Precipitate Microstructure Research Articles

Effects of Microalloying Tin and Combined Addition of Silver and Tin on the Formation of Precipitate Free Zones and Mechanical Properties in Al-Zn-Mg Alloys

The additions of microalloying tin and (silver and tin)-combination were performed to modify the precipitate microstructure in the vicinity of grain boundaries with precipitate free zones (PFZs) and mechanical properties in Al-Zn-Mg alloys. In the Sn-containing alloy, TEM observation showed that some precipitates were sparsely formed within the region of PFZs of the Al-Zn-Mg ternary alloy. The quantitative analysis of the chemical compositions in precipitates showed that tin mainly contributes to nucleation in the vicinity of grain boundaries through the suppression of the vacancy depletion. This is well explained by the favorable interaction of tin with vacancies. The (Ag+Sn)-containing alloy formed a very narrow PFZ width and corresponding tensile properties were remarkably improved, which shows that tin enables to reduce the amount of microalloyed silver in Al-Zn-Mg alloys. [doi:10.2320/matertrans.L-MZ201123]

Effects of microalloying tin and combined addition of silver and tin on the formation of precipitate free zones and mechanical properties in Al–Zn–Mg alloys

The additions of microalloying tin and (silver and tin)-combination were performed to modify the precipitate microstructure in the vicinity of grain boundaries with precipitate free zones (PFZs) and mechanical properties in Al–Zn–Mg alloys. In the Sn-containing alloy, TEM observation showed that some precipitates were sparsely formed within the region of PFZs of the Al–Zn–Mg ternary alloy. The quantitative analysis of the chemical compositions in precipitates showed that tin mainly contributes to nucleation in the vicinity of grain boundaries through the suppression of the vacancy depletion. This is well explained by the favorable interaction of tin with vacancies. The (Ag+Sn)-containing alloy formed a very narrow PFZ width and corresponding tensile properties were remarkably improved, which shows that tin enables to reduce the amount of microalloyed silver in Al–Zn–Mg alloys.

CaO-SiO2绿光玻璃陶瓷荧光体的沉积组织结构对发光性质的影响

Calcium silicate glass-ceramic phosphor with green emission was evaluated. The effect of crystals precipitation microstructure on spectral properties was investigated by changing the holding time at melting temperature. With the in crease of holding duration, the amount ratio of luminous β-Ca2SiO4∶ Eu^2+ crystalline phase to glassy phase tended to be decreased, leading to a red-shift of the emission band and exhibiting more transparent of the glass-ceramic phosphor. The precipitation of SiO2 crystals in priority was proposed. The broad excitation band of the green-emitting glass-ceramic phosphors covers the wavelength from 270 to 440 nm, showing the potential for the high-power phosphor-converted LED.

Radiation-induced segregation and phase stability in ferritic–martensitic alloy T 91

Radiation-induced segregation in ferritic–martensitic alloy T 91 was studied to understand the behavior of solutes as a function of dose and temperature. Irradiations were conducted using 2MeV protons to doses of 1, 3, 7 and 10dpa at 400°C. Radiation-induced segregation at prior austenite grain boundaries was measured, and various features of the irradiated microstructure were characterized, including grain boundary carbide coverage, the dislocation microstructure, radiation-induced precipitation and irradiation hardening. Results showed that Cr, Ni and Si segregate to prior austenite grain boundaries at low dose, but segregation ceases and redistribution occurs above 3dpa. Grain boundary carbide coverage mirrors radiation-induced segregation. Irradiation induces formation of Ni–Si–Mn and Cu-rich precipitates that account for the majority of irradiation hardening. Radiation-induced segregation behavior is likely linked to the evolution of the precipitate and dislocation microstructures.

