High strain rate compression of titanium and some titanium alloys

A modified Hopkinson bar was used to compress specimens of commercially-pure titanium, IMI 125 and titanium alloys, AMS 4911B and AMS 4916B, at natural strain rates of between 3 × 10³ s⁻¹ and 3 × 10⁴ s⁻¹. All three materials deformed in a viscous manner with a linear increase of flow stress with strain rate and a macroscopic viscosity of 4·5 kPa s, 5·7 kPa s and 9·2 kPa s, respectively. At strain rates above about 1 × 10⁴ s⁻¹ there was a decrease in macroscopic viscosities to 0·9 kPa s and 2·5 kPa s for the titanium and 4911B alloy respectively, which is attributed to changes in the dislocation drag mechanisms. The results for the 4916B alloy at the higher strain rates were too scattered to give a definite trend. It is suggested that the increase in the macroscopic viscosity with alloying is due to an increase in the dislocation drag coefficient by solution hardening and to a reduction in the density of mobile dislocations by dislocation pinning.

It is also proposed that the susceptibility of these materials to catastrophic shear failure which occurs without prior linear work-softening is due to the higher propensity for titanium and titanium alloys to shear failure as a consequence of their thermo-mechanical properties. The decrease of strain at which shear occurred with increasing strength is in accord with this suggestion.     

Effect of sliding speed on friction and wear behaviour of nanocrystalline nickel tested in an argon atmosphere

The sliding speed dependence of the coefficient of friction (COF) and wear rate (W) of a nanocrystalline (nc) Ni with a grain size of 15 ± 3 nm and a hardness of 5.09 ± 0.11 GPa was compared to that of a microcrystalline (mc) Ni with a grain size of 20 ± 5 μm and a hardness of 1.20 ± 0.05 GPa. The sliding wear tests were performed in an argon environment under a constant normal load of 2 N using three different sliding speeds of 0.2 × 10⁻², 0.8 × 10⁻² and 3.0 × 10⁻² m/s. The lesser wear damage in the nc Ni at any given speed was attributed to its higher hardness and its greater elastic depth recovery ratio compared to the mc Ni. The mc Ni's COFs and wear rates were independent of the sliding speed over the relatively small range used. However, the same small increase in sliding speed caused an 86% reduction in the nc Ni's wear rate, from 3.44 × 10⁻³ to 0.47 × 10⁻³ mm³/m, and a 31% increase in its COF, from 0.49 ± 0.05 to 0.64 ± 0.06. A modified Archard equation was proposed to predict wear rates of Ni as a function of grain size and sliding speed. Increasing the sliding speed made it increasingly difficult for surface damage by plastic deformation to occur in the nc Ni, because the grain-boundary-induced deformation mechanisms are more difficult to operate at higher strain rates. At the highest speed, the smallest amount of debris was generated, which was not sufficient to form protective tribolayers leading to a high COF value.     

The mean dynamic yield strength of copper and low carbon steel at elevated temperatures from measurements of the “mushrooming” of flat-ended projectiles

Mean dynamic yield strengths for copper and mild steel are deduced from strain measurements on the “mushroomed” ends of flat-ended projectiles, after impact on a flat, nominally rigid anvil. The kinetic energy at impact is equated with plastic work, to give a mean dynamic yield strength averaged over the deformed specimen. Experiments are carried out over the temperature range 20–700°C, with impact velocities in the region of 600 ft/sec, giving a mean strain rate estimated at 5 × 10³/sec. The yield stress-temperature results obtained show an abrupt increase in dynamic/static mean yield stress ratio at homologous temperatures, T/T_M, of 0·4 for steel and 0·5 for copper. These results agree generally with the findings from other investigations into high-speed blanking⁵ and indentation.⁷

Existing theories for the mushrooming of flat-ended projectiles^{1, 3} do not predict the profiles actually obtained in the present experiments.     

