A study on age hardening in cast Fe-Mn-Al-Si-C alloys

Phase transformations in a group of cast and solution treated (at 1100° C) Fe-Mn-Al-Si-C alloys during isothermal ageing have been investigated as a function of (a) ageing temperature (500 to 800° C), (b) aluminium (5 to 10%) and carbon (0.27 to 0.9%) contents for fixed manganese ( 30%) and silicon ( 1.5%) levels. Irrespective of the ageing temperature and carbon content, 5% Al alloys do not undergo appreciable ageing. Ageing tendency increases with increase in aluminium and carbon contents. Hardening is caused by the precipitation of Al(Fe, Mn)C _x phase inside the austenite grain and Mn₁₂Si₇Al₅ phase at the grain boundaries. The sequence of precipitation of the phases depends on the ageing temperature and composition. Cold working by rolling, after homogenization at 1100° C, accelerates ageing.     

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.     

Impact of pre-ageing on age hardening response of twin-belt cast AlMg1SiCu sheet

The potential of twin-belt cast (TBC) AlMg1SiCu sheet for structural automotive applications was investigated with a particular emphasis on the impact of pre-ageing on its age hardening response. The optimum T6 process for the TBC AlMg1SiCu sheet is identified to be a water-quench from the solution heat treatment at 540 °C and a subsequent ageing treatment at 180 °C. This process gives hardness values as high as 120 HV within several hours when ageing at 180 °C is performed shortly after the solution treatment. The age hardening capacity is impaired, however, when the sheet is stored at room temperature before the artificial ageing cycle. Pre-ageing at 100 and 140 °C is effective in improving the age hardening response of the naturally aged 6061 sheet. Pre-ageing suppresses natural ageing and clustering activities and gives lower T4 yet a much higher T6 hardness.     

Optimization of hardening of Al–Zr–Sc cast alloys

The effects of composition, cooling rate after the end of solidification, and annealing regime on the structure and hardening of binary and ternary alloys of the Al–Sc–Zr system are studied. The liquidus in Al–Sc–Zr alloys is experimentally assessed in order to facilitate the correct choice of casting temperatures. The precipitation during slow cooling after the end of solidification causes hardening in the as-cast state and decreases the hardening effect during annealing. It is shown that the full hardening ability of precipitates can be achieved only upon their homogeneous distribution in the matrix. The optimum total concentration of Sc and Zr in aluminium alloys should be about 0.3 wt% at the ratio Zr:Sc ≥ 2. That allows conventional casting temperatures and considerable hardening during annealing.     

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。     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Case hardening nickel alloys

Boriding, or boronizing, can be used to create a hard, tough surface ‘case’ on nickel alloys, extending the application of these materials into more severe environments. The case layer is a complex nickel boride layer.     

Laser-arc hardening of aluminium alloys

The characteristics of a joint laser-arc hardening of the aluminium alloys V124 and Al25 are investigated. Optimum parameters for the process are established. The laser-arc source gives a greater hardening effect in comparison with that of the arc source at treatment rates over 17 mm s. A microstructural analysis of the solidified melt zone showed that it represents α-phase dendrites surrounded by the eutectic Al-Si in which silicon has a fibre structure. With the aid of quantitative metallography, the parameters of the melt crystallization are determined for the case of fusion by the laser-arc source.     

Microstructure and mechanical properties of high power diode laser (HPDL) treated cast aluminium alloys

The appliance and development of modern technologies in the areas of surface engineering can be extended by laser surface treatment, especially using high power diode laser (HPDL) for remelting, feeding and/or alloying. The purpose of this work was to determine technological and technical conditions for tungsten carbide (WC) ceramic powder feeding into the surface layer of the laser treated Al–Si–Cu cast aluminium alloys with high power diode laser, as well as to investigate the microstructure and ceramic powder particle distribution in the surface layer. Special attention was devoted to monitoring of the layer morphology of the investigated material and on the particle occurred. Light and scanning electron microscopy as well as X‐Ray diffraction were used to characterize the microstructure of the remelted zone. A wide range of laser powers was choose and implicated by different process speed rates. Also one powder in form of tungsten carbide was used for feeding with the middle particle size of 80 µm. As the main findings there was found that, the obtained surface layer is without cracks and defects as well as has a comparably higher hardness value compared to the non remelted material. The hardness value increases according to the laser power used so that the highest power applied gives the highest hardness value in the remelted layer. Also the distribution of the tungsten carbide particles is good, but there are still possibilities for further modelling. The major purpose of this work is to study the effect of a high power diode laser melting on the cast Al–Si–Cu alloys structure to provide application possibilities for automotive and aviation industry.     