Microstructure modifications induced by a laser surface treatment in an AA7449 aluminium alloy

This work investigates the modification of the precipitate microstructure induced by laser surface treatments in an AA7449 aluminium alloy in T7651 temper. Microhardness maps in the cross-section below the laser lines, as well as maps of the precipitate size and volume fraction obtained by Small-Angle X-ray Scattering, show that a significant precipitate dissolution and coarsening has been induced by the laser treatment. Integrated modelling is carried out to quantify this effect, including a thermal finite element model, a size class precipitation model and a precipitation hardening model. The precipitation model is calibrated using separate reversion experiments, and then coupled to the thermal and mechanical models, allowing a quantitative description of the modification of the microstructure.

Orowan strengthening at low temperatures in bcc materials studied by dislocation dynamics simulations

In this paper, the first investigation of the Orowan strengthening mechanism in the thermal plastic regime of body-centered cubic (bcc) materials is made with dislocation dynamics simulations. In bcc crystals, the mobility of dislocations strongly depends on temperature and dislocation line characters. Unlike the classical picture of the Orowan mechanism, simulations show that the difference in mobility between screw and non-screw characters is the key parameter. Simulations at different temperatures and with different precipitate microstructures illustrate the contribution of two mechanisms to Orowan strengthening: a thermally activated mechanism induced by the length dependency of the screw dislocation mobility and an athermal mechanism associated with dislocation line tension. The influence of the particle distribution is studied with a comparison between regular and random particle arrangements. In most cases, distribution effect can be accounted for by calculating effective particle spacing in the screw dislocation directions.

Mapping the microstructure of a friction-stir welded (FSW) Al-Li-Cu alloy

The friction-stir welding (FSW) process induces both heat and deformation which lead to inhomogeneous precipitation microstructures in structural hardening alloys. A map of this microstructure can be obtained by 2D scanning SAXS in alloys where (a) a single phase is precipitating and (b) the precipitates are roughly isotropic. In Al-Li-Cu alloys, these conditions are not fullfilled. Very anisotropic precipitates are forming (T1 and θ′) with aspect ratios in the range 10 to 100. The inhomogeneity in the texture of the material (due to the deformation and recrystallisation) is a strong obstacle to the interpretation of the SAXS signal. This paper is an attempt to apply systematic simple interpretation models to characterise the precipitation microstructure across the weld area. It shows that in certain condition, it is possible to apply simple Guinier-type plot to extract the morphologies (length and thickness) of the particles.

Enhanced SCC resistance of AA7005 welds with appropriate filler metal and post-welding heat treatment

Stress corrosion cracking (SCC) resistance for weldment of Al–Zn–Mg alloys is enhanced with appropriate filler metal and post-welding heat treatment while maintaining the weld strength integrity. Tungsten insert gas (TIG) arc welding is used to join the AA7005-T4 plates with three high-Mg fillers, ER5183, ER5556 and ER5356. Four heat treatments, NA (natural ageing), T6, T73, and RRA, were applied to the welded plates. Micro-hardness test, tensile test and stress corrosion test were conducted on the heat-treated specimens. Differential scanning calorimetry (DSC) analyses and transmission electron microscopy (TEM) observations were also performed on the specimens cut from the fusion zone. For the three high-Mg filler metals, only the ER5356 filler with low Mn content showed improvement in the tensile property after different post-welding heat treatments. Weld strength improvement were ideal for the T7 post-welding heat treatment. No improvements were observed for the ER5183 and ER5556 filler wires. The ultimate tensile strength of ER5183, ER5556 and ER5356 weld were found to fall within the, respective, ranges of 250–280, 270–300, and 280–360 MPa corresponding to the different heat treatments. Post-welding heat treatment improves the SCC resistance of welds made with ER5356 and specimens with T7 and RRA heat treatments. DSC analyses showed that only the specimen fabricated from the weld made with ER5356 has an endothermic dissolution reaction peaked at 141 °C. The TEM images from the fusion zone (FZ) of T7 and RRA welds with ER5356 filler wire showed similar micro-structure and distribution of precipitates. They both have relatively large and non-continuous precipitates at the grain boundary. The size of precipitates differed in the matrix. The combination of ER5356 filler metal and post-welding T7 treatment can offer the AA7005 weld with better SCC resistance and high tensile strength.

