Dilatometry Tests Research Articles

Austenite Reverse Transformation in a Q&P Route of Mn and Ni Added Steels

In this work, four low carbon steels with different contents of Mn and Ni were heat treated by quenching and partitioning (Q&P) cycles where high partitioning temperatures, in the range of 550 °C–650 °C, were applied. In order to elucidate the effect of applying these high partitioning temperatures with respect to more common Q&P cycles, the materials were also heat treated considering a partitioning temperature of 400 °C. The microstructure evolution during the Q&P cycles was studied by means of dilatometry tests. The microstructural characterization of the treated materials revealed that austenite retention strongly depended on the alloy content and partitioning conditions. It was shown that the occurrence of austenite reverse transformation (ART) in the partitioning stage in some of the alloys and conditions was a very effective mechanism to increase the austenite content in the final microstructure. However, the enhancement of tensile properties achieved by the application of high partitioning temperature cycles was not significant.

Nanostructured Carbide-Free Bainite Formation in Low Carbon Steel

It is an important challenge to reduce the carbon content in nanostructured bainitic steels for commercialization purposes while still being able to gain the desired microstructural characteristics in nanoscale and not to deteriorate the strength–ductility combinations. That is the point at which an appropriate heat treatment procedure design would be an important parameter. This article aims to investigate how to obtain nanostructured bainite in steel with 0.26 wt% carbon content by applying multi-step austempering procedures. One-, two- and three-step austempering processes have been implemented, and proper heat treatment temperatures and approaches were selected based on dilatometry tests. Results indicated that it has become possible to achieve bainitic ferrites and austenite films with overall thicknesses of 164, 145 and 132 nm and 134, 105 and 90 nm at the end of one-, two- and three-step austempering heat treatments, respectively. Meanwhile, microstructural characteristics resulted in enhanced mechanical properties with ultimate tensile strength (UTS) of 1435, 1455 and 1428 MPa in combination with elongation levels of 15.4, 13.6 and 11.4% after implementing those heat treatments. Finally, it has been shown that applying the multi-step austempering heat treatments resulted in enhanced yield strength and impact toughness values due to the microstructural characteristics and proper heat treatment procedure design.

Modeling of Temperature Gradients During Short Time Dilatometry Testing

Dilatometry test systems are commonly used for characterizing the transformation behavior of steels. For rapid heating and quenching, small specimens are commonly utilized in combination with induction heating and gas quenching. In these systems, the steel test article is assumed to have a uniform temperature throughout the sample. This is a good assumption for slow heating and cooling rates; however, for simulating induction hardening cycles, this assumption may not be accurate. Using computer models, it is possible to predict the temperature dynamics of the sample, both radially and axially, during the thermal processing cycle (heating and cooling). O1 tool steel specimens 4 mm diameter by 10 mm long were utilized for heating and quenching tests on a DIL805L dilatometer to characterize and model heating and cooling temperature gradients. Specimens instrumented with multiple thermocouples were induction heated and helium gas quenched. The test data and geometry were evaluated with 1-D and 2-D induction heating models along with 3-D CFD cooling models to characterize transient temperature gradients. The goal of the modeling is to better characterize temperature corrections required when rapid heating and cooling processes are used to determine transformation temperatures in induction hardenable steels. There were limited temperature gradients during heating at 50 °C/s, but large temperature variations were developed during quenching with helium. An investigation was undertaken to evaluate alternate cooling system designs to provide improved temperature uniformity during the cooling process.

Thermoanalytical and dilatometric studies of the Al2O3\u2013Cu\u2013Mo hybrid composite

The present research is focused on the characterization of the composites from Al2O3–Cu–Mo system. The composites were prepared by slip casting method and subsequent sintering of green bodies in a reduced atmosphere. Two series of samples with different volume content of metallic powders were produced in the work: 10 vol% and 15 vol% of metal content with respect to the total solid-phase content. The sintering process of the composites was analyzed in detail. The linear shrinkage of the composites was measured. The shrinkage curves were obtained by dilatometry test in a heating mode. The composites were characterized by DTA/TG analyses and thermal conductivity. The composites were characterized by XRD, SEM, and EDX. The hardness was measured by Vickers hardness tester. Moreover, the Brazilian test was performed to determine the tensile mechanical properties of the composites. Fractography investigation was carried out as well. Dilatometric tests showed that the increase of the metallic phase volume in the ceramic matrix causes a decrease in the starting temperature of densification. Simultaneously, dilatometric experiments indicated that an increasing amount of metallic particles into the ceramic matrix increases the temperature of maximum densification and decreases the total shrinkage of the composites during sintering. The DTA/TG showed the characteristics of the dispersant decomposition to the atmosphere during thermal treatment and increase of mass connected with the oxidation of metals. Fractography results revealed good adhesion between Al2O3 matrix and the metallic phase. The observation allowed to conclude that the Al2O3 matrix surface is characterized by the brittle fracture mechanism.

