LaB6 Addition Research Articles

Effect of LaB6 addition on the in-situ synthesis of TiBx/TixNiy/TiC reinforced Ni-based composite coatings

The TiBx/TixNiy/TiC reinforced Ni60 composite coatings were successfully prepared on the surface of 35CrMoV substrate using the laser cladding technology by adding different content of LaB6 (0, 0.5, 1.5, 2.5 wt%) powders into the mixture of TC4 (Ti6Al4V) and Ni60 (94 wt% Ni60, 6 wt% TC4). The phase composition, microstructure, element distribution and chemical composition of the cladding layer were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) with energy spectrum analysis (EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), respectively. The reinforced phases in the cladding layer were mainly composed of TiB2, TiC, TiB, Ti2Ni, NiTi and Ni3Ti. The addition of LaB6 did not affect the phase composition, but the grains are obviously refined and the distribution of Ni, Cr and Ti elements is uniform at the addition of 1.5 wt% LaB6 addition. The cladding layer obtained excellent tribological and mechanical performance with the addition of 1.5 wt% LaB6, and the hardness, average friction coefficient and wear volume of the cladding layer reach 808 HV1, 0.183, and 8943 μm3, respectively. The reasonable addition of LaB6 significantly improves the friction and mechanical properties of the cladding layer.

Effect of LaB6 doping on the microstructure, microhardness and corrosion behavior of laser cladded FeCoNiCrMo coating on Ti6Al4V

In order to improve the microhardness and corrosion resistance of Ti6Al4V alloy, LaB6 powders was added to the FeCoNiCrMo high entropy alloy coatings fabricated by laser cladding on Ti6Al4V. The microstructure, corrosion resistance and microhardness of LaB6/HEA coatings were analyzed by SEM, EDS, XRD, TEM and performance characterization equipment. The results showed that the appropriate content of LaB6 (2 wt%) could significantly reduce the cracking sensitivity of the coating, and in-situ synthesized La2O3 refined the microstructure and achieve fine grain strengthening. The reduction of cracks and refinement of the microstructure improved the corrosion resistance of HEA. At the LaB6 content of 2 wt%, the corrosion current density of the coating was 2.164 × 10−6 A·cm−2, passive potential was 1.339 V, and the number and size of pitting on the surface of the sample were the lowest, indicating that the passivation film had the strongest resistance to corrosive media. In addition, at the amount of LaB6 addition of 2 wt%, the microhardness of the coating could reach up to 987 HV0.3, which could be attributed to the in-situ synthesis of the ceramic phase TiBX and the refinement of the grains.

Microstructure and Mechanical Properties of LaB6/Ti-6Al-4V Composites Fabricated by Selective Laser Melting

In this study, TiB + La2O3/Ti-6Al-4V composites were successfully prepared by in situ reaction using selective laser melting technology. The effect of LaB6 content on the fabrication quality, microstructure evolution and mechanical properties of the composite samples was investigated. The results show that the relative density of the sample gradually decreased from 98.56% to 96.57% as LaB6 content increased from 0 wt% to 3 wt%. With increasing LaB6 content, TiB precipitates gradually aggregated and grew from a discrete needle-like structure to a dendritic structure, before eventually developing a cell-like structure. The dislocations piled up around the TiB and La2O3 reinforcements, which impeded the motion of the dislocations and led to the enhancement of the tensile strength of the samples. Different from the addition of a single reinforcement due to the combined strengthening effect of the micrometer-scale and nanoscale reinforcements, the strength of the samples was increased significantly. The Ti-6Al-4V sample with 3 wt% LaB6 addition showed the most significant strengthening effect. Compared to the pure Ti-6Al-4V sample, the 3 wt% LaB6 addition sample gained a 35.71% increase in hardness and a 14.5% increase in tensile strength. Additionally, wear volume was reduced by 47.5%. The results revealed that the addition of LaB6 was a potential way to improve the mechanical performance of the titanium alloys in the additive manufacturing process.

