Effect of hot isostatic pressing on mechanical properties and microstructure of 70–30 cupronickel castings

Abstract

The present work is part of an investigation into the use of hot isostatic pressing to recover 70–30 cupronickel castings. These alloys have particularly good corrosion resistance and, when strengthened with silicon and chromium, produce a material capable of use in very severe conditions of stress and massive corrosion. However, it is not possible to recover such castings by the application of repair welding, because of the possibility of reduced corrosion resistance in the vicinity of the weld. Hot isostatic pressing represents an alternative method of casting recovery. The results reported in the present work refer to the effect of hot isostatic pressing on mechanical properties, microstructure, and the level of segregation in the alloys. Hot isostatic pressing may be used to remove casting defects in the form of fine pores up to total porosity of 5%. However, in cases where porosity takes the form of very large defects, the mechanical properties of the recovered region are inferior to those of the originally sound material. This effect is probably associated with the presence of very finely distributed oxide particles in the originally defective parts of the casting. The optimum hot isostatic pressing temperature for the best overall combination of properties was 950°C.

MST/1732     

Processing and mechanical properties of porous Ti–7.5Mo alloy

Titanium (Ti) and its alloys continue to be utilized extensively for skeletal repair and dental implants. Most metallic implant materials including pure Ti and Ti alloys used today are in their solid forms and are often much stiffer than human bone. However, the elastic modulus of Ti and Ti alloys can be reduced through the introduction of a porous structure, which may also provide new bone tissue integration and vascularization abilities. In the present study, porous Ti–7.5Mo alloy scaffolds made from ball-milled alloy particles and sintered at 1100 °C for 10, 15 and 20 h respectively were successfully prepared through a space-holder sintering method. In the sintered Ti–7.5Mo, no obvious diffraction peaks of elemental Mo remained after the sintering, and a duplex α + β microstructure was confirmed from the XRD pattern. The samples made from BM15 (the alloy particles ball-milled for 15 h) had higher relative density, compressive strength and elastic modulus performance than those from BM3 and BM30 (the alloy particles ball-milled for 3 and 30 h, respectively) when they were sintered under the same conditions. Moreover, the longer sintering time lead to the higher relative density and the greater compressive strength and modulus of the sample. In this work, the strength and modulus of the sintered porous Ti–7.5Mo conforms to the basic mechanical property requirement of cancellous bones.     

Design and mechanical characterization of a Zr–Nb–O–P alloy

In this study, Sn-free Zr–1.5Nb–O–P alloys were manufactured and their mechanical properties were characterized. The ultimate tensile strength (UTS) of cold rolled Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (680 MPa) were close to that of a commercially available Zr–1Nb–1Sn–0.1Fe alloy (720 MPa), achieving a good mechanical strength without the addition of Sn, an effective solution strengthening element. The UTS of recrystallized Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (533 MPa) was far greater than that of a commercially available Zr–1Nb–O (323 MPa) because of the strengthening due to higher Nb and oxygen content combined with phosphorous strengthening. The activation volumes for the cold rolled Zr–1.5Nb–P alloys were not much different from those of annealed Zr–1.5Nb–P alloys despite the higher dislocation density in the cold rolled alloys. Insensitivity of the activation volume to the dislocation density and the decrease of the activation volume with the addition of phosphorous support the suggestion linking the activation volume with the activated bulge of dislocations limited by segregation of oxygen and phosphorous atoms.     

Influence of oxygen content on microstructure and mechanical properties of Ti–Nb–Ta–Zr alloy

The influence of oxygen content on microstructure and mechanical properties of Ti–22.5Nb–0.7Ta–2Zr (at.%) alloy was investigated in this work. According to experiments, the grains were refined apparently when the oxygen content was between 1.5% and 2.0%. The ultimate tensile strength (UTS) increased and elongation decreased with increasing oxygen content. But at the content of 1.0%, the elongation was nearly the same to that of the original alloy (about 16%). The elastic modulus remained comparatively low (<65 GPa) when the content was lower than 1.5%, and then increased dramatically. Therefore, there existed the best oxygen content-1.0%, at which fine grains were obtained, as well as UTS of 750 MPa, elongation of 16% and elastic modulus of 65 GPa. The Ti–22.5Nb–0.7Ta–2Zr–1.0O alloy maintained typical ductile fracture characteristics of beta titanium alloy, and had a little superelasticity.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Influence of hot pressing temperature on the microstructure and mechanical properties of 75% Cu–25% Sn alloy

