Hardness Increment Research Articles

Effect of boron carbide and Kaoline reinforcements on the microstructural and mechanical characteristics of aluminium hybrid metal matrix composite fabricated through powder metallurgy technique

ABSTRACT In the present work Al- 10% B4C- X% Kaoline (X = 0, 2, 4, 6, 8) hybrid metal matrix composite (HMMC) was fabricated through powder metallurgy technique. Physical and mechanical properties like density, Hardness, Porosity, Ultimate tensile strength (U.T.S) and Impact energy were studied for the fabricated composite samples. Results concluded that there was a 91.93% increment in hardness and 75% increment in U.T.S of the fabricated samples was observed for Al-10%B4C-4%Kaoline HMMC. However, incorporation of kaoline reinforcement beyond 4% leads to the formation of agglomerates in the composites reducing the U.T.S from 238 MPa to 186 MPa and hardness from 119 VHN to 101 VHN. The fabricated composite specimens were characterised using an X-ray Diffractometer (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The XRD patterns reveal the presence of Aluminium, B4C and kaoline particles. In addition to this, the presence of intermetallic phases was observed for Al-10%B4C-6% Kaoline and Al-10%B4C-8% kaoline HMMC. Uniform distribution of reinforcements and formation of agglomerations for 6% and 8% kaoline reinforcements was confirmed by SEM and their respective EDS mapping. Results showed that the Al-10 % B4C- 4% Kaoline has superior mechanical properties compared to monolithic aluminium

Design, phase equilibria, and coarsening kinetics of a new [formula omitted] precipitation-hardened multi-principal element alloy

Multi-principal element alloys (MPEAs), with dispersed nanometric precipitates in a ductile matrix, are attracting a considerable amount of literature’s attention. Understanding the coarsening kinetics and potential properties of such alloys is crucial for future developments in the field. In the present study, we report a new precipitation-hardened multi-principal element alloy, Cr29.7Co29.7Ni35.4Al4.0Ti1.2 (at%), developed with the aid of the CALPHAD method. The alloy has a tough Cr-Co-Ni FCC matrix and nanometric L12 precipitates. After standard heat treatments, the alloy was aged at 850 °C for different aging times and the coarsening kinetics was investigated. The evolution of mechanical properties as a function of aging time was evaluated through microhardness tests. An increment of 106 HV 0.5 was observed compared to the sample in the solution-treated condition. This hardness increment was higher than that of conventional superalloys with a similar volume fraction of precipitates. Experimental data indicate that particle growth during coarsening is governed by diffusion controlling mechanism. The coarsening rate constant was lower than that of traditional wrought superalloys aged at the same temperature, indicating better thermal stability of the current MPEA. Furthermore, experimental particle size distributions show a good correlation with the curve predicted by the Lifshitz–Slyozov–Wagner (LSW) theory. Deviations and the applicability of this theory for MPEAs are discussed.

Influence of friction stir processing and aging heat treatment on microstructure and mechanical properties of selective laser melted Mg-Gd-Zr alloy

The low ductility of Mg alloy fabricated by selective laser melting (SLM) technique usually restricts its application in several fields. Friction stir processing (FSP) is a promising post-treatment method to solve this problem. A detailed study of how FSP affect pore defects, molten pool boundary, grain morphology, aging precipitates and mechanical properties of SLMed Mg-10Gd-0.2Zr (G10K, wt.%) alloy was conducted. The results show that FSP can lead to porosity reduction from 0.779% to 0.015%, vanishment of molten pool boundary, columnar to equiaxed transition and grain refinement. Thus, a significant enhancement of room temperature tensile properties is reported with a remarkable increase of elongation (EL) from 2.2% to 7.5% without the sacrifice of strength after FSP (while yield strength (YS) and ultimate tensile strength (UTS) increased from 180 MPa, 228 MPa to 202 MPa, 272 MPa respectively). After peak aging heat treatment, the plate aspect ratio and area number density of β' aging precipitates in the FSP-T5 G10K alloy are higher than those of SLM-T5 G10K alloy, resulting in a stronger aging hardening response (hardness increment 28.3 HV versus 22.7 HV). As a result, the FSP-T5 G10K alloy exhibits YS of 285 MPa, UTS of 356 MPa and EL of 1.3%, while those of the SLM-T5 G10K alloy are only 243 MPa, 260 MPa and 0.3%. These findings demonstrate that FSP is very effective for modifying microstructure and enhancing mechanical properties of the SLMed alloys.

