Room Temperature Microhardness Research Articles

Mechanical Behavior of Oxide Dispersion Strengthened Steel Directly Consolidated by Rotary Swaging

Among the main benefits of powder-based materials is the possibility of combining different constituents to achieve enhanced properties of the fabricated bulk material. The presented study characterizes the micro- and sub-structures and related mechanical properties of ferritic steel strengthened with a fine dispersion of nano-sized Y2O3 oxide particles. Unlike the typical method of preparation via rolling, the material presented herein was fabricated by direct consolidation from a mixture of powders using the versatile method of hot rotary swaging. The mechanical properties were evaluated at room temperature and also at 1300 °C to document the suitability of the prepared steel for high-temperature applications. The results showed that the imposed shear strain, i.e., swaging ratio, is a crucial parameter influencing the microstructure and, thus, material behavior. The workpiece subjected to the swaging ratio of 1.4 already exhibited a sufficiently consolidated structure with ultra-fine grains and featured high room-temperature microhardness values (up to 690 HV0.5), as well as a relatively high maximum flow stress (~88 MPa) when deformed at the temperature of 1300 °C with the strain rate of 0.5 s−1. However, the dispersion of oxides within this sample exhibited local inhomogeneities. Increasing the swaging ratio to 2.5 substantially contributed to the homogenization of the distribution of the Y2O3 oxide particles, which resulted in increased homogeneity of mechanical properties (lower deviations from the average values), but their lower absolute values due to the occurrence of nucleating nano-sized recrystallized grains.

Pt Schwarz crystals stabilized by minimal-surface grain boundaries and twins at the grain size limit

Schwarz crystals with uniformly distributed twins enclosed by general grain boundaries (GBs) resembling triply minimal-surfaces were observed in high stacking fault energy metal Pt which is believed not readily to twin. By refining the grains of pure Pt from 140 μm to several nanometers via plastic deformation technique, deformation twins were found to emerge numerously and the structural morphologies of Schwarz crystals can be identified at grain sizes below 4 nm. Both experiments and atomistic simulations suggest that Pt Schwarz crystals are thermally stable at temperatures approaching the thermal equilibrium melting point. The measured room temperature microhardness as a function of grain size, on the other hand, may increase monotonically up to 12.9 GPa, revealing a sustained Hall-Petch relationship. The observed outstanding mechanical stabilities can be attributed to that, regardless of the critical twin size and the stacking fault energy of the material, the plastic deformation at the grain size limit is solely limited by twinning and both the nucleation and bowing-out of twinning partial dislocations from GB sources are notoriously difficult according to simulations and theory.

Towards enhancing ODS composites in laser powder bed fusion: Investigating the incorporation of laser-generated zirconia nanoparticles in a model iron–chromium alloy

Oxide dispersion-strengthened (ODS) steel is a sought-after composite material known for its high demand in high-temperature and corrosive environments. Achieving the desired ODS steel properties requires specific conditions for the size and nanoparticles (NP) distribution in the printed part. Laser ablation in liquid (LAL) enables precise NP size adjustment. At the same time, the dynamic melt pool solidification in the Laser Powder Bed Fusion (PBF-LB/M) process complements this by creating favorable conditions for successful ODS processing. In this study, ZrO2 NP with a small and narrow particle size distribution (d50 = 3.8 nm; d90 = 10 nm) is produced by LAL. Dielectrophoretic deposition achieves the homogeneous, deformation-free coating of the binary Fe20Cr (wt.-%) matrix powder with NP. PBF-LB/M printed parts out of the oxide-additivated powder exhibit a crack-free structure and a density of up to 98%. Expectedly, the metal matrix grain sizes and room temperature microhardness (~ 220 HV) are not affected by NP addition. NP tracing by 2D simulation indicates a homogeneous NP distribution and less than 10% NP to be agglomerated in the solidified part. Thus, a promising perspective for a complete laser-based process chain for generating and processing ODS alloys is outlined.Graphical abstractTable of Contents

Enhancing Thermoelectric and Mechanical Properties of p-Type (Bi, Sb)2Te3 through Rickardite Mineral (Cu2.9Te2) Incorporation

