Measured Hardness Values Research Articles

Assessing strength of ferrite and martensite in five dual phase and two martensitic steels via high throughput nanoindentation to elucidate origins of strength

This paper describes the main findings from an experimental investigation into overall and local strength of four dual-phase (DP) steels classified based on their tensile strength (TS) levels from DP590, to 780, to 980, to 1180, a martensitic steel (MS) 1700, and a high-strength low-alloy (HSLA) steel in rolled and additively manufactured (AM) conditions. The DP steels and the MS steel consist of ferrite and martensite, while the HSLA steel is solely martensitic. High throughput nanoindentation mapping is employed to measure the mechanical hardness of individual phases contributing to strength of the steels. A clustering methodology for correlating measured hardness and phase maps is conceived to infer hardness per phase. With increasing fractions of martensite, the hardness values of both ferrite and martensite are found to increase with more rapid increase in the hardness of ferrite than martensite. The phases increasingly dislocate with the fraction of martensite as rationalized by crystal plasticity modeling of strength of the steels. Initial slip resistances and a set of hardening parameters associated with the slip in ferrite and martensite are established to model the flow stress behavior of the steels. In modeling the co-dependent nature of crystallographic slip in ferrite and martensite, the initial dislocation densities are inferred and correlated with the measured hardness values. The hardness of martensite in the rolled HSLA steel is comparable to the hardness of martensite in the MS steel, while martensite in the AM HSLA steel is softer owing to the tempering occurring during the AM process.

Enhancing the Structural and Mechanical Properties of Ti-Zr Alloy through Boron Doping

This study aims to improve the structural strength of the commonly used Ti-15Zr alloy in dental applications by investigating the effects of low boron additions. Ti-15Zr alloys containing 1-4% boron have been produced by vacuum arc melting. The phase ratios in the microstructure of the produced alloys vary according to the boron content. With increasing boron content, the ratio of TiB compound in the phase structure increases. The hardness of Ti-Zr-B alloys exhibited a notable increase in correlation with rising boron content. Measured hardness values of 36.31, 39.50, 44.14, and 53.40 displayed a clear upward trend with higher boron percentages. The tensile strength of the Ti-Zr-B alloys exhibits a trend of initially increasing with boron content, reaching its highest value of 888 MPa at 1% boron. The yield strength follows a similar with tensile strengh, with an initial rise from 449 MPa at 0% boron to a peak of 562 MPa at 1% boron content. Beyond this point, the yield strength slightly decreases to 469 MPa at 2% boron but sharply drops to 186 MPa at 4% boron content. As boron content increases in the Ti-Zr-B alloys, the percentage elongation, indicating the material's plastic deformation capacity before fracture, consistently decreases from 17.03% at 0% boron to 0.70% at 4% boron content.

Low temperature synthesized high entropy carbide composites: A potential high temperature anti-wear material

Bulk Al2O3/(NbTaMoW)C composites with uniform distribution of Al2O3 were designed and fabricated based on thermite reaction. The optimized composites were subjected to mechanical/tribological performance testing and directly compared to (NbTaMoW)C, revealing a significantly enhanced mechanical properties and wear resistance at high temperature. Compared to (NbTaMoW)C, the flexural strength and fracture toughness of Al2O3/(NbTaMoW)C composites significantly enhanced. The measured hardness value is 8% higher than that of (NbTaMoW)C. For the first time, the tribological behavior of Al2O3/(NbTaMoW)C and (NbTaMoW)C was studied systematically at temperatures from 25 °C to 800 °C. Al2O3/(NbTaMoW)C and (NbTaMoW)C present excellent wear resistance below 600 °C, while the wear rate exhibited an obvious increasing at 600 °C–800 °C. The specific wear rate of Al2O3/(NbTaMoW)C is much lower than that of (NbTaMoW)C at 600–800 °C. Al2O3/(NbTaMoW)C exhibits remarkable anti-wear performance at high temperature because the presence of Al2O3 improves oxidation resistance and decreases oxidation wear.