Dynamic tensile characteristics of TRIP-type and DP-type steel sheets for an auto-body

This paper is concerned with the dynamic tensile characteristics of transformation-induced plasticity (TRIP)-type and dual phase (DP)-type steel sheets at intermediate strain rates ranging from 0.003 to 200 s⁻¹. The dynamic responses of TRIP600, TRIP800, DP600 and DP800 steel sheets are investigated with the evaluation of stress–strain curves, the strain rate sensitivity, the fracture elongation and the effect of pre-strain. The dynamic responses were acquired from dynamic tensile tests at the intermediate strain rates with a high-speed material testing machine developed. Experiments were carried out with specimens whose dimensions were carefully determined by finite element analyses and experiments to induce uniform deformation in the gauge section at the intermediate strain rates. The tensile tests provide stress–strain curves and the strain rate sensitivity. Experimental results show two important aspects for TRIP-type and DP-type steel sheets quantitatively: The flow stress increases as the strain rate increases and the fracture elongation and the formability of TRIP-type sheets are better than those of DP-type sheets at the intermediate strain rates. The pre-strain effect was also investigated for two types of metals at the intermediate strain rates. TRIP600 and DP600 steel specimens pre-stained by 5% and 10% were elongated at the strain rate of 0.003 s⁻¹ for quasi-static loading, and then tested at strain rates of 0.003, 1, 10 and 100 s⁻¹. The results demonstrate that the mechanical properties of TRIP600 and DP600 steels are noticeably influenced by the pre-strain when the strain rate is over 1 s⁻¹. The ultimate tensile strength as well as the yield stress increases due to the pre-strain.     

Mechanical characterization at intermediate strain rates for rate effects on an epoxy syntactic foam

An intermediate strain-rate mechanical testing technique was developed through proper modifications of a hydraulically driven loading frame (MTS 810) and a split Hopkinson pressure bar (SHPB). The modified MTS and SHPB were used to obtain valid stress–strain data for an epoxy syntactic foam at intermediate strain rates in the order from 10⁻¹ to 10² s⁻¹. Additionally, lower and higher strain-rate characterization of the foam material was conducted, such that the compressive stress–strain data of the syntactic epoxy foam were obtained at strain rates from 0.005 to 2150 s⁻¹ without any gap in the intermediate strain-rate range. The syntactic epoxy foam exhibited nonlinear strain-rate dependency of failure strength.     

Repeated point contact loading on Ce-TZP

The deformation and fragmentation behaviour in a toughened ceria-stabilized zirconia ceramic have been investigated by using unlubricated repeated metallic point contact loading at room temperature to explore the possibilities of cyclic fatigue effects. All tests were conducted on a purpose-designed and built computer-controlled apparatus. 120° hardened silver steel cones were cyclically loaded on the polished Ce-TZP substrate, and the damage was observed and analysed as a function of the number of cycles for loads of 19.6 ± 9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the substrate. A tetragonal → monoclinic martensitic transformation occurred in the zirconia beneath, and adjacent to, the contact zone. This transformation zone increased in size as the number of cycles increased, even though there was virtually no change in the diameter of the flattened tip. The expansion associated with this phase change in the zirconia caused granular lifting from the surface, at the edge of the contact zone, that resulted in intergranular fragmentation and spelling of the substrate. The hardness of the substrate in the contact zone increased by approximately 15% after 2×10⁵ cycles. Traces of metal transfer onto the ceramic substrate could be observed only at 2×10⁵ cycles and above.     

Mechanical behavior of erosion-corrosion scales on steels as characterized by single-particle impacts

Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s⁻¹. The steel with higher silicon content showed significantly lower metal loss rates under all conditions.

The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s⁻. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.     

Study of lubricated impact using optical interferometry

An experimental study of lubricated impacts between a steel ball and a flat glass surface has been performed. The experimental set-up consists of a Ø70 mm ball mounted on a pendulum which impacts onto a lubricated glass disc. The contact region is studied by means of optical interferometry using a monochromatic light source, a microscope and a high-speed video recording equipment.

The lubricants are of PAO type and the viscosity ranges from 27 to 2600 mm² s⁻¹ at the test temperature. The impact velocity is varied between 0.08 and 0.29 m s⁻¹.

A dimple occurs at the centre of the contact where the lubricant is trapped. The influence of viscosity and impact velocity on the dimple's depth and diameter is studied.     

Fretting properties of SUS304 stainless steel in a vacuum environment

In this paper the effect of the ambient pressure on the friction and wear of SUS304 stainless steel during fretting at room temperature is described.

The ambient pressure was varied from 10⁻³ to 10⁵ Pa. The experiment was carried out under the following conditions: normal load, 14 N; slip amplitudes, 35 and 110 μm; frequency, 8.3 Hz; number of fretting cycles, 6 × 10⁴.