Bi2Te3-based alloys are widely utilized in Peltier coolers owing to their highest thermoelectric performance at near-room-temperatures. However, their peak dimensionless thermoelectric figure of merit, zT, is limited to a narrow temperature window due to minority carrier excitation emerging upon heating at around 400 K. Here, we show how this issue can be overcome by incorporating a synthetic rickardite mineral, Cu3–xTe2, in p-type (Bi, Sb)2Te3. The significant enhancement of the electronic and thermal properties could be achieved due to small Cu incorporation into the crystal structure of (Bi, Sb)2Te3 and homogeneous precipitation of Cu3–xTe2 at the grain boundaries. This leads to a high average zT value (zTave) of 1.22 between 350 and 500 K for two compositions, Bi0.5Sb1.5Te3 (BST-5) and Bi0.3Sb1.7Te3 (BST-3), with peak zT values of 1.32 at 467 K and 1.30 at 400 K, respectively. These high zT values result in a considerably high maximum device ZT of ca. 1.15 and a theoretical efficiency of up to 7% between 325 and 525 K. Additionally, room-temperature micro-hardness is substantially improved, which is desirable for constructing reliable and durable thermoelectric modules.

High entropy steel processed through mechanical alloying and spark plasma sintering: Alloying behaviour, thermal stability and mechanical properties

In present investigation, a non-equiatomic high entropy steel (HES) was synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). The Fe40Mn14Ni10Ti10Al15Si10–C1 HES milled up to 30 h was found to have a metastable dual-phase containing a major BCC (a = 2.872 Å; cI2) and a γ-brass type (a = 8.890 Å; cI52) phase having nanostructured grains of ∼10 ± 2 nm. The differential scanning calorimetry (DSC) thermogram exhibited the four exothermic heating events at 530 °C, 690 °C, 860 °C and 1000 °C. The phase transformation corresponding to the heating events was correlated with the ex-situ XRD of the as-milled powder. The phases evolved were Fe–Si solid solution, FCC, Fe5Si3 type and TiC phases. Fe40Mn14Ni10Ti10Al15Si10–C1 milled HES powder was consolidated through SPS at 900 °C and at 50 MPa. The SPSed HES were found to have a dual-phase structure containing a major FCC phase (a = 3.602 Å; cF4) and a minor BCC phase (a = 2.876 Å; cI2) along with the intermetallic phases like Fe5Si3 type (a = b = 6.808 Å, c = 4.745 Å; hP16) and TiC (a = 4.301 Å; cF8). The mechanical properties of these SPSed samples were discerned through instrumented microhardness and compression tests. The microhardness, elastic modulus, ultimate compressive strength and strain were found to be ∼10.4 GPa, ∼209 GPa, ∼2305 MPa and ∼15% respectively. This SPSed HES showed an excellent room temperature microhardness and strength which can be attributed to the co-existence of FCC phase along with the minor phases and hard intermetallics. The strengthening mechanism suggested that the grain boundary, dislocation, and precipitates strengthening were dominant. Attempts have been made to understand the phase evolution and the consequent mechanical properties in these HES.

Bulk nanocrystalline W-Ti alloys with exceptional mechanical properties and thermal stability

Nanocrystalline (NC) W metals and alloys often exhibit higher radiation tolerance and strength than their coarse-grained counterparts. However, their thermal stability is low, making it difficult to achieve bulk NC W metals and alloys by consolidation using conventional techniques such as pressure-less sintering, hot-explosive-compaction sintering, and spark plasma sintering. Here we report the synthesis and mechanical properties of bulk NC W100-xTix (x = 10 at.%–30 at.%) alloys prepared by consolidating mechanically alloyed NC powders under a high-temperature/high-pressure condition. Adding 20 at.%–30 at.% Ti largely improves the sinterability of NC W-Ti alloy powders. The room-temperature microhardness and compressive yield strength of consolidated bulk NC W80Ti20 alloy are ∼ 16.9 and 6.0 GPa, respectively, which are mainly caused by grain boundary strengthening and significantly higher than those of previously reported W and W alloys. The ultimate compressive strength of bulk NC W80Ti20 measured between 900 and 1100 °C deceases with increasing temperature. This behavior can be explained by the activation of Rachinger grain boundary sliding. No grain growth is observed in bulk NC W80Ti20 after compression at 1000 °C. Theoretical calculation suggests that it is the segregation of Ti at grain boundaries that decreases the specific grain boundary free energy and makes the NC W80Ti20 alloy thermodynamically stable.