Phase transitions of carbon nanotube bundles under non-proportional triaxial compressions

Lateral compressions of (n, n) and (n, 0) single-walled carbon nanotube (SWCNT) bundles are simulated by density functional theory. Feasible transition pathways are verified by the body-centered tetragonal C4 (bct-C4) phase from the (4, 4) bundle and the carbon-centered orthorhombic C8 (Cco-C8) phase from (6, 6) and (8, 8) bundles. Three new phases, the sp3-hybridized phases Cco-C16 from the (4, 0) bundle and Cco-C32 from the (8, 0) bundle, the hexagonal phase Hex-C24 composed of sp2- and sp3-hybridized carbons from the (6, 0) bundle, under non-proportional triaxial loading are predicted. Measured hardness values for Cco-C16/C32 exceed 40 GPa, indicating super-hardness. Electronic band structures of Cco-C16/C32 exhibit a 3.64 eV bandgap, while Hex-C24 exhibits metallic carbon properties. Our results provide the potential phase transition pathways of SWCNT bundles under non-proportional compression.

Determination of ef ective thickness of cemented steel layer

The problems of increasing the strength of high-loaded steel parts of transmissions of energy-saturated machines are considered. It is noted that, along with hardness, an important parameter of the hardened cemented steel layer is its effective thickness the distance from the surface of the sample (part) to the zone with hardness 50 HRC (529 HV0.5). It is shown that great infl uence on accuracy of its defi nition have unavoidable banding of metal in the area of hardness measurement and an instrumental error of hardness measurement. A method of increasing the accuracy of determining the effective thickness of the cemented layer, based on interpolation by polynomial of the second degree of distribution of experimentally measured hardness values HV0.5 in an extended range of changing distances from the surface of the article to the hardness measurement HV0.5 is offered. Error of defi nition of an effective thickness of the cemented layer is reduced at the expense of expansion of area of change of hardness HV0.5 in an analyzed zone with approximation of its monotonous change in this area and the subsequent solution of the received square equation. Approximating polynomial statistically correctly refl ects the character of hardness change HV0.5 of cemented steel in the analyzed area depending on the distance from the workpiece surface to the hardness measurement place. The effectiveness of the method was confi rmed when determining the effective thickness of the grouted layer of a sample of 18CrNiGt steel after carburizing and hardening. The obtained results will be useful in the development of optimal regimes of carburizing of high-loaded gearwheels of automobile and tractor transmissions.

Correlation of microstructure, hardness, and electrical conductivity of hypereutectic Al-Si/B4C composites manufactured by hot pressing technique and subjected to hot extrusion

ABSTRACT B4C (5, 10 and 15 wt %) particles were added to hypereutectic Alumix-231® metal powders to produce composite material. The mixed powders were blended in a triaxial mixer. The production of compact samples was carried out by a hot press process. A hot extrusion secondary treatment was applied to some of the produced samples. Density measurements, microstructure examinations, and measurements of hardness and electrical conductivity were conducted on the samples. Primary Si particles and Cu-, Mg- rich intermetallic phases were observed within the microstructure of the sample produced from pure Alumix-231®. B4C particles seemed to be concentrated at the grain boundaries in the hot press samples. Through the secondary process of extrusion, primary Si particles and B4C particles exhibited mechanical fiberization-like dispersion. The samples with a density of over 97% were produced. The hardness of the samples increased with increasing wt % B4C particle ratio. The measured hardness values of the extruded samples were higher than the hot press samples. The extruded samples also showed high electrical conductivity values.

Hardness Measurements and Interface Behavior of SiC-B4C-Si Multiple Phase Particulate Composites Made with Melt Infiltration and Additive Manufacturing

Reaction-bonded SiC-B4C-Si ceramic composites were binder jet 3D-printed and subsequently pressureless-melt-infiltrated with molten Si. The addition of B4C aided the Si infiltration to produce a highly dense composite. The microstructures and phases of these composites were examined. The measured hardness values of each constituent with Vickers and nanoindentation matched the bulk values, and the macro-hardness values with Knoop and spherical indentation represented the bulk, composite hardness values of all three phases together, which was close to a rule of mixtures value. For particulate-based composites, this is a significant finding. The interfacial bonds of SiC and Si were imaged using scanning transmission electron microscopy to view intimacy, whereas the crack propagation was examined with carefully placed indents. This work demonstrated that pressureless melt infiltration with a reactive particle provides a method to shape non-wetting reaction-bonded ceramic composites with limited shrinkage and high density and provides insights into the mechanical behavior with numerous indentation techniques.