The relationship between the frictional behaviour and the number of cycles is affected by the pressure. The coefficient of friction at steady state increases with a decrease in the pressure to below 10 Pa. From the point of view of the wear mechanism, the transition is determined as that from oxidative wear at the higher pressure to adhesive wear at the lower pressure. The transition pressure depends on the slip amplitude, i.e.it is 2.7 Pa at 35 μm and 10⁻¹ Pa at 110 μm. The wear volume is greater in the oxidative wear regime than in the adhesive wear regime. Oxide wear debris is removed easily from the interface in the former regime. In contrast, metallic wear debris is retained at the interface and adhesive transfer occurs from one surface to the opposing surface in the latter regime. The characteristic feature of the wear damage at 10⁻³ Pa and at 35 μm is the formation of a vertical crack at the boundary between the fretted and the unfretted areas.     

Preliminary considerations in the use of industrial sonic nozzles

Preliminary considerations applying to the necessary instrumentation for the use of sonic nozzles is reviewed. A way to make sonic nozzles for industry, together with a method to determine the discharge coefficient of a convergent nozzle operating in the sonic regime in conditions of ideal flow is suggested. The results obtained for the behaviour of the discharge coefficient of sonic nozzles, in the range from 4×10⁴ to 2×10⁵ are compared with the standard ISO 9300 (Standard ISO 9300. Measurement of gas flow by means critical flow Venturi nozzles, 1990) and the curve proposed by Ishibashi et al. (M. Ishibashi, M. Takamoto, N. Wanatabe, Y. Nakao. Precise calibration of critical nozzles of various shapes at the Reynolds number of 0.2–2, 5×10⁵. Proceedings of FLOMEKO 94).     

The wear of artificial finger joints using different lubricants in a new finger wear simulator

A new design of finger wear simulator has been manufactured. The simulator is a dual cycle machine, interspersing dynamic flexion–extension motion, where the loads were 10–15 N, with periods of a static 100 N ‘pinch’ load. Also, a two-piece finger prosthesis has been designed and manufactured from silane cross-linked polyethylene. Using the simulator, a comparison of the wear of the cross-linked polyethylene prosthesis was undertaken, with Ringer solution, distilled water and dilute bovine serum as the lubricants. Each test was run at 37°C and included a control prosthesis to account for lubricant uptake. All prostheses came from the same batch, having a gel content of 87%. In total, testing exceeded 27 million cycles. With bovine serum, the total wear factor for the prosthesis was 0.07×10⁻⁶ mm³/N m. With Ringer solution it was 0.98×10⁻⁶ mm³/N m and with distilled water the wear factor was 0.60×10⁻⁶ mm³/N m. This order of results matched that found with pin on plate wear tests using these same three lubricants. The lower wear found with bovine serum may have been due to the positive boundary lubricating effects of the proteins within the serum. Lubricant uptake was greater and more significant in the bovine serum test.     

A portable three-dimensional stylus profile measuring instrument

This paper describes the development of an inexpensive, portable three-dimensional (3-D) stylus-based surface profiler with a scan range of 4.5 mm × 5.5 mm and a maximum vertical range of 150 μm (limited by signal conditioning electronic range) with a resolution of better than 0.9 μm and a range of 30 μm with a resolution of better than 30 nm at the highest vertical sensitivity. The instrument occupies a volume with dimensions of approximately 75 × 75 × 40 mm (L × B × H) with scan speeds up to 0.5 mm s⁻¹. Construction of instrument components from similar material results in a thermally balanced design with a reasonably low thermal coefficient of 0.27 μm K⁻¹ and a first-order time constant of approximately 40 min. The stylus probe sensor has a free resonant frequency of 49 Hz and a low damping ratio of 0.006. When in stationary contact with a steel surface, this increases to above the transducer bandwidth of 900 Hz. Calibration of the probe sensor is achieved through direct comparison against a standard stylus gauge. Lateral calibration of the specimen carriage position has been assessed by the measurement of standard gratings and laser interferometry. Planar errors caused by the motion of the carriage have been assessed by measuring an optical flat and inferring deviation from a perfect plane as an indication of the worst-case error, which in this case, was 0.2 μm (P-V). The design and construction of the internal datum and the portability of the instrument to facilitate in situ measurement of components are emphasized. Images illustrating the surface mapping capabilities of the profiler are presented.     

A comparison of the wear of cross-linked polyethylene against itself under reciprocating and multi-directional motion with different lubricants

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of ‘pin-on-plate’ wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40 N rubbing against XLPE plates. All the XLPE came from the same batch.

