated that by using IR at the ageing stage, the hardening rate enhanced because the rapid heating via this method leads to faster diffusion of the alloying elements. Moreover, the high density of nano-sized precipitates formed during ageingleads to higher strength and suitable ductility. Cryogenic treatment showed a negligible effect on both microstructure and tensile properties; however, it improved ductility. Overall, the combination of a high heating rate and cryogenic treatment led to the highest mechanical properties. SEM micrograph of the fracture surface of alloy demonstrated that in Cryogenic treatment+Artificial Ageing (CAA) condition, the surface had been fully covered by deep dimples in contrast to the Cryogenic treatment+Infrared Heating (CIR) and Water-Quench+ Infrared Heating (QIR) conditions which their dimples were shallow and also some facets were observed.