Microstructure evolution and aging hardening in a Cu-25Ni-25Mn alloy

The microstructure evolution and mechanical property of Cu-25Ni-25Mn alloy after solution treatment and aging treatment are investigated via TEM observation, XRD analysis and Vickers hardness test. The effect of the NiMn precipitates on hardness and its strengthening mechanism in the Cu-25Ni-25Mn alloy is quantitatively analyzed. The results show that in 450 °C aging process, Ni and Mn precipitate from the copper matrix and form nanoscale NiMn phase particle with a face-centered tetragonal (FCT) structure. The XRD analysis indicates that the NiMn precipitates have a lattice constant of a = b = 0.3693 ± 0.0004 nm, c = 0.3570 ± 0.0006 nm, which is fully coherent with the copper matrix. The precipitation of NiMn phase lead to a precipitation strengthening, which provides significant increase in hardness in the peak-aged Cu-25Ni-25Mn alloy. Compared to the solution treated sample, the hardness of peak-aged sample has been increased by 367 HV. The coherency strengthening and modulus strengthening are the dominant strengthening mechanisms. The hardness increment predicted by coherence strengthening and modulus strengthening mechanism has good consistency with the experiment results.

Microcracks Reduction in Laser Hardened Layers of Ductile Iron

A study of surface hardening of Ductile Iron (DI) with and without austempering heat treatment was developed. The chemical composition of the material contains alloying elements such as Cu and Ni, that allow to obtain a Ductile Iron Grade 120-90-02, based on ASTM A536, which was heat treated to be transformed to Austempered Ductile Iron (ADI). Specimens of 10 × 10 × 5 mm3 were obtained for application of surface hardening by Nd:YAG UR laser of 150 W maximum power. The parameters used were advance speed of 0.2 and 0.3 mm/s and power at 105, 120, 135 and 144 W; the departure microstructures were fully pearlitic in the samples without heat treatment, and ausferrite for austempered samples. Microstructural characterization of hardened samples was performed were analyzed and martensite and undissolved carbides were identified in the pearlitic samples, while in ausferrite samples it was found finer martensite without carbides. The depth of hardened surface to different conditions and their respective microhardness were measured. The results indicate that the surface hardening via laser is a suitable method for improving wear resistance by means of hardness increment in critical areas without compromising the core ductility of DI components, but the surface ductility is enhanced when the DI is austempered before the laser hardening, by the reduction of surface microcracks.

Microstructural stability and intermetallic embrittlement in high Al containing FeCrAl-ODS alloys

Abstract The effect of Al addition on the microstructural stability and intermetallic embrittlement in FeCrAl oxide dispersion strengthened (ODS) alloys was investigated in 1423 K solutionized and 748 K aged conditions by comparing their phase stability and room temperature mechanical properties. Microstructural characterization revealed that the microstructures of solutionized FeCrAl-ODS alloys containing 10 wt% Al. Contrary to this, the ordered B2 phase observed in alloys containing >20 wt% Cr and 16 wt% Al remained B2, even after prolonged ageing at 748 K. Ageing at 748 K up to 1000 h resulted in the precipitation of the ordered Fe3Al intermetallic but insignificant hardness increment in FeCrAl-ODS alloys as compared to the hardness increment due to α’ phase separation. The hardness values reached to their maxima after 100 h ageing at 748 K and thereafter either it attained a plateau or decreased slightly. Ageing also led to increase in strength and decrease in ductility due to intermetallic embrittlement; however, the loss of ductility was limited to 2–4% only.

Roller crushers in iron mining, how does the degradation of Hadfield steel components occur?

