Structure and Performance Changes of Ni-Co-Al Shape Memory Alloys in Relation to Co/Al Atomic Ratio

As a potential ferromagnetic shape memory alloy,Ni-Co-Al has excellent mechanical properties,large magentic-field-induced strain and high martensitic transformation temperature.The relationship between microstructure and performance(mechanical and magnetic properties)of Ni-Co-Al with different Co/Al atomic ratios(RCo/Al)was investigated.Samples exhibitβandγdual-phase structure.Theγphase grows coarse and the volume fraction ofγphase increases with the rise of RCo/Al.Besides,sample with high amount ofγphase content has smallerβgrains owing to the pinning effect ofγphase.The martensite,transformed fromβphase,is tetragonal L10 structure with a(111)twinning plane.The martensitic transformation temperature of samples ascends with increasing RCo/Alowing to more Co embedded into the cell,which makes the valence electron concentration(e/a)of system rise.The saturation magnetization(Ms)of samples increases as RCo/Alrises because Co-richγphase has excellent magnetic property.Meanwhile,both compressive and micro-hardness tests reveal that the samples containing moreγphase have excellent ductility due to the intrinsic good ductility nature ofγphase.     

Hot Ductility of Ti-V Bearing Microalloyed Steel in Continuous Casting

Hot ductility of Ti-V bearing steel was studied by theoretical calculation and thermal simulation experiment.Meanwhile,microsegregation and precipitates were analyzed.The results showed that elements S,Ti and O were enriched at the grain boundaries,while the hot ductility was deteriorated by inclusions of(Fe,Mn,Si,Al)(S,O)in the interdendritic region.At the temperature of 1 100℃,large TiN particles had little effect on hot ductility.In the temperature range from 975 to 925℃,reduction of area(R.A)declined rapidly from 81.88% to 63.16% with the size of particles decreasing from 40 to 20nm and quantity increasing from 1.4 to 11.9μm-2,respectively.In the temperature range from 875 to 825℃,R.Areduced unobviously from 35.14% to 28.6% with ferric films thickening gradually.The critical stress,88.79 MPa,was equal to tensile strength at 825 ℃.Intergranular fracture was easy to occur with higher critical stress below 825℃.     

The grain size dependence of flow and fracture in a Cr-Mn-N austenitic steel from 300 to 1300K

The influence of polycrystal grain size in the range 18 μm to 184 μm on the tensile behavior of an austenitic stainless steel containing by wt pct 21 Cr, 14 Mn, 0.68 N and 0.12 C has been investigated over the temperature range 298 to 1273 K. Decreasing grain size has been shown to increase the flow stress at small strains in accordance with the Hall-Petch relationship at temperatures up to 873 K. The variation of the Hall-Petch constants with temperature is influenced by dynamic strain ageing between 575 and 775 K. Above 875 K, especially at low strain-rates a reversal of the Hall-Petch correlation occurs and the flow stress decreases with decreasing grain size. The relationship between ductility and temperature is marked by a minimum ductility at about half the absolute melting temperature and intergranular cavitation is observed. A decrease in grain size generally enhanced the ductility in this temperature regime whilst at fine grain sizes this trend was reversed. These results are explained in terms of a combination of a Griffith-Orowan type fracture criterion and an intergranular void sheet mechanism of fracture.     

Verification of Interpretation of Dynamic Strain Aging for Intermediate Temperature Embrittlement in Ni-Bi Alloy

Intermediate temperature embrittlement(ITE)is a general phenomenon in Ni alloys and recently was interpreted by dynamic strain aging(DSA).The relationship between ITE and DSA was studied by a binary Ni-Bi alloy.The experimental alloy of well-controlled purity was produced by vacuum induction melting and then heat-treated properly.Tensile tests were performed at various tensile temperatures,and the elongation at fracture was used to indicate the ductility.In order to identify the mechanisms of fracture and ITE,fracture morphologies of the samples of low ductility were observed by scanning electron microscopy.According to the tensile ductility,Ni-Bi alloy shows an obvious embrittlement behavior in the intermediate temperature range(700-750 ℃).However,the stress-strain curves of Ni-Bi alloy and the fracture morphologies indicate that DSA does not exist over the whole temperature range.Based on the experimental results and literatures,the interpretation of DSA was then discussed and proved to be invalid for elucidating the general feature of ITE in Ni-Bi alloy and Ni-based superalloys.     