Effect of LaB6 on the microstructure evolution and mechanical properties of Ti-45Al-8Nb alloy

High-Nb-containing TiAl alloys have been considered an excellent candidate for demanding high-temperature structural components due to their attractive mechanical properties and low density. However, the intrinsic brittleness at room temperature remains a great barrier limiting its processability. Here a novel alloy design strategy was introduced to improve its ductility from the perspective of grain refinement and removal of oxygen within the matrix. A minor content of LaB6 was introduced into Ti-45Al-8Nb alloy prepared by vacuum arc melting. La2O3 and TiB ceramic phases were in-situ synthesized owing to the decomposition of LaB6 additive. The effect of LaB6 addition on the microstructure and compressive properties of TiAl-based alloys was investigated. The LaB6 inoculation not only refines the lamellar colonies but also promotes the generation of single γ phases, contributing to the transformation from coarse full-lamellar to refined near-lamellar and even dual-phase colonies. The major fracture mode transformed from inter-lamellar to trans-lamellar to inter-granular with LaB6 addition increasing. With 0.4 wt% LaB6 addition, both the ultimate compress strength and fracture strain increased from 1065 MPa and 17.1 % to 1503MPa and 26.98 % respectively.

From crack-prone to crack-free: Unravelling the roles of LaB6 in a β-solidifying TiAl alloy fabricated with laser additive manufacturing

The 4th generation of intermetallic γ-TiAl based alloy, the so-called β-solidifying TiAl, has attracted great attentions due to its promising high-temperature properties. But it is intrinsically brittle and crack-prone, which lowers the processability of laser additive manufacturing (LAM) that offers an effective pathway for material processing and design. Thus, improving the mechanical properties, particularly the room temperature plasticity, of LAMed β-solidifying TiAl alloy is of technical interest and theoretical importance. Based on previous results, the present work investigates the synergistic roles of La and B in grain refinement and crack elimination in a LAMed β-solidifying TiAl alloy with LaB6 addition (0–2 wt%). Results show that La2O3 and TiB in-situ formed via reaction between the β-solidifying TiAl and the added LaB6 nanoparticles. Microstructural feature, phase transformation texture and mechanical properties at both room and high temperatures can be optimized through controlling the LaB6 addition level. 0.5 wt% LaB6 has been determined as the optimized addition, at which the balanced mechanical properties at both room temperature and 800 °C can be achieved for the LAMed β-solidifying TiAl alloy. Excessive LaB6 addition led to agglomeration of in-situ formed precipitates, reducing density and therefore deteriorating mechanical properties. The study offers an in-depth understanding of the effective grain refining approach to overcome the intrinsic brittle nature of β-solidifying TiAl through optimized doping of LaB6.

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2‐SiC composites

AbstractA study has been carried out to examine the effect of LaB6 addition on the compressive creep behavior of ZrB2‐SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB2‐20 vol% SiC composites containing LaB6 (10% in ZSBCL‐10 and 14% in ZSBCL‐14) besides 5.6% B4C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL‐14 composite has exhibited a lower steady‐state creep rate at 1300°C than the ZSBCL‐10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB2 grain boundaries and ZrB2‐SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL‐10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL‐14 composite at 1300–1400°C suggest that ZrB2 grain boundary diffusion is the rate‐limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation‐induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep‐based deformation and damage at the inner core of samples.

In-situ dual-deoxidation design of advanced titanium matrix composites by pressureless sintering

We establish a methodology of in-situ dual-deoxidation to fabricate advanced titanium matrix composites (TMCs) by pressureless sintering (PLS). Specifically, the in-situ dual-deoxidation of impurity O in Ti matrix was designed as the in-situ formation of H2O and La2O3 particles, which was accomplished by PLS of advanced TMCs from powder mixtures containing Ti–6Al–4V matrix of TiH2 and 60Al40V together with LaB6 addition. Interestingly, the advanced TMCs with in-situ La2O3 particle and TiB whisker have various merits, including the significantly reduced O content in the Ti matrix, the refined equiaxed α-Ti grain with monolayer nanosized L phase grain boundary, and especially the nearly full-density evading Kirkendall's pore. Resultantly, the advanced TMCs circumvent the strength–ductility trade-off of representative Ti–6Al–4V TMCs by various materials processing methods. Our results provide a novel strategy for fabricating cost-effective advanced TMCs by PLS to yield superior mechanical properties.