Testing length-scale considerations in mechanical characterization of WC-Co hardmetal produced via binder jetting 3D printing

The extreme versatility of additive manufacturing (AM) as processing technology results in “AMed pieces” with intrinsic characteristics linked to the shaping route followed, which are also key for defining mechanical integrity. The latter requires validation by measuring the mechanical properties, at both macroscopic (global) and microscopic (local) levels; and thus, consideration of specific testing length-scale aspects. This work aims to study the correlation between microstructure and mechanical properties for a WC-12%wt.Co hardmetal grade produced via binder jetting 3D printing (BJT) and subsequent sintering. In doing so, macro- and micro- Vickers hardness as well as scratch tests, using different loads and indenter tips, are conducted. It is found that studied samples processed by means of BJT exhibit a microstructure consisting of a relatively wide carbide size distribution, including a significant volume fraction (higher than 15%) of carbides larger than 3 μm. This is a direct consequence of the relatively high sintering temperature needed for getting full dense specimens, when manufactured following this AM route. Meanwhile, mechanical properties are found to be isotropic, with hardness and scratch resistance values falling within ranges of those expected for hardmetals with similar binder content and mean carbide grain size. Very interesting, length-scale effects on testing are observed in terms of dispersion of measured hardness value as applied load decreases. These findings, together with similar ones linked to length-scale influence on scratch response, point out that effective selection of mechanical testing parameters become critical for studying and understanding phenomena such as elastic/plastic and deformation/fracture transitions in AMed hardmetals.

Preparation of fully dense boron carbide ceramics by Fused Filament Fabrication (FFF)

Boron carbide is the third hardest material known, with a high melting point (2450 °C) and poor sintering ability. Therefore, boron carbide is a challenging material for shaping by conventional processing routes and can still be considered as unsuitable for commercial production of ceramics parts by additive manufacturing technologies. This work reports the first successful preparation of boron carbide ceramics fabricated by fused filament fabrication from a newly developed composite filament containing 65 wt% of micron-sized boron carbide powder dispersed in a thermoplastic binder. A commercial FFF desktop printer with a 0.40 mm nozzle was used for manufacturing of complex-shaped green bodies. Almost fully dense boron carbide ceramics with printed parts sized up to 4 centimeters and relative density higher than 96% after sintering were prepared. The DTA/TG analysis of composite filament and heat microscopy technique were used to set the debinding temperature program with critical temperature at 140 °C, due to the thermal decomposition of the binder. Microstructure SEM images after sintering showed excellent material homogeneity, while micro-CT images showed very well retained experimental shapes of collimator-like printed grids. The x-ray diffraction proved the presence of boron carbide phase with the free carbon phase at the level of about 1 wt% without significant influence on the measured hardness value of 29.88 ± 1.27 GPa.

Solid solution strengthening of high-entropy alloys from first-principles study

• Solid solution strengthening (SSS) effects in a series of conventional alloys and high-entropy alloys are investigated. • Agreement between predicted and measured hardness is satisfactory for both alloys. • Size misfit parameters and shear modulus misfit parameters are derived from first-principles calculations. • The predicted host/alloy family-dependent fitting constants can be used to estimate the hardness of these SSS alloys. Solid solution strengthening (SSS) is one kind of strengthening mechanisms and plays an important role in alloy design, in particular for single-phase alloys including high-entropy alloys (HEAs). The classical Labusch–Nabarro model and its expansions are most widely applicable to treating SSS of solid solution alloys including both conventional alloys (CAs) and HEAs. In this study, the SSS effects in a series of Fe-based CAs and HEAs are investigated by using the classical Labusch–Nabarro model and its expansions. The size misfit and shear modulus misfit parameters are derived from first-principles calculations. Based on available experimental data in combination with empirical SSS model, we propose fitting constants ( i.e. , the ratio between experimental hardness and predicted SSS effect) for these two families of alloys. The predicted host/alloy family-dependent fitting constants can be used to estimate the hardness of these SSS alloys. General agreement between predicted and measured hardness values is satisfactory for both CAs and HEAs, implying that the proposed approach is reliable and successful.