This work shows, for the first time, a systematic wear and damage analysis on a novelty roller crusher component used in the iron ore mining industry. Crusher suffers from complex abrasive-impact load during the comminuting the mineral ore in processing plants, which induce severe damage and defects that can result in a decreased performance or in a crusher breakage. The wear mechanisms and the extent of deformation have been studied by microhardness measurements, optical microscopy (OM), scanning electron microscopy (SEM) including electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). It was found that the abrasive-impact contact causes a significant hardness increment of over 700HV, around three times than initial state of the base material, and the strain hardening extends up to a depth in the extremely deformed region above 18 mm from the worn surface. SEM results of worn surfaces showed the impact and abrasion damages. Microstructure cross-section analyses exhibited the deformed microstructure is composed of bands and deformation twins. Also, it was observed the presence of crack propagated along with the large carbides at the grain boundaries. EBSD analysis of cracked and non-crack areas revealed that the high-distorted Taylor factor grains accompanied by grains oriented {111} parallel to the abrasive-impact dynamic load direction were susceptible to fatigue crack formation and propagation. Near to the cracks, a significant increase in dislocation density was found compared to other deformed regions, suggesting that these regions had a high level of stored energy resulting in an exhaust of the ability of plastic deformation. TEM results confirm the formation of nanoscale grains on the deformed surface layer.

Microstructural Evolution upon 10-wt% Mo Alloying and Laser Surface Melting of M2 High Speed Steels Hardfacings

The article aims to comprehend the microstructural changes, in Plasma Transfer Arc (PTA) deposited M2 high speed steel (HSS) hardfacings upon incorporation of 10 wt% Mo alloying during deposition followed by laser surface melting. PTA deposited hardfacings were produced over 4140 steel. Then Mo alloyed and unalloyed PTA deposits were subjected to laser surface melting (LSM) process. A comprehensive microstructural characterization for all the resultant structures was carried out. Optical metallography using appropriate etching reagents and SEM microscopy in conjunction with XRD techniques were employed to ascertain the matrix structure and carbides morphology. The PTA microstructure was close to equilibrium structure of M2 HSS containing mixture of ferrite/austenite/martensite along with MC, M2C and M6C type carbides. While the LSM of M2 HSS caused higher fraction of martensite and finer grains in the structure resulting in increment in hardness. 10-wt% Mo addition changes the carbides from MC and rod like M2C to fibrous M2C and fishbone like M6C carbides. The LSM of Mo alloyed M2 HSS PTA deposits led to an overall decrease in the fraction of M6C carbides and fibrous M2C carbides accompanied by a decrease in hardness.

Precipitation behavior and age hardening effect of the precipitates in a Fe-13Cr-4Al-2Mo-1.2Nb alloy

Precipitation and growth behavior of Fe2Nb-type Laves precipitates are crucial for the mechanical behavior of FeCrAl-based cladding alloys containing Mo and Nb. In this study, the age hardening response of a Fe-13Cr-4Al-2Mo-1.2Nb alloy at 600–800 °C and the corresponding precipitation behavior were systematically investigated. The alloy exhibited a typical two-stage hardening behavior separated by a plateau during aging at all the temperatures. For the alloy aged at 700 °C, the total hardness increment by aging reached ~102.7 HV (40%), with an aging time of 60 min to reach the peak-aged state. Based on detailed transmission electron microscopy (TEM) investigations, the precipitation sequence of the alloy was elucidated as: Supersaturated solid solution → γ″ → γ' → C14-Fe2(Nb,Mo) Laves phase. As the aging proceeded, the Fe2(Nb,Mo) precipitates also evolved from a needle-like morphology with a semi-coherent interfacial structure into a short-rod-like and finally a blocky morphology with a non-coherent interfacial structure. The JMA equation describing the aging kinetics of the alloy was established. The apparent activation energy for the precipitation of the Fe2(Nb,Mo) phase was calculated as 141.5 KJ/mol.