The alloy of 75% Cu–25% Sn was utilised and hot-pressed for 4 min at 421, 520 and 600 °C to obtain a self-sharpening bond for diamond honing stones at low sintering temperature. Densification and mechanical tests were performed, and structures were investigated by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. Results showed that the porous structures changed into microporous structures when the hot pressing temperature was increased from 421 °C to 600 °C. The mechanical properties improved from HRB 79.1 to HRB 105.1 in hardness and from 104.2 MPa to 201.4 MPa in transverse rupture strength. After hot pressing at 600 °C, the microstructure consisted of α(Cu) + δ eutectoid and micropores, which meets the requirements of bonds for honing stones.     

Effect of thermomechanical processing on superplasticity in Ti–Al–Mo–Zr alloy

Abstract

Superplasticity in terms of total tensile elongation was studied in a titanium alloy of nominal composition Ti–6·5Al–3·3Mo–1·6Zr (wt-%) for three strain rates (1·04 × 10^?3, 2·1 × 10^?3, and 4·2 × 10^?3s^?1) and in the temperature range 1123–1223 K for microstructures obtained by different processing schedules. Fine equiaxed microstructure with a low aspect ratio of 1·15 was accomplished in this alloy by combining two types of deformation. While the first step consists of heavy deformations for refining and intermixing the phases, a second step, consisting of light homogeneous reductions in several stages, was necessary to remove the banding that developed during the first step. The resulting microstructure underwent enormous tensile elongation (1700–1725%), even under relatively high strain rates (1·04 × 10^?3 and 2·1 × 10^?3s^?1), making this alloy most suitable for commercial superplastic forming. The present investigation also revealed that the usual sheet rolling practice of heavy reductions to refine the microstructure leads to localised banding which could not be removed by annealing; therefore, the tensile elongation was limited to 770% only. The reason for this may be attributed to the resistance in grain boundary sliding and rotation encountered in microstructures with shear bands and grains with high aspect ratio. Strain enhanced grain growth was also greater in these microstructures.

MST/555     

Investigation on WC–Al composite coatings of AZ91 alloy by mechanical alloying

In this paper, WC–Al composite coatings of AZ91 alloy prepared by mechanical alloying have been investigated in detail. It was found that the premixing process of composite powders has no significant effect in promoting the formation of uniform coating of WC–Al powders. Under the optimised conditions, i.e. the composition of composite powders of (10 g WC–6·5 g Al–3 g AZ91–0·5 g Mg), ball-to-powder weight ratio of 14∶1 and milling duration of 12 h, the average thickness of composite coating can be remarkably increased to 38·01 μm. Compared with the bare substrate, the Brinell hardness of the specimen with WC–Al composite coating can be significantly increased by about 90·01%.     

Effects of heat treatment on microstructure and mechanical properties of twin roll casted Al–5.5Mg–0.02Ti alloy

In the present investigation the Al–5.5Mg–0.02Ti alloy produced by twin roll casting (TRC) process (varying rolling speed, i.e., 3, 4, and 5 rpm) has been subjected to heat treatment for microstructure modification. Grain coarsening at the center of the strip has been observed during heat treatment process. Homogeneous microstructure of the alloys has been achieved by heat treatment process, and it has been found that the time to achieve homogeneous structure depends on the rolling speed. Transmission electron microscope (TEM) studies revealed that undesired Mg rich phase (Mg₅Al₈) has been successfully eliminated by heat treatment process. Fine and equi-axed grains in the alloys obtained by heat treatment process shows high strength and elongation.     

Microstructural and mechanical characteristics of Cu–Cu2O composites compacted with pulsed electric current sintering and hot isostatic pressing

This paper reports the influence of applied sintering process – pulsed electric current sintering (PECS) and hot isostatic pressing (HIP) – on the microstructure and mechanical properties of Cu–Cu₂O composites. In PECS fine-grained structure was obtained while in HIPing the grain growth was more noticeable, mostly due to the longer process time. The studies also showed that Cu₂O-phase distributed in Cu-matrix increased microhardness; at a fixed grains size Cu–Cu₂O structure had higher hardness than Cu so that 20% higher microhardness was obtained when Cu₂O was doubled from 19.1 to 37.2 vol%. At best, 99.1% density with 690 nm grain size and 1.35 GPa hardness were achieved by PECS whereas by HIP the same density with 1860 nm grain size gave 1.02 GPa hardness. The grain growth and the effect of second phase clustering on the grain growth were evaluated experimentally.     