Enhancing the crystallization phenomena and strength of porcelain stoneware: the role of CaO

Limestone was used to modify the fluxing action of two potash feldspars (a pure potash feldspar and a soda-potash feldspar) labeled P and C, respectively, in the formulation of porcelain stoneware based on Cameroonian raw materials. The effect of limestone addition (0–10 mass%) was investigated in the range of sintering temperature between 1125 and 1300 °C. Characterization of sintered samples including thermal behavior (DTA, TG and dilatometry test), phase evolution, densification parameters, flexural strength, morphology as well as pores structure was investigated in details. The maximum flexural strength (138 MPa) was obtained at 1175 °C with P series (7 mass% addition of limestone) and at 1200 °C (122 MPa) for C series. The maximum density (≈ 3.1 g cm−3) and lower water absorption (≈ 0%) were obtained at 1200 °C. Mullite and anorthite were identified as main crystalline phases. Starting from 1175 °C and mostly at 1200 °C, the two series compositions (with 7 mass% addition) presented a self-glazing phenomenon which gave them significant brightness and high aesthetic quality; these properties were accomplished only at 1300 °C for reference samples with no lime addition (P0 and C0). A proper addition of limestone (3–7 mass%) with potash feldspar significantly reduced the sintering temperature (~ 150 °C) and permits the production of high strength (122–138 MPa), low energy and sustainable porcelain stoneware.

Experimental study and modelling of the phase transformation of Zircaloy-4 alloy under high thermal transients

Dilatometry tests were performed to study the phase transformation kinetic of nuclear fuel claddings made of Zircaloy-4 upon fast heating rates (up to 2000 °C/s). These tests highlighted that from equilibrium to 500 °C/s the temperature at which the transformation starts shifts towards higher temperatures with increasing heating rates. Above 500 °C/s, no impact of the heating rate was observed. The temperature at which the transformation ends remains close to 960 °C with no clear dependence on heating rate. Metallurgical analyses were carried out and were in agreement with the results obtained by dilatometry. A phase transition model was identified from equilibrium to 2000 °C/s.

Development of laser welding of high strength aluminium alloy 2024-T4 with controlled thermal cycle

An innovative process design, to avoid thermal degradation during autogenous fusion welding of high strength AA 2024-T4 alloy, based on laser beam welding, is being developed. A series of instrumented laser welds in 2 mm thick AA 2024-T4 alloys were made with different processing conditions resulting in different thermal profiles and cooling rates. The welds were examined under SEM, TEM and LOM, and subjected to micro-hardness examination. This allowed us to understand the influence of cooling rate, peak temperature, and thermal cycle on the growth of precipitates, and related degradation in the weld and heat affected area, evident as softening. Although laser beam welding allows significant reduction of heat input, and higher cooling rates, as compared to other high heat input welding processes, this was found insufficient to completely supress coarsening of precipitate in HAZ. To understand the required range of thermal cycles, additional dilatometry tests were carried out using the same base material to understand the time-temperature relationship of precipitate formation. The results were used to design a novel laser welding process with enhanced cooling, such as with copper backing bar and cryogenic cooling.

Modeling Solid-State Phase Transformations of 13Cr-4Ni Steels in Welding Heat-Affected Zone

Fatigue damage is commonly encountered by operators of Francis type hydraulic turbine runners made of 13Cr-4Ni soft martensitic stainless steel. These large and complex welded casting assemblies are subjected to fatigue crack initiation and growth in the vicinity of their welded regions. It is well known that fatigue behavior is influenced by residual stresses and the microstructure. By including solid-state phase transformation models in welding simulations, phase distribution can be evaluated along with their respective volumetric change and their effect on residual stresses. Thus, it enables the assessment of welding process on fatigue crack behavior by numerical methods. This paper focuses on modeling solid-state phase transformations of 13Cr-4Ni soft martensitic stainless steel, used for manufacturing hydraulic turbine runners, occurring upon welding. It proposes to determine the material parameters of the models for both the austenitic and the martensitic transformation by nonisothermal dilatometry tests. The experiments are conducted in a quenching dilatometer with applied thermal conditions as experienced in the heat-affected zone of homogeneous welds. The activation energy and the kinetic parameters of the austenitic transformation from fully martensitic state are measured from the experimental results. The martensitic transformation modeling from a fully austenitic domain is also presented.