(TiB+La2O3)/Ti-V-Al lightweight high temperature shape memory composites with high strength fabricated by reaction hot pressing and hot rolling

In the present study, lightweight high-strength Ti-V-Al based high temperature shape memory composites were fabricated to broaden the application of shape memory alloys. In-situ TiB whiskers and La2O3 particles were introduced and the distribution of reinforcements was optimized by the combination of low-energy milling, hot-press sintering and subsequent hot rolling. The in-situ reinforcements formed a network structure, which evolved from an equiaxed shape to an ellipsoidal shape. Furthermore, the direction of the reinforcement was inclined to the rolling direction. In the reinforcement-lean area, the phase constitution evolved from the β parent phase to the α″ martensite phase, and the width of the α″ martensite gradually widened with increasing distance to the in-situ reinforcements. The composites maintained high martensitic transformation temperature characteristics. Excellent mechanical properties with a high tensile strength of 885 MPa and a large elongation of 13.1% are obtained in the composite with a 1.0 wt% LaB6 addition and 70% deformation ratio. Meanwhile, the composite realized 3.49% recoverable strain under 6% pre-strain conditions.

Effects of LaB6 on the high-temperature oxidation behavior of TiC+TiBx reinforced titanium matrix composite coatings fabricated by laser cladding

The effects of LaB6 addition on the oxidation behavior of TiC+TiBx reinforced titanium matrix composite coatings at the temperature of 600 °C were investigated. The results showed that severe oxidation occurred on the Ti-6Al-4V after oxidation at 600 °C for 50 h. The oxidation weight gain of the (TiC+TiBx)/Ti coating without LaB6 addition (2.43 mg/cm2) was reduced to 49.6% that of the Ti-6Al-4V substrate (4.9 mg/cm2). Though (TiC+TiBx)/Ti coatings improved the oxidation resistance for the surface of Ti-6Al-4V substrates, cracks were easily formed between the coarse rod-like TiB2 and the titanium matrix during the high temperature resulting in less high-temperature oxidation protection. When the LaB6 addition gradually increased from 0 wt% to 1.0 wt%, the oxidation weight gain decreased from 2.43 mg/cm2 to 1.30 mg/cm2, and the linear rate constant Kp1 decreased from 0.04752 to 0.02703. The oxidation law gradually changed from the linear law to the parabolic law when the LaB6 addition increased to 1.5 wt%, which indicated the oxide layer gradually became dense and protective. The oxidation weight gain decreased from 1.23 mg/cm2 to 0.87 mg/cm2 when the LaB6 addition increased from 1.5 wt% to 2.0 wt%, while increased instead when the LaB6 addition increased to 2.5 wt%. An appropriate amount of LaB6 (2.0 wt%) addition significantly improved the oxidation resistance of the coating with the lowest parabolic velocity constant Kp2 = 0.01418 due to the refinement of grain sizes of the coating, especially the refinement of the coarse rod-like TiB2 phase, which decreased the crack forming sensibility and increased the compactness and continuity of the oxidation layer. A small amount of in-situ formatted La2O3 at the grain boundaries acted as a barrier also blocked the ingress of oxygen.

Effect of LaB6 addition on mechanical properties and irradiation resistance of 316L stainless steels processed by selective laser melting

The 316L stainless steels (SS) have become one of the most common structural materials in nuclear system applications due to their excellent physicochemical performance. Selective laser melting (SLM) preparation, mechanical properties, and irradiation resistance of 316L SS reinforced by second-phase particles have aroused extensive attention. Here, the mechanical properties of 316L SS with different LaB6 contents processed by SLM before irradiation and the irradiation-induced swelling after 50 keV He+ irradiation at room temperature were investigated. Results showed that the microstructure was refined with the LaB6 addition, and the size of 316L SS subgrains decreased with the increase of LaB6. LaB6 reinforced 316L SS (316L-LaB6) exhibited higher hardness and yield strength compared with 316L SS without LaB6 addition. After 50 keV He+ irradiation, the irradiation-induced swelling rate of 316L-LaB6 was lower than that of 316L SS without LaB6 addition, indicating that LaB6 improved the irradiation resistance of 316L SS. Consequently, our study indicated the potential of using 316L-LaB6 as irradiation-resistant material.