High-temperature abrasive wear characteristics of H13 steel modified by laser remelting and cladded with Stellite 6 and Stellite 6/30% WC

The study is aimed to analyse the comparative behaviour of the high-temperature abrasive wear of H13 steel surfaces modified by laser melting and cladding with Stellite 6 and Stellite 6 + 30 wt% WC. 3-body abrasive tests were conducted at room temperature, 450 °C, 550 °C, and 650 °C. The microstructural evolution, microhardness, wear surface morphology and mechanisms, and various phases formed during laser surface modifications were also studied. The laser remelting of H13 steel surface increased its room temperature microhardness to 750 ± 35 HV0.01, whereas laser cladding of Stellite 6 powder yielded hardness of around 600 ± 20 HV0.01 in the clad layer; and Stellite 6/WC composite clad layer had marginally higher hardness than the Stellite 6 clad layer in the matrix and much higher hardness of ~3000 HV0.01 at the sporadically distributed WC particle sites. Though the room temperature microhardness of laser remelted H13 surface is the highest, the volumetric wear loss in it was comparable to that of the Stellite 6 cladding. However, Stellite 6/WC composite layer recorded a relatively less volumetric loss as WC particles resisted the abrasive wear. With increasing temperature, the wear loss in laser remelt surfaces increased at a fast rate, while that in Stellite 6 and composite clad layers varied marginally with no definite trend. Overall, Stellite 6/WC composite cladding performed better than others in the current temperature range.

Process Parameters Study on Structure and Properties of Zr-Based Bulk Metallic Glasses

The effects of different process parameters on the microstructure and mechanical properties of Zr-based bulk metallic glass were studied by using four factors and four levels of orthogonal experiment. Through the observation and analysis of the X-ray diffraction pattern, room temperature compressibility, microhardness distribution and transmission electron micrograph of the [Zr0.72-x(Cu0.59Ni0.41)0.28+x]88-yAl12+y (atomic percent) amorphous alloy with different process parameters, the results show that the compressive strength of the amorphous alloys with composition of [Zr0.73(Cu0.59Ni0.41)0.27]88Al12 and [Zr0.73(Cu0.59Ni0.41)0.27]87Al13 amorphous alloy under the corresponding processing parameters has reached 2411MPa and 1993MPa, and the plastic strain of the alloys is 33.8% and 19.3%. At the same time, through orthogonal analysis, in the four process parameters, the superheat temperature of the melt during the preparation of the alloy mainly affects the compressive strength of the alloy, while the cooling rate of the melt during the preparation of the alloy has a great influence on the plastic properties of the alloy.

The precipitation of η phase during the solution treatments of Allvac 718Plus

The precipitation of η phase during the solution treatments of Allvac 718Plus was investigated in this study. The results showed that there was free-γ′ zone appearing around η phase which was consistent with the hexagonal η-Ni3(Al/Ti)0.5Nb0.5 after heat treatments. The granular η phase was found to be the transition of the plate-like η phase and have no pinning effect against the grain boundary, while the plate-like η phase has a pinning effect on the grain boundary and leads to the formation of serrated grain boundaries. With the solution temperature increasing, the size and fraction of η phase were observed to increase and then decrease with the morphology changing from granular to plate-like and then to granular. With the solution time prolonging, the fraction and size of η phase were increased, and the η phase formed in a cellular mode and then grow in a Widmanstätten morphology into the grain with its morphology changing from the dominant granular/plate-like to Widmanstättan structure. It was found that the increase in room temperature microhardness with the solution time changing from 6 h to 24 h was caused by the formation of Widmanstätten η phase.

Development of a new integrated severe plastic deformation method

ABSTRACT A novel one-pass integrated severe plastic deformation method, entitled extrusion compression angular pressing forward extrusion (ECAP-FE), was designed and used for fabricating a fine-grained as-cast AZ31 Mg alloy. Subsequently, mechanical (room temperature compression, tensile and microhardness) and microstructural properties of the processed sample were investigated in detail. In addition, a finite element simulation of the proposed method was carried out for evaluating the equivalent plastic strain and strain rate distribution. The results showed that the ECAP-FE method is a powerful method for processing the ultrafine-grained as-cast AZ31 Mg alloy in a single pass to achieve a uniform and fine microstructure with enhanced mechanical properties. Therefore, it appears that the proposed method has a great potential to a wide range of industrial applications.