Microstructure analysis of martensitic low alloy carbon steel samples subjected to deformation dilatometry

Low alloy martensitic steels are commonly used in structural and wear resistant applications due to their excellent mechanical properties and abrasion resistance. Martensite phase is generally achieved by rapid cooling, and prior deformation in the austenite region also affects the martensite transformation. It is important to understand the martensite transformation when there is deformation above Ae3. Deformation and quenching simulations have been performed using dilatometry on a low alloy carbon steel. The aim was to determine the influence of deformation above Ae3 (prior deformation) on, firstly, the austenite grain size and shape, and secondly, the martensitic microstructure and variant selection. In addition, the hardness of the martensitic structure due to prior deformation has been investigated. The experimental results obtained from electron backscatter diffraction and microhardness tests on the deformation dilatometry test samples were analysed. The orientation relationship Kurdjumov-Sachs has been used to analyse the martensitic variants. The results revealed a deeper understanding of prior austenite grain structure's effect on the martensitic transformation kinetics and its morphology. The martensite laths' misorientation interval 15–48° were used to visualise the prior austenite grain size. The martensitic lath structure is more refined due to increased prior deformation. Shorter martensite formation time promotes a single dominating packet within the prior austenite grain.

Study on Non-Isothermal Transformation of Ti-6Al-4V in Solution Heating Stage

In order to understand the non-isothermal transformation behavior of Ti-6Al-4V titanium alloy in the continuous heating stage of solution treatment, thermal dilatometry tests with heating rates of 0.1~0.8 °C/s were designed. The conversion between the expansion amount and the transformed volume fraction was realized by the lever principle, and the transformation characteristics of α + β → β were quantified based on the Kissinger-Akahira-Sunose (KAS) theory and the modified Johnson–Mehl–Avrami (JMA) model. The results show that the phase transformation kinetics curves present typical “S” patterns, and the element diffusion transformation controls the nucleation and growth of new grains during the transformation of α + β → β. The phase transformation interval gradually moves to high temperature regions with the increase of heating rates, and the phase transformation activation energy is 445.5 kJ·mol−1. The phase transformation process is divided into three stages according to the relationship between the Avrami exponent n and the transformed volume fraction fT. These three stages correspond to different stages of grain nucleation or growth.

Every midwife matters

Low alloy martensitic steels are commonly used in structural and wear resistant applications due to their excellent mechanical properties and abrasion resistance. Martensite phase is generally achieved by rapid cooling, and prior deformation in the austenite region also affects the martensite transformation. It is important to understand the martensite transformation when there is deformation above Ae3. Deformation and quenching simulations have been performed using dilatometry on a low alloy carbon steel. The aim was to determine the influence of deformation above Ae3 (prior deformation) on, firstly, the austenite grain size and shape, and secondly, the martensitic microstructure and variant selection. In addition, the hardness of the martensitic structure due to prior deformation has been investigated. The experimental results obtained from electron backscatter diffraction and microhardness tests on the deformation dilatometry test samples were analysed. The orientation relationship Kurdjumov-Sachs has been used to analyse the martensitic variants. The results revealed a deeper understanding of prior austenite grain structure's effect on the martensitic transformation kinetics and its morphology. The martensite laths' misorientation interval 15–48° were used to visualise the prior austenite grain size. The martensitic lath structure is more refined due to increased prior deformation. Shorter martensite formation time promotes a single dominating packet within the prior austenite grain.

Thermal expansion behavior of Al–B4C composites by powder metallurgy

In the present investigation, Al/B4C composite samples were prepared by hot press method. Thermal properties of the samples containing different reinforcement contents were evaluated. XRD characterization did not reveal the presence of any detrimental phase. Density and microhardness measurements confirmed proper compaction and high hardness of the samples, respectively. SEM observations showed no evidence of cracks at the matrix/reinforcement interface and it was seen that B4C particles had high wettability with Al matrix. Dilatometry tests demonstrated that by improving the reinforcement content, the coefficient of thermal expansion declined in a way that Al/25% B4C sample had the lowest thermal expansion coefficient (8 ppm/C), which makes it suitable for heat sink applications. Moreover, the experimental results were in good agreement with thermoelastic models in specific temperature ranges.

