Effects of Ca addition on as-cast microstructure and mechanical properties of Mg–3Ce–1.2Mn–1Zn (wt.%) magnesium alloy

The effects of Ca addition on the as-cast microstructure and mechanical properties of the Mg–3Ce–1.2Mn–1Zn (wt.%) alloy were investigated by using optical and electron microscopes, differential scanning calorimetry (DSC) analysis, and tensile and creep tests. The results indicate that the additions of 0.3–0.9 wt.%Ca to the Mg–3Ce–1.2Mn–1Zn alloy do not cause an obvious change in the morphology and distribution for the Mg₁₂Ce phase in the alloy. However, the grains and secondary dendrite arm spacings of the Ca-containing alloys are refined, and an increase in Ca amount from 0.3 wt.% to 0.9 wt.% causes the grain size and secondary dendrite arm spacings to gradually decrease, respectively. In addition, the additions of 0.3–0.9 wt.%Ca to the Mg–3Ce–1.2Mn–1Zn alloy can effectively improve the as-cast tensile and creep properties of the alloy, and an increase in Ca amount from 0.3 wt.% to 0.9 wt.% causes the as-cast tensile and creep properties to gradually increase, respectively.     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.     

Effects of heat input on microstructure and mechanical properties of Fe–2Cr–Mo–0.12C steel

ABSTRACT

Thermal simulated specimens with the heat inputs of 20, 50 and 80?kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at ?40°C) and hardness for the heat input of 50?kJ/cm were 102?J and 346?HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath.

This paper is part of a thematic issue on Nuclear Materials.     

Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al₃(Sc,Zr)?+?α-Al?+?Al₃(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.     

Decomposition of Fe–(6–20) at.% Mo alloys

Binary iron alloys containing 6, 10, 15 and 20 at.% Mo were investigated. It was found that the “ordering–phase separation” transition is a time depending process: at the beginning of isothermal aging at 550 °C, a modulated structure is formed in the solid solution as a result of the tendency toward phase separation; with time, Fe₂Mo particles precipitate in the modulated structure dissolving as a result of the tendency toward ordering. Further aging at 550 °C leads to the complete dissolution of the modulated structure and Fe₂Mo particles remain the only phase precipitated in the solid solution.     

Microstructures of binary Cr–xNi alloys (0 ≤ Ni ≤ 50 wt.%) in their as-cast state and after high temperature exposure

Metallic materials designed for use at very high temperature must be based on elements with high melting points. In this work, several binary alloys, chosen in the Cr-rich part of the Cr–Ni diagram, were elaborated by foundry and characterised by metallography in the as-cast condition and after exposure at 1200 °C. Many of the obtained alloys are composed of imbricated body-centred cubic chromium phase saturated in Ni and face-centred cubic nickel phase saturated in Cr. These structures may evolve more or less at high temperature. High values of hardness were obtained for some of these alloys, suggesting high strength at elevated temperature. The hardness evolution vs. the Cr content was well represented by a law of mixture of the volume fractions and the hardness of the separated phases. Already intrinsically resistant to oxidation at high temperature, most alloys are also not sensitive to internal nitridation, contrary to pure Cr. Such Cr–Ni alloys may be considered as possible bases for heat-resistant alloys.     

The effect of annealing on the deformation behaviour and microstructure of crystallized Mg–23.5Ni (wt.%) alloy

Rapidly solidified amorphous Mg–23.5Ni (wt.%) ribbons were crystallized at 300 and 400 °C for 90 min. After annealing at 300 °C the microstructure was heterogeneous, consisting of rounded eutectic–lamellar domains, which contained magnesium grains smaller than 500 nm. In the case of ribbons annealed at 400 °C the microstructure, however, was homogenous, and composed of well-formed magnesium grains and Mg₂Ni particles. At room temperature both crystallized materials were brittle due to the high volume fraction of Mg₂Ni particles, but they exhibited some ductility with increasing test temperature. Above 200 °C, the microstructure of the ribbons annealed at 300 °C was characterised by the formation of particle free zones during the tensile test. This structure was not observed in the material annealed at 400 °C. Deformation behaviour and changes in the microstructure during plastic flow of both crystallized materials were explained according to grain boundary sliding mechanisms.     