Applying a Neural Network-Based Machine Learning to Laser-Welded Spark Plasma Sintered Steel: Predicting Vickers Micro-Hardness

This paper presents an artificial neural network (ANN) approach to the estimation of the Vickers hardness parameter at the weld zone of laser-welded sintered duplex stainless steel. The sintered welded stainless-steel hardness is primarily determined by the sintering conditions and laser welding processing parameters. In the current investigation, the process parameters for both the sintering and welding processes were trained by ANNs machine learning (ML) model using a TensorFlow framework for the microhardness predictions of laser-welded sintered duplex stainless steel (DSS 2507 grade). A neural network is trained using a thorough dataset. The mean absolute error (MAE), mean square error (MSE), root mean square error (RMSE), and R2 for the train and test data were calculated. The predicted values were in good agreement with the measured hardness values. Based on the results obtained, the ANN method can be effectively used to predict the mechanical properties of materials.

Defect mediated structural, optical, electrical and mechanical properties of mechano-synthesized PbTe nanostructure as a superior thermoelectric material: Correlation among electrical, mechanical and optical properties

This work reports on the formation of anti-site defects in mechanically alloyed PbTe samples and their anomalous effects on the optical and electrical properties of the sample. PbTe nanoparticles have been synthesised by mechanical alloying of the elemental Pb and Te powders mixed in stoichiometric ratio under an inert (Ar) atmosphere for different time durations. The structural and microstructural characterisations have been carried out by analysing X-ray diffraction (XRD) patterns, field-emission scanning electron microscope (FESEM), and high-resolution transmission electron microscope (HRTEM) images. The elemental composition has been revealed from the energy-dispersive X-Ray (EDX) spectrum. The phase pure cubic PbTe is formed within just 30 min of milling. The Rietveld analysis of XRD patterns, FESEM, and HRTEM images revealed the crystallite size reduction and the increase in r.m.s. lattice strain with the increase in milling durations. Inclusions of more and more anti-site defects of type [PbTe] with increasing milling time are reflected in the expansion of the cubic lattice parameter with milling durations. Fourier-transform infrared (FTIR) spectrum reveals that PbTe is a narrow bandgap semiconductor, and the bandgap shows a redshift with increased milling time and reduced crystallite size. This anomalous behaviour of the optical bandgap with reduced particle size is well explained by Urbach tailing due to increased defects in the more extended milled sample. The positions of the conduction and valence bands in the milled PbTe samples are determined for different milling times. The DC conductivity study also shows anomalous behaviour; electrical conductivity increases with particle size reduction in more extended milling time samples. It is resolved by considering that the carrier concentration increases with a longer milling time due to induced anti-site defects while milling. It results in bandgap reduction with milling durations and shows the predominant effect of electronic conductivity over ionic conductivity. The conduction mechanism is explained as per the Pertritz model, and the dependence of activation energies on particle size and temperature has been explained in detail. The Vickers hardness of the milled samples is determined, and its variation with grain size is explained with the Hall-Petch relation. The improved electrical conductivity with increased milling is also manifested in the measured hardness values.

Use of AFM topography images to determine microindentation hardness of cast tungsten carbide powders

Hardness is defined as the resistance of a material to localized plastic deformation. Owing to their non-destructive nature, static indentation hardness tests are widely used in industry. Hardness testing is particularly useful for the mechanical characterization of materials that cannot be tested otherwise, e.g. powdered materials. In this study, challenges related to Vickers microindentation hardness testing of hard brittle cast tungsten carbide (CTC) powders were extensively investigated. Test load was optimized to obtain sufficiently large crack-free indentations allowing for precise measurement of the diagonal lengths. The influence of the operator and imaging technique on the measured hardness value was evaluated. Topography of residual imprints was investigated using atomic force microscopy (AFM) and a systematic and operator bias-free method to locate the indentation vertexes was developed. Results suggested that measurement variability introduced by AFM scanning and post-processing was as low as 3.1% and 1.3% with respect to the mean hardness value, respectively. Since the variability due to the measuring system can be isolated, the homogeneity of powders can be reliably evaluated from the hardness measurements thus obtained.