Heat Treatment of a Candidate Material for 700 ℃ A-USC Power Plants

The preliminary chemical composition optimization and heat treatment parameters of Inconel 740 H were investigated systematically.Six heats of experimental alloys(designated as heat 401 to heat 406)with varied chemical compositions were subjected to standard heat treatment at four different solution temperatures.Due to its superior combination of strength,ductility,and toughness together with its minor mechanical property changes at different solution temperatures,heat 405 was chosen as an optimized one to conduct further heat treatment investigations.The evolution of grain size with solution temperature for heat 405 was studied and the optimal solution temperature was determined.After solution treatment at the optimal temperature,four different cooling rates were applied to investigate the effect of cooling rate on the size distribution modal and mean radius of theγ′precipitates.No bi-modal size distribution ofγ′precipitates was found for all cooling rates and water cooling was recommended for Inconel740 H.In addition,the effect of pre-aging at 800 ℃for 16 hon the microstructure of Inconel 740 Haged for a long time was studied.It is found that pre-aging at 800℃ would result in bi-modal size distribution ofγ′precipitates after long time aging at 750℃compared with mono-modal size distribution in samples without pre-aging,which might be caused by the difference in initial size distribution modal before Ostwald ripening ofγ′precipitates in samples with and without pre-aging at 800 ℃.     

Characteristics of Hot Ductility in Steels Containing Nb,Ti

The hot ductility was measured for a series of steels containing Nb and/or Ti with Gleeble-3500 thermo-mechanical simulator.It is found that the effect of austenitizing temperature on hot ductility of Nb-containing steel is significant.As the austenitizing temperature rises up,the shape of embrittling curve (SEC) from V-shape gradually changes to U-shape,and the range of embrittling temperature widens out and the embrittling degree deepens.The SEC of Nb-containing steel is U-shaped and the embrittling temperature range is about 600-1050℃.The initial temperature of embrittlement on the right is nearly independent of carbon content.The SEC of Ti-containing steel is N-shaped and two lowest points are at about 600℃ and the A-F transformation temperature.The SEC of Nb-Ti-containing steel is basically the superposition of two shapes mentioned above.The crack on straightening in continuous casting belongs to intergranular fracture,and the crack on rolling (crack expansion) belongs to transgranular fracture.     

Rietveld refinement,microstructure,mechanical properties and oxidation characteristics of Fe-28Mn-xAl-1C(x=10 and 12 wt.%)low-density steels

The quantitative relationship between microstructure and properties of austenitic Fe-28Mn-xAl-1C(x=10 and 12 wt.%)low-density steels was evaluated using Rietveld method to refine X-ray diffraction(XRD)patterns.The results showed that a typical three-phase austenitic steel was obtained in the forged Mn28Al10(i.e.Fe-28Mn-10Al-1C)steel,which included about 92.85 wt.% γ-Fe(Mn,Al,C)(austenite),5.28 wt.%(Fe,Mn)_3AlC_(0.5)(κ-carbide),and 1.87 wt.% α-Fe(Al,Mn)(ferrite).For the forged Mn28Al12(i.e.Fe-28Mn-12Al-1C)steel,nevertheless,only about 76.64 wt.% austenite,9.63 wt.%κ-carbide,9.14 wt.%ferrite and 4.59 wt.% Fe_3Al(DO_3)could be obtained.Nanometerκ-carbide and DO_3 were mainly distributed in austenite grains and at the interface between austenite and ferrite,respectively.The forged Mn28Al10 steel had a better combination of strength,ductility and specific strength as compared with the forged Mn28Al12 steel.The ductility of the forged Mn28Al12 steel was far lower than that of the forged Mn28Al10 steel.The oxidation kinetics of Mn28Al10 steel oxidized at 1323 Kfor 5-25 h had two-stage linear rate laws,and the oxidation rate of the second stage was faster than that of the first stage.Although the oxidation kinetics of Mn28Al12 steel under this condition also had two-stage linear rate laws,the oxidation rate of the second stage was slower than that of the first stage.When the oxidation temperature increased to 1373K,the oxidation kinetics of the two steels at 5-25 hhad only onestage linear rate law,and the oxidation rates of the two steels were far faster than those at 1323K for5-25 h.The oxidation resistance of Mn28Al12 steel was much better than that of Mn28Al10 steel.Ferrite layer formed between the austenite matrix and the oxidation layer of the two Fe-Mn-Al-C steels oxidized at high temperature.     