Strengthening of solid solution (Ti,La)(C,N)-based cermets by LaB6 addition

(Ti,La)(C,N)-based solid solution cermets are fabricated from self-prepared (Ti,La)(C,N) carbonitrides by powder metallurgy with LaB6 addition in the solid solution. Based on the results of XRD and SEM analyses, it is confirmed that there is complete dissolution of (Ti,La)(C,N) powder. The presence of La can effectively suppress grain overgrowth and thus enhance the homogeneity of porcelain microstructure, consequently increasing the particle size of black phase. On the basis of equal mass fraction of La, the performance of (Ti,La)(C,N)-based cermets is better than that of Ti(C,N)-La-based cermets prepared by simple addition of LaB6 to Ti(C,N)-based cermet powder. The discrepancy can be directly related to the effect of adding LaB6 in the solid solution. The physical and mechanical properties of (Ti,La)(C,N)-based cermets comes to optimum when the content of dissolved La is 0.25 wt%, and there is hardness elevation of 20%. With suitable La addition, there is decrease of oxygen content in the solid solution of cermets. It is hence deduced that LaB6 can improve the performance of cermets as a deoxidizer.

Uncovering the roles of LaB6-nanoparticle inoculant in the AlSi10Mg alloy fabricated via selective laser melting

The effects of inoculation treatment with LaB6 nanoparticles (0–2 wt% additions) on the microstructural evolution and mechanical performance in a selective laser melted AlSi10Mg alloy were comprehensively investigated. The addition of 0.2–0.5 wt% LaB6 nanoparticles was identified to be optimal to achieve substantial grain refinement, microstructural homogeneity and thus remediation in the mechanical property anisotropy in the AlSi10Mg alloy. The substantial grain refinement was attributed to the coherent Al/LaB6 interfaces, which facilitated the heterogeneous nucleation of Al on the LaB6 nanoparticles during solidification. Increasing the LaB6 addition up to 2 wt% only marginally further refined the equiaxed grains, which can be understood in terms of the concept of nucleation free zone formed in the liquid at front of the growing solid-liquid interfaces. The LaB6 nanoparticles within the nucleation free zone could not be activated to be nucleants for α-Al. As a result, random orientation relationships between LaB6 nanoparticles within the nucleation free zone and the Al matrix were determined. Those excessive LaB6 nanoparticles weakened the melt pool boundaries, and therefore deteriorated the longitudinal ductility of the SLMed AlSi10Mg alloy.

Effects of LaB6 on microstructure evolution and properties of in-situ synthetic TiC+TiBx reinforced titanium matrix composite coatings prepared by laser cladding

Different amounts of LaB6 (0, 0.5, 1.0, 1.5, 2.0 and 2.5 wt%) were added into Ti-6Al-4V + B4C powder mixtures for fabricated TiC+TiBx reinforced titanium matrix composite coatings on Ti-6Al-4V surfaces by laser cladding processing. The effects of LaB6 addition on the phase composition, microstructure, microhardness, and wear resistance of the coatings were investigated in detail. XRD and SEM results proved that the coatings mainly comprised three kinds of reinforcements such as granular TiC, rod-like TiB2, and acicular TiB. With LaB6 added, La2O3 formed in the grain boundary of rod-like TiB2 through in-situ reaction and impeded the TiB2 growth. The SEM showed that an appropriate amount (≤2.0 wt%) of LaB6 addition significantly refined the coarse rod-like TiB2 phases that were distributed in the upper area of the coatings, however, the excessive LaB6 (≥2.5 wt%) addition reduced the grains refinement effect. The microhardness of the coating was increased with the microstructure refined. And the coating with 2 wt% LaB6 addition had the highest microhardness (1243.4 HV0.2) which increased to 1.23 times more than that of the coating without LaB6 addition, while the wear mass loss was the least (0.97 × 10−3 g) which decreased to 29.3% that of the coating without LaB6 addition.