The evolution of microstructure in Cu–8.0Ni–1.8Si–0.15Mg alloy during aging

The microstructure of precipitates in Cu–8.0Ni–1.8Si–0.15Mg (wt.%) alloy with super-high strength and high electrical conductivity was investigated by transmission electron microscope and selected-area electronic diffraction. The sequence of phase transformation in the alloy with aging time is that: spinodal decomposition + ordering(β-Ni 3Si) → ordering(β-Ni 3Si) + δ-Ni 2Si → δ-Ni 2Si + β-Ni 3Si. And the relationship between the matrix and precipitates is that: ( 1 1 0 ) m / / ( 1 1 0 ) β / / ( 2 1 1 ¯ ) δ , [ 1 1 2 ¯ ] m / / [ 1 1 ¯ 2 ] β / / [ 3 2 4 ] δ . Spinodal decomposition, ordering (β-Ni 3Si) and precipitates (β-Ni 3Si + δ-Ni 2Si) are attributed to the strengthening during aging, whose multiple interactions results in the variation of micro-hardness. After solution treatment at 970 °C for 6 h, cool rolled by 50%, and aged at 450 °C for 180 min, the Cu–8.0Ni–1.8Si–0.15Mg alloy has an average tensile strength of 1005 MPa, 0.2% proof strength of 768 MPa, elongation of 5.6% and an average electrical conductivity of 31.5.0% IACS. The morphology of the fracture surface of the alloy contains shallow dimple and quasi-cleavage pattern.

Understanding the mechanical properties of 2198 Al-Li-Cu alloy in relation with the intra-granular and inter-granular precipitate microstructure

3rd generation Al-Li-Cu alloys such as AA2198 are known to present extremely high strength levels because of the precipitation state of the material. The final precipitation state is reached via a complex sequence that can involve notably solute clusters, GP zones, θ', T1 and δ' phases. In this work, we have studied the evolution of mechanical properties during heat treatment. Microhardness and tensile tests have shown a direct link with the evolution of the size of intra-granular T1 precipitates. The evolution of the fracture mode in relation with the precipitation state has been followed by the so-called short bar test that allows to load relatively thin plates in the short transverse direction, which is particularly sensitive to the inter-granular fracture mode. Along the heat treatment, from the naturally aged T351 state and along ageing at 155°C, we show a clear transition from a ductile transgranular fracture mode to a ductile inter-granular fracture mode, controlled by the precipitation of the T1 phase at the grain boundaries.

Evolution of precipitate microstructures during the retrogression and re-ageing heat treatment of an Al–Zn–Mg–Cu alloy

Using a combination of experimental techniques, including anomalous small-angle scattering and atom-probe tomography, the evolution of precipitate microstructures during the different steps of retrogression and re-ageing (RRA) heat treatments of an Al–Zn–Mg–Cu alloy has been systematically evaluated. Quantitative information on the morphology, scale and chemistry of the precipitates provide new insight into the mechanisms at work during this process. It is shown that both the final chemistry and precipitate size distribution are different in the final RRA temper compared to classical heat treatments, with the presence of small clusters nucleated during the re-ageing step, and an average precipitate composition richer in Cu, together with a matrix enrichment in Zn, related to the difference in diffusivity between the two solute atoms. The mechanisms of precipitate evolution during the reversion and re-ageing steps are discussed in light of the influence of the process parameters.