Correlation of Cooling Rate, Microstructure and Hardness of S34MnV Steel

A study of the hardenability, the microstructure and the phase transformations as a function of the cooling rate, of a marine crankshaft S34MnV steel has been performed using a Jominy end-quench test and dilatometry tests. The cooling rate has a distinct influence on the hardness and microstructure characteristics. As the cooling rate decreases, the hardness drops from HRC53 at the quenched end to HRC22 at the top end of the Jominy bar. The corresponding microstructures change from hard martensite to bainite, then soft pearlite and ferrite. The critical temperatures Ac1, 740 °C and Ac3, 830 °C were determined, and a continuous cooling transformation diagram was constructed from the dilatometric data. The important role of cooling rate on final hardness, phase transformation and microstructures are analyzed.

Reduced Thermal Expansion and Enhanced Redox Reversibility of La0.5Sr1.5Fe1.5Mo0.5O6−δAnode Material for Solid Oxide Fuel Cells

High performance anode materials with suitable thermal and chemical expansions are highly desirable for solid oxide fuel cells. In this work, we report a promising anode material La0.5Sr1.5Fe1.5Mo0.5O6-δ (LSFM) synthesized in nitrogen at 1050 °C. Its phase stability, mechanical behavior, redox stability, and electrochemical performance were studied. The electrical conductivity of LSFM reaches 23 S cm–1 in 5% H2–95% N2 at 800 °C with excellent reversibility over three redox cycles. After lanthanum doping, the coefficient of thermal expansion (CTE) is reduced from 17.12 × 10–6 K–1 (SF1.5M) to 15.01 × 10–6 K–1 (LSFM), and this value can be lowered further with a higher lanthanum content. Dilatometry testing at 800 °C shows that the chemical expansion behavior of LSFM is highly reversible during the oxidation–reduction cycling. These results indicate that the thermal and chemical expansion of the crystal lattice can be reduced by a stronger metal–oxygen (M–O) bond strength, leading to an improvement in redox reversibility. The polarization resistance of the LSFM symmetrical cell at 800 °C in humidified hydrogen is 0.16 Ω cm2, and the active region is ∼4.5 μm. The half-tear-drop-shaped impedance spectroscopy indicates an oxygen bulk diffusion and surface reaction colimited process. The maximum power density of the LSFM single cell reaches 1156 mW cm–2 at 800 °C within humidified H2. The new ceramic material LSFM is a promising anode for high performance solid oxide fuel cells.

Investigating the effect of SiC particles on the physical and thermal properties of Al6061/SiCp composite

In the present study, the effect of volume fraction of SiC reinforcement particles on the physical and thermal properties of Al matrix composite (6061) was investigated. In this regard, composite powders with different reinforcement contents were prepared using hot press molding. It was seen that with increasing the amount of the reinforcement, the density of the samples decreased, while the porosity increased. Microhardness tests indicated that there was a direct relationship between the hardness and the amount of the reinforcement. SEM images confirmed the proper distribution of the reinforcing particles in the matrix. Dilatometry tests exhibited the decline of coefficient of thermal expansion by improving the amount of reinforcement. XFA test was also used to determine the thermal conductivity of the samples and it was seen that with increasing SiC content in the composite samples, the thermal conductivity declined to 61.5 W/m.K due to enhancement of crystalline defects in the phase boundaries. Finally, the experimental results were compared to thermoelastic models.

The influence of temperature during water-quench rapid heat treatment on the microstructure, mechanical properties and biocompatibility of Ti[sbnd]6Al[sbnd]4V ELI alloy

This study investigates the influence of a rapid heat treatment followed by water-quenching on the mechanical properties of Ti6Al4V ELI alloy to improve its strength for use in implants. Prior to the experiment, a dilatometry test was performed to understand the progressive α-to β-phase transformation taking place during heating. The results were then used to carry out heat treatments. Microstructure was analysed using SEM, EBSD, EDX and XRD techniques. Vickers micro-hardness, tensile and high cycle rotating bending tests were used to analyse the influence of the $\\alpha'$-phase fraction on the strength of the studied alloy. Results show that this process can provide a Ti6Al4V ELI alloy with a better Yield Strength (YS)/uniform deformation (εu) ratio and improved high cycle fatigue strength than those observed in the current microstructure used in medical implants. Lastly, cytotoxicity tests were performed on two types of human cells, namely MG63 osteoblast-like cells and fibroblasts. The results reveal the non-toxicity of the heat-treated Ti6Al4V ELI alloy.