Microstructure evolution and mechanical properties of LaB6-modified Ti2AlNb alloy fabricated by blended elemental powder metallurgy

This study was conducted to evaluate the microstructure evolution of the blended elemental powder metallurgy Ti2AlNb alloy with a 2.0 wt% LaB6 addition and the effects of LaB6 doping on the microstructure and mechanical properties of the alloy. The results showed that the microstructure evolution could be divided into two stages. During the first stage, the reaction between Ti and Al powders was dominant. During the second stage, the Nb-enriched area disappeared gradually and microstructural homogeneity was enhanced with the diffusion of Nb. It was demonstrated that the LaB6 phase could be transformed into La2O3, (Ti, Nb)B and the (Ti, Nb)3B2 transition phase in the sintering process. LaB6 addition inhibited the precipitation of the brittle α2 phase and refined the microstructure. Compared with the alloy without LaB6, the Ti2AlNb alloy with 2 wt% LaB6 had 31.5% and 38.4% improvements in the fracture strain at 25 °C and 650 °C, respectively, and its yield strength was also increased.

Effect of LaB6 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Si3N4 ceramics was hot-pressed with LaB6–MgO as sintering additives in the temperature range from 1550 °C to 1750 °C for 2 h under the external mechanical pressure of 60 MPa, followed by post-sintering treatment at 1900 °C for 2 h under the N2 pressure of 2 MPa. The effect of sintering temperature on the densification behavior, mechanical properties, thermal conductivity, phase composition and microstructure evolution were investigated in detail. Compared to the Si3N4 ceramics sintered with La2O3–MgO, the addition of LaB6–MgO was beneficial for the improvement of fracture toughness, Vickers' hardness and thermal conductivity of Si3N4 ceramics. The maximum Vickers’ hardness of 18.98 ± 0.63 GPa was achieved at 1550 °C. The bending strength of 1039.1 ± 70.1 MPa and fracture toughness of 8.46 ± 0.23 MPa m1/2 could be obtained at 1750 °C. After post-sintering treatment, the Si3N4 ceramics exhibited improved thermal conductivity of 75.63 W m−1 K−1 and fracture toughness of 8.72 ± 0.11 MPa m1/2 due to the repeated dissolution-precipitation process.

Effect of trace lanthanum hexaboride on the phase, grain structure, and texture of electron beam melted Ti-6Al-4V

This study investigated the effect of trace lanthanum hexaboride (LaB6) addition on the microstructure and mechanical properties of an electron beam melting (EBM) processed Ti-6Al-4V component. LaB6 exhibited a significant effect on the grain structure, phase, and texture of the EBM-processed Ti-6Al-4V alloys. Although prior-β columnar grains were observed in both Ti-6Al-4V and LaB6-modified Ti-6Al-4V (Ti-6Al-4V-LaB6), the width of the columnar grains decreased significantly with LaB6 addition. Alternating acicular α′ martensite and acicular α laths were distributed in the Ti-6Al-4V, whereas refined lamellar α + β structures were observed in the Ti-6Al-4V-LaB6. The texture intensity of α and β phases was also reduced in Ti-6Al-4V-LaB6. We propose that the addition of LaB6 provided a large amount of heterogenous nucleation sites for solidification and α phase formation. Consequently, high tensile strength with considerable elongation was achieved in the EBM-processed Ti-6Al-4V modified by trace LaB6 addition.

Effect of LaB6 addition on the microstructure and properties of (Ti3Al + TiB)/Ti composites by laser cladding

An amount of LaB6 was added into the (Ti3Al + TiB)/Ti composites prepared on Ti6Al4V by laser cladding. The results showed that nano-scale La2O3 particles were formed through the in-situ reaction between La and O elements in the material system. With appropriate LaB6 addition (3.0%), the microstructure was refined and the second phase reinforcements were distributed more homogeneously. In addition, the oxygen elements in the coating were bound through the formation of La2O3, and the residual stress concentration was obviously reduced compared with that in the coating without LaB6 addition. However, the beneficial effect would be disappeared because of the tendency to create clusters when the additive content of LaB6 was excessive. During the solidfication process, the first generated La2O3 particles had a significant hetregeneous nucleation effect especially on the TiB phase in the coating. The grain refinement effect played the equally and even higher important role as the dispersion strengthening in the system. The microhardness and wear resistance of the coating with 3.0% LaB6 addition were improved. In addition, its high temperature oxidation resistance was also obviously increased, and the related mechanisms were discussed.

