Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Design,Processing, Microstructure,Properties, and Applications of Advanced Intermetallic TiAl Alloys

After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ‐TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high‐temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi‐phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo‐mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ‐TiAl based alloys, but concentrates on β‐solidifying γ‐TiAl based alloys which show excellent hot‐workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application‐oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro‐ and nanostructure during hot‐working and subsequent heat treatments.     

Influence of pre-compression on the ductility of AA6xxx aluminium alloys

Reversed loading experiments were conducted to study the influence of pre-compression on the ductility of three aluminium alloys. Diabolo-shaped specimens were machined from extruded profiles along the transverse direction, and heat treated to peak strength (T6 temper). The specimens were subjected to five different levels of pre-compression (0, 10, 20, 30, 40%), i.e., the specimens were first compressed to a prescribed strain and then pulled to fracture in tension. Using a laser-based measuring system, the minimum diameter in the extrusion direction and thickness direction were continuously measured during the tests until fracture. The three aluminium alloys AA6060, AA6082.25 and AA6082.50 had different grain structure and texture. The AA6060 and AA6082.50 alloys had recrystallized grain structure with equi-axed grains and large elongated grains, respectively. The AA6082.25 alloy had a non-recrystallized, fibrous grain structure. It was found that pre-compression has a marked influence on the ductility of the aluminium alloys, which depends on the microstructure and strength of the alloy. Using the compressed configuration as the reference configuration, the relative failure strain could be calculated. For the AA6060 alloy, the relative failure strain increased for increasing pre-compression, and was approximately doubled for 40% pre-compression compared to pure tension. For the AA6082.25 alloy, a slight increase in the relative failure strain was observed for increasing pre-compression, while for the AA6082.50 alloy the relative failure strain was low and approximately constant for different levels of pre-compression.     

Grain refinement in nanostructured Al–Mg alloys subjected to high pressure torsion

Nanostructures of three binary Al–Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion at room temperature were comparatively investigated using transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray line profile analysis. Grain size distributions, dislocation densities, and densities of planar defects including stacking faults and microtwins were quantified. The average subgrain size decreased considerably from 120 to 55 nm as the Mg content increased from 0.5 to 4.1 wt%. The average dislocation density in the alloys first increased to a maximum and then decreased as the Mg content increased and the average subgrain size decreased. The role of Mg solute on these features and the refinement mechanisms associated with the typical nanostructures and faults were interpreted.     

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s⁻¹. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.     

Influence of heat treatment on microstructure and tensile properties of GlidCopR dispersion strengthened copper alloy

Dispersion strengthened copper is a family of commercial engineering alloys comprised of fine aluminum oxide particles dispersed in a pure copper matrix. In this study, the influence of heat treatment on microstructure and tensile properties of GlidCop^R dispersion strengthened copper alloy is examined. Heat treating the as-rolled alloy was found to have no appreciable influence on grain structure and intrinsic microstructural features. The strength of the material decreases with heat treatment with concomitant improvement in ductility. The material maintains a high value of yield strength retention ratio. The influence of heat treatment on tensile properties is detailed.     

Effect of preheat treatment on ingot structure and recrystallisation of brass component of rolling texture in hot rolled AA 5182

Abstract

Two different preheating treatments were used to vary the Al₆ Mn dispersoid characteristics in AA 5182. High temperature followed by slow cooling produced coarse, needlelike dispersoids which allowed rapid recrystallisation following hot rolling, whereas a low temperature preheat produced afine dispersion of low aspect ratio particles which retarded recrystallisation. Long, unrecrystallised bands persisted in the hot rolled material subected to the low temperature preheat even after holding for 24 h at the final pass temperature. Micro-orientation determinations within the bands showed that they were predominantly of the brass (Bs) orientation, {110}〈112〉, confirming the results of X-ray bulk texture analyses. The results are considered conclusive evidence of the widely reported higher resistance to recrystallisation of the Bs orientation compared with other orientations in rolled high stacking fault energy fcc metals. The Bs oriented bands could be removed by annealing at a temperature substantially greater than that experienced in the final hot rolling pass.

MST/1475     

Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25-980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. Abrupt decrease of ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation is not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.     

The effect of boron on the microstructure and physical properties of chemically vapour deposited nickel films

The solid solution addition of boron greatly enhances the strength and hardness of chemically vapour deposited (CVD) nickel while dramatically changing the microstructure. The solid solubility of boron in nickel is limited, and single-phase alloys containing in excess of 0.3 at% B are supersaturated with respect to the formation of one or more intermetallic boride phases. Single-phase Ni-B alloys containing 0 to 13.0 at% B were produced by CVD on polycrystalline copper substrates at 155° C in an atmospheric pressure process. The microstructure, mechanical and physical properties were characterized for the alloys both as-deposited and after various thermal treatments by using optical microscopy, transmission electron microscopy, X-ray diffraction and micro-indentation hardness testing with a diamond pyramid indentor. The grain size of the alloy was found to decrease sharply with rising boron content. Concomitantly, the defect density of the material rose significantly, the microhardness increased and the ductility decreased. With annealing at a temperature of 300° C or greater, precipitation of the Ni₃B intermetallic phase, recovery and grain growth occurred.     

