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

authors: Nadakuduru, Vijay Navaratna

subjects: ultrafine grained

abstract: 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.

text: 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.

year: 2009

title: Rates of photospheric magnetic flux cancellation measured with Hinode

authors: Park, Soyoung

subjects: magnetic fields

abstract: Photospheric magnetic flux cancellation on the Sun is generally believed to be caused by magnetic reconnection occurring in the low solar atmosphere. Individual canceling magnetic features are observationally characterized by the rate of flux cancellation. The specific cancellation rate, defined as the rate of flux cancellation divided by the interface length, gives an accurate estimate of the electric field in the reconnecting current sheet. We have determined the specific cancellation rate using the magnetograms taken by the Solar Optical Telescope (SOT) aboard the Hinode satellite. The specific rates determined with SOT turned out to be systematically higher than those based on the data taken by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory. The median value of the specific cancellation rate was found to be 8 × 106 G cm s-1—a value four times that obtained from the MDI data. This big difference is mainly due to a higher angular resolution and better sensitivity of the SOT, resulting in magnetic fluxes up to five times larger than those obtained from the MDI. The higher rates of flux cancellation correspond to either faster inflows or stronger magnetic fields of the reconnection inflow region, which may have important consequences for the physics of photospheric magnetic reconnection.

text: Rates of photospheric magnetic flux cancellation measured with Hinode Photospheric magnetic flux cancellation on the Sun is generally believed to be caused by magnetic reconnection occurring in the low solar atmosphere. Individual canceling magnetic features are observationally characterized by the rate of flux cancellation. The specific cancellation rate, defined as the rate of flux cancellation divided by the interface length, gives an accurate estimate of the electric field in the reconnecting current sheet. We have determined the specific cancellation rate using the magnetograms taken by the Solar Optical Telescope (SOT) aboard the Hinode satellite. The specific rates determined with SOT turned out to be systematically higher than those based on the data taken by the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory. The median value of the specific cancellation rate was found to be 8 × 106 G cm s-1—a value four times that obtained from the MDI data. This big difference is mainly due to a higher angular resolution and better sensitivity of the SOT, resulting in magnetic fluxes up to five times larger than those obtained from the MDI. The higher rates of flux cancellation correspond to either faster inflows or stronger magnetic fields of the reconnection inflow region, which may have important consequences for the physics of photospheric magnetic reconnection.

year: 2009

title: How many deaths were caused by the Tarawera eruption?

authors: Lowe, David J.

abstract: Most texts and brochures describing the Tarawera eruption of 10 June, l 886, record that it resulted in about 150 fatalities (e.g., Scott & Nairn, 1989; Ansell & Taber, 1996). Indeed, a figure of 153 deaths is recorded at the excellent new visitors' centre at the Buried Village, Te Wairoa, and in a number of other contexts. Elsewhere, numbers ranging from as few as about one hundred to possibly 'thousands' of deaths have been suggested (e.g., Houghton & Johnston, 1999).

text: How many deaths were caused by the Tarawera eruption? Most texts and brochures describing the Tarawera eruption of 10 June, l 886, record that it resulted in about 150 fatalities (e.g., Scott & Nairn, 1989; Ansell & Taber, 1996). Indeed, a figure of 153 deaths is recorded at the excellent new visitors' centre at the Buried Village, Te Wairoa, and in a number of other contexts. Elsewhere, numbers ranging from as few as about one hundred to possibly 'thousands' of deaths have been suggested (e.g., Houghton & Johnston, 1999).

year: 2001