Polyimide capping layer on improving electrochemical stability of silicon thin-film for Li-ion batteries

Silicon (Si) is a promising anode material for LIBs due to its high theoretical gravimetric capacity of 3570 mAh g−1, which is about ten times higher than that of graphite. However, the large volume expansion of Si during lithiation gives rise to non-trivial mechanical challenges and constraints which hinder widespread commercial adoption. In particular, electrode cracking and delamination destroy the intrinsic electrical connectivity in the Si electrodes, leading to rapid capacity degradation upon cycling. Herein, we demonstrate a novel approach of capping Si thin-film electrodes with a high strength polyimide by electro-spraying to overcome the pulverization and mechanical issues during cycling. The polyimide layer provides mechanical support to the Si thin-film and maintains its capacity during cycling, with a high capacity of 2610 mAh g−1 at 100 mA g−1 and no capacity loss after 300 cycles at 3500 mA g−1. Furthermore, in situ dilatometry tests reveal that the polyimide capping layer enables reversible electrode thickness change during lithiation and de-lithiation, thus improving the cycling performance. This work provides a practical manufacturing strategy for commercializing silicon-based electrode films.

Effect of coiling conditions on the strengthening mechanisms of Nb microalloyed steels with high Ti addition levels

Steels alloyed with high Ti addition levels present an interesting combination of high strength and formability, principally due to the high levels of precipitation hardening that can be attained. However, their mechanical properties can be highly sensitive to variations in the processing route. In this work, dilatometry tests were performed to study the effect of coiling conditions on the microstructures and hardening mechanisms of a reference Nb microalloyed steel (0.03%Nb) and two high Ti-Nb steels (0.05%Ti-0.03%Nb, 0.1%Ti-0.03%Nb). Coiling temperatures from 550 °C to 675 °C and cooling rates of 0.01 °C/s and 0.03 °C/s were considered. A significant increase in hardness was observed for the high Ti-Nb steel samples. While the grain size and dislocation hardening were similar for all steels, much higher precipitation strengthening values in the range of 69–163 MPa and 100–307 MPa were calculated for the 0.05%Ti and 0.1%Ti steels, respectively. As a consequence, high yield strength values (over 700 MPa) were estimated for coiling temperatures greater than 625 °C for the Ti10Nb3 steel. However, it was also observed that the mechanical behavior of this steel greatly depended on coiling conditions: maximum mechanical strength was achieved at 625–650 °C, while it decreased significantly for temperatures between 550 °C and 600 °C and at 675 °C. The small size and density of the precipitates detected with TEM support the large precipitation hardening effect calculated in these conditions.

Study of Structural and Electric Properties of the PZT 52/48 Doped With Er+3

The scientific community has shown a growing interest in relating to the ferroelectric materials and it has made an effort to develop them. Among these several ferroelectric materials, we proposed to investigated Pb(ZrTi)O3 doped with distinct Er+3 concentrations. Our aim was to investigate how the erbium affects the PZT electrical properties. To determine the ideal sintering temperature of our ceramic samples, the dilatometry tests were performed. These results have showed that the erbium caused a change in sample densities (91.9% < ρ < 99.0%) as compared to undoped PZT sample. Furthermore, the X-ray diffraction results indicated that all samples are monophasic and peaks are indexed to tetragonal crystalline structure. According to these also results, there was an increment of the parameter a and a reduction of parameter c as compared to undoped PZT sample. On the other hand, the average grain size have decreased with the increment of erbium content. The electrical characterization of ours ceramics showed a peculiar ferrelectric-paraeletric phase transition around 390 oC. Finally, the ferroelectric hysteresis measurements for doped samples in site A have revealed that remnant polarization and coercive field values depend on content. Whereas, doped samples in site B, the corresponding values are similar.

Semi-solid Sintering of Ti6Al4V/CoCrMo Composites for Biomedical Applications

A composite of Ti6Al4V reinforced with CoCrMo particles is fabricated by conventional powder metallurgy and semi-solid sintering. Composites are fabricated by mixing 20% wt. of CoCrMo reinforcing particles into the Ti6Al4V matrix. Sintering is studied by dilatometry tests between 1050 and 1130 °C and compact characterization is performed by SEM and XRD. Microhardness is evaluated on the polished surface in order to obtain the mechanical properties. It is found that densification is achieved by the formation of a liquid phase due to a eutectic point formed by the Ti and Co at around 1130 °C. Composites with 93% of relative density are obtained. Their microstructure is composed by the Ti6Al4V matrix surrounded by a Ti2Co phase. Average microhardness of composites is higher than that obtained for monolithic samples of both alloys. It is assessed that the control of the liquid phase is the key to obtain highly dense composites that could be used as dental or orthopedic implants.