High-Temperature Mechanical Properties of Shipbuilding Steel BC in Continuous Casting

The mechanical property of shipbuilding steel BC has been studied by means of tensile test at various temperatures from 700 ℃ to 1000℃ with theGleeble-1500D thermo-mechanical simulator.The results indicate that the yield strength and tensile strength of steel have an analogous change pattern as temperature decreasing,and the transition in variation rate of strength occurs at 800℃ in both of them;the hot ductility trough of steel BC is a temperature range from 725℃ to 800℃,while the best hot ductility ranges from 875℃ to 1000℃ with the ductility value over 80%.For the sake of understanding the fracture mechanism of the steel,fracture surface and microstructure of the specimens have been examined by scanning electron microscope and metalloscope correspondingly.The results show that both the second phase particles and the pro-eutectoid ferrite surrounding the austenite boundaries play a significant role to the variation of hot ductility of steel BC.Deservedly,the research is important to the improvement and the further studies on the quality of steel during slab continuous casting process.     

Investigation on the hot ductility of Fe-36Ni invar alloy with cerium addition

The hot ductility of Fe-36 Ni invar alloy with different additions of the element cerium was investigated using a Gleeble-3800 thermal-mechanical simulator over the temperature range 850- 1 050 ℃,and the improvement mechanism of the hot ductility was analyzed using a combination of SEM,EDS,and OM. The results indicated that Fe-36 Ni invar alloy exhibited poor hot ductility below 1 050 ℃,which was mainly attributed to weak grain boundaries and the action of grain boundary sliding. However,the alloys with cerium contents of 0. 016% and 0. 024% both demonstrated substantial improvement in the hot ductility over the entire testing temperature range. The observed improvement of the hot ductility of the alloy with 0. 016% cerium at 950 ℃ and the alloy with 0. 024% cerium at 900℃ was associated mainly with the grain boundary strengthening and the restriction of the grain boundary sliding because the addition of cerium reduced the segregation of sulfur at grain boundaries and refined the grain structure.The occurrence and acceleration of dynamic recrystallization were found to be responsible for the high hot ductility of the alloy with 0. 016% cerium at 1 000 ℃ and the alloy with 0. 024% cerium at 950- 1 000 ℃ as a result of the refinement of the grain structure by addition of cerium.     

Age hardening and the potential for superplasticity in a fine-grained Al-Mg-Li-Zr alloy

Experiments were conducted to determine the age-hardening characteristics and the mechanical properties of an Al-5.5 pct Mg-2.2 pct Li-0.12 pct Zr alloy processed by equal-channel angular (ECA) pressing to give a very fine grain size of ∼1.2 μm. The results show that peak aging occurs more rapidly when the grain size is very fine, and this effect is interpreted in terms of the higher volume of precipitate-free zones in the fine-grained material. Mechanical testing demonstrates that the ECA-pressed material exhibits high strength and good ductility at room temperature compared to conventional Al alloys containing Li. Elongations of up to ∼550 pct may be achieved at an elevated temperature of 603 K in the ECA-pressed condition, thereby confirming that, in this condition, the alloy may be a suitable candidate material for use in superplastic forming operations.     

Thermodynamic Analysis of Desulfurization and Its Connection With Deoxidation and Temperature for SPHC in JISC During LF Refining Process

The thermodynamic characteristics of desulfurization reaction (CaO)+[S]=(CaS)+[O] is analyzed based on the detailed composition of liquid steel and slag of Steel Plate Hot Commercial (SPHC) in Jiuquan Iron & Steel Corporation(JISC), where the activities of CaO, CaS and Al2O3 in molten slag are calculated by thermodynamic software FactSage for a more accurate result. The critical values of [O%]/[S%] for desulfurization at different temperature is are obtained, typically 0.09 at 1873K, which shows directly that it should deoxidize adequately for obtaining a favorable desulfurization condition. In addition, the thermodynamic analysis indicates that the actual dissolved O is much higher than that of equilibrium calculation which shows Al-O reaction in LF is far away from equilibrium, but it is perfect agreement with the computing results when taking the activity of Al2O3 as 1 that due to the inclusion component in LF is mainly Al2O3. Besides, with the temperature rise, the sulfur partition ratio increases softly meanwhile the reaction between Al and O is limited to a great degree resulting in the increase a dissolved oxygen in liquid steel that decreases the sulfur partition ratio seriously. As a result, the sulfur partition ratio appears to decrease with temperature increase in Al killed steel.     

Influence of Aluminum Content on Mechanical Properties and Cold Workability of Fe-33Ni-15Co Alloy

 The influence of 001% to 025% of Aluminum content (in mass percent) on the mechanical properties and cold workability of low thermal expansion Fe-33Ni-15Co sheets in the annealed and hot rolled condition was investigated. Addition of Al up to 015% resulted in the increase of ductility and toughness and improvement of workability. Besides, achievement of fine grain structure and increase in the amount of annealing twins caused ductility to improve. Addition of Al higher than 015% led to the increase in oxidation reaction during melting process and to refine under argon atmosphere. This caused the volume fraction of Al containing inclusions to increase and consequently the ductility and toughness to decrease.     