Enhanced thermoelectric performance of Bi0.3Sb1.7Te3 based alloys by dispersing TiC ceramic nanoparticles

Bismuth antimony telluride (BiSbTe) composites are the most studied thermoelectric materials around room temperature. However, the extension of commercial applications is mostly restricted by its inferior conversion efficiency and mechanical stability. In this work, we report that the introduction of nano-sized ceramic titanium carbide (nano-TiC) can achieve a high dimensionless figure of merit (ZT) value up to 1.3 at 400 K and an increment in hardness over 37.93% at room temperature in Bi0.3Sb1.7Te3 (BST) composites. On one hand, nano-TiC creates BST/TiC interfaces so that total thermal conductivity is reduced. Meanwhile, the Seebeck coefficient increases due to the scattering of carriers through the TiC-driven energy filtering effect. On the other hand, the hardness increases substantially with the addition of TiC resulted from the pinning effect induced hardening by grain boundaries and secondary phases. The results indicate that nano-TiC dispersion into BST matrix is effective for the enhancement in both thermoelectric performance and mechanical properties, which is favorable to practical applications. Moreover, the method of introducing nano-sized ceramic materials can also be extended to other thermoelectric systems.

Hydrogen uptake and its influence in selective laser melted austenitic stainless steel: A nanoindentation study

The effect of hydrogen (H) charging on the nanoindentation response of a selective laser melted (SLM) 316L austenitic stainless steel was investigated and compared with its conventionally manufactured (CM) counterpart. Results show that the hardness increment in the SLM samples due to H charging is relatively smaller. Thermal desorption spectroscopy analysis suggests that the charged SLM alloy has not only a smaller H content but a lower apparent H diffusivity in comparison to the CM alloy. This was attributed to the ultrafine solidification cell structure in the SLM alloy. Through the low-load nanoindentation experiments and forward-scattered electron imaging analysis, statistical distributions of the hardness of the cell walls and interiors were assessed. The cell walls, consisting of high-density dislocations with segregated elements, were relatively insensitive to H charging than the cell interiors. These results are discussed in terms of the apparent H solubility and diffusivity in the SLM alloy.

Effect of 9 wt% Mn addition on cold rolling and annealing behaviour of Cu-31Zn alloy

The primary material to make bullet shell, cartridge brass (with 28–32% Zn), is required to have proper ductility and controlled recrystallization. These properties will reduce cracks when bullet shells are fabricated and deformed up to 70%. In order to increase ductility, 9 wt% Mn is added to Cu-31Zn alloy. In this research, Cu-31Zn-9Mn was fabricated by gravity casting and homogenized at 800 ℃ for 2 h. Furthermore, the alloy was cold-rolled with 20, 40, and 70% deformation. Samples with 70% deformation then annealed at 300, 400, and 600 ℃ for 30 min. The results showed that the increments in deformation for 20, 40, dan 70% lead to second phase piping with L/T ratios of 4, 16, and 18, followed by the increment in hardness to 147, 171 and 205 HV, respectively. On the other hand, annealing treatment at 300, 400, and 600 ℃ for 30 min caused decreasing hardness (201, 128, and 171 HV) due to the recovery phenomenon, recrystallization (d grain ∼ 5 μm), and grain growth (d grain ∼ 40 μm), respectively. In order to achieve complete recrystallization in a brass cartridge with the addition of Mn, higher annealing temperature or longer holding duration is required than that of the base Cu-Zn alloy.

Ageing kinetics of SiCp reinforced Al metal matrix composites

Metal Matrix composites have been used widely in structural application due to their excellent load sharing mechanism, strength and durability. These properties get amplified when matrices of age hardenable alloys are used. In present study, extruded Al6061 matrix composites with SiCp as reinforcements were subjected to T6 heat treatment. The hardness (VHN) was determined for composite and base alloy specimens for different ageing times. SEM and TEM characterization were carried out to observe matrix- reinforcement interface and precipitate formation. A 30% increase in hardness was observed due to addition of SiC reinforcements. The hardness increment in aged samples of Al6061 alloy and Al-SiC composite when compared to their untreated samples were 52% and 55% respectively. Peak ageing of the composite and its base alloy occurred at 3 h and 5 h respectively.