In situ observation on microstructure evolution of 22MnB5 in hot stamping process

High temperature confocal laser scanning microscopy (CLSM) was used to investigate the microstructure evolution of high-strength boron steel 22MnB5 during hot stamping. The experimental results show that it is complete austenitized at temperatures about 810 °C during the heating process. Most of the initial austenite grain size is small and locally coarse. At 920 °C to 1000 °C, the phenomenon of B remelted and solidified was observed which played a very good pinning role at the austenite grain boundary, preventing coarsening of austenite grains. The segregation of B and the addition of Mn result in a significant reduction in both the minimum boron reverse melt content and the final solidification temperature. In the continuous cooling stage, martensitic transformation occurs at the cooling rate of 60 °C/s, and the martensite start point is 400 °C and martensite finish point is 280 °C. A large number of bursts are concentrated from 380 °C to 330 °C. There are two main forms of martensitic transformation: first, martensite begins to appear at the coarse austenite grain boundary, and grows transgranularly. Second, the new martensite laths starts from the previously formed laths and grows at a certain angle into the austenite grains. The main factor in the increase of martensite in continuous cooling is the formation of variable temperature martensite rather than the growth of martensite laths. At the cooling rate of 20 °C/s, the bainite and ferrite transformation appeared and the conversion temperature of bainite was about 600 °C. The cooling speed has a great influence on the performance of the 22MnB5 hot stamping component. The room temperature microhardness at cooling rates of 5 °C/s, 20 °C/s, and 60 °C/s was 194 HV, 243 HV, and 430 HV, respectively. Therefore, ensuring sufficient cooling rate is a key condition for obtaining ultra-high strength hot stamping components.

Selective Laser Melting of an Al–Fe–V–Si Alloy: Microstructural Evolution and Thermal Stability

Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si (wt%). The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al12(Fe,V)3Si and AlmFe metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 °C resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al12(Fe, V)3Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 kJ/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min, which was resistant to coarsening exposed at 425 °C up to 500 h. The Al12(Fe,V)3Si and AlmFe phases were coarsened at 475 and 525 °C with increasing the exposure time. Coarsening of Al12(Fe,V)3Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600 °C resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which significantly degraded the room temperature microhardness.

Physical and Mechanical Properties of GdBa2Cu3O7−δ Added with Nanosized CoFe2O4

Superconductor samples of type (CoFe2O4) x GdBa2Cu3O7−δ , 0.0≤x≤0.1 wt.%, were synthesized by the conventional solid-state reaction technique. The prepared samples were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), electrical resistivity, AC magnetic susceptibility, and Vickers microhardness. Phase examination by XRD indicated that the orthorhombic structure of Gd-123 is not affected by nanosized ferrite CoFe2O4 addition, whereas the volume fraction of Gd-123 increased with x up to 0.01 wt.%. Morphological investigations by SEM revealed that the porosity among grains decreased as x increased from 0.0 to 0.1 wt.%. The superconducting transition temperature T c was estimated from electrical resistivity and the AC magnetic susceptibility measurements. A reduction in T c was observed as x increased from 0.0 to 0.1 wt.%, consisting with the effect of adding magnetic impurities to HTSCs. Room temperature microhardness measurements were carried out at different applied loads (0.5–10 N) for the study of the mechanical performance of the studied samples. The experimental results were analyzed using Meyer’s law, Hays–Kendall approach, elastic/plastic deformation model, and proportional specimen resistance model. The results showed that all the samples exhibited normal indentation size effect, and Vickers microhardness values were increased as nanosized CoFe2O4 addition increased. The analysis showed that the proportional specimen resistance model is the best model for computing the load-independent microhardness values of our samples.

Impact of microstructural evolutions during thermal aging of Alloy 625 on its monotonic mechanical properties

Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500 ∘ C that may impact its mechanical properties. In this study, the impact of its microstructure evolutions occurring upon thermal aging on the monotonic mechanical properties has been studied. Thermal exposures of up to ∼2000 hours in the 550 ∘ C – 900 ∘ C temperature range have been investigated. TTT diagrams of the δ and γ′′ phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature micro-hardness after overagings performed at 650 ∘ C and 700 ∘ C due to the precipitation of the γ′′ phase. Moreover, it is clearly demonstrated a strengthening effect of the δ phase observed after long term thermal exposures at temperatures in excess of 700 ∘ C. Finally, the impact of a thermal aging in the γ′′ precipitation domain on the tensile properties was evaluated from room temperature up to 800 ∘ C. It is shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides.