Influence of the Heat Treatments on Creep Behaviour of Nickel Aluminide with Boron

The nickel aluminide with boron alloy is being considered for elevated-temperature structural application in particular for jet turbine engine components. The alloy is attractive due to its ease of production, the low cost of its components, and its property advantages relative to superalloys. Therefore, if alloys based on Ni₃Al are successfully developed, substantial increases in engine performance and efficiency may be realized.The creep characteristics of an intermetallic Ni₃Al alloy containing boron produced by hot isostatic pressing were investigated in the temperature range 800 to 900^°C. Various heat treatments were used to produce different initial grain sizes of this alloy.Parameters studied were steady state strain rate, time to fracture, ductility and Larson-Miller parameter. The stress exponent, activation energy for creep and grain size exponent were calculated.It was found that by increasing the temperature of the heat treatment, the grain size increased. The results showed that the creep behaviour for this alloy improved as grain size increased. Furthermore, a comparison of the resulting creep data with data obtained from references is discussed.     

Artificial ageing of Al-Si-Cu-Mg casting alloys

The T6 heat treatment is commonly used for gravity cast Al-Si-Cu-Mg alloys. The influence of the alloying elements Cu and Mg and the artificial ageing temperature on the age hardening response were investigated. Artificial ageing was conducted at 170 °C and 210 °C for various times for three alloys, Al-7Si-0.3Mg, Al-8Si-3Cu and Al-8Si-3Cu-0.5Mg, cast with three different solidification rates (secondary dendrite arm spacing of about 10, 25 and 50 μm). The coarseness of the microstructure has a small influence on the yield strength, as long as the solution treatment is adjusted to obtain complete dissolution and homogenisation. The peak yield strength of the Al-Si-Mg alloy is not as sensitive to the ageing temperature as the Al-Si-Cu and Al-Si-Cu-Mg alloys are. The ageing response of the Al-Si-Cu alloy is low and very slow. When 0.5 wt% Mg is added the ageing response increases drastically and a peak yield strength of 380 MPa is obtained after 20 h of ageing at 170 °C for the finest microstructure, but the elongation to fracture is decreased to 3%. The elongation to fracture decreases with ageing time in the underaged condition as the yield strength increases for all three alloys. A recovery in elongation to fracture of the Al-Si-Cu-Mg alloy on overageing is obtained for the finest microstructure, while the elongation remains low for the coarser microstructures. The quality index, Q = YS + K?, can be used to compare the quality of different Al-Si-Mg alloys. This is not true for Al-Si-Cu-Mg alloys, as K depends on the alloy composition. Overageing of the Al-Si-Mg alloy results in a decrease in quality compared to the underaged condition.     

High-strength magnesium alloys for degradable implant applications

This article describes the design principles deployed in developing high-strength and ductile Mg-Zn-Zr-Ca-Mn(-Yb) alloys based on a concept, which aims to restrict grain growth considerably during alloy casting and forming. The efficiency of the development approach is discussed. Moreover, the microstructure and phase analysis of the alloys subjected to different thermal treatments are presented and the influence of the alloy composition, particularly the addition of Yb, on the evolution of the microstructure is discussed in connection with the mechanical properties of the materials. The newly developed alloys exhibit high strength (yield stress of up to 350 MPa) at considerable ductility (elongation to fracture of up to 19%) in the as-extruded state and reveal age hardening potential (increase in hardness of 10-15% compared to that in the recrystallization heat-treated state). Appropriate heat treatments enable tailoring of the strength-ductility relation. Thermal annealing of the material resulted in a remarkable increase in ductility (elongation to fracture of more than 20% for all heat-treated samples) while high strength is retained (yield stress ranging from 210 to 315 MPa). We attribute the attractive mechanical properties of the developed alloys to their fine-grained microstructure, where the grain boundaries and lattice defects are stabilized by second phase particles formed during casting and thermal treatments.     

Fundamentals of the New Rheocasting Process for Magnesium Alloys

Casting of high strength, ductile and pressure tight components at low cost is the prerequisite for the introduction of magnesium alloys into hydraulic and structural applications. This paper introduces the New Rheocasting process (NRC) as a novel approach for semi‐solid casting of light metals, in which the slurry is prepared from normal casting alloys directly at the foundry machine. The specialties of the process and the alloy requirements are explained. Ways for increasing ductility and process stability with slight alloy modifications and proper heat treatment are shown. The resulting mechanical properties are compared with data received from classical high pressure die casting parts. New Rheocasting of the alloy AZ71proves to be superior in strength and ductility, and shows excellent K_JC values.     