Investigation of microstructure,hardness and wear properties of Al–4.5 wt.% Cu–TiC nanocomposites produced by mechanical milling

The present work deals with studies on the manufacturing and investigation of mechanical and wear behavior of aluminum alloy matrix composites (AAMCs), produced using powder metallurgy technique of ball milled mixing in a high energy attritor and using a blend–press–sinter methodology. Matrix of pre-mechanical alloyed Al–4.5 wt.% Cu was used to which different fractions of nano and micron size TiC reinforcing particles (ranging from 0 to 10 wt.%) were added. The powders were mixed using a planetary ball mill. Consolidation was conducted by uniaxial pressing at 650 MPa. Sintering procedure was done at 400 °C for 90 min. The results indicated that as TiC particle size is reduced to nanometre scale and the TiC content is increased up to optimum levels, the hardness and wear resistance of the composite increase significantly, whereas relative density, grain size and distribution homogeneity decrease. Using micron size reinforcing particulates from 5% to 10 wt.%, results in a significant hardness reduction of the composite from 174 to 98 HVN. Microstructural characterization of the as-pressed samples revealed reasonably uniform distribution of TiC reinforcing particulates and presence of minimal porosity. The wear test disclosed that the wear resistance of all specimens increases with the addition of nano and micron size TiC particles (up to 5 wt.%). Scanning electron microscopic observation of the worn surfaces was conducted and the dominant wear mechanism was recognized as abrasive wear accompanied by some delamination wear mechanism.     

Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.     

Isothermal aging effect on the microstructure and dry sliding wear behavior of Co–28Cr–5Mo–0.3C alloy

In this study, an attempt to investigate the role of isothermal aging on the microstructure and dry sliding wear behavior of Co–28Cr–5Mo–0.3C alloy was made. Regarding the results, it is clear that isothermal aging at 850 °C for 8 and 16 h contributed to the formation of lamellar type carbides (γ-fcc + M₂₃C₆) at the grain boundary regions. Moreover, at higher aging times (24 h), the lamellar type carbides decreased whereas severe precipitation of carbides was found to occur on the stacking faults. Furthermore, according to X-ray diffraction results, 24 h isothermal aging of solution treated specimens did not lead to complete fcc → hcp transformation. The wear properties of as-cast and heat treated samples were determined at 0.5 ms⁻¹ speed several under normal applied loads such as 50, 80, and 110 N. At the lowest load applied (50 N), isothermally aged specimens for 8 and 16 h have higher wear resistance probably due to more volume fraction of lamellar-type carbides when compared to as-cast for both 4 and 24 h aged specimens. But, at higher applied loads (80 and 110 N) due to the formation of adhesive oxide layer on the as-cast specimen surface, the wear rate of as-cast samples is lower compared with all heat treated specimens.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

The effect of Y on the hot-tearing resistance of Al–5 wt.% Cu based alloy

It is well known that the continuing quest for Al–Cu alloy with improved casting defects is driven by the need in aerospace industries. Especially, the wide crystallization range of Al–5 wt.% Cu based alloys results in generation of hot-tearing. In Al–5 wt.% Cu alloy, the hot-tearing susceptibility is influenced by the larger crystallization range, the quantity and distribution of the eutectic microstructure in the grain boundary and the size of primary α (Al) particles. The effect of Y content on the hot-tearing resistance in Al–5 wt.% Cu based alloys was studied by ZQS-2000 twin Bar Tester of Thermo-Cracking-Linear Contraction Tester. The hot-tearing behavior was studied by thermal analysis, optical microscopy (OM) and scanning electron microscopy (SEM). It was found that Y promoted the end-solidification temperature and decreased the quantity of eutectic in grain boundary. Y-rich phase precipitation caused a depression of the begin-solidifying temperature of α (Al), narrowed the crystallization range, increased the hot-tearing resistance and decreased the hot-tearing susceptibility significantly.     