The effect of the friction stir welding tool shape on tensile properties of welded Al 6061-T6 joints

This study employs various Friction Stir Welding (FSW) tools to investigate mechanical properties of Al 6061-T6 joints. Welding tools were cut to form one-sided conical shape pin with a groove to enable material flow in the stirred zone over welding passes. Weld joints were made through six different FSW tool shapes. One-sided conical shape pins consisted of different cone angles 0º (simple cylinder pin), 5º, 10º, 15º, 20º, and 25º. Tensile tests were then carried out to evaluate elastic modulus, ultimate tensile strength, and elongation of joint samples. Measured hardness values and grain sizes within weld regions were affected by the welding tool shape. Tensile properties of aluminum joint samples were improved as cone angle increased from 0º to 20º through different welding tools. This was attributed to excess of plastic flow evolution over the weld zones during welding operation. Beyond cone angle of 20º, tensile strength and elongation of the joint sample dropped noticeably. Drop in tensile properties of joint sample welded with cone angle of 25º was attributed to small contact surface of conical tool with welded slabs resulting in weak connection on the opposite side of the weld joint, while lower cone angles of pins showed no such technical challenges.

In situ resource utilization of structural material from planetary regolith

Maximizing in situ resource utilization (ISRU) will be a necessary precept for all space faring nations wishing to establish a sustainable habitat on the Moon and Mars. As off-world infrastructure development advances and longer sustainable habitats are produced, what determines ISRU will also need to evolve. In this paper we explore a period in lunar colonization following the NASA Artemis program, where the available energy for manufacturing is still limited, where lunar chemical processing plants have not yet been constructed, and where the mass/weight of material transport from Earth is still confined to super heavy rockets. We present a multilayer construct, called a Regishell, that utilizes surface regolith mixed with a binder material. The Regishell is a robotic-build system that lands intact and is utilized multiple times. The focus of this paper is not the robot system but the binder materials that could be taken from the Artemis landers or produced in situ that when mixed with the regolith form a hardened structure under lunar solar radiation. The hardened material when integrated into the Regishell forms a protective structural support for a human habitat. We present examples of fused material along with measured hardness values for a polymer and a geopolymer binder. Moreover, we have conducted Monte Carlo simulations using the Regishell construct with the regolith mixture that includes a water layer to prove viability as a radiation shield. We find that the lunar regolith layer alone is adequate to substantially reduce astronaut space radiation dose due to solar particle events and galactic cosmic rays for a 14-day lunar surface mission.

Analysis of heat treatment parameters on the properties of selected tool steels M390 and M398 produced with powder metallurgy

This paper deals with the comparison of two tool steels M390 and M398 produced by powder metallurgy of the MICROCLEAN type. The primary use of steel is in the plastics industry to produce screws for injection moulding machines. The main purpose of the paper is to analyze the hardness after different stages of heat treatment. The first part of the article evaluates the hardness of steels after hardening, which was performed using a hardening dilatometer DIL805A. A total of 8 different controlled cooling measurements were performed, for both types of powdered tool steels, from a maximum rate of 100 °C/s to a minimum of 0.01 ° C/s. Based on the results of the first set of samples from the hardening dilatometer, the optimal cooling time is evaluated to achieve the highest hardness of both materials. This rate was used to heat treat a second set of samples that had undergone a quenching process from which a selected group was quenched to -78 °C. Freezing was performed to reduce the amount of residual austenite. Before measuring the hardness, the samples were tempered at temperatures of 200, 400, 600 °C for 2x2h. The measured hardness values were then compared with data from the manufacturer BÖHLER.