Effects of LaB6 content on microstructure and mechanical properties of TaC-LaB6 composites

Tantalum carbide (TaC)-based composites containing 0.1–30 mol% lanthanum hexaboride (LaB6) were prepared by hot pressing at 1900 °C for 30 min under 30 MPa pressure. Effects of LaB6 content on densification, microstructure and mechanical properties of the consolidated TaC-LaB6 composites were investigated. Sinter-forge was performed to test creep resistance of the TaC-LaB6 composites at temperatures up to 1950 °C. Addition of LaB6 increased densification of the TaC-LaB6 composites and the compositions with ≥1 mol% LaB6 almost reached full densities. LaB6 reacted with TaC and the Ta2O3 impurity to form minor phases of TaB2, B2O3 and C. Average size of the TaC grains in the consolidated composites showed a decrease trend with increase in the LaB6 content. The TaC-LaB6 composites with 1–2 mol% LaB6 addition had relatively good flexural strength, Vickers hardness and fracture toughness with a value of 530 MPa, 15.6 GPa and 5.29 MPa m1/2, respectively. Addition of LaB6 modified interfacial bonding strength of the TaC grains, hence increase in creep resistance under sinter-forge up to 1950 °C.

In situ synthesis and strengthening of ultra high-carbon martensitic stainless steels in addition of LaB6

A powder metallurgy route which is characterized by the vacuum solid state sintering is utilized to fabricate Fe-20Cr-3C and Fe-20Cr-3C-2Mo-3V-2Co ultra high-carbon martensitic stainless steels. A sintering window from 1190 °C to 1210 °C is chosen to obtain high density samples and avoid liquid phase simultaneously. The densification process is promoted by solubility effect during formation of M7C3 and grain boundary diffusion. TEM analysis demonstrates three types of reaction products: the La(BO2)3 phase, the La2O3 phase and the La(Fe0.5V0.5)O3 phase, after adding 0.1 wt% LaB6. The impurity elements such as S and O are absorbed following the LaB6 addition to form non-gaseous products, which promotes densification by decreasing the sintering resistance and thus contributes to the improvement of bending strength and hardness. Both of the electrochemical and immersion results lead to the fact that the corrosion resistance of the steels are improved by 0.1 wt% LaB6 addition, in 3.5 wt% NaCl solution. However, the corrosion resistance of Fe-20Cr-3C-0.1LaB6 still can not compete with that of 316L stainless steel. Nevertheless, Fe-20Cr-3C-2Mo-3V-2Co and Fe-20Cr-3C-2Mo-3V-2Co-0.1LaB6 after quenching at high temperature present comparable corrosion resistance with that of 316L in 3.5 wt% NaCl solution.

In situ synthesis and strengthening of powder metallurgy high speed steel in addition of LaB6

A novel technology which was characterized by the vacuum solid state sintering was developed for powder metallurgy high speed steels production. During sintering, both the WC and Mo2C reacted with Fe and transformed to W and Mo rich M6C carbides which were the common hard phases in high speed steels. Also, a high number of W, Mo and Fe were dissolved in VC, forming the MC carbides. The densification of the material mainly relied on the solubility effect during the M6C and MC carbides formation. By alloying with a 0.1 wt% of LaB6 to the steel, the bending strength and the fracture toughness were improved from 3290 MPa and 25.6 MPam1/2 to 4018 MPa and 29.4 MPam1/2, respectively. The TEM analysis demonstrated three types of reaction products by the LaB6 addition: the amorphous phase, the core-shell structure and the La2O3 phase. The impurity elements such as the Mg, Al, Si, S, Ca, and O were absorbed following the LaB6 addition. Moreover, the deoxidization effect caused by the LaB6 addition promoted the sintering at a high-temperature period which contributed to the bending strength and fracture toughness improvement.