Hardening and microstructural reactions in high-temperature equal-channel angular pressed Mg–Nd–Gd–Zn–Zr alloy

The use of high-temperature (330 °C) equal-channel angular pressing (ECAP) is demonstrated to promote precipitation of a fine and uniform dispersion of the FCC β 1 phase in an Mg–Nd–Gd–Zn–Zr alloy. Significantly, this process induces a hardening reaction in the alloy, where isothermal ageing at this temperature leads only to softening. The evolution of microstructure is characterized using transmission electron microscopy and scanning electron microscopy. The nucleation and growth of precipitates during the high-temperature ECAP are discussed. This research highlights a new approach to engineer precipitate microstructures via the application of severe plastic deformation so as to extend the property space of high-temperature Mg alloys.

Multimodal Precipitation in the Superalloy IN738LC

IN738LC is a polycrystalline, nickel-base superalloy, which is used in aggressive environments at high temperatures. The required strength is provided by precipitate strengthening. Both unimodal and multimodal precipitate distributions are observed in IN738LC. After reaching a critical size, a unimodal precipitate microstructure transforms to a bimodal one. This transformation is controlled by the precipitate-matrix interface, which is under compression in IN738LC. As the unimodal precipitates grow, the strained interface, due to differential lattice parameter of the matrix and the precipitate phase, stops solute diffusion into the growing precipitates. Hence, the solute atoms, entrapped in the matrix, saturate the matrix and form new, fine precipitates. Dissolution of some large precipitates also supplies solute to supersaturate the matrix. On the other hand, a multimodal precipitate distribution tends to become unimodal at low aging temperatures and bimodal at high aging temperatures. Interestingly, the activation energy is calculated for the coarsening of large precipitates in multimodal distribution and is found to vary with aging time.

New approach to predict the long-term creep behaviour and evolution of precipitate back-stress of 9–12% chromium steels

Modern advanced 9–12 % Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. The mechanical properties of these steels are significantly influenced by the presence and stability of various precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service, which leads to a varying obstacle effect of these precipitates on dislocation movement. In this work, the experimentally observed creep rupture strength of an modified 9–12% Cr steel developed in the European COST Group is compared to the calculated maximal obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after long time service. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.

Effects of Microstructure on the Incipient Fatigue and Fretting Crack Processes in Al-Cu-Li Alloys

Abstract The influence of microstructure on the crack nucleation and growth under fatigue and fretting loadings was investigated on two different Al-Cu-Li alloys used for aerospace applications (2050-T8 and 2196-T8) and containing different hardening precipitates (mainly T1 precipitates for 2050-T8 and T1+δ′ for 2196-T8). Fatigue tests have been carried out on specimens with a central hole to take into account the stress concentration present in riveted plates. The number of cycles for crack initiation in fatigue is found to be smaller in the plate which contains larger intermetallic particles. Concerning the fretting tests, cracking seems to be essentially controlled by contact conditions and no influence of precipitation microstructure was observed for the experimental conditions investigated.

The relationship between the microstructure and the magnetic properties of nano-scale magnetic particles in a Cu–Fe–Co ternary alloy

Abstract The relationship between the microstructures and the properties of nano-scale Fe–Co magnetic particles formed in a copper alloy was investigated. Several structural aspects and physical properties of Fe–Co precipitates were investigated simultaneously using transmission electron microscopy, high resolution transmission electron microscopy, and superconducting quantum interference device magnetometry. The microstructure and magnetic properties of precipitates comprising both Fe and Co atoms evolved with isothermal annealing at 873 K. We found that the evolution of small coherent precipitates obeys the Ostwald ripening law; however, the coarsening rate of the particles increased after coherent particles transformed into incoherent precipitates. The coercive force varied, correlating with the evolution of precipitates.