Strain rate sensitivity of the commercial aluminum alloy AA5182-O

The mechanical behavior of the commercial aluminum alloy AA5182-O is investigated at temperatures ranging from −120 to 150 °C and strain rates from 10⁻⁶ to 10⁻¹ s⁻¹. The strain rate sensitivity parameter is determined as a function of temperature and plastic strain, and the strain rate and temperature range in which dynamic strain aging leads to negative strain rate sensitivity is mapped. The effect of dynamic strain aging on ductility and strain hardening is investigated. The sensitivity of the measured quantities to the experimental method employed and their dependence on grain shape are discussed. The experimental data are compared with the predictions of a model constructed based on a recently proposed mechanism for dynamic strain ageing. The mechanism is based on the effect solute clustering at forest dislocations has on the strength of dislocation junctions. The model is shown to reproduce qualitatively the experimental trends.     

Production of Al-(12–25) wt% Si alloys by rapid solidification: melt spinning versus centrifugal atomization

Al-Si-x(x=Cu, Mg, Ni, Co or Sr) alloys with Si content in the range 12–25 wt% were quenched from the liquid state using two methods: melt spinning and centrifugal atomization. The powders obtained were degassed followed by hot extrusion. Effects of chemical composition, quenching conditions, hot extrusion and heat treatment on the variation in the microstructure were examined. The present results show the necessary conditions for supersaturated solid solution, and those required for mechanism of solute trapping by moving the solid/liquid interface. Also, the mechanical properties of the products obtained were evaluated. It is observed that melt spun ribbons with Si concentrations of more than 12% possess high yield strength with low ductility. These materials undergo softening on ageing at temperatures above 150 °C. The properties of extruded alloy powders are markedly improved as compared to those made by ingot metallurgy. This effect is mainly due to the silicon particle refinement brought about by rapid solidification with cooling rates higher than 10⁵ Ks^–1.     

Fabrication,properties and structure of a high temperature light alloy composite

Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.     

Multipass cold rolling and resultant mechanical properties of a Cr-containing nickel aluminide

The difficulties encountered in fabricating Ni₃Al-based intermetallic alloys into final structural components, due to their limited workability as a result of their inherent high yield strength and low ductility at elevated temperatures, are an important issue that have restricted the commercial applications of these materials. The Osprey spray deposition process is capable of delivering near-net-shape preforms, thereby avoiding the technical problems related to the hot working of these materials, e.g. hot rolling of slabs. The present work concerns an investigation of the cold rollability of a chromium-containing Ni₃Al intermetallic alloy produced with the Osprey process. The sliced preform with a thickness of 7 mm was successfully cold rolled through multipasses into sheets with a thickness of 0.7 mm and a good surface finish. The material has been found to have a high working hardening rate at room temperature. The maximum total reduction permissible without resulting in rolling defects is 30%. Thus, for larger reductions, intermediate annealing between rolling passes is necessary and it has been optimized to be at 1100°C for one hour. The repeated cold rolling and the recrystallization occurring during intermediate annealing change the initial microstructural features and grain size of the Osprey-spray-deposited material. The cold-worked and annealed intermetallic sheets with a thickness of 0.7 mm have a yield strength of 570 and 730 MPa and a elongation value of 33 and 7%, at room temperature and at 700°C, respectively. Fractography shows a transition from the transgranular fracture mode at low temperatures to the intergranular fracture mode at temperatures above 650°C.     

Effect of thermomechanical treatment on the properties of Fe-11Al and Fe-14Al alloys

Fe-Al alloys have the potential to be relatively inexpensive soft magnetic materials if their formability could be improved. An investigation has been made on the effect of thermomechanical treatment on the properties of Fe-11 wt%Al and Fe-14 wt%Al alloys (designated Fe-11Al and Fe-14Al respectively). For the former the room temperature mechanical properties were found to be determined principally by the recrystallised grain size. A good combination of properties for Fe-11Al, i.e. high strength and ductility, was obtained when the grain size was less than about 100 m. The small grain size was produced by warm rolling at 600°C followed by 1 hour annealing at 600–700°C. On the other hand hot rolling followed by annealing resulted in large grain size, hence rendered the alloy brittle. The cold formability also exhibited a grain size dependence, with the Fe-11Al alloy with a fine recrystallised grain size having good cold rollability. In contrast Fe-14Al was brittle irrespective of the treatment given; ductility of less than 1% was observed in all cases and the cold rollability was limited. Ordering was not seen to be a factor affecting the observed mechanical properties and rollability of either alloy as all the thermomechanical treatments, other than an ordering treatment of 500 hours at 400°C, resulted in a disordered structure. The stress required to work these alloys at elevated temperatures were estimated from compression tests and it is apparent that for Fe-11Al the stress is greatly reduced (50%) from the room temperature value at 600°C and that at 750°C both alloys required a similar stress which was about 15% of the room temperature value. The magnetic properties of Fe-11Al compared favourably with Fe-14Al; the former has a higher saturation induction, a similar coercive force but a lower permeability than Fe-14Al.