Synthesis, microstructure and mechanical properties of bulk ultrafine grained Ti-47Al-2Cr (at%) alloy

Hot isostatic pressing (HIP) and powder compact forging of Ti/Al/Cr composite powders of composition Ti-47Al-2Cr (at%) have been carried out to synthesize bulk ultrafine grained Ti-47Al-2Cr alloy. The Ti/Al/Cr composite powders were produced using high energy mechanical milling of elemental Ti, Al, Cr powders in a retsch planetary mill. The microstructure and mechanical properties of the bulk consolidated alloy produced using different processing techniques has been investigated. The mechanical properties of the alloy were studied in tension and compression both at room and elevated temperatures especially to know the formability of the material. The bulk alloy samples produced by HIP for 2 hours at 1000 degrees C had porosity of approximately ~ 5%, indicating that the HIP time was not sufficient to close the pores. The microstructure mainly consisted of TiAl as the major phase and Ti(Al) and Ti3Al as minors, the unreacted Ti(Al) phase in the microstructure was mainly due to the initial powder condition, in which a small fraction of powder particles were rich in Ti. Tensile testing of the alloy samples was carried out at different temperatures. At room temperature the alloy was fairly brittle, without any plastic deformation, and had a fracture strength of ~ 100 MPa. At elevated temperatures the samples became ductile, as reflected by considerable amounts of tensile elongations at 800 degrees C and above. The maximum amount of elongation was found to be between 70 - 80% at 900 degrees C. The tensile yield strength at 800 degrees C was in the range of 84-90 MPa and decreased to 55-58 MPa with the testing temperature of the samples to 900 degrees C. In compression the alloy showed plastic yielding and yield strength of ~ 1.4 GPa at room temperature. Compression testing at 900 degrees C revealed that compressive deformations equivalent to a height reduction of 50% could be easily achieved without cracking. Direct powder compact forging using canned powder compacts of the Ti/Al/Cr composite powder was successfully used to produce bulk consolidated Ti-47Al-2Cr alloy samples. It has been observed that the density of the bulk consolidated alloy sample after forging varied from the centre to the periphery. XRD analysis showed that the forged samples, consisted of TiAl (as major phase) along with Ti(Al) and Ti3.3Al phases. Mechanical testing of the samples showed that the samples exhibited brittle type of fracture both in tension and compression at room temperature and the fracture strength of the samples was in the range of 115 - 130 MPa in tension and 1.38-1.4 GPa in compression without any yielding. When being tested at 900 degrees C, the samples became very ductile showing yield strength in the range of 70-90 MPa and elongation to fracture between 80-165% in tension, and a yield strength of ~ 65 MPa and 50% deformation in compression was easily achievable. Nearly fully dense Ti-47-2Cr alloy samples with density of ~98% were produced by using HIP at 1000 degrees C for a duration of 3 hours. TEM observations revealed equiaxed grains with grain sizes in the range of 200-500 nm. The tensile testing of the alloy samples at different temperatures revealed that the brittle to ductile transition temperature of the alloy was in the range of 700 and 750 degrees C, similar to that reported from literatures. The alloy showed significantly higher strengths both at room and at elevated temperatures, due to the low level of porosity in the sample. Elongation of 95 - 117% at 750 degrees C and 70-100% at 800 degrees C was observed. The ultrafine grained Ti-47Al-2Cr alloy produced using a combination of mechanical milling and HIP/powder compact forging has demonstrated good formability at elevated temperatures leaving a large space for secondary processing to improve the quality of the material

The mechanical behaviour of an ultrafine grained Ti-47Al-2Cr (at%) alloy in tension and compression and at different temperatures

A bulk ultrafine grained (UFG) Ti-47Al-2Cr (at%) alloy has been produced using a powder metallurgy process that combines high energy mechanical milling (HEMM) of a mixture of Ti, Al and Cr powders to produce a Ti/Al/Cr composite powder and hot isostatic pressing (HIP) of the composite powder compact. The purpose of the present study is to determine the mechanical behaviour of the alloy in tension and compression at room temperature (RT) and elevated temperatures, and also to compare the compression behaviour of the material with its tensile behaviour. It has been found that due to the residual pores, lack of full level interparticle bonding and high oxygen content (0.87wt%) in the consolidated samples, the UFG TiAl based alloy has a very low room temperature tensile fracture strength of 100 MPa and shows no tensile ductility. However these microstructural defects and high oxygen content have much less significant effect on the room temperature compressive mechanical properties, and the alloy shows a high compressive yield strength of 1410 MPa, and some ductility (plastic strain to fracture 4%). At elevated temperatures of 800oC and above, the alloy shows high tensile and compressive ductility as demonstrated by 75% tensile elongation to fracture and no cracking in upset forging with a height reduction of 50% at 900oC. The yield strength of the alloy at 900oC is 55 MPa in tension and 33 MPa in compression, both of which are lower than those of coarse grained TiAl based alloys with similar compositions at 900oC. This is due to a higher creep rate of the UFG alloy caused by the small grains. The good formability of the UFG TiAl based alloy as reflected by the lower critical temperature above which the alloy becomes highly formable indicates that the material can be used as a suitable precursor for secondary thermomechanical processing and super-plastic forming

Tensile properties and fracture behaviour of an ultrafine grained Ti–47Al–2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti–47Al–2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 °C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 °C, respectively, but very high ductility (elongation to fracture 70–100%) when tensile tested 800 °C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700–800 °C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 °C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 °C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 °C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 °C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing

Mechanical behaviour of Titanium, Ti-6Al-4V (wt %) alloy and Ti-47Al-2Cr (at %) alloy produced using powder compact forging

Powder compact forging was used to produce bulk consolidated titanium and Ti-6Al-4V (wt %) and Ti-47Al-2Cr (at%) alloy disks from hydrogenated and dehydrogenated (HDH) and gas atomised powders (GA) powders (in the case of titanium and Ti-6Al-4V) and a mechanically milled powder (in the case of Ti-47Al-2Cr alloy). The bulk titanium and Ti-6Al-4V (wt %) alloy have been produced by forging of the powder compacts. The Ti-47Al-2Cr (at %) alloy was produced using canned powder compact forging of a Ti/Al/Cr composite powder. The purpose of the present study is to investigate the deformation and fracture behaviour of the bulk consolidated as-forged materials, by conducting tensile testing at room temperature (RT) and examination of the fractured specimens which had near-α, α + β and γ phase structures, respectively. It was found that as-forged bulk titanium disk produced using HDH powder showed a yield point with a yield strength of ~700 MPa and with a considerable amount of ductility. While the as-forged Ti-6Al-4V (wt %) alloy produced using HDH powder, fractured prematurely without any yielding. On the other hand yielding was observed in the as-forged Ti-6Al-4V (wt %) alloy produced using GA powder, showing a yield strength of ~970 MPa and a considerable amount of plastic strain to fracture. The bulk consolidated Ti-47Al-2Cr (at %) alloy also fractured prematurely with fracture strength of ~125 MPa. The mechanical behaviour of the as-forged bulk materials was found to be dependent on several factors such as initial powders used, green density of the powder compact, forging parameters used during forging. It was expected that the entrapped gas in green compacts, absorbed oxygen, porosity and inter-particle bonding play an important role on the quality of the as-forged material, which in turn affected the mechanical behaviour of the bulk material