Solution Treatment Cycles Research Articles

Thermo-mechanical behaviour of microbial induced carbonate precipita-tion (MICP) sand for geothermal pavements

Introduction
 Ground source heat pump (GSHP) or shallow geothermal energy systems are gaining attention for helping combat global warming and the negative effects of urbanisation caused by human activities. In recent decades, energy geo-structure techniques have developed by using subsurface infrastructures to exchange heat with the ground. These techniques can provide space heating and cooling while still preserving the primary structural function. As a result, they have become a valuable part of geothermal energy systems. Researchers have developed an understanding of incorporating ground heat exchangers into foundation piles, retaining walls, and tunnel linings with modest additional cost [1-4]. Pavements are also structures in contact with the ground that have the potential to be used as energy geo-structures (i.e., geothermal pavements), yet, their use has not been extensively studied [5-8].
 Soil thermal conductivity is an important factor influencing the efficiency of geothermal pavements since heat transfer in soils occurs primarily by conduction [9]. Microbially induced calcium carbonate (CaCO3) precipitation (MICP) is an innovative technique for strengthening sandy soils by coating and binding soil grains with calcium carbonate crystals. The distribution and arrangement of the CaCO3 within the pore spaces play a crucial role in determining the resulting strength of the treated sand. In addition, these crystals can act as thermal bridges to enhance the soil's thermal conductivity [10]. Combining geothermal pavements with MICP sand is still nascent, and the limited number of studies that exist mainly focus on the associated thermal property changes [11]. However, since the principal function of pavements is transmitting (dynamic) loads to the subbase and the underlying soil, the thermal conductivity of the MICP-treated pavement may vary as a result of the applied mechanical loads (e.g., due to the partial or total loss of thermal bridges and/or particle rearrangement). This research thus investigates the changes in the thermal conductivity of MICP-treated sands as they are subjected to quasi-static triaxial compression. The experimental results collected can deepen our understanding of the thermo-mechanical behaviour of MICP-treated sands and provide practical insights for using MICP to reinforce the subbase or underlying soil of geothermal pavements.
 Methodology
 This research performed a series of quasi-static triaxial tests on MICP-treated Houston sand, fine-grained, high-purity silica sand. Sporosarcina pasteurii (strain designation DSM 33) was used for the MICP treatment of the soil specimen. To study the effect of CaCO3 content on the thermo-mechanical performance of MICP-treated sand, three cementation solution treatment cycles were applied, yielding theoretical CaCO3 contents of 0.6%, 1.6% and 2.7% by weight, respectively. Details on the MICP treatment of the samples can be found in [12]. Samples for triaxial testing were treated in cylindrical tubes of 50mm inner diameter and 100mm height. To investigate the influence of the CaCO3 content on the soil thermal conductivity, MICP-treated specimens were air-dried prior to triaxial testing Triaxial tests were subsequently conducted in dry conditions to isolate the effect of the MICP and avoid the influence of water content on soil thermal conductivity (λ). Furthermore, a new miniaturised transient sensor was embedded in the triaxial samples to monitor the λ changes during the sharing phase [12].
 Results
 An example of the evolution of the deviatoric stress with axial strain under 50kPa confining stress (σ3) in the triaxial cell is shown in Figure 1a. Compared to the untreated sand, MICP-treated samples lead to higher peak strength and stiffness. Importantly, λ changes during triaxial testing for different CaCO3 contents are summarised in Figure 1b. Results indicate that the increase in CaCO3 content can significantly improve λ, and that λ rapidly decreases post-peak strength due to dilation and CaCO3 bond breakage. Once the samples reach its ultimate state, λ remains unchanged.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of 18Ni-300 Maraging Steel Produced by Additive-Subtractive Hybrid Manufacturing.