Characteristics of Deoxidation and Desulfurization during LF Refining Al-killed Steel by Highly Basic and Low Oxidizing Slag简

Steel and slag samples were taken at the start and the end of LF refining for steel plate cold common （SPCC）, in the compact strip production （CSP） process, and at the same time, the temperature and oxygen activity a[o] were measured by using an oxygen sensor. Furthermore, inclusions in steel samples were monitored by scanning electron microscopy （SEM） combined with energy dispersive spectroscopy （EDS）. It was confirmed that a [o] in liq- uid steel was in equilibrium with inclusion rather than with top slag during LF refining. Desulfurization was related to deoxidation since a[o] at slag-steel interface was clarified to be very close to that in liquid steel under the specific con- dition in LF with intense stirring by argon blowing and refined by highly basic low oxidizing slag for Al-killed steel. Sulfur partition ratio （Ls） was very sensitive to a[o]. Since a[o] increased rapidly with temperature rise, it not only offset promotion to desulfurization reaction with temperature rise but decreased Ls. For Al-killed steel, the.modifica- tion of Al2O3 for lowering the activity of Al2O3 in inclusion was believed to be favorable for both deoxidation and desulfurization during LF refining.     

Effect of intercritical annealing temperature and cooling rate on the microstructure and mechanical properties of low-carbon aluminum killed steel

In this study,the intercritical annealing process for a typical low-carbon aluminum killed steel is investigated.A cold-rolled sheet was annealed at intercritical temperatures ranging from 730 ℃ to 770 ℃ and then cooled in air or water.The annealed steel was then baked at 210 ℃,and its mechanical properties and microstructures were analyzed in detail.It is shown that after the air-cooling process,the strength of steel decreased and ductility increased with an increase in the annealing temperature.However,after water-cooling,the strength and ductility both increased with the increase of annealing temperature.These results are attributed to the propertyoptimization of the steel.     

Effect of Microalloying Additions on the Microstructure and Mechanical Properties of Low Carbon Steel

Low carbon steels are characterized by good weldability,formability and fracture toughness properties.However,the low strength levels of these steel grades limit their wide applications.On the other hand,increasing the strength by increasing the carbon content and alloying elements deteriorates the other properties.In this study,the microalloying technique was used to examine the possibility of attaining low carbon steels with good combination of strength,ductility and impact properties.A low carbon steel microalloyed with single addition of vanadium and another one microalloyed with combined addition of vanadium and titanium were used in this investigation and their properties were compared with non-microalloyed low carbon steel having the same base composition.Furthermore,other two nonmicroalloyed and V-microalloyed steels with higher carbon,silicon and manganese contents were also investigated to reveal the effect of base composition.Tensile,hardness,room and zero temperature Charpy V-notch impact tests were conducted to evaluate the variations in the mechanical properties of low carbon hot forged steel containing vanadium and combinations of vanadium and titanium.In addition,the microstructures of the different investigated steels were observed using both optical microscope and scanning electron microscope.Furthermore,the hardness of the ferrite phase was also determined using micro-hardness technique.The results showed improvement of the mechanical properties of the investigated steels by both single V-and combined V + Ti-microadditions.Tensile,hardness and impact tests results indicated that good combinations of strength,ductility and impact properties can be achieved by V-microalloying addition.Steel with combination of V and Ti microaddition has much higher hardness,yield strength,ultimate tensile strength and impact energy at both room and zero temperatures compared with non-microalloyed and single Vmicroalloyed steels.Higher C,Si and Mn contents result in increasing the strength accompanied with decrea     

Synthesis of Al2O3/TiCN-0.2%Y2O3 Composite by Hot Pressing

Al2O3/TiCN composites were synthesized by hot pressing.The influences of components and HP temperature on mechanical properties,such as bending strength,breaking tenacity and Vickers hardness were investigated.The results showed that the mechanical properties of Al2O3/TiCN composite increased with temperature when hot pressing temperature is below 1650 ℃.The mechanical properties reached their maximums when the composites were sintered at 1650 ℃ for 30 min under hot pressing pressure of 35 MPa,the value of bending strength,breaking tenacity and Vickers hardness was 1015 MPa,6.89 MPa·m1/2,and 20.82 MPa,respectively.When hot pressing temperature was above 1650 ℃,density decreased because of decomposition with increased temperature,and mechanical properties dropped because of rapid growth of grains in size at high temperature.Microstructure analysis showed that the addition of Y2O3 led to the formation of YAG phase so as to inhibit the growth of crystals.This helped to improve breaking tenacity of the composites.TiCN particles with diameters of 1 μm dispersed at Al2O3 grain boundaries,inhibited grain growth and enhanced mechanical properties of the composites.SEM study of the propagation of indentation cracks showed that the bridge linking behavior between matrix and strengthening phase might lead to the formation of the coexisted field of crack deflection,branching and bridge linking.The mechanism of this phenomenon was that the addition of Y2O3 improved the dispersion of TiCN particles so as to enhance the tenacity of the composites.The breaking tenacity was changed from 5.94 to 6.89 MPa·m1/2.     