Hybrid composites of Al5083- graphene - fly ash by friction stir processing: machining studies

Hybrid composites are the modern engineered materials which exhibit presence of two or more than two reinforcements in a matrix. In the current work, Al5083 - Graphene and fly ash were dispersed in Al5083 alloy to develop hybrid composites by friction stir processing (FSP). Microstrucutral studies revealed band structures in the stir zone. Increased hardness was measured for the composites due to the grain refinement resulted from FSP and also due to the presence of reinforcements (Graphene and fly ash). The variations within the measured hardness values were observed as higher for the composites due to the variations in the concentration of the Graphene and fly ash in the stir zone. Lower cutting forces were recorded for the composites during drilling experiments conducted on the base materials and the composites. Here, the role of graphene as solid lubricant can be noticed as an important factor. The overall results demonstrate that Al5083-Graphene-fly ash hybrid composites can be successfully developed by FSP which exhibit better mechanical properties and machining behaviour.

Decomposition mechanism of AlMgB14 during the spark plasma sintering

In this work, AlMgB14-based materials were obtained from an (Al–Mg)–B powder mixture by spark plasma sintering. The microstructure, phase composition, and properties of the obtained materials were investigated. The sample, which was sintered at a temperature of 1400 °C, has the highest average hardness of 29.5 GPa with a maximum measured hardness value of 34.55 GPa. The density of the obtained sample is 2.621 g/cm3 (theoretical density of pure AlMgB14 is 2.59 g/cm3). A mechanism for the decomposition of AlMgB14 at local overheating of the (Al–Mg)–B powder mixture during spark plasma sintering was proposed. In the first stage, AlMgB14 decomposes into AlB12, an intermetallic compound Al12Mg17 and Mg. In the second stage, the decomposition products react with oxygen to form spinel MgAl2O4 and aluminum borate Al4B2O9. Finally, the use of isothermal holding for 5 min at a temperature of 1470 °C leads to an increase in the MgAl2O4 spinel content, further reaction of Al2O3 and boron oxide B2O3 and, as a consequence, the formation of borate Al18B4O33.

Application of two-step diffusion couple technique in high-throughput screening of optimal composition and aging temperatures for alloys design: A demonstration in binary Ni-Al system

In this paper, four binary Ni-13.4 at.% Al/Ni-17.7 at.% Al diffusion couples were first prepared and subjected to homogenization at 1573 K for 10,800 s, from which a continuous concentration profile formed. The three diffusion couples were then cooled down for aging at respective temperatures, i.e., 1173, 1123, and 1073 K, for 14400 s. The effect of composition and aging temperature on the aging microstructure was studied in detail by means of different experimental techniques and statistical analysis. The volume fraction, grain size, and shape factor of ?? precipitates in the three diffusion couples were plotted as a function of alloy composition and annealing temperatures. Together with the previously proposed evaluation function in which the phase fraction, grain size, and shape factor of ?? precipitates were chosen as the evaluation indicators, the optimal alloy composition and aging temperature for binary Ni-Al alloys with the best mechanical properties were evaluated, and finally validated by the measured hardness values. The successful demonstration of alloy design in the present binary Ni-Al alloys indicated that the two-step diffusion couple, together with the evaluation function for mechanic properties, should be of generality for high-throughput screening of optimal alloy composition and heat treatment process in different alloys.

Role of plunge depth on the joint formation and mechanical behavior of Al6063‐AZ91 dissimilar lap joint produced by friction stir welding

AbstractDissimilar lap joint of Al6063 aluminium and AZ91 magnesium alloys was successfully produced by friction stir welding. Three different plunge depths (3.2 mm, 3.25 mm, and 3.3 mm) were adopted during welding. Similar Al6063–Al6063 lap joints were also produced along with the dissimilar Al6063–AZ91 joints for the purpose of comparing the joint formation. With the increased plunge depth, the width of the similar Al6063 ‐ Al6063 lap joint was increased. On the contrary, joint width was decreased for the dissimilar joint with increased plunge depths. The dissimilar joint was formed with a strong metallurgical bonding between the Al6063 and AZ91 alloys, which is attributed to the mechanical mixing of these alloys in the nugget zone. Additionally, the formation of intermetallics was also observed from the x‐ray diffraction analysis. The variations within the measured hardness values were higher at the joint interface due to the mixing of aluminium and magnesium alloys in the nugget zone. From the tensile shear tests, increased strength and decreased elongation were measured with the increased plunge depth. The results demonstrate the importance of the plunge depth on the lap joint formation between dissimilar Al6063–AZ91 alloys during friction stir welding.