Hyperfine interactions and structural features of Fe–44Co–6Mo (wt.%) nanostructured powders

Nanocrystalline Fe–44Co–6Mo (wt.%) powders have been prepared by high-energy ball milling from elemental Fe, Co and Mo pure powders in a P7 planetary ball mill. The obtained powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mössbauer spectrometry techniques. The influence of milling process and Mo substitution for Co in equiatomic FeCo have been examined in order to study structural evolution and formation mechanism of nanostructured Fe(CoMo) solid solution. XRD results show the formation of a BCC Fe(CoMo) solid solution (a = 0.2874 nm) where unmixed nanocrystalline Mo with a BCC structure is embedded. Disordered Fe(CoMo) solid solution is characterized by a broad hyperfine magnetic field distribution with two regions centered at B₁ = 35.0 T and B₂ = 30.7 T, respectively, attributed to disordered Fe(Co) solid solution and CoMo enriched environments. Prolonged milling and Mo addition cause the decrease of average hyperfine magnetic field while the average isomer shift remains nearly constant.     

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.     

Hydrogen sorption characteristics of the composites 90 wt.% Mg (MgH2)–10 wt.% V0.855Ti0.095Fe0.05

The hydrogen absorption–desorption characteristics of composites containing 90 wt.% Mg or MgH₂ and 10 wt.% of the intermetallic compound V_0.855Ti_0.095Fe_0.05 obtained by mechanical alloying for 1 and 5 h in an inert medium were investigated. Absorption measurements were performed under a hydrogen pressure P = 1 MPa at temperatures of 623, 573, 523, and 473 K. Dehydriding was studied at 623 and 573 K and a pressure of 0.15 MPa. It was established that the presence of the additive improved significantly the hydriding kinetics of magnesium while the effect of the duration of mechanical alloying was less pronounced. Due to the small difference in specific surface areas and crystallite sizes, both composites investigated showed no substantial difference in behavior during absorption and desorption of hydrogen. The best absorption–desorption properties were found with the composite 90 wt.% Mg–10 wt.% V_0.855Ti_0.095Fe_0.05 mechanically activated for 5 h.     

Effects of zirconium addition on as-cast microstructure and mechanical properties of Mg–3Sn–2Ca magnesium alloy

At present, the mechanical properties of the Mg–3Sn–2Ca magnesium alloy are not satisfying and further enhance needs to be considered via further alloying/microalloying additions. The effects of Zr addition on the as-cast microstructure and mechanical properties of the alloy were investigated by using optical and electron microscopies, differential scanning calorimetry (DSC) analysis, and tensile and creep tests. The results indicate that adding 0.41, 0.76 or 1.18 wt.% Zr can refine the grains of the alloy, and the primary CaMgSn phases in the Zr-containing alloys are changed from coarse needle-like net to relatively fine short block and/or particle-like shapes. As a result, the tensile and/or creep properties of the Zr-containing alloys are improved. Among the Zr-containing alloys, the alloy with the addition of 0.76 wt.% Zr exhibits the relatively optimum mechanical properties.     

Improvement of magnetic properties of an Fe–6.5 wt.% Si alloy by directional solidification

The magnetic properties of an Fe–6.5 wt.% Si alloy can be improved through texture and microstructure control during directional solidification process. With the increasing of directional solidification rate, the main texture of the Fe–6.5 wt.% Si alloy along specimen withdrawing direction evolved in the way of < 130> → < 100> → < 142>, and the coercivity initially decreased and then increased. For the directional solidification rate of 1 mm/min, a homogeneous microstructure of the Fe–6.5 wt.% Si alloy specimen with low energy boundaries between columnar grains was obtained. The main texture of the specimen was < 100>, and the coercivity of the alloy was reduced by 44% compared with that of the alloy consisting of equiaxed grains.     

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (1 0 0) substrate

β-Ga₂O₃ films were grown on double-side polished MgAl₆O₁₀ (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD) at 600, 650 and 700 °C. The refractive index dispersive behaviors of Ga₂O₃ films have the typical shape of the normal dispersion curve. Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)–green emission from 300 to 650 nm. A minor deep UV emission around 275 nm (∼4.51 eV) was observed for the sample prepared at 700 °C. The intensity of the emission increased markedly when measured at low temperature. The corresponding PL mechanisms were discussed in detail and a schematic diagram was proposed.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.