The present work investigates the effectiveness of two heat treatment cycles-solution treatment + aging (STA) and direct aging (DA)-on optimizing the microstructure and enhancing the mechanical properties of 18Ni-300 maraging steel (300 MS) produced by additive-subtractive hybrid manufacturing (ASHM). The STA treatment led to a fully martensitic microstructure with minor remnants of the cellular substructures associated with the solidification conditions in ASHM. DA resulted in some reverted austenite and partial dissolution of the cellular morphologies into shorter fragments. Despite the contrasting microstructures, the tensile strength and the macro- and micro-hardness were comparable between STA and DA conditions. By contrast, the potential for improving the ductility was higher with the DA heat treatment. This is attributed to the higher reverted austenite content in the samples treated by DA, i.e., up to a maximum of 13.4% compared to less than 3.0% in the STA samples. For the DA sample with the highest reverted austenite content of 13.4%, the highest local and global fracture strain values of 30.1 and 5.9 ± 0.6% were measured, while the respective values were 23.4 and 4.4 ± 0.1% for the corresponding STA sample. This work suggests that DA of 300 MS produced by ASHM is sufficient to achieve comparable hardness and tensile strength to STA, whilst maintaining reasonable ductility. Avoiding the solution treatment cycle, with its appreciably higher temperatures, could benefit the dimensional stability and surface quality that are important for ASHM of 300 MS parts.

From as-cast to heat treated X-40 superalloy: Effect of cooling rate after partial solution treatment on microstructural evolutions and mechanical properties

Abstract This research study aims at investigating the influence of partial solution treatment and the subsequent cooling strategy on the microstructure and mechanical properties of an as-cast Co–Cr–W superalloy. Three different cooling scenarios were employed to explore the effect of cooling rate on the carbide reprecipitation potential from the \solid solution formed during the heat treatment. The partial dissolution/breakdown of the continuous network of the Cr-rich M7C3 carbides during the partial solution treatment cycle along with their transformation to M23C6 carbides and formation of secondary carbides during the cooling stage were taken into account to discuss the obtained hardness values, and high-temperature creep results. Regardless of the applied cooling rate, the secondary carbides were formed during the cooling stage, and the heat-treated samples showed almost the same microstructure and mechanical properties, which eliminates the need for the aging cycle. This can alleviate the concerns associated with the heat treatment of the large-size X-40 industrial components since the gradient in the cooling rate after the heat treatment cycle would not lead to non-uniform parts. From the hardness and creep viewpoints, all of the samples could meet the minimum requirements of the X-40 superalloy. Mechanical properties suggest that the applied partial solution treatment cycle can be considered as a suitable candidate for high-temperature applications of X-40 superalloy.

Microstructure-performance relationships in gas tungsten arc welded Hastelloy X nickel-based superalloy

This study aims at understanding the metallurgical and mechanical response of Hastelloy X nickel-based superalloy to the thermal cycle of gas tungsten arc welding (GTAW), post welding solution treatment (ST), and aging. In the as-weld condition, the weldment was characterized by the formation of the M6C carbide, sigma phase, and micro-solidification cracks in the interdendritic zones of the weld metal, and constitutional liquation of M6C carbide in the heat-affected zone. It is demonstrated that neither the weld metal nor the heat-affected zone could adversely affect mechanical properties of the welded joint in as-weld condition. However, to alleviate the microstructural heterogeneity, tensile residual stresses, and non-equilibrium microstructure developed due to the welding thermal cycle, the weldment was subjected to the solution treatment cycle (1177 °C/5 min) as post-weld heat treatment. The post-weld heat treatment enabled complete elimination of sigma phase. The tensile and creep properties of the weldment after the solution treatment cycle were compared to that of the as-weld condition. Despite the 8% decrease in the strength of the weldment after the ST cycle, the sample not only still met the minimum mechanical requirements but also benefited from a homogenized and nearly equilibrium microstructure. Microstructural observations of the weldment after the aging cycle (800 °C/8h), as well as its mechanical response, revealed that the formation of continuous Cr-rich carbides at the grain boundaries of the base metal deteriorates the room temperature ductility and creep life of the joints.