Structure and mechanical properties of Ti–5Cr based alloy with Mo addition

The effects of molybdenum (Mo) on the structure and mechanical properties of a Ti–5Cr-based alloy were studied with an emphasis on improving its strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti–5Cr and a series of Ti–5Cr–xMo (x = 1, 3, 5, 7, 9 and 11 wt.%) alloys were prepared by using a commercial arc-melting vacuum-pressure casting system, and investigated with X-ray diffraction (XRD) for phase analysis. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that Ti–5Cr–7Mo, Ti–5Cr–9Mo and Ti–5Cr–11Mo alloys exhibited ductile properties, and the β-phase Ti–5Cr–9Mo alloy exhibited the lowest bending modulus. However, the Ti–5Cr–3Mo and Ti–5Cr–5Mo alloys had much higher bending moduli due to the formation of the ω phase during quenching. It is noteworthy that the Ti–5Cr–9Mo alloy exhibited the highest bending strength/modulus ratios at 26.0, which is significantly higher than those of c.p. Ti (8.5) and Ti–5Cr (13.3). Furthermore, the elastically recoverable angle of the Ti–5Cr–9Mo alloy (30°) was greater than that of c.p. Ti (2.7°). The reasonably high strength (or high strength/modulus ratio) β-phase Ti–5Cr–9Mo alloy exhibited a low modulus, ductile property, and excellent elastic recovery capability, which qualifies it as a novel implant materials.     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.     

Sc and Sc–Zr effects on microstructure and mechanical properties of Be–Al alloy

Herein, we investigated the effects of Sc and Sc–Zr on the microstructure and mechanical properties of Be–Al alloy, showing that Sc alloying resulted in Be grain refinement and reduced the secondary dendritic arm spacing (SDAS) of these grains by 1/3, whereas Sc–Zr alloying further decreased the SDAS to 7.5?µm and afforded equiaxed/cellular-like morphology with further refined Be grains. The above alloying resulted in the formation of intermetallic compounds (Be₁₃Sc, Be₁₃Zr, and Al₃(Sc_1–xZr_x)), increasing the macrohardness of the Be–Al alloy, with the microhardness and elastic modulus of the Be phase increasing to a larger extent than those of Al. Importantly, Sc–Zr alloying resulted in better microstructure modification and mechanical reinforcement than Sc alloying.     

Thermodynamic characterization of Al–Cr,Al–Zr,and Al–Cr–Zr alloy systems

Abstract

The equilibrium phase diagrams of Al–Cr, Al–Zr, and Al–Cr-Zr, with particular reference to aluminium-rich alloys, have been critically reviewed. On the basis of these, and consistent with measured thermodynamic values, the binary systems have been thermodynamically characterized. Using these characterizations, phase equilibria have been extrapolated in the ternary, with the intention of augmenting the sparse experimental information concerning the equilibrium liquidus (0–10 at.%Cr, Zr) and solid solution range of aluminium in Al–Cr–Zr. Using the same parameters that define the equilibrium phase relationships, metastable phase relationships can also be extrapolated into the ternary.

MST/418     

Microstructure evolution and mechanical properties of a Ti–35Nb–3Zr–2Ta biomedical alloy processed by equal channel angular pressing (ECAP)

In this paper, an equal channel angular pressing method is employed to refine grains and enhance mechanical properties of a new β Ti–35Nb–3Zr–2Ta biomedical alloy. After the 4th pass, the ultrafine equiaxed grains of approximately 300 nm and 600 nm are obtained at pressing temperatures of 500 and 600 °C respectively. The SEM images of billets pressed at 500 °C reveal the evolution of shear bands and finally at the 4th pass intersectant networks of shear bands, involving initial band propagation and new band broadening, are formed with the purpose of accommodating large plastic strain. Furthermore, a unique herringbone microstructure of twinned martensitic variants is observed in TEM images. The results of microhardness measurements and uniaxial tensile tests show a significant improvement in microhardness and tensile strength from 534 MPa to 765 MPa, while keeping a good level of ductility (~ 16%) and low elastic modulus (~ 59 GPa). The maximum superelastic strain of 1.4% and maximum recovered strain of 2.7% are obtained in the billets pressed at 500 °C via the 4th pass, which exhibits an excellent superelastic behavior. Meanwhile, the effects of different accumulative deformations and pressing temperatures on superelasticity of the ECAP-processed alloys are investigated.     