Research on the Cold Rolling Mechanical Character of Low Carbon Base Plate Produced by CSP

With cold rolling base plate of low carbon steel by CSP process, the cold deformation experiments were carried out by the two-roller reverse-mill in the laboratory. The work-hardening was studied for different deformation plates through the room temperature tensile and microhardness measured, and the microstructure was also studied after deformation. It was found that the steel yield and tensile strength increased and work-hardening marked with the deformation augment, the test steel microstructure is ferrite with mingle small amount pearlite, its grain is refined and elongated with deformation increasing.

Mechanical properties of (Cu 0.5Tl 0.5)-1223 added by nano-SnO 2

In this work, superconducting samples of type (Cu 0.5Tl 0.5)-1223 added by nano-SnO 2 (0.0–1.0 wt.%) were prepared by solid-state reaction technique. The prepared samples were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) for phase analysis and microstructure examination, respectively. Room temperature microhardness measurements were carried out at different applied loads (0.49–2.94 N) for studying the performance of the mechanical properties of the studied samples. Furthermore, the experimental results of microhardness measurements were analyzed using different models in order to show the effect of nano-SnO 2 addition on microhardness and elastic modulus.

High-temperature tensile properties of Mg/Al 2O 3 nanocomposite

Mg/1.1Al 2O 3 nanocomposite was synthesized using solidification process called disintegrated melt deposition technique followed by hot extrusion. Microstructural characterization showed that reasonably uniform distribution of reinforcement leads to significant grain refinement of commercially pure magnesium matrix and effectively restricted the grain growth during high-temperature tensile test. Physical properties characterization revealed that addition of nano-Al 2O 3 particulates as reinforcement improves the dimensional stability of pure magnesium. Mechanical properties characterization revealed that the presence of thermally stable nano-Al 2O 3 particulates as reinforcement leads to a significant increase in room temperature microhardness, dynamic elastic modulus, 0.2% yield strength, UTS and ductility of pure magnesium and efficiently maintained the strengthening effect up to 150 °C. Fractography revealed that fracture behavior of magnesium matrix change from brittle to mixed ductile mode with activation of non-basal slip system in room temperature to complete ductile mode at high temperature due to the presence of nano-Al 2O 3 particulates.

Anisotropy in room temperature microhardness and indentation fracture of xenotime

Microhardness and fracture anisotropy of natural single crystal xenotime were studied using Vickers and Knoop indentations. Indentations with loads from 10 to 500 g were applied on (0 0 1) and (1 0 0) crystallographic planes with various orientations of the indenter diagonal around the plane normal. The indentations were characterized with optical microscopy and SEM for cleavage, the anisotropy of microhardness, and indentation fracture toughness. Crack geometry was studied using FIB. The observations are interpreted using the crystallographic structure of xenotime.

Micro-indentation creep of porous nanocrystalline metallic and composite compacts

Experimental observations concerning the time dependent variation in the room temperature microhardness of porous compacts of nanocrystalline Palladium are presented. These data and the earlier findings on the elevated temperature response of compacts of nanocrystalline composites Fe-63 vol% TiN and Ni-58 vol% TiN are interpreted as due to power law creep. The equations developed are validated using the experimental results.

Gamma prime coarsening and age-hardening behaviors in a new nickel base superalloy

By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), phase extraction, small-angle X-ray scattering (SAXS), and microhardness testing, the coarsening kinetics of gamma prime and age-hardening behavior have been investigated in a newly developed nickel base superalloy aged at elevated temperatures for times of up to 4000 h. The precipitates of the alloy as standard precipitation treatment are γ′, MC, and M 23C 6. Coarsening of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion of solute atoms in the alloy. The activation energy for γ′ coarsening is evaluated as 247 kJ mol −1. Due to the different growth rate of γ′ precipitates, room temperature microhardness of the alloy decreases notably with an increase in ageing temperature, while it decreases slowly with ageing time from 500 to 4000 h.