Heat Treatment of Low Cost Beta Titanium Alloy Produced by Investment Casting

Beta (β) Ti-alloys are promising class of Ti-alloys for high performance structural applications, thanks to their high strength to weight ratios and outstanding corrosion resistance. In the current work, Ti-4.5Fe-7Cr low cost beta Ti-alloy was produced by investment casting. DSC test was carried out to determine the transformation temperatures. The influence of solution treatment and aging temperatures on microstructure and mechanical properties were studied. Six heat treatment cycles were carried out, two solution treatment cycles in β- and α+β-range at 900 °C and 750 °C, respectively, and four precipitation hardening cycles, i.e. α+β-aging at 450 °C and 650 °C after solution treating at both 900 °C and 750 °C. Microstructure examination, XRD analysis and compression test had been conducted to investigate the influence of the different heat treatment regimes on the proposed alloy. Solution treatment at 900 °C results in coarser β-grain size than at 750 °C; hence lower strength. Strengthening by α-phase precipitation and grain refinement was achieved during aging. Aging at 650 °C results in higher strength than at 450 °C although coarser grain size, this is attributed to stronger effect of precipitation hardening of α-phase at 650 °C as a result of its higher amount than at 450 °C. The designed alloy shows high strength and excellent malleability, i.e. the alloy reveals superior plastic deformability, where specimens did not reveal any fracture; and the compression tests were interrupted at a compressive strain of 33%; therefore, this alloy may find applications as a healthcare, automotive or aerospace material.

The influence of heat treatment on microstructure and wear properties of friction stir welded AA6061-T6/Al2O3 nanocomposite joint at four different traveling speed

In the present study, 6mm thick AA6061-T6 alloy was friction stir welded (FSWed) at different traveling speeds while Al2O3 nano-particles were incorporated between adjoining plates. To investigate the influence of post weld heat treatment (PWHT) on microstructure and wear properties, solution treatment cycle was applied to FSWed specimens for 1h at 540°C then aged for 18h at 180°C. Distribution of Al2O3 nano-particles in the stir zone (SZ) was observed via scanning electron microscopes (SEM). After heat treatment, grain size changed imperceptibly. Surprisingly, all specimens exhibited inferior hardness to the as-received AA6061-T6 alloy which was attributed to precipitates solution during FSW. After heat treatment, hardness value of the specimen FSWed at 40mm/min improved. In the meantime, the uniform dispersion of Al2O3 particles in this specimen resulted in refinement and homogenous distribution of strengthening precipitates. Thanks to Al2O3 nano-particles, wear loss reduced remarkably (62%). Additionally, after heat treatment, the specimen FSWed at 40mm/min showed improved wear behavior.

Effects of Solution Heat Treatment on Microstructure and Mechanical Properties of the Mg-4.5Zn-4.5Sn-2Al-0.6Sr Alloy

Microstructure and mechanical properties of the Mg-4.5Zn-4.5Sn-2Al-0.6Sr alloy are investigated both in the as-cast condition and after the different three-step solution heat treatments (a solution heat treatment of 310 °C × 4 h + 340 °C × 28 h followed by a high-temperature solution treatment) to explore the optimal solution treatment cycle. The as-cast alloy contains a microstructure consisting of the α-Mg matrix, Mg2Sn, Mg51Zn20, Mg32(Al, Zn)49, and MgSnSr phases. After the solution heat treatment, all the Mg51Zn20, the Mg32(Al, Zn)49 phases, and most of the Mg2Sn phase are dissolved into the matrix, only the MgSnSr phase and a minority of the Mg2Sn phase are remained in the granular form or the fine dot-like. The volume fraction of the residual second phases decreases from 5.61 to 1.84% with the increasing solution time from 0 to 4 h at 420 °C and it decreases from 2.9 to 0.4% with the increasing solution temperature from 420 to 480 °C for 2 h. The alloy that experiences the solution treatment of 310 °C × 4 h + 340 °C × 28 h + 460 °C × 2 h exhibits the highest strength and the best plasticity among all the solution-treated alloys. Therefore, the optimal solution treatment is 310 °C × 4 h + 340 °C × 28 h + 460 °C × 2 h. The residual second phases in the alloy that experiences the optimal solution treatment are confirmed to be the Mg2Sn phase and the MgSnSr phase which are related to their relatively high thermal stability. The ultimate tensile strength and the elongation to rupture of the as-solutionized alloy are 238 MPa and 12%, respectively, about 25 MPa and 2.4% higher than the counterparts of the as-cast alloy.