Hot Working of High Nitrogen Austenitic Stainless Steel

With hot rolling in laboratory and Gleeble thermal simulator,the hot working of a high nitrogen austenitic stainless steel(HNASS)was researched.The results showed that dynamic recovery(DRV)and dynamic recrystallization(DRX)in HNASS occurred during hot working,and both of them had well-defined stress peaks in flow curves under different conditions.During hot rolling experiment at temperature from 950 to 1050 ℃,recrystallization phenomenon does not take place in test material until the deformation ratio is up to 40%.Recrystallization influences remarkably the strength and ductility of material,and the test HNASS possesses better combination of strength with ductility.According to the curve of θ-σ(strain hardening rate-steady state stress),the DRX critical strain of test material was determined.Also,the activation energy of hot working was calculated to be 746.5 kJ/mol and the equation of hot working was obtained.     

Effects of Mn and Cr contents on microstructures and mechanical properties of low temperature bainitic steel

The effects of Mn and Cr contents on bainitic transformation kinetics,microstructures and mechanical properties of high-carbon low alloy steels after austempered at 230,300 and 350 ℃ were determined by dilatometry,optical microscopy,scanning electron microscopy,X-ray diffraction and tensile tests. The results showed that Mn and Cr can extend bainitic incubation period and completion time,and with the increase of Mn and Cr content,the bainitic ferrite plate thickness decreased and the volume fraction of retained austenite increased. TRIP( transformation induced plasticity) effect was observed during tensile testing which improved the overall mechanical property. The increase of Mn concentration can improve the strength to a certain extent,but reduce the ductility. The increase of Cr concentration can improve the ductility of bainitic steels which transformed at a low temperature. The low temperature bainitic steel austempered at 230 ℃ exhibited excellent mechanical properties with ultimate tensile strength of( 2146 ± 11) MPa and total elongation of( 12. 95 ± 0. 15) %.     

Effect of Oxygen Partial Pressure on Phase Equilibria and Liquidus in CaO-Al2O3-FeOx System

 A high temperature equilibration experiment was carried out to investigate the effect of oxygen partial pressure on the phase equilibria and liquidus in CaO-Al2O3-FeOx system with the intermediate oxygen partial pressures of 10.13 Pa and 1.01×10-3 Pa. The equilibrated phases and their compositions of the quenched samples were analyzed by using SEM/EPMA (Scanning Electron Microscope/Electron Probe Micro-Analysis) and XRD (X-Ray Diffraction). The phase equilibrium results include two cases, the two-phase coexistence and the three-phase coexistence in the high Al2O3 region with oxygen partial pressure of either 10.13 Pa or 1.01×10-3 Pa. Effects of oxygen partial pressure and temperature on the liquidus along the primary phase fields of CaO·Al2O3 and CaO·2Al2O3 were notable. With the decrease of oxygen partial pressure, the liquid area expands and the liquidus of CaO·Al2O3 and CaO·2Al2O3 primary fields moves to the Al2O3-FeOx region. On the other hand, the liquid area of CaO-Al2O3-FeOx system extends extremely to the high Al2O3 region with the temperature increasing from 1400 to 1500 ℃, especially at lower oxygen partial pressure. The present experiment results are in good agreement with the calculated ones by FactSage.     

Tensile and fracture properties of an Mg-RE-Zn alloy at elevated temperatures

Magnesium alloy EZ10(Mg-RE-Zn) was deformed in tension at temperatures from 20 up to 520 °C. A rapid decrease of the yield and tensile strength with temperature was observed at temperatures higher than 300 °C. On the other hand, ductility of samples rapidly increased in the same temperature range. Light microscopy and scanning electron microscopy was used to reveal the reason for these behaviours. Intermetallic particles in grain boundaries are responsible for excellent mechanical properties at lower temperatures. Diffusional processes occurring at temperatures higher than 300 °C significantly influenced the deformation mechanism as well as the fracture character.