Transition in wear behavior and mechanical properties of novel high nitrogen martensitic steel in cryogenic temperature regimes

In space engines with liquid cryogenic LH2/ LO2 propellants, bearings are used in cryopumps subjected to harsh working conditions. High nitrogen martensitic stainless (HNMS) steel is a newly developed steel modifying currently used AISI 440C steel by partial substitution of C with N and is a potential replacement for AISI 440C as an antifriction bearing material In the present study, HNMS samples are austenitized at 1075 °C followed by cooling to −80 °C (CT) and then double tempering at 500 °C followed by cryogenic treatment at −196 °C for 8 h and 1 h soft tempering at 100 °C (CTC). Hardness increment by 7.9% and 7.6%, wear rate decrement by 11.11% and 7.69% and shift in DBTT from − 60 °C to − 90 °C was observed for CTC samples compared to CT samples at −196 °C and RT respectively. The improved properties for CTC samples might be due to precipitation and uniform distribution of tertiary nano precipitates.

Room and High Temperature Sliding Wear Characteristics of Laser Surface Melted Stellite 6 and Mo-Alloyed Stellite 6 Hardfacings

In this work, a post treatment of laser surface melting (LSM) has been employed on Stellite 6 and 10 wt.% Mo-alloyed Stellite 6 hardfacings deposited by plasma transferred arc (PTA) process. LSM process refined the microstructures of both hardfacings, while favoring a network-like complex carbide dominated microstructure in the Mo-alloyed version. With reference to the PTA Stellite 6 hardfacing, LSM process led to an increment in surface hardness albeit a subsequent reduction of wear loss at room temperature, where abrasive wear mechanism was dominant. At 500 °C, oxidative wear contributed to the progress of wear by favoring CoO and Co3O4 type tribo-oxides on the contact surfaces of the PTA and LSM’ed hardfacings, respectively. However, Co3O4 type tribo-oxides exhibited poor mechanical stability, than CoO, which led to easier removal from the contact surface and aggravated the wear loss by abrasive wear mechanism. In this respect, LSM’ed hardfacings exhibited higher wear loss than PTA Stellite 6 hardfacing at 500 °C, unlike room temperature.

Enhanced age hardening response and precipitation evolution of elastic stress aged Mg–Zn alloys

How to improve the age hardening response of Mg alloys in economical and effective ways is an important and interesting issue. The age hardening response and precipitation evolution of Mg–Zn alloys with and without external elastic stress are investigated. Microstructural observations of stress-free aged samples show that the low hardness is mainly due to the low number density and relatively small aspect ratio of β1′ precipitates, while a large number of nano-scale strengthening β1′ precipitates appear in the α-Mg matrix at the early stage of stress aging. In addition, the aspect ratio of β1′ precipitates in the stress aged samples is evidently higher than that of the stress-free aged samples at the same peak aged stage. Abundant dislocations introduced by the applied stress act as nucleation sites of precipitates and rapid diffusion path of solutes, which are mainly responsible for the remarkable increment in hardness and promoted kinetics. Precipitate-free zones ( PFZs) are observed in both samples with and without stress. In the stress aged samples, the formation of PFZs is mainly controlled by the motion of dislocations, which is different from the solute-vacancy depletion mechanism in the stress-free samples. Results imply that the external stress plays a similar role to that of commonly used methods in reducing the size, increasing the density, and promoting the precipitation kinetics, but has a negligible stress-orienting effect on the precipitate evolution.

Effects of nanosized precipitates on irradiation behavior of CoCrFeNi high entropy alloys

Evolution of nanosized precipitates in typical (CoCrFeNi)94Ti2Al4 high entropy alloys under irradiation and its effect on the irradiation resistance were investigated in detail. The as-solutionized samples and the aged samples with massive nanosized precipitates were irradiated at ambient temperatures with a dose ranging from 10 to 49 dpa of 4 MeV Au ions. It was found that the ordered precipitates became disordered quickly at the early stage under 10 dpa irradiation, accompanying with the dissolution process which became more and more severer at higher doses. After irradiation, the hardness slightly changed in the aged specimens containing massive precipitates, but increased by> 30% in the as-solutionized ones with no precipitates. The obvious hardness increment in the as-solutionized samples is due to the creation of defects while the small variations of the hardness in the aged specimens can be ascribed to the disordering and dissolution of precipitates. In addition, the size of dislocation loops induced by the irradiation in the aged specimens is much smaller than that in the as-solutionized samples. We confirmed that presence of the nanosized precipitates delayed defect evolution and reduced the sizes of dislocation loops.