Improvement of the tensile properties of an Al–Si–Cu–Mg aluminum industrial alloy by using multi stage solution heat treatments

Novel multi-stage solution heat treatments were applied to an Al–Si–Cu–Mg industrial aluminum alloy to improve its tensile properties. Two and three-stage solution treatments were tested, varying the secondary dendritic arm spacing (SDAS) and the Sr content of the alloy. The temperature of the solution heat treatments refer to the onset temperature of incipient melting (Tim). In the two stage cycles proposed, the hold temperature of the first stage was always set above Tim, while the isothermal treatment of the second stage was performed at a temperature below Tim. This is an inverse sequence compared with the sequence used by other authors in which the solution heat treatments start below Tim and then increase above Tim. It was found that with the proposed solution treatments, it was possible to obtain similar or better alloy tensile properties with shorter treatments times than the properties obtained today in industry by conventional solution treatment. For example, in the case of an alloy of 26μm of average SDAS with 100ppm Sr, it was found that a two-stage solution treatment cycle (4h at 520°C followed by 1h at 490°C) produced better tensile properties than the conventional single stage solution cycle (5h at 485°C) applied in industry, increasing the UTS, YS and elongation percentage values by 15, 10 and 82%, respectively. Even a solution treatment cycle consisting of 1h at 530°C followed by 1h at 490°C produced better tensile properties of the alloy than those obtained by the conventional industrial solution treatment in approximately half the time. In general, larger values of SDAS and higher Sr additions (200ppm) in the alloy both resulted in a decrease in the tensile properties, primarily affecting the elongation to fracture.

Rapid Heat Treatment of Aluminum High-Pressure Diecastings

Recently, it has been demonstrated that common high-pressure diecasting (HPDC) alloys, such as those based on the Al-Si-Cu and Al-Si-Mg-(Cu) systems, may be successfully heat treated without causing surface blistering or dimensional instability. In some compositions, the capacity to exploit age hardening may allow the proof stress values to be doubled when compared to the as-cast condition. This heat treatment procedure involves the use of severely truncated solution treatment cycles conducted at lower than normal temperatures, followed by quenching and natural or artificial aging. The potential therefore exists to develop and evaluate secondary HPDC alloys designed specifically for rapid heat treatment, while still displaying high castability. This article reports results of an experimental program in which responses of various alloy compositions to age hardening have been investigated with the primary aim of further reducing the duration and cost of the heat treatment cycle while maintaining high tensile properties. Composition ranges have been established for which values of 0.2 pct proof stress exceeding 300 MPa (i.e., increases of ~100 pct above as-cast values) can be achieved using a procedure that involves a total time for solution treatment plus age hardening of only 30 minutes. This rapid aging behavior is shown to be related to precipitation of the complex Q′ phase, which forms primarily when Mg contents of the alloys are above ~0.2 wt pct.

Optimisation of the Solution Heat Treatment of Rheo-Processed Al-Cu-Mg-(Ag) Alloys A206 and A201

The traditional solution treatment cycles that are currently applied to rheo-processed A201 are mostly those that are used for conventional castings. These solution treatments are not necessarily the optimum solution treatments for rheo-processing. As a result, DSC analysis was done to optimize this heat treatment. The new solution treatment, which consists of higher temperatures and shorter times (515°C/5h, followed by 570°C/10h), resulted in slightly higher hardness values for both alloy A206 and A201.