Fabrication and characterization of porous Ti–7.5Mo alloy scaffolds for biomedical applications

Porous titanium and titanium alloys are promising scaffolds for bone tissue engineering, since they have the potential to provide new bone tissue ingrowth abilities and low elastic modulus to match that of natural bone. In the present study, porous Ti–7.5Mo alloy scaffolds with various porosities from 30 to 75 % were successfully prepared through a space-holder sintering method. The yield strength and elastic modulus of a Ti–7.5Mo scaffold with a porosity of 50 % are 127 MPa and 4.2 GPa, respectively, being relatively comparable to the reported mechanical properties of natural bone. In addition, the porous Ti–7.5Mo alloy exhibited improved apatite-forming abilities after pretreatment (with NaOH or NaOH + water) and subsequent immersion in simulated body fluid (SBF) at 37 °C. After soaking in an SBF solution for 21 days, a dense apatite layer covered the inner and outer surfaces of the pretreated porous Ti–7.5Mo substrates, thereby providing favorable bioactive conditions for bone bonding and growth. The preliminary cell culturing result revealed that the porous Ti–7.5Mo alloy supported cell attachment.     

Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

Al–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3?wt.%Si powder at the rotational speed ranging from 250 to 350?rpm for 5?h.     

Formation of calcium phosphates on low-modulus Ti–7.5Mo alloy by acid and alkali treatments

This study investigated the hydroxyapatite (HA) coating on metal implants in order to enhance their bioactive properties. In this study, HA coatings were formed on the surfaces of commercially pure titanium (c.p. Ti) and Ti–7.5Mo which were acid-etched and subsequently alkali-treated before samples were soaked in simulated body fluid (SBF). Specimens of c.p. Ti and Ti–7.5Mo were etched in either H₃PO₄ or HCl, and subsequently treated in NaOH. The surfaces of acid-etched c.p. Ti showed a porous structure, whereas those of acid-etched Ti–7.5Mo showed some grinding marks, but no porosity. After subsequent alkali treatment in NaOH, the surfaces of both the c.p. Ti and Ti–7.5Mo substrates exhibited microporous network structures. The specimens were then immersed in SBF at 37 °C for 28 days. Apatite began to deposit on acid-etched and NaOH-treated Ti–7.5Mo within 1 day after immersion in the SBF. After 28 days of immersion in the SBF, a dense and uniform layer was produced on the surfaces of all samples. The HA formation rate was the highest for HCl and NaOH-pretreated samples, and the results of EDS and XRD showed that much more intensive peaks of HA appear on the specimens of HCl and NaOH-treated Ti–7.5Mo than on any other sample. Thus, this method of apatite coating Ti–7.5Mo appears to be promising for artificial bone substitutes or other hard tissue replacement materials with heavy load-bearing applications due to their desirable combination of bioactivity, low elastic modulus, and low processing costs.     

Structure,mechanical properties,and grindability of dental Ti–Zr alloys

Structure, mechanical properties and grindability of a series of binary Ti-Zr alloys with zirconium contents ranging from 10 to 40 wt% have been investigated. Commercially pure titanium (c.p. Ti) was used as a control. Experimental results indicated that the diffraction peaks of all the Ti-Zr alloys matched those for alpha Ti. No beta-phase peaks were found. The hardness of the Ti-Zr alloys increased as the Zr contents increased, and ranged from 266 HV (Ti-10Zr) to 350 HV (Ti-40Zr). As the concentration of zirconium in the alloys increased, the strength, elastic recovery angles and hardness increased. Moreover, the elastically recoverable angle of Ti-40Zr was higher than of c.p. Ti by as much as 550%. The grindability of each metal was found to be largely dependent on the grinding conditions. The Ti-40Zr alloy had a higher grinding rate and grinding ratio than c.p. Ti at low speed. The grinding rate of the Ti-40Zr alloy at 500 m/min was about 1.8 times larger than that of c.p. Ti, and the grinding ratio was about 1.6 times larger than that of c.p. Ti. Our research suggested that the Ti-40Zr alloy has better mechanical properties, excellent elastic recovery capability and improved grindability at low grinding speed. The Ti-40Zr alloy has a great potential for use as a dental machining alloy.