Thermomechanical behavior of Al-Cu-Si commercial alloy modified with rare earths

The effect of cerium/lanthanum rare earths on the microstructural and mechanical behavior of an automotive grade AA319 aluminum alloy was studied at room and hot working conditions. The microstructural results in the modified AA316 aluminum alloys showed that Ce/La additions formed stable micrometric phases at room and hot working conditions, whilst a reductionin the total area of the eutectic-Si phase of up to 75\% was observed in comparison with the reference alloy. The effect of the Ce/La addition in the aluminum alloys produced an increment in hardness and UTS under room temperature; however, their mechanical behavior was improved in hot working conditions. This improvement is attained to the presence of well dispersed Ce/La phases increasing the amount of the $\theta$'-phase (Al$_2$Cu) across the aluminum matrix. The $\theta$'-phase (Al$_2$Cu) interferes directly with crack diffusion of the solid in two crystallographic directions. Additionally, it was observed through transmission electron microscopy, that Ce/La presence alters the kinetics of precipitation in the formation of the coherent $\theta$-phase from the $\theta$'-phase.

Influence of fillers on mechanical behaviour of carbon fiber/vinyl ester hybrid composites

In this experimental work, vinyl ester resin was reinforced with high strength carbon fiber (CV) and the mechanical properties were further improved by incorporating SiC/graphite particles. Overall, the enhancements of both the hardness and strengths of CV composites with SiC/graphite were successfully achieved by fabricating composites using hand lay-up stacking followed by hot pressing. Mechanical tests demonstrate that, the tensile and flexural strengths/moduli of CV improved by increasing SiC/graphite from 2 to 6 wt%. The mechanical performances of CV/graphite hybrid composites were found to be supreme with the incorporation of 6 wt% of graphite particles. Meantime, the use of higher surface area and rigid flake kind graphite particles considerably showed improvement in the overall mechanical performance of CV hybrid composites. Furthermore, the CV/SiC hybrid composites show superior hardness. The utmost increment of hardness was found for the CV composite with 6 wt% SiC. The information bestowed during this paper is accustomed to develop load bearing hybrid composites for structural applications.

Effects of roll pattern and reduction ratio on optical characteristics of A1008 cold–rolled steel specimens: analytical approach and experimental correlations

Cold rollings of A1008 steel specimens are carried out in this study to analyze the influences of roll’s surface pattern and specimen’s reduction ratio on surface morphologies, optical properties, and mechanical properties of rolled samples. A 3D fractal formula, along with dedicated algorithms developed, is applied to solve the periodic (characteristics) lengths and fractal dimensions for roll and rolled surfaces along and perpendicular to the rolling directions. The correlation for every fractal parameter between roll and the rolled specimen is developed to be a function of roll’s surface pattern and reduction ratio first. The light tracking simulations were then carried out for rolled samples only. Rockwell hardness and tensile tests were conducted to evaluate the specimen’s hardness and Young’s modulus. A roll having a higher fractal dimension, or a longer periodic length, can result in a higher fractal dimension and a longer periodic length of the rolled specimen, respectively. A larger reduction ratio also results in the rolled surface with higher fractal dimensions and longer periodic lengths. Increasing the fractal dimensions of a rolled specimen can elevate the maximum illuminance and, conversely, lower the minimum illuminance, while an increase in periodic lengths can increase the minimum illuminance and decrease the maximum illuminance. A decrease in the fractal dimensions and/or increase in the periodic lengths of rolled surfaces in the two directions can raise the illuminance uniformity. For the specimens prepared with the same roll pattern, an increase in reduction ratio can bring an increment in illuminance uniformity and hardness of rolled samples. This study provides an efficient way to determine the roll pattern and reduction ratio that satisfies the specific demands in optical properties.