In all tests, pin wear was much less than plate wear. Under reciprocation only, the least wear was found when bovine serum was the lubricant (k=0.6×10⁻⁶ mm³/N m) and the maximum wear was when distilled water was the lubricant (k=5.8×10⁻⁶ mm³/N m). When multi-directional motion was applied to the test pins, increased wear occurred under lubrication with bovine serum (k=3.4×10⁻⁶ mm³/N m). Surprisingly, wear decreased when distilled water was used (k=0.7×10⁻⁶ mm³/N m), yet wear factors remained similar in the ‘dry’ test (k=0.7×10⁻⁶ mm³/N m). The dry tests had remarkably low wear.     

HIGH STRAIN RATE SHEAR EVALUATION AND CHARACTERIZATION OF AISI D2 TOOL STEEL IN ITS HARDENED STATE

High strain rate mechanical testing on fully hardened AISI D2 tool steel (at 62 HRc) was performed utilizing the Compressive Split Hopkinson Bar technique (CSHB) incorporating a punching shear strain state. The high strain rate conditions were comparable to those encountered in machining processes, with shear strain rates on the order of 5 × 10⁴ s^-1 and shear strains in excess of unity (100% mm/mm). The tests were performed at various initial temperatures ranging from 296-873 K to investigate the flow stress behavior of the hardened tool steel as a function of temperature. The high strain rate experimental shear stress-strain data was used to fit the flow stress by; i) an empirically based constitutive law in the form proposed by Johnson and Cook; as well as, (ii) a physically based constitutive law proposed by Zerilli and Armstrong which accounts for strain, strain rate, and temperature dependence of flow stress. The data incorporated the adiabatic temperature rise in the shear zone and was used in the constitutive law modeling. The deformed microstructure was investigated using optical and scanning electron microscopy to determine the extent of the shear localization zone and the final fracture mode.     

Tribological behavior of select MAX phases against Al2O3 at elevated temperatures

The layered M_n+1AC_n ternary carbides – MAX phases – Ta₂AlC, Ti₂AlC, Cr₂AlC and Ti₃SiC₂ were tested under dry sliding conditions against alumina at 550 °C and 3 N load (for a stress of ≈0.08 MPa) using a pin-on-disk tribometer. Ta₂AlC and Ti₂AlC exhibited low specific wear rates, SWRs, (≤1 × 10⁻⁶ mm³/N m), while the coefficients of friction, μ, were 0.9 and 0.6, respectively. At 0.4, μ of Ti₃SiC₂ was the lowest measured, but the SWR, at ≈2 × 10⁻⁴ mm³/N m, was high. With a μ of 0.44 and a SWR of 6 × 10⁻⁵ mm³/N m the Cr₂AlC sample was in between. No visible wear of Al₂O₃ counterparts was observed in all the tribocouples. Tribofilms, which were mainly comprised of X-ray amorphous oxides of the M and A elements and, in some cases, unoxidized grains of the corresponding MAX phases, were formed on the contact surfaces. The correlations between observed tribological properties and tribofilm characteristics are discussed.     

Determination of the wear coefficient of tungsten carbide by a turning operation

Turning operation was carried out, by using tungsten carbide inserts and a CNC lathe on low carbon and medium carbon steels, to determine the wear coefficient of tungsten carbide. The nominal (starting) workpiece diameter was 118 mm and the cutting speeds used were 70, 100, 130 and 160 m/min. The thrust and turning forces acting on the insert were measured from a force dynamometer. The turning distance was obtained from the diameter of the workpiece and its rotational speed. Calculations were made on the flank and crater wear volumes using an OMIS machine. The average wear coefficient of tungsten carbide was found to be 10×10⁻⁸. This value was obtained by averaging the wear coefficient values determined from the flank wear on turning both low and medium carbon steels; as well as from a statistical analysis of the wear coefficient values obtained between a turning temperature of 453 and 664°C. As compared with the average value of 106×10⁻⁸ obtained from the moving pin-on-disc test conducted earlier, it is lower by about one order of magnitude. It is suggested that the high turning temperature at the tool-chip interface may have lowered the hardness of the work materials during the turning operation to give the lower wear coefficient values.     