Analysis of fatigue crack initiation and S-N response of model cast aluminium piston alloys

Fatigue crack initiation and S-N fatigue behaviour of hipped model Al7Si-Sr and Al0.7Si piston alloys have been investigated after overaging at 260 °C for 100 h to provide a practical simulation of in-service conditions. The results show that hipping did not affect the S-N behaviour of Al7Si-Sr. This is attributed to the lack of significant change in porosity distribution in this alloy because of its low porosity levels even in the unhipped state. However, hipping profoundly improved the fatigue performance of alloy Al0.7Si due to the significant reduction in porosity. In this investigation, it was observed that porosity was rendered impotent as a fatigue crack initiator in both hipped alloys. Instead, fatigue cracks were observed to originate mainly from intermetallic particles (particularly the Al₉FeNi phase) in both alloys and sometimes from oxide particles in Al0.7Si alloy. Fatigue cracking was also frequently observed at intermetallic clusters in hipped Al0.7Si. The observed scatter in fatigue life is discussed in terms of the size of fatigue crack initiating particles and the overall particle size distribution which follows a power law distribution function.     

Order hardening in nickel-molybdenum and nickel-tungsten alloys

Order hardening characteristics were studied in Ni-20% Mo, Ni-25% Mo and Ni-20% W. They were processed through quench and reheat cycles and the progress of order hardening was monitored through microhardness measurements. A correlation with the microstructure as observed with polarized light microscopy and transmission electron microscopy was established. Among the various factors, such as degree of order, domain size, coherency strains and dislocations, which can contribute to strengthening, domain size seems to play a major role.     

Age hardening and precipitation in a cast magnesium-rare-earth alloy

The precipitation sequence responsible for the age-hardening behaviour of a cast Mg-1.3 wt% rare-earth alloy has been investigated by analytical electron microscopy. Very fine intermediate precipitates formed at an early stage of ageing. Plate-shaped Mg₃MM precipitates (MM = misch metal) and hexagonal prism-shaped Mg₁₂MM precipitates were primarily responsible for age hardening. Precipitate morphologies, crystal structures and crystallographic orientation relationship were determined for the various types of precipitates that formed during ageing at different temperatures.     

Structural refinement of cast magnesium alloys

Abstract

The structure of cast magnesium alloys (grain size and precipitate morphology and size) affects the properties of the products and the scope for use of the alloys. The structure can be controlled by minor additions of inoculants, which are largely determined on the basis of the composition of the alloy concerned. The present paper reviews the scientific background of structural refinement by inoculation and its application to Mg–Zn, Mg–Al, and Mg–Al–Si alloys.     

Fracture toughness of cast aluminium alloys

An investigation was carried out to evaluate the fracture toughness of cast aluminium alloys of different microstructural complexity, brought about by alloy constitution and cooling rate of castings. In all cases the three-point bend specimens, which had a thickness of 15 mm, did not provide valid plane — strain stress intensity factor values. The fracture susceptibility at a given stress level reckoned in terms of the conditional plane strain stress intensity factor (K _Q) was found to be lowest in aluminium-4.5% copper alloy castings and the susceptibility increased with increase in microstructural complexity. Casting cooling rate in these castings is likely to affect the damage potential of a given defect at yield stress to a greater extent than the fracture susceptibility at a given stress.     

Accelerated age hardening response by in-situ ultrasonic aging of a WE43 alloy

Custom built in-situ ultrasonic aging equipment was used to investigate the precipitation hardening response in a commercial WE43 alloy at 210°C. Three power levels (100, 300, 400?W) with time intervals of 15, 30, and 60?s between sonication were used to measure the change in hardness. Aging response was 24 times faster, decreasing the time to achieve peak hardness from 48?hrs during conventional aging to 2?hrs under the influence of ultrasonic waves. While higher power increased the peak hardness, shortening the interval time from 30 to 15?s did not have any influence. Transmission electron micrographs revealed well-developed β₁ variants in as little as 15 minutes for ultrasonically aged samples, which was the reason for enhanced age hardening response.     

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.     

An investigation of the age hardening behavior of PM 2024Al-Fe-Ni alloys and the effect of consolidation conditions

The aging ability of two PM alloys based on 2024Al with Fe and Ni addition has been investigated by means of EDX, XRD, DSC, TEM and Vickers hardness analysis, and compared with that of the base alloy PM 2024 aluminum. Effect of consolidation temperature and powder size on the aging behavior of the 3F5N alloy was also studied. The results showed that the 3F0N alloy, PM 2024Al with 3 mass% Fe single addition, exhibited poor aging ability compared to 2024 alloy. In this alloy, the amount of solid solution Cu was found to decrease by forming Al₇Cu₂Fe compound during solidification, resulting in a lower amount of Cu dissolved into the α-Al matrix of the extrusions during the solution treatment. Whereas the 3F5N alloy, PM 2024Al with a 3 mass% Fe and 5 mass% Ni combined addition, showed almost the same age hardenability compared to PM 2024 alloy. Due to the addition of Ni, the amount of insoluble compound Al₇Cu₂Fe was decreased by the formation of Al₉FeNi phase in the 3F5N alloy. Thus, more Cu could be dissolved into the matrix during the solution treatment. A quantity of GPB zone could be formed in the 3F5N alloy during the aging resulting in higher age hardenability than the 3F0N alloy. The extrusion temperature and powder size were found to affect the aging hardenability of the 3F5N extrusions. Although higher age hardenability could be obtained in the 3F5N specimen extruded from powders with the relatively larger diameter, it was found that with decreasing extrusion temperature the higher aging ability could also be obtained in the 3F5N alloy extruded from finer powders. This revised version was published online in September 2006 with corrections to the Cover Date.