Precipitate Behavior and Microstructure Characterization of An Al-Zn-Mg-Cu Alloy Prepared by Spray Forming Process

A high Zn content Al-Zn-Mg-Cu alloy was prepared by spray forming process and the precipitate behavior and microstructure of the extruded alloy were also investigated. The precipitate sequence of the spray-formed alloy could be described as “α-solid solution → GPI zone → GPII zone (also called Metastable ′ )→ Stable  (MgZn2)” during artificial ageing treatment. In the early stage of artificial ageing treatment, the GPI zone was the main strengthening phase and kept coherent relationship with the matrix. With the increasing of ageing time, ′ phase dominate strengthening phase and kept semi-coherent relationship with the matrix. With the further increasing of ageing time,  phase took the place of ′ phase, and dominated the strengthening phase in the alloy. The grain size of the spray deposit is finer than that of cast alloys. The ultimate tensile strength of the alloy is over 810MPa in peak ageing condition.

Effect of PFZ and Grain Boundary Precipitate on Mechanical Properties and Fracture Morphologies in Al-Zn-Mg(Ag) Alloys

The effect of PFZ and grain boundary precipitates formed in aging processes on the macroscopic mechanical properties in Al-Zn-Mg(-Ag) alloys were evaluated using TEM, SEM, tensile test and nanoindentation. Decreases in width of PFZ and smaller size of grain boundary precipitates aging at lower temperatures and/or the addition of Ag processes improved the tensile properties, and the presence of PFZ was found to be harmful to the fracture. Nanoindentation hardness results clarified that the hardness within PFZ is smaller than that in grain interiors, indicating that, in the alloy with large width of PFZ, preferential deformation occurs within PFZ in the initial stage of deformation, and this causes lower elongation regardless of the same level of proof stress and the same sizes of grain boundary precipitates. From the quantitative correlation between precipitate microstructures in the vicinity of grain boundaries, mechanical properties and fracture morphologies, the deformation process of the alloys is considered to divided by three types; i.e. in case of the alloys with the small width of PFZ and the small size of grain boundary precipitates, in case of the alloys with the large width of PFZ and the small size of grain boundary precipitates and in case of the alloys with the large width of PFZ and the large size of grain boundary precipitates.

The microstructure and properties of 17-4PH martensitic precipitation hardening stainless steel modified by plasma nitrocarburizing

17-4PH martensitic precipitation hardening stainless steel was plasma nitrocarburized at 430 °C and 460 °C for 8 h. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope, X-ray diffractometer, microhardness tests, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The results show that the microstructure of plasma nitrocarburized layer is characterized by a compound layer with no evident diffusion zone. The phases in the 430 °C treated layer are mainly of γ′-Fe 4N, nitrogen and carbon expanded martensite (α′ N), and some incipient CrN phases. When the temperature increases up to 460 °C, there is no evidence of α′ N phase. The processes of bulk precipitation hardening and surface treatment by plasma nitrocarburizing can be successfully combined in a single-step process on this steel. The hardness of modified layer can reach up to 1186HV, which is 3 times higher than that of untreated steel. The wear and corrosion resistance of the specimens can be apparently improved by plasma nitrocarburizing. The 460 °C/8 h treated specimen has the best wear and corrosion resistance in the present test conditions.

Effects of concentration of chloride anion on the morphology and microstructure of precipitates from lead nitrate solutions

Lead nitrate mixed with alkali in solution generates insoluble precipitates. The morphology and microstructure of the precipitates are dramatically influenced by the chloride anion concentration in the lead nitrate precursor solution. To intensively investigate the effect of the chloride anion concentration, we performed a series of controllable experiments with different concentrations of chloride anions (the concentration ratios of Pb2+ : Cl− = 1 : 0, 1 : 0.2, 1 : 0.5, 1 : 1, 1 : 2, and 1 : 3 in the solution) at different solution temperatures of 90 and 80 °C. The results reveal that, with the increasing concentration of chloride anions, the colour of the precipitates changes from yellow to red and finally to white; the morphology of the precipitates changes from plate-like to rod-like; the crystal structure of the precipitates changes from β-PbO to a mixture of β- and α-PbO (or α-PbO), and finally to lead hydroxide. These results suggest that adding chloride could lead to the formation of rod-like morphology and simultaneously could restrain the dehydration of Pb(OH)2.