Tribological behavior of aluminum alloys surface layer implanted with nitrogen ions by plasma immersion ion implantation

Some 2014 and 2024 aluminum alloys were implanted with nitrogen ions (N⁺) by Plasma Immersion Ion Implantation (PIII), and dose range was from 2×10¹⁷ to 1×10¹⁸ N⁺ cm⁻². The microstructure of surface layer was studied by Transmission Electron Microscopy (TEM). The depth profile of the implanted layer was investigated by Auger Electron Spectrometry (AES). The wear test was carried on a pin-on-disk wear tester. The micro-morphology of wear was observed by Scanning Electron Microscopy (SEM). The results reveal that: after implanted with nitrogen ions, the friction coefficient of surface layer decreased, and the relative wear resistance increased with the increase of the nitrogen dose. The tribological mechanism was mainly adhesive, and the adhesive wear tended to become weaker gradually with the increase of nitrogen dose. The upper two effects were mainly attributed to the formation of hard AlN precipitation and supersaturated solid solution of nitrogen in the surface layer.     

The wear behaviour of high-chromium white cast irons as a function of silicon and Mischmetal content

The sliding wear behaviour of high-chromium white cast iron (16.8% Cr) has been examined as a function of silicon and Mischmetal alloy additions (1, 2, 3 and 5% Si and 0.1 and 0.3% Mischmetal). Such additions are known to modify the structure, but there is considerable controversy as to the exact effect. Silicon was found to refine the dendritic structure and increased the eutectic carbide volume fraction. However, for contents above 3%, transformation of the austenitic matrix to pearlite occurred in preference to martensite. Mischmetal additions reduced the austenite dendrite arm spacing, but did not have a significant effect on the carbide structure. The wear behaviour was investigated for each alloy in the as-cast (austenitic matrix) and hardened (martensitic) conditions using a block on ring configuration in pure sliding in the load range 42–238 N for a distance of 70 km against a hardened M2 steel counterface. For low loads (42 and 91 N), all the alloys showed a similar wear rate (3×10⁻⁴ to 4×10⁻⁴ mm³/m), associated with the formation of a thin (3 μm) oxide film of Fe₂O₃, the formation of very fine debris and a small depth of deformation below the worn surface (7 μm). For higher loads, wear was a strong function of microstructure, and was associated with a thicker film of the oxides Fe₂O₃ and Fe₃O₄ and greater depths of deformation. The iron with 2% silicon exhibited the best performance with a wear rate of 7×10⁻⁴ mm³/m and this was attributed to its finer structure and the formation of a thicker oxide film. In contrast, the iron with 5% silicon exhibited the worst performance, with a wear rate of 14×10⁻⁴ mm³/m, attributed to the pearlitic matrix. A linear relationship was observed between the depth of carbide fracture and the wear rate. The relationship between microstructure and wear mechanism is discussed.     

3-D analysis of semiconductor dopant distributions in a patterned structure using LEAP

This work presents the first 3-D analysis of lateral dopant diffusion in a patterned structure using a pulsed laser-assisted local electrode atom probe (LEAP). A structure similar to a device channel was created for this work by performing a 3 keV, 1×10¹⁵ cm⁻² As⁺ implant on a poly-Si line patterned wafer with 70 nm line width and 200 nm line pitch. The wafer was subsequently annealed at 950 °C for 1 s. LEAP samples were made using a site-selective in-situ focused ion beam (FIB) process. The results from LEAP analysis were then compared with high-resolution transmission electron microscopy (HRTEM) and Florida object-oriented process simulator (FLOOPS) results. Good structural agreement was found between the LEAP and HRTEM results. Several 1-D As concentration profiles extracted from the LEAP data were also found to be in good agreement with FLOOPS process simulation results. These profiles also represent for the first time that results from a 3-D process simulator have been able to be confirmed experimentally using a single sample.     

Erosion of an aligned alumina-stainless steel composite

Steady-state erosion of a fully aligned composite consisting of a regular array of alumina rods embedded into a 304 stainless steel matrix has been investigated at room temperature using alumina erodents whose mean diameters varied from 37 to 390 μm. Impact angles varied between 15° and 90° with velocities in the range of 60–100 m s⁻¹. All experiments were performed with the axis of the alumina rods perpendicular to the erosion surface. Experiments under identical conditions were performed on bulk samples of 304 stainless steel and on the same type of alumina. The composite results can be described, to a reasonable approximation, using a model whose basis is that constrained steady-state erosion occurs such that the bulk stainless steel erosion rate determines the form of a stationary erosion-surface profile. The physical basis of the constraints are discussed in